WEATHER AND CLIMATE: WINDS, AIR MASSES, FRONTS

WEATHER AND CLIMATE

types of clouds

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 Seasons

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.

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.

Winds

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 Jet 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 are fast-flowing, narrow rivers of air high in the atmosphere that blow from west to east. The Earth has two main types: the polar jet stream and the subtropical jet

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

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.

Heat

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.

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