PRECIPITATION: TYPES, HOW IT FORMS, EXTREMES

PRECIPITATION

Types of precipitation

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

Severe storm that dropped large hail
in southern Texas on March 2, 2023 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

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.

Monsoons

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.

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