Precipitation

The falling of water from the sky in various forms is known as precipitation.

They're all formed by clouds in the troposphere, which are around 8 to 16 kilometers (4 to 11 miles) above the ground.

Precipitation happens when any form of water particle falls from the upper atmosphere to the earth's surface. Frictional drag and gravity induce the drop to the ground. When a particle falls from the cloud, it creates a turbulent wake behind it, promoting faster and more frequent drops.

Depending on the atmospheric conditions, the crystallized ice may reach the ground as ice pellets or snow, or it may melt and convert into raindrops before reaching the earth's surface.

Rain, snow, sleet, freezing rain, hail, snow grains, and diamond dust are all examples of diverse types of precipitation. They are several types of water that fall from the frozen clouds in the sky.

Process, Forms, and Types 

Process: Condensation of water vapors in the air mass causes precipitation.

Due to adiabatic cooling, the rising air mass containing a significant number of water vapors becomes saturated.

Condensation of water vapors causes clouds to develop. In every cloud, there are updrafts and downdrafts.

The updraft determines cloud growth and height. The stronger the updraft, the taller the cloud. As the liquid water accumulates, the updraft weakens and the downdraft strengthens. Thereafter, Precipitation results.

While all clouds contain water, only some generate precipitation. Precipitated moisture from clouds sometimes evaporates before reaching the earth's surface. Precipitation happens when cloud droplets or ice crystals grow large enough to overcome atmospheric updrafts. It signifies that specific processes are at work in a cloud that produces rain.

These mechanisms can be explained by two processes:

1. Bergeron Process

The clouds contain ice crystals and super-cooled water droplets. When an ice crystal collides with super-cooled water, it freezes the water. This method uses two properties of water.

First property- A cloud's water droplets do not freeze at 0°C but remain liquid until -40°C termed as Super-cooled water If disturbed, super-cooled water will freeze. It needs nuclei to freeze on. But freezing nuclei are rare in the atmosphere. 

Hence, with an increase in temperature, some water droplets will turn into ice. Encouraging the formation of all-ice clouds, a single ice crystal is added to a supercooled water droplet cloud.

Second property- Over ice crystals, the saturation vapor pressure (es) is lower than over water. Between water and ice crystals, a vapor pressure gradient is created. Ice crystals are formed at the expense of supercooled water. When these ice crystals get large enough, they begin to tumble out of the cloud. These ice crystals melt and fall as rain before reaching the ground.

2. Collision – Coalescence Process

This method is suited to clouds whose base does not exceed the freezing point.

Warm clouds are these. These clouds have several various-sized cloud droplets.

Large drops expand at the expense of smaller ones. So they crash with the smaller droplets and form part of it.

Updrafts and downdrafts repeatedly lift and lower cloud droplets in a huge cloud. So these drops grow swiftly. Raindrops must have a diameter of at least 100µ.

Cloud droplets collide to generate the size of a drizzle droplet. More collisions produce larger drops and rain. Small droplets of uniform size may be present in clouds that do not produce precipitation. A condition like this could lead to colloidal stability in the clouds.

Because of the small size of the droplets, a collision may not occur and these cloud droplets can fall slowly and uniformly without colliding. As a result, any clouds that lack the required size of cloud droplets may fail to produce precipitation.

Forms: The different forms of precipitation are as follows:

1. Rain

Rain is a form of precipitation that is in the form of water drops of a size that is larger than 0.5mm. The maximum raindrop size is about 6mm. Drops of larger size break up into smaller drops as it falls down on the Earth’s surface.

Rainfall is the predominant form of precipitation and therefore, the term precipitation is used synonymously with rainfall. The magnitude of the rainfall shows high temporal and spatial variation. This variation causes the occurrence of hydrologic extremes like floods and droughts.

2. Snow

Snow consists of ice crystals in a flaky form, having an average density of 0.1g/cc. It is also an important form of precipitation that usually forms in colder climates and higher altitudes.

3. Drizzle

Drizzle is a fine sprinkle of tiny water droplets that have a size less than 0.5mm and an intensity greater than 1mm/h. The tiny drops that form a drizzle appear floating in the air.

4. Glaze or Freezing Rain

The glaze is formed when rain or drizzle comes in direct contact with the cold ground at around 0 degrees celsius. This water drops freeze to form an ice coating known as glaze.

5. Sleet

Sleet is frozen raindrops that are formed when rainfall passes through the air in the atmosphere at subfreezing temperatures.

6. Hail

Hail is a kind of showery precipitation in the form of pellets or lumps that have a size greater than 8mm. Hail occurs during violent thunderstorms. These are examples of precipitation. Let us now look at what are the different types of precipitation.

7. Sun shower

A sun shower occurs when rain falls while the sun shines.

In the absence of clouds, raindrops fall from the sky when rain-bearing winds blow several miles away.

As a result, a sun shower occurs when a single rain cloud crosses the earth's surface, allowing the sun's rays to flow through. It is usually accompanied by a rainbow.

8. Grains of Snow

Snow grains are tiny white ice grains. Snow grains are flat, with a diameter of about 1mm. They are almost as big as a drizzle. 

9. Diamond Dust

Diamond dust is made up of tiny ice crystals that occur at low altitudes and temperatures. The dazzling effect caused by light reflecting off ice crystals in the air gave diamond dust its name.

10. Ice Crystal

This is a common occurrence in colder regions of the globe. The crystals resemble fog, with water particles freezing into ice. Ice crystals look similar to needles, plates, or columns in shape.

11. Virga

Virga looks like a strip of rain that comes down from the cloud's base and evaporates before it reaches the ground. Dry or warm air is in the way when rain falls through it.

Types of Precipitation: Precipitation occurs when the moist air mass undergoes the process of condensation. This process occurs when the air is cooled and saturated with the same moisture amount. This process of cooling the air mass occurs only when the air mass moves up to the higher altitudes. The air mass can be lifted to higher altitudes primarily by three methods based on which there are three different types of precipitation which are as follows:

• Cyclonic Precipitation

• Convective Precipitation

• Orographic Precipitation

1. Cyclonic Precipitation

A cyclone is a region in the atmosphere that has a large low pressure having circular wind motion. The cyclonic precipitation is caused due to the movement of moist air mass to this region by the difference in pressure. Cyclonic precipitation is two types: frontal and non-frontal precipitation.

• Frontal Precipitation

A frontal is known as the hot moist air mass boundary. This precipitation is caused due to the expansion of air near the frontal surface.

• Non-Frontal Precipitation

This is a cold moist air mass boundary that moves and results in precipitation.

2. Convective Precipitation

The air above the land area gets heated up due to some cause. Most of this warmer air rises up, cools, and precipitates. Convective precipitation is showery by nature. This type of precipitation occurs in varying intensities.

3. Orographic Precipitation

Moving air masses have a chance to strike barriers such as mountains. Once they strike, they rise up causing condensation and precipitation. The precipitation that occurs is greater on the windward side of the barrier when compared to the leeward side of the barrier.