What is Precipitation?

Precipitation forms the basis of the water cycle on the Earth. The air is packed with water molecules in the form of moisture or water vapor. This water vapor usually rises to the atmosphere due to density differences. Precipitation represents any form of water, in liquid form or frozen form that forms in the earth's atmosphere. The water vapor up in the atmosphere condenses due to low temperature and it leads to the formation of clouds. The water vapor that forms clouds on account of condensation, is preferably known as cloud droplet. These cloud droplets when become heavy and can no longer sustain the weight of condensed water droplets, they come to the Earth in the form of rain, snow, ice pellets, and hail. Precipitation patterns, evaporation, and condensation are the three parameters that decide the global water cycle, climate change, and weather. Additionally, smoke and dust particles are equally important parameters for precipitation, as they provide the surface area for the water vapors to get condensed and to form water droplets. So they are known as condensation nuclei. The presence of other pollutants in the air also serves as condensation nuclei. Precipitation always results in the pure water supply to earth, as the salt from the seawater does not evaporate with the water.

Precipitation in the form of rainfall
CC BY-SA 3.0 | Image credits: https://commons.wikimedia.org | Saperaud-Commonswiki

Mechanism of raindrop formation

The clouds present in the atmosphere bear water vapor and cloud droplets, these drops are small in size and hence in weight. Due to their small weight, these droplets do not easily fall back to Earth on their own. Due to continuous evaporation and condensation, these droplets form visible clouds which float in the air. But most of these water droplets are suspended in the air in the form of clouds and do not possess the potential to drip down due to large updrafts that hold the clouds. Precipitation events occur when the fall velocity of the water droplets overcomes the updrafts. This occurs when the tiny water drops which are stuck on the surface of the dust and smoke particles collide with each other to form a heavier drop. The collisions should be such that a larger fall velocity is achieved than the updraft of the clouds.

A major cause of heavy rainfall is the raised levels of moisture in the air or the presence of highly moist air. This moist air allows the moisture to move along three-dimensional temperature and moisture zones, preferably known as weather fronts. Due to the presence of greater degree of moisture, the condition permits heavy precipitation, which results in the formation of thunder clouds. Thunder clouds are responsible for higher annual precipitation because of the formation of rain bands in a particular area. Heavy precipitation in mountainous regions is the result of the movement of upslopes that force the moist air to move at higher elevations and fall as rain along the mountainsides.

The Bergeron-Findeisen process

The Bergeron-Findeisen process, also known as Wegener–Bergeron–Findeisen process, is a mechanism proposed by Tor Bergeron and Walter Findeisen, which explains the mechanism of ice crystal growth in mixed-phase clouds. This theory states that ice crystals can grow directly from water vapor deposition. The water vapor may be the result of evaporation of supercooled water and deposition as ice pellets, or the water vapor can be supersaturated with respect to ice. The saturation vapor pressure of water is always greater than the saturation vapor pressure of the ice. This will result in the growth of the ice crystals. This is a rapid conversion of water vapor into ice crystals. This quick change permits additional crystal growth and accounts for the development of large ice crystals with the adjacent formation of supercooled clouds. In less than a minute, these supercooled clouds turn into snowflakes from ice pellets. The weight and density of these snowflakes are high enough to have a sufficient fall velocity regardless of the updrafts.

Ice is the solid form of water and is transparent. It sometimes appears as a bluish tint depending upon the impurities that affect the weather and climatic conditions. Water forms ice when it reaches a temperature of zero degrees, and the molecules of water pack and bind up together due to the fall in temperatures. Snow on the other hand is frozen precipitation that majorly is composed of ice crystals known as snowflakes. Snow is a soft and fluffy white substance that forms when water vapor freezes under extremely cold conditions. Precipitation of snow from clouds results in snowfall. Nimbostratus clouds are responsible for the best snowfall in a particular region depending upon the weather conditions. But all the snow that is composed of ice crystals have similar crystal features, that is, they all have six sides.

Sleets vs Hail

Freezing of liquid water droplets like raindrops results in the formation of small ice pellets, these are known as sleets. Sleets are common during winter storms when during snowfall, the snow melts into water droplets, and the water droplets re-freeze into small ice pellets resulting in sleets. Re-freeze of water drops occurs due to the cold temperature conditions which is a favorable condition for the water to reach its freezing point. On the other hand, hail is common during the non-winter seasons when water drops freeze and grow in size before hitting the ground as hail storms. Hail is also known as frozen precipitation.

Context and Applications

This topic is extensively taught in many undergraduate and postgraduate degree courses of:

  • Bachelors in Civil Engineering
  • Bachelors in Science in Physics
  • Bachelors in Science in Biology
  • Bachelors in Science in Chemistry
  • Bachelors in Geography
  • Masters in Environment Engineering
  • Masters in Science in Geo-technology

Practice Problems

1. Which of the following precipitation is the result of the combination of many ice crystals?

  1. Snowfall
  2. Freezing rain
  3. Rain
  4. Hail storm

Answer: Option a

Explanation: Snowfall is the result of the combination of many ice crystals.

2. The Bergeron-Findeisen process forms the basis of which of the following mechanisms?

  1. Precipitation
  2. Precipitation patterns
  3. Precipitation events
  4. Ice crystal formation and growth

Answer: Option d

Explanation: The Bergeron-Findeisen process is responsible for explaining the mechanism of ice crystal formation and growth.

3. What is the other name given to air if it contains water vapor suspended in it?

  1. Precipitation
  2. Precipitation pattern of air
  3. Moist air
  4. Cloud droplets

Answer: Option c

Explanation: If the air contains suspended water vapor in it, it is known as moist air.

4. What is the name given to precipitation when water re-freezes?

  1. Hail
  2. Snow
  3. Annual precipitation
  4. Sleet

Answer: Option d

Explanation: When the snow melts into water drops, these water drops before falling in the ground re-freeze into tiny ice crystals known as sleets.

5. Which of the following factors results in the initiation of condensation nuclei?

  1. Atmospheric conditions regarding water content.
  2. Atmospheric conditions regarding the content of supercooled liquid drops.
  3. Number cloud droplets.
  4. Presence of dust and smoke particles.

Answer: Option d

Explanation: The presence of dust and smoke particles in the air provides a surface area to allow the initiation of the formation of condensation nuclei.

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