The term weather describes the state of the atmosphere at a given point in time and geographic location. Weather forecasts provide an estimate of the conditions we expect to experience in the near future and are based on statistical models of similar conditions from previous weather events. Temperature, amount and form of airborne moisture, cloudiness, and strength of wind are all different components of our weather. Severe weather events such as tornadoes, tropical storms, hurricanes, floods, lightning strikes and extremes of heat or cold can be costly and deadly. [Source: NOAA]

Climate is the long-term prevailing pattern of temperature, precipitation and other weather variables at a given location, described by statistics, such as means and extremes. The climate includes conditions in the atmosphere and ocean, and is often described in terms of the intensity, frequency, and duration of severe and non-severe weather events. Over Earth's history, indications of climate change have been recorded in fossils and ice core samples. Climate change can result in extended periods of heat and drought at one extreme and extensive glaciation at the other. Current patterns in climate data show that our planet's global surface temperature is rising. This change is linked to the dramatic increase in greenhouse gases in the atmosphere that has occurred over the past two centuries. Understanding climatic processes and the resulting impacts of a changing climate are important as every living organism on Earth is affected.

In other words, weather reflects short-term conditions of the atmosphere while climate is the average daily weather for an extended period of time at a certain location. Think about it this way: Climate is what you expect, weather is what you get. Weather is what you see outside on any particular day. So, for example, it may be 75° degrees and sunny or it could be 20° degrees with heavy snow. That’s the weather. Climate is the average of that weather. For example, you can expect snow in the Northeast in January or for it to be hot and humid in the Southeast in July. This is climate. The climate record also includes extreme values such as record high temperatures or record amounts of rainfall. If you’ve ever heard your local weather person say “today we hit a record high for this day,” she is talking about climate records. So when we are talking about climate change, we are talking about changes in long-term averages of daily weather. In most places, weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season. Climate, however, is the average of weather over time and space.

Weather-related disasters cost the world economy an estimated $60 billion a year in the 2000s. All weather is a product of moisture, heat and air pressure. What causes different kinds of weather are the amounts of each and how they mix and react with one another. A basic mechanism behind weather is: Air that is heated becomes less dense and rises. As it rises it cools and the moisture it caries condenses into clouds. If there is enough of it it falls precipitation.

World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA)

Position of the Earth and Weather

Robert Stewart wrote in the “Introduction to Physical Oceanography”: The Earth in Space Earth’s orbit about the sun is nearly circular at a mean distance of 1.5×108 kilometers. The eccentricity of the orbit is small, 0.0168. Thus earth is 3.4 percent further from the Sun at aphelion than at perihelion, the time of closest approach to the sun. Perihelion occurs every year in January, and the exact time changes by about 20 minutes per year. In 1995, it occurred on 3 January. Earth’s axis of rotation is inclined 23.45 degrees to the plane of earth’s orbit around the sun (figure 4.1). The orientation is such that the sun is directly overhead at the Equator on the vernal and autumnal equinoxes, which occur on or about 21 March and 21 September each year. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

The latitudes of 23.45 degrees North and South are the Tropics of Cancer and Capricorn respectively. The tropics lie equatorward of these latitudes. As a result of the eccentricity of earth’s orbit, maximum solar insolation averaged over the surface of the earth occurs in early January each year. As a result of the inclination of earth’s axis of rotation, the maximum insolation at any location outside the tropics occurs around 21 June in the northern hemisphere, and around 21 December in the southern hemisphere.


Wind is simply the air in motion. Usually when we are talking about the wind it is the horizontal motion we are concerned about. If you hear a forecast of west winds of 10 to 20 mph that means the horizontal winds will be 10 to 20 mph FROM the west. Sunlight is the primary energy source driving the atmosphere and ocean. It is also the power behind winds. There is a boundary layer at the bottom of the atmosphere where wind speed decreases as the boundary is approached, and in which fluxes of heat and momentum are constant in the lower 10–20 meters.

Although we cannot actually see the air moving we can measure its motion by the force that it applies on objects. For example, on a windy day leaves rustling or trees swaying indicate that the wind is blowing. Officially, a wind vane measures the wind direction and an anemometer measures the wind speed.

The vertical component of the wind is typically very small (except in thunderstorm updrafts) compared to the horizontal component, but is very important for determining the day to day weather. Rising air will cool, often to saturation, and can lead to clouds and precipitation. Sinking air warms causing evaporation of clouds and thus fair weather.

Causes of Winds

Earth’s orbit around the sun and its rotation on a tilted axis results in some parts of Earth to receive more solar radiation than others. This uneven heating produces global circulation patterns. For example, the abundance of energy reaching the equator produces hot humid air that rises high into the atmosphere. A low pressure area forms at the surface and a region of clouds forms at altitude. The air eventually stops rising and spreads north and south towards the Earth's poles. About 2000 miles from the equator, the air falls back to Earth's surface blowing towards the pole and back to the equator. Six of these large convection currents cover the Earth from pole to pole.

Winds are mainly caused by 1) differences in temperatures of the atmosphere (and corresponding pressure changes); 2) the rotation of the Earth and 3) unequal heating of the oceans and the continents. Heated air become light and expands. Cooler air is denser. Wind are caused when the cool denser air pushes the lighter warm air and causes it be displaced. In this case the wind travels from the cooler air towards the warmer air. This goes somewhat contrary to simple reasoning. You would think that because the warm air is expanding it would displace the cooler air. What happens here is that the warm moves upwards as it expands. the Earth’s rotation causes air current to shift to the right. Mountain ranges, ocean currents and high attitude jet stream winds — that can reach 200 mph — influence winds and air movements.

Small scale localized winds include sea breezes caused when the cooler air over the cooler ocean moves in to the displace the warmer air over the warm land. The directions change at night when the land is cooler and the sea is warmer. In the mountains breezes often head up the slopes in the day as warm arm is displaced upwards.

On a larger scale winds are mostly caused by the movement of air from high pressure areas to low pressure areas. The closer the isobar lines — which measure difference in pressure — on a weather map the stronger the wind. Most of the time this means cooler warm is moving towards hotter air. Occasionally warm air expands into areas of cold air, causing winds to move in that direction.

Isobars, Air, Pressure, Friction and Winds

You have probably seen a surface map marked with H's and L's which indicate high and low pressure centers. Surrounding these "highs" and "lows" are lines called isobars. "Iso" means "equal" and a "bar" is a unit of pressure so an isobar means equal pressure. We connect these areas of equal pressure with a line. Everywhere along each line is constant pressure. The closer the isobars are packed together the stronger the pressure gradient is.

Pressure gradient is the difference in pressure between high and low pressure areas. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest.

Pressure gradient force from high pressure to low pressureAlso, notice that the wind direction is clockwise around the high pressure system and counter-clockwise around the low pressure system. In addition, the direction of the wind is across the isobars slightly, away from the center of the high pressure system and toward the center of the low pressure system. Why does this happen? To understand we need to examine the forces that govern the wind.

There are three forces — pressure, the Coriolis force and pressure — that cause the wind to move as it does. All three forces work together at the same time. The pressure gradient force (Pgf) is a force that tries to equalize pressure differences. This is the force that causes high pressure to push air toward low pressure. Thus air would flow from high to low pressure if the pressure gradient force was the only force acting on it.

So why does air spiral out from highs and into lows? This is where friction comes in. The surface of the Earth is rough and it not only slows the wind down but it also causes the diverging winds from highs and converging winds near lows. What happens to the converging winds near a low? A property called mass continuity states that mass cannot be created or destroyed in a given area. So air cannot "pile up" at a given spot. It has to go somewhere so it is forced to rise. As it rises it cools. When air cools it can hold less water vapor so some of the invisible vapor condenses, forming clouds and precipitation. That is why there is often inclement weather near low pressure areas.

What about the diverging air near a high? As the air spreads away from the high, air from above must sink to replace it. Sinking air warms. As air warms it can hold more water vapor, which means that clouds will tend to evaporate. That is why fair weather is often associated with high pressure.

Coriolis Force

Because of the Earth's rotation, a force known as the Coriolis force, affects the direction of wind flow. Named after Gustav-Gaspard Coriolis, the French scientist who described it mathematically in 1835, this force is what causes objects in the northern hemisphere to turn to the right and objects in the southern hemisphere to turn to the left.

One way to see this force in action is to see what happens when a straight line becomes a curve. Picture the Earth as a turntable spinning counter-clockwise. A ruler is placed over the turntable and a pencil will move in a straight line from the center to the edge while the turntable spins underneath. The result is a curved line on the turntable.

How the corilois force works on the Earth When viewed from space, wind travels in a straight line. However, when viewed from the Earth, air (as well as other things in flight such as planes and birds) is deflected to the right in the northern hemisphere. The combination of the two forces would cause the wind to blow parallel to straight isobars with high pressure on the right.

Air Masses, Fronts and Streams

These global wind patterns drive large bodies of air called air masses. Each of these large bodies of air extends across large areas of the Earth and is thousands of feet thick. The location over which an air mass forms will determine its characteristics. For example, air over the tropical ocean becomes exceptionally hot and humid. Air over a high latitude continent may become cold and dry. You have probably noticed the temperature rapidly dropping on a nice warm day as a cold air mass pushed a warm one out the way. Fronts

The location where two air masses meet is called a front. They can be indirectly observed using current weather maps, which can be used to track them as the move across the Earth. Cold fronts, generally shown in blue, occur where a cold air mass is replacing a warm air mass. Warm fronts, shown in red, occur where warm air replaces cold air. Jet Streams

The local weather conditions that we experience at the Earth's surface are related to these air masses and fronts. However the environment far above us impacts their movement. High in the atmosphere, narrow bands of strong wind, such as the jet streams, steer weather systems and transfer heat and moisture around the globe.

As they travel across the Earth, air masses and global winds do not move in straight lines. Similar to a person trying to walk straight across a spinning Merry-Go-Round, winds get deflected from a straight-line path as they blow across the rotating Earth. In the Northern Hemisphere air veers to the right and in the Southern Hemisphere to the left. This motion can result in large circulating weather systems, as air blows away from or into a high or low pressure area. Hurricanes and nor'easters are examples of these cyclonic systems.

Air Masses

North American airmassesAn air mass is a large body of air with generally uniform temperature and humidity. The area from which an air mass originates is called a "source region." Air mass source regions range from extensive snow covered polar areas to deserts to tropical oceans. The United States is not a favorable source region because of the relatively frequent passage of weather disturbances that disrupt any opportunity for an air mass to stagnate and take on the properties of the underlying region. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.

The four principal air mass classifications that influence the continental United States according to their source region are: 1) Polar latitudes - Located poleward of 60̊ north and south; 2) Continental - Located over large land masses between 25̊N/S and 60̊N/S; 3) Maritime - Located over the oceans between 25̊N/S and 60̊N/S; 4) Tropical latitudes - Located within about 25̊ of the equator.

As these air masses move around the Earth they can begin to acquire additional attributes. For example, in winter an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass (mP) - one that is still fairly cold but contains moisture. If that same polar air mass moves south from Canada into the southern U.S. it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. This is called a continental polar air mass (cP).

The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the summer. Continental tropical (cT) air is dry air pumped north, off of the Mexican Plateau. If it becomes stagnant over the Midwest, a drought may result. Maritime tropical (mT) air is air from the tropics which has moved north over cooler water.

Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.


Fronts are classified as to which type of air mass (cold or warm) is replacing the other. For example, a cold front demarcates the leading edge of a cold air mass displacing a warmer air mass. A warm front is the leading edge of a warmer air mass replacing a colder air mass. If the front is essentially not moving (i.e. the air masses are not moving) it is called a stationary front.

Fronts don't just exist at the surface of the Earth, they have a vertical structure or slope as well. Warm fronts typically have a gentle slope so the air rising along the frontal surface is gradual. This usually favors the development of widespread layered or stratiform cloudiness and precipitation along and to the north of the front. The slope of cold fronts are more steep and air is forced upward more abruptly. This usually leads to a narrow band of showers and thunderstorms along or just ahead of the front, especially if the rising air is unstable.

Cold fronts typically move faster than warm fronts, so in time they "catch up" to warm fronts. As the two fronts merge, an occluded front forms. In the occluded front, the cold air undercuts the cooler air mass associated with the warm front, further lifting the already rising warm air.

Fronts are usually detectable at the surface in a number of ways. Winds usually "converge" or come together at the fronts. Also, temperature differences can be quite noticeable from one side of the front to another. Finally, the pressure on either side of a front can vary significantly.

Here is an example of a location that experiences typical warm frontal passage followed by a cold frontal passage: Clouds lower and thicken as the warm front approaches with several hours of light to moderate rain. Temperatures are in the 50s with winds from the east. As the warm front passes, the rain ends, skies become partly cloudy and temperatures warm into the mid 70s. Winds become gusty from the south. A few hours later, a line of thunderstorms sweeps across the area just ahead of the cold front. After the rain ends and the front passes, winds shift to the northwest and temperatures fall into the 40s and skies clear.


In bad heat waves many of those who are killed are elderly. A heat wave in Guangzhou in July 2004 in which temperatures reached 39̊C, killed at least 39 people, most of them elderly.

A summer heat wave sets in many areas of Japan after the June rainy season is over. In many places it is very hot with little relief many days in a row. Areas near mountains sometimes experience high temperatures associated with the foehn wind effect. The hottest areas in the summer in Japan are on the Japan Sea and in Saitami Prefecture near Tokyo. Heat generated in Tokyo is blown against the mountains in Saitama.

Unusually high summer temperatures have been attributed to global warming, rising air currents and very strong high pressure over the Pacific Ocean. In 2007 high temperatures were blamed on rising air currents created by the La Nina phenomena in the Pacific and rising air currents in India, creating a funneling effect that strengthened the rising air currents over Japan.

When extremely hot days and tropical nights continue for extended periods the asphalt of the roads and walls of buildings do not cool down sufficiently at night, resulting in high room temperatures in office buildings from early in the morning, This boosts demand for air conditioning and electricity.

During heat waves in Japan the sale of air conditioners, beer soft drinks and watermelons increases. Japanese, Chinese and Koreans crave watermelons when the weather is hot and give them as summer present at Bon events. Demand was so strong for watermelons in the heat wave of 2007 that shortages were reported and prices were significantly higher than what they were in 2006 and 2005.

Urban Weather and the Heat Island Effect

Large urban areas suffer from the “heat island — effect. In some cities, temperatures in some cities have increased 5.2 degrees between 1900 and 2000, five times more than global warming. In other cities, the seasons have occurred as many as 20 days earlier than they have in the suburbs.

In the summer the increases have been greatest between midnight and 5:00am, causing more “tropical nights,” and has produced a microclimate that traps air pollution and makes storms more likely downwind from the city. In the winter, it means no snow in places that once had snow. In the autumn, leaves that used to change color in November now change in December. In the spring, flowers bloom when there uses to be snow.

Heat is emitted from air conditioners and vehicles. The highest temperature readings in cities usually occur along streets and are lowest in parks, gardens and rivers.

Large cities act like radiators. During the day asphalt roads, car exhaust, roofs, car bodies and concrete building absorb heat. At night they release it, making the nights much warmer than they otherwise would be. Some cities are particularly bad because they don’t have many trees or parks. Cities near large bodies of water draw in warm moist air in the afternoon as the heat trapped by the city starts to be released and warm air rises. The moist air and the warm city air collide and push other air higher. As it rises it cools and creates clouds and rain. Prevailing winds blow the clouds so that down wind areas of large cities get more rain than areas upwind.

The Japanese are attempted to battle heat island effect by replacing dirt fields in school yards with grass ones and using water-retentive blocks made from crushed, recycled asphalt and concrete as paving material. The water-retentive blocks are just as strong as ordinary paving blocks but reduce heat and produce a cooling effect when water is evaporated from them. Because the blocks are made from recycled material they are also environmentally friendly.

Tokyoites battle the heat island effect by splashing water all over the place on pavement and concrete. Around Tokyo station trees were planted on rooftops, water-retentive pavement has been installed and a building was knocked down to create a wind channel to dissipate some of the heat that builds up there. People living in top floor apartments are combating the “heat island — effect by installing roof-top gardens. New technology, water systems and grass allow gardens to built without damaging existing roofs. City planners have discussed building pipelines under the cities to bring in cool water from local bays. Air conditioner sales have boomed in recent years.


In order for precipitation to form, particularly over a large area, several ingredients are necessary. First there must be a source of moisture. The primary moisture sources in the U.S. are the Atlantic and Pacific Oceans as well as the Gulf of Mexico. Winds around high and low pressure systems (a subject of another lesson) transport this moisture inland.

Once the moisture is in place, clouds still need to form. The most effective way to do this is by lifting the air. This can be accomplished by forcing the air up and over mountains or, more commonly, by forcing air to rise near fronts and low pressure areas.

Cloud droplets and/or ice crystals are too small and too light to fall to the ground as precipitation. So there must be a process(es) for the cloud water, or ice, to grow large enough to fall as precipitation. One process is called the collision and coalescence or warm rain process. In this process, collisions occur between cloud droplets of varying size, with their different fall speeds, sticking together or coalescing, forming larger drops.

Finally the drops become too large to be suspended in the air and they fall to the ground as rain. The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation it is "easier" for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface it may snow, otherwise the snowflakes may melt to rain.

The vertical distribution of temperature will often determine the type of precipitation (rain vs. snow vs. sleet vs. freezing rain) that occurs at the surface during the wintertime. More often than not, the temperature does not decrease with height but increases, many times by several degrees, before decreasing. This increase, then decrease is called an inversion. In winter, an inversion can be critical in determining the type or types of weather.

As snow falls into the layer of air where the temperature is above freezing, the snow flakes partially melt. As the precipitation reenters the air that is below freezing, the precipitation will re-freeze into ice pellets that bounce off the ground, commonly called sleet. The most likely place for freezing rain and sleet is to the north of warm fronts. The cause of the wintertime mess is a layer of air above freezing aloft.

Freezing rain will occur if the warm layer in the atmosphere is deep with only a shallow layer of below freezing air at the surface. The precipitation can begin as either rain and/or snow but becomes all rain in the warm layer. The rain falls back into the air that is below freezing but since the depth is shallow, the rain does not have time to freeze into sleet. Upon hitting the ground or objects such as bridges and vehicles, the rain freezes on contact. Some of the most disastrous winter weather storms are due primarily to freezing rain.


Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice. How fast does hail fall? We really only have estimates about the speed hail falls. One estimate is that a 1cm hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at 48 m/s (171 km/h). However, the hailstone is not likely to reach terminal velocity due to friction, collisions with other hailstones or raindrops, wind, the viscosity of the wind, and melting. Also, the formula to calculate terminal velocity is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere!

How does hail form? There are two ideas about hail formation. In the past, the prevailing thought was that hailstones grow by colliding with supercooled water drops. Supercooled water will freeze on contact with ice crystals, frozen rain drops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

Recent studies suggest that supercooled water may accumulate on frozen particles near the back-side of the storm as they are pushed forward across and above the updraft by the prevailing winds near the top of the storm. Eventually, the hailstones encounter downdraft air and fall to the ground.

Hailstones grow two ways: by wet growth or dry growth processes. In wet growth, a tiny piece of ice is in an area where the air temperature is below freezing, but not super cold. When the tiny piece of ice collides with a supercooled drop, the water does not freeze on the ice immediately. Instead, liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.

Dry growth hailstones grow when the air temperature is well below freezing and the water droplet freezes immediately as it collides with the ice particle. The air bubbles are "frozen" in place, leaving cloudy ice.

Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are "balanced" in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together - forming large and very irregularly shaped hail.

The different ways precipitation is formed determines what type of precipitation it becomes. Hail is larger than sleet, and forms only in thunderstorms. Hail formation requires air moving up (thunderstorm updraft) that keep the pieces of ice from falling. Drops of supercooled water hit the ice and freeze on it, causing it to grow. When the hailstone becomes too heavy for the updraft to keep it aloft, ot it encounters downdraft air, it falls. Sleet forms from raindrops that freeze on their way down through a cloud. Snow forms mainly when water vapor turns to ice without going through the liquid stage. There is no thunderstorm updraft involved in either of these processes.

Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the Earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed!

Winter Weather

Three basic ingredients are necessary to make a winter storm. 1) Cold air — below freezing temperatures in the clouds and near the ground are necessary to make snow and/or ice. 2) Lift — something to raise the moist air to form the clouds and cause precipitation. An example of lift is warm air colliding with cold air and being forced to rise over the cold dome. The boundary between the warm and cold air masses is called a front. Another example of lift is air flowing up a mountainside. 3) Moisture — to form clouds and precipitation. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of moisture.

Snow — Most precipitation that forms in wintertime clouds starts out as snow because the top layer of the storm is usually cold enough to create snowflakes. Snowflakes are just collections of ice crystals that cling to each other as they fall toward the ground. Precipitation continues to fall as snow when the temperature remains at or below 0 degrees Celsius from the cloud base to the ground.

Types of snow eather. 1) Snow Flurries — Light snow falling for short durations. No accumulation or light dusting is all that is expected. 2) Snow Showers — Snow falling at varying intensities for brief periods of time. Some accumulation is possible. 3) Snow Squalls — Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. Snow squalls are best known in the Great Lakes Region. 4) Blowing Snow — Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind. 5) Blizzard — Winds over 35mph with snow and blowing snow, reducing visibility to 1/4 mile or less for at least 3 hours.

Sleet occurs when snowflakes only partially melt when they fall through a shallow layer of warm air. These slushy drops refreeze as they next fall through a deep layer of freezing air above the surface, and eventually reach the ground as frozen rain drops that bounce on impact.

Freezing Rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are "supercooled," they instantly refreeze upon contact with anything that that is at or below O degrees C, creating a glaze of ice on the ground, trees, power lines, or other objects. A significant accumulation of freezing rain lasting several hours or more is called an ice storm.


Snow forms when water vapor condenses into a crystal. Water molecules (H²0) have a V-shape — with the hydrogen atoms forming 104̊-apart Micky Mouse ears on a head of oxygen. When water vapor freezes, the water molecules link up and form “puckered hexagons.” What happens after that depends on temperature, humidity and other factors.

In relatively dry air, the sides of a “puckered” hexagon grow relatively slowly and evenly. In humid conditions, the corners of a snowflake grow more quickly and have larger more elaborate branches. Before a snowflake lands on earth it is blown by winds tossed around by clouds and struck by other snowflakes, all of which affect a snowflakes structure and appearance.

Minutes bumps on the hexagon’s corners attract more and more water (flat surfaces tend to deflect molecules). The bumps grow into branches and they in turn develop their own formations.. Because all six sides are exposed to the same changes, they grow the same way.

A typical snowflake has a billion, billion water molecules. Its shape is unique because each snowflake has followed it own unique route between the cloud, where it forms and its finale destination. On the matter of snowflake uniqueness, Dr. Kenneth Libbrechts, a snowflake expert at Cal Tech, told the New York Times, “We can calculate the odds of identical snowflakes: they are astronomically small.”

Bacteria ride in clouds. A British zoologist once suggested that some bacteria actually trigger clouds to form as a means of distributing themselves.

Solar Dimming Can Trigger Freezing Winters -Study

In 2011, Reuters reported: “A cyclical drop in the sun's radiation can trigger unusually cold winters in parts of North America and Europe, scientists say, a finding that could improve long-range forecasts and help countries prepare for blizzards. Scientists have known for a long time that the sun has an 11-year cycle during which radiation measured by sunspots on the surface reaches a peak then falls. But pinning down a clear link to weather has proved harder. [Source: David Fogarty, Reuters, October 9, 2011]

"Our research confirms the observed link between solar variability and regional winter climate," lead author Sarah Ineson of the UK Met Office told Reuters in an email. The study was published in the journal Nature Geoscience on Monday.Her team focused on data from the recent solar minimum during 2008-10, a period of unusual calm for the sun and intense winters in the United States and parts of Europe that shut down air travel and disrupted businesses.

The researchers found that a reduction in ultraviolet (UV) radiation from the sun can affect high-altitude wind patterns in the Northern Hemisphere, triggering cold winters. "While UV levels won't tell us what the day-to-day weather will do, they provide the exciting prospect of improved forecasts for winter conditions for months and even years ahead. These forecasts play an important role in long-term contingency planning," Ineson, a climate scientist, said.

Ineson and colleagues from the Imperial College London and the University of Oxford used satellite data that more accurately measures UV radiation from the sun and found a much greater variability than previously thought. They found that in years of low activity, unusually cold air forms high in the atmosphere over the tropics. This causes a redistribution of heat in the atmosphere, triggering easterly winds that bring freezing weather and snow storms to northern Europe and the United States and milder weather to Canada and the Mediterranean.

Water Vapor, Humidity, and Dewpoint, and Relationship to Precipitation

Water Vapor: Water is a unique substance. It can exist as a liquid, solid (ice), and gas (water vapor). A primary way water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Winds in the atmosphere then transport the water vapor from one place to another. A major source of water vapor in Kentucky is the Gulf of Mexico. Most of the water vapor in the atmosphere is contained within the first 10,000 feet or so above the Earth's surface. Water vapor also is called moisture.

Absolute Humidity: Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount of water vapor, the higher the absolute humidity. For example, a maximum of about 30 grams of water vapor can exist in a cubic meter volume of air with a temperature in the middle 80s. SPECIFIC HUMIDITY refers to the weight (amount) of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of air). Absolute and specific humidity are quite similar in concept.

Relative Humidity: Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

Dewpoint: Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare. While dewpoint gives one a quick idea of moisture content in the air, relative humidity does not since the humidity is relative to the air temperature. In other words, relative humidity cannot be determined from knowing the dewpoint alone, the actual air temperature must also be known. If the air is totally saturated at a particular level (e.g., the surface), then the dewpoint temperature is the same as the actual air temperature, and the relative humidity is 100 percent.

Relationship of Dewpoint and Relative Humidity to Clouds and Precipitation

If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air). However, for precipitation to occur, the air must be rising at a sufficient rate to enhance condensation of water vapor into liquid water droplets or ice crystals (depending on air temperature) and to promote growth of water droplets, supercooled droplets, and/or ice crystals in clouds. Droplets grow through a process called "collision-coalescence" whereby droplets of varying sizes collide and fuse together (coalesce). Ice crystal processes (including deposition and aggregation) also are important for particle growth. In thunderstorms, hail also can develop. Once the suspended precipitation particles grow to sufficient size, the air can no longer support their weight and precipitation falls from the clouds. In humid climates, thunderstorms often cause heavier rain than general wintertime rainfall since moisture content in the air typically is higher in the spring and summer, and since air usually rises at a much more rapid rate within developing thunderstorms than in general winter systems. "Cloud microphysics" is the study of droplet and ice crystal production and growth within clouds and their relationship to precipitation.

Precipitable Water: Meteorologists are not just interested in dewpoint or absolute humidity at the surface, but aloft as well. Precipitable water (PW) is a measure of the total amount of water vapor contained in a small vertical column extending from the surface to the top of the atmosphere. However, as mentioned above, the majority of moisture in the atmosphere is contained roughly within the lowest 10,000 feet. Precipitable water values around or above 1 inch are common in the spring and summer east of the Rocky Mountains (including Kentucky). Values of 2 inches in the summer indicate a very high moisture content in the atmosphere, typical of a tropical air mass. In general, the higher the PW, the higher the potential for very rain from thunderstorms if they were to develop. However, another very important consideration is not only the amount of ambient moisture in a particular location, but also the amount of moisture advection and convergence which provides additional moisture to an area. If significant, these added factors help explain why rainfall totals from thunderstorms can exceed actual PW values of the air in which the storms are occurring. The movement of thunderstorms, called propagation, also is very important in determining the actual amount of rainfall in any one location. The slower the movement of thunderstorms, the higher the rainfall potential in one area.

Heat Index: While dewpoint is a more definitive measure of moisture content, it is the relative humidity that commonly is used to determine how hot and humid it "feels" to us in the spring and summer based on the combined effect of air temperature and humidity. This combined effect is called the " Heat Index." The higher the air temperature and/or the higher the relative humidity, the higher is the heat index and the hotter it feels to our bodies outside.

Wind Chill Index: In the winter, there is another index we use to determine how cold our bodies feel when we are outside. This is called the "Wind Chill Index" (also known as "Wind Chill Factor"). This index combines the effect of the air temperature with the speed of the wind. When it is cold outside and the wind is blowing, the wind carries away heat from our bodies faster than if the wind was not blowing. This makes it feel colder to us. Therefore, the stronger the wind is in the winter, the colder it feels to us and the lower is the wind chill index.

High humidity/dewpoints in the summer and cold wind in the winter are important because they affect how our bodies "feel" when we are outside. If the heat index is very high or the wind chill index is very low, then we must take safety measures to protect our bodies from possible effects of the weather, including heat exhaustion, sunstroke, and heat stroke in the summer, and frostbite in the winter.

Seasonal Changes

Birds migrate from the North Atlantic to the southern tip of South America. Whales and other marine mammals swim thousands of miles across the ocean. Seasonal changes in precipitation and temperature affect soil moisture, evaporation rates, river flows, lake levels, and snow cover. Leaves fall and plants wither as cold and dry seasons approach. These changes in vegetation affect the type and amount of food available for humans and other organisms. Only with the recent advent of rapid transportation are fresh fruits and vegetables available in grocery stores during the winter in cold regions. Animals do not shop at grocery stores, they must find alternate food sources, move to warmer locations, or hibernate.

As we go about our everyday activities it is not obvious that the Earth is tilted 23.5 degrees on its axis and that we orbit the sun. Nevertheless, these factors result in changes to the distribution of the sun’s energy across the surface of the Earth causing the seasons.

As the Earth orbits the sun every 365 ¼ days, the axis is always pointing in the same direction into space with the North Pole toward Polaris, the North Star. Around June 21st, the northern hemisphere is angled towards the sun, and receives the most direct radiation and the most energy. This is the start of summer in the northern hemisphere and winter in the southern hemisphere. Six months later, in December, the Earth has made half a revolution around the sun. The northern hemisphere is now angled away from the sun and receives less energy than the southern hemisphere; this is the beginning of winter in the northern hemisphere, and summer in the southern hemisphere. From north to south the results of the distribution of solar energy can be seen in the changing vegetation (see the accompanying diagram), animal behaviors, and by examining the clothes people wear.

What are some ways that animals adapt to seasonal changes in your region? How does this compare to other areas? How do different groups of people adjust to the seasonal changes in your region? How do the seasons impact the use of energy in your community? Investigating questions such as these may help provide relevance of seasonal changes for students. Inquiry lessons based around these types of phenomena can be used in any grade level and can help educators differentiate instruction.


The word “monsoon” is derived from “mausim”, the Arabic word for season. It describes climatic changes brought about by a change in wind direction that affects half the world's population. There are two distinct monsoon seasons. the first occurs from roughly June to September when a southwest wind brings moisture-laden air from the Indian Ocean, Arabian Sea and Bay of Bengal to the Indian Subcontinent and Southeast Asia. The same wind also brings in moisture from the Atlantic to Central Africa. The second monsoon occurs from December to February, when a northeast wind brings cold dry air from Tibet and Siberia to India and moist rainy air from the Pacific to the Philippines, Indonesia and Australia. [Source: Priit Vesilind, National Geographic, December 1984]

The monsoon rains are generated by a massive circulation of air caused by scorching summer heat. If there was no monsoon India and Southeast Asia would resemble the Sahara. The process begins when hot air over the land rises. Moisture-laden air over the ocean moves in to takes its place. As this air rises it sheds its moisture on the moisture on the land. The risen air moves back over the sea on wind going the opposite direction of the one bringing air from the ocean. After the moisture laden-air has moved off it is displaced by a third of air arrives from the sea and the whole cycle begins again.

India is well-known for its monsoons. There the monsoon begins in June and ends in August, September or October with most of the rain falling in the first three months.But that isn’t always the case. Some monsoon-drenched regions receive most of their precipitation between mid-October and the end of December. The monsoon generally breaks at the beginning of June on the southwest coat of India, and gradually breaks in all parts of the country, except the southeastern areas.

Rains tend to fall in short afternoon downpours during the monsoon season, when the countryside is lush and green and beautiful but the jungles are full of leeches and dirt roads in remote areas become impassable. Often times the high humidity is more or a problem than the rain. Clothes don’t dry; fungus grows in shoes; with the weather seeming hotter than it really is. There are also more mosquitos and other insects buzzing around than in drier periods of the year.

In much of southern India there are two monsoons: the southwest monsoon from mid-June to early September; and the northeast monsoon from mid-October to the end of the November. The rest of the year is dry with occasional showers. In West Bengal and Bangladesh the monsoon season lasts longer than other other parts of South Asia. It can extend from April to mid-November.

The monsoon season in India more less coincides with the rainy seasons in Vietnam, Cambodia, Thailand, China and Japan but is different from the rainy seasons on the west coast of Malaysia (from September to November) and rainy season in Singapore, Borneo, Indonesia and the east coast of Malaysia (November to January).

The worst time of the year in monsoon-affected countries is in the scorching months preceding the monsoon's arrival. If for some reason the monsoon doesn't come it can mean famine for millions.

Image Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), Wikimedia Commons

Text Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated January 2023

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