One may think that precipitation is just about rain, but that’s not the case. In simple terms, precipitation is any type of water particles, liquid or solid, that form in the atmosphere and falls on the earth’s surface.
METAR weather codes are an international standard for weather forecasting and reporting used by meteorologists and aircraft pilots.
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Types of Precipitation
Precipitation is of several types and can be grouped into three broad categories, liquid, freezing, and frozen precipitation. Liquid precipitation includes rain, drizzle, and dew. When the rain (or drizzle) occurs under subfreezing conditions, the resultant precipitation becomes known as freezing rain or freezing drizzle.
Under certain atmospheric conditions, precipitation can also fall in the form of frozen particles, such as snow, sleet, graupel, and hail.
A raindrop on a leaf
METAR code: RA
Rain is perhaps the most common type of precipitation that falls on the earth’s surface in the form of water droplets, also called raindrops. The formation of raindrops begins with the absorption of the atmospheric water vapor by a mass of dry air.
As the given air mass absorbs more and more water vapor, it gradually reaches towards saturation point, a phase or stage beyond which it cannot retain any more water vapor. Once the saturation point is achieved, the air mass transforms into a cloud.
How much humidity can any given dry air mass contain, before reaching its saturation point, depends on its temperature. The warmer the temperature, the more water vapor it can hold.
The next phase is called coalescence, in which smaller water droplets combine to form larger ones. Once these water droplets become massive enough to overcome the air resistance on the cloud, they fall as rain. The process of coalescence takes place in clouds with warmer or above freezing temperatures.
The size of raindrops varies from less than 2mm to as much as 9mm. Smaller droplets are almost spherical in shape. However, they become oblate as they grow larger.
In certain hot and dry climates, such as deserts, precipitation falls from clouds but evaporates mid-air before reaching the ground. These clouds often have a funnel-like appearance. This phenomenon is called Virga.
Glaze ice on tree branches in Canada | Image Courtesy: Nicolas M. Perrault
Precipitation is classified as freezing rain (METAR code: FZRA) when the falling raindrops have below-freezing temperatures but are entirely liquid (supercooled liquid). These raindrops then freeze upon impact on any surface, including trees, cars, wires, and aircraft, to form glaze ice.
What Causes Rain?
A. Convective precipitation is a principal cause of rainfall in the tropics. In essence, convection precipitation occurs when a warm air mass, with moisture, is lifted by an underlying cold mass of air. The differences between the two fronts cause weather changes and rain.
B. In many mountainous regions of the world, rainfall occurs due to orographic causes. Such rainfall takes place when a mass of flowing air carrying moisture rises on the windward side of the mountain. As the air mass reaches a certain altitude, it goes through adiabatic cooling, causing the air mass to attain maximum relative humidity. It then leads to precipitation.
C. Air pollutants, such as carbon, sulfur dioxide, and particulates, act as cloud condensation nuclei (CCNs), promote frequent rainfall. Global warming, and, more locally, the urban heat island effect is the principal cause of a drastic increase in rainfall in the urban area.
Drizzle fall in Bournemouth town, England | Image Courtesy: David Lally
METAR code: DZ
Drizzle is another type of liquid precipitation that frequently occurs in the subtropics, mostly in the colder regions. The size of the water droplets in drizzles is smaller (less than 0.5mm) than those of rain.
Drizzle is strongly associated with cumulus and stratocumulus clouds covering large parts of the subtropics and polar regions. Due to its low-intensity nature, drizzles rarely have any significant impact on human society.
Freezing drizzles (METAR code: FZDZ), on the other hand, are far more dangerous. Even a thin layer of ice formation on roads due to freezing drizzle, also known as black or clear ice, can cause accidents and vehicle crashes. The supercooled water droplets present in stratocumulus clouds pose a danger to aircraft as well. By freezing onto the airframe and other expose components, it can significantly degrade the in-flight performance of an aircraft.
There have been a handful of recorded aircraft incidents related to freezing drizzle. One such incident occurred on October 31, 1994, when a domestic American flight carrying passengers from Indianapolis to Chicago crashed in a field near Roseland, Indiana.
It was found out that a severe ice buildup due to a freezing drizzle damaged several critical instruments of the airplane, which eventually led to the crash.
Stranded traffic during 2011 Chicago Snowstorm | Image Courtesy: Victor Grigas/Wikimedia Commons
METAR code: SN (+/-)
Snow is a type of frozen precipitation that falls in the form of ice crystals. It all starts with the freezing of minute supercooled droplets that transforms into ice crystals within clouds. At the time of their formation, ice crystals are no more than 10 micrometers in diameters. Over time, these crystals grow in size of up to a few millimeters by absorbing the surrounding water droplets.
When individual ice crystals fall through precipitation, they often collide and combine with each other to form clusters. These clusters are what we call snowflakes.
Variation of Snow Fall
Based on the intensity and duration, snowfall can be classified as snow flurry (light snowfall), snow shower, snowstorm (medium intensity snowfall), and blizzard. In the United States and Canada, a snowfall is classified as a blizzard if; the sustained wind speed reaches 56 km/h (35 miles per hour), and visibility is reduced to less than 400 meters (0.2 miles).
If the size of the falling snow particles is less than 1mm in diameter, it is classified as snow grains. These small frozen particles are feather-like and flat that do not break after impact.
Apart from the polar regions, snowfalls are prevalent in the mountainous regions of the world. At 48 degree latitude in the Northern Hemisphere, permanent snow cover is found at about 2,500 meters above sea level. In Greenland, located between 58 degrees and 83 degrees Northern latitudes, this level is between 100-500 meters.
4. Graupel or Sleet
Graupel formation (on the right) from snowflake (on the left)
METAR code: GS
Graupel, also known as snow pellets, or soft hail, is a distinct type of precipitation consisting of crisp, fragile, and opaque snowflakes (rime ice) of circular shape. They are usually between 2-5 mm in diameter.
The formation of graupel takes place when ice crystals come in contact with supercooled water droplets at high altitudes. These supercooled droplets are minute, no more than 10 micrometers in diameter, and can remain in a liquid state even at -40 degrees celsius.
Upon contact, the supercooled droplets freeze instantly onto the surface of ice crystals. This formation is known as rimed or rime ice. Once the snow crystals are entirely covered with the formation and losses their original shape, they become known as graupel.
Graupel differs from hail and ice pellets by its shape, size, and the mechanism under which it falls. Hail, which only occurs during thunderstorm or hailstorm, is hard and have a uniform texture compared to fragile graupel.
A large hailstone | Image Courtesy: NOAA
METAR code: GR
Hail is another type of frozen precipitation that falls in the form of relatively large and irregular chunks of ice, known as hailstones. On average, hailstones have a diameter of more than 5 millimeters and are larger than graupel. Large pieces can measure up to 15 cm in diameter.
The formation of hailstones takes place within thunderstorm clouds with below freezing temperature in higher altitudes. Large hailstones are composed of multiple layers of ice of varying thickness. This is due to the movement of hailstones inside the cloud, driven by a strong updraft wind.
As the hailstones move, if they come in contact with a region inside the cloud with a high concentration of supercooled water droplets (which freezes on the hailstone’s surface), they acquire a new, thick layer. On the other hand, hailstones acquire a thin and glassy layer of ice when they move through areas with high water vapor content within the cloud.
Hailstones are most frequent along the mountain ridges in the mid-latitudes, between the tropics and the Arctic Circle (Northern Hemisphere).
Most of China’s southwestern region receives intense hailstorms that often lead to deaths and property damages. In the United States, a vast region extending from Wyoming to Texas experiences periodic thunderstorms and hail precipitation. This region is otherwise known as “Hailstorm Alley.”
Other Types of Precipitation
Megacryometeor is a rare type of precipitation that occurs under strange atmospheric conditions. It is similar, in many ways, to large hailstones. They can weigh anywhere between less than a kilogram (about 1 pound) to several kg.
The largest megacryometeor on record, which weighed nearly 400 kilograms (881 pounds), was found in 2004, near the city of Toledo, Spain. Another massive chunk, with over 200 kg in weight, was found in Brazil in 1997. Due to their size and impact on the earth’s surface, megacryometeors are often confused with meteors.
Unlike other precipitation types, the exact mechanism behind the formation of megacryometeors is still unclear. Studies, however, have shown certain similarities between them and hailstones.
Frequently Asked Questions
What Causes Precipitation?
In simple terms, precipitation occurs when water droplets or ice crystals inside clouds become large enough that air underneath them can no longer hold their weight, resulting in fall.
In most cases, particularly in high altitudes, precipitation begins as snow (as water droplets freeze in freezing temperatures). However, when the snow crystals fall through a warmer air mass, they become raindrops.
Is Fog a Precipitation?
Fog in London | Image Courtesy: George Tsiagalakis/Wikimedia Commons
Rather than precipitation, fog is a low-lying cloud that carries minute water droplets and even ice crystals. They can produce light precipitation in the form of drizzle and snow.
Fogs appear under extremely humid conditions when gaseous water, or water vapor, condenses into tiny droplets. For water vapor to condense in the atmosphere, the presence of tiny solid particles, such as dust, sea salt, or impure carbon (cloud condensation nuclei) is required. Fog can be classified into different types, including radiation, advection, valley, and freezing fog.
How is Precipitation Recorded or Measured?
The traditional method of measuring liquid precipitation uses a rain gauge, a small cylindrical vessel with an open upper end, and an attached scale. Rain gauges are of different types, including tipping-bucket type, recording weighing type, and non-recording cylindrical type.
A weather radar display | Image Courtesy: NOAA
Another, more sophisticated method of measuring precipitation involves the use of radar technology. By sending and listening to backscattering signals from the atmosphere, weather radars can indirectly determine the type, particle size, and quantity of the precipitation. Most modern weather radars are pulse-Dopplers.
One such instrument is the Cloud Profiling Radar, a component of NASA’s CloudSat Earth observation satellite.