How precipitation forms

Overview

Dew point, stable and unstable air and how orographic or relief precipitation forms.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is precipitation, and in what forms does it reach the ground?

   Model answer

   Precipitation is any form of water, liquid or solid, that falls from cloud and reaches the Earth's surface. It includes drizzle, with droplets under 0.5 millimetres, rain, sleet, which is partly melted snow, snow, and hail, which is layered ice formed in vigorous cumulonimbus updraughts. Dew and hoar frost are not precipitation, because they condense directly onto surfaces rather than falling from cloud.

2. 2

   What has to happen before precipitation can fall?

   Model answer

   Air containing water vapour must be made to rise. As it rises it expands and cools, and once it has cooled to its dew point it is saturated. The vapour then condenses onto tiny hygroscopic condensation nuclei such as dust, salt or pollution particles to form cloud droplets. Those droplets must grow large and heavy enough, by colliding and coalescing or by ice crystals growing at the expense of supercooled droplets, to overcome the updraught and fall. Remove any one of these stages and there is no precipitation.

3. 3

   What is the dew point?

   Model answer

   The dew point is the temperature to which a parcel of air must be cooled, at constant pressure, for it to become saturated, that is for its relative humidity to reach 100 per cent. Cooling it any further forces condensation. Warm air can hold far more moisture than cold air, so it can reach a much higher dew point, but the dew point of any particular parcel depends on how much vapour it actually holds: warm dry air has a low dew point, and saturated air has a dew point equal to its own temperature.

4. 4

   What is relative humidity and why does it rise as air is lifted?

   Model answer

   Relative humidity is the amount of water vapour actually in the air expressed as a percentage of the maximum the air could hold at that temperature. Lifting the air does not change how much vapour it contains, but the air expands and cools, and cooler air has a smaller capacity. The same quantity of vapour therefore represents a larger fraction of a shrinking capacity, so relative humidity climbs until it reaches 100 per cent and condensation begins.

5. 5

   What is the dry adiabatic lapse rate and why does it have the value it does?

   Model answer

   The dry adiabatic lapse rate is the rate at which a parcel of unsaturated rising air cools purely because it expands into the lower pressure around it, with no heat exchanged with its surroundings. It is close to 10 degrees Celsius per 1000 metres. A descending unsaturated parcel warms at exactly the same rate as it is compressed.

6. 6

   What is the saturated adiabatic lapse rate and why is it lower than the dry rate?

   Model answer

   The saturated adiabatic lapse rate is the rate at which a parcel of air that has already reached dew point cools as it continues to rise, and it is roughly 5 to 7 degrees Celsius per 1000 metres. It is lower than the dry rate because condensation is taking place inside the parcel and releases latent heat, which partly offsets the cooling caused by expansion. This is why saturated air, once it starts to condense, can keep rising more easily.

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