Atmospheric moisture and precipitation

Atmospheric moisture and precipitation

When water vapour is changed into either a liquid or solid, due to air being cooled to its saturation point. The cooling can occur in one of four ways:

  1. Adiabatic or convective cooling: When air that has been warmed in the day rises as 'thermals'. The air expands as it rises which uses energy. This process is known as adiabatic cooling because it is the loss of pressure with height that leads to cooling.
  2. Orographic cooling and frontal uplift: When moist air rises due to crossing a mountain barrier or meeting a mass of air that is colder and denser than itself.
  3. Advection cooling: When warm, moist air is cooled as it crosses over a cooler sea or land surface.
  4. Radiation cooling: When skies are clear at night, the ground loses heat very quickly. As a result, the air directly above the ground also cools quickly. Fog and dew may form.

This is the amount of water vapour in the air. Cold air is unable to hold as much water vapour as warm air, and so is more easily saturated. Relative humidity is expressed as a percentage and is the amount of water vapour that is in the air at a particular temperature against the amount of water vapour that the air could hold. Air is saturated if the RH is 100%.

At the most comprehensive level, precipitation includes sleet, fog and frost, but it is only really rain and snow that make significant contributions to amounts of precipitation.

Rainfall types:

There are three main types of rainfall, all of which are dependent on the following factors:

  1. Air rising and cooling.
  2. Condensation and development of clouds.
  3. Presence of moisture.
  4. Growth of cloud droplets.

The two theories that attempt to explain rainfallformation in more depth are Ice-crystal mechanism and Collision theories.

Convectional rainfall

This is most commonly associated with equatorial areas, where intense insolation occurs. The ground becomes heated to such an extent that the air above it (which is heated by conduction) becomes warmer than its surroundings expands and then rises. As the air continues to rise, it cools further, becomes unstable and large cumulonimbus clouds develop. Convectional thunderstorms are commonly associated with this type of rainfall.

Atmospheric moisture and precipitation

Orographic rainfall

This type of rainfall is also known as relief rainfall and occurs when moist air is forced to rise over a mountain barrier, such as on the western side of the UK. As the air is forced to rise, its ability to hold water vapour decreases and rainfall can result. As air falls back down under the influence of gravity on the leeward side of the mountain, it is warmed and its ability to hold water vapour subsequently increases. This is known as the rain shadow effect and helps to account for the lower amounts of rainfall on the eastern side of the UK.

When moist air is forced to rise over a mountain range, clouds and rain (often heavy) occur:

Atmospheric moisture and precipitation

Frontal, depressional or convergent rainfall

This type of rainfall occurs as a result of the meeting of two different air masses with significantly different characteristics. The lighter, warmer air is forced to rise along a front and steady rainfall results. The process is explained in more detail under the section on depressions.

Snow forms when temperatures are so low that water vapour condenses directly to a solid.

Sleet is formed when there is a difference in temperature between the upper and lower layers of the troposphere. At upper levels, the temperature is below freezing, but closer to the Earth it is slightly above freezing (approximately 2 degrees). The snowflakes that formed in the upper atmosphere melt slightly as they travel downwards.

Hail occurs when raindrops become frozen, often in cumulonimbus clouds.

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