1. Unit Weather
Precipitation

2. What is precipitation?
Precipitation is the release of water from the atmosphere to the earth’s surface as a solid or liquid.
It includes rain, snow, hail and sleet.
Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet.
Approximately 505,000 km3 of water falls as precipitation each year, 398,000 km3 of it over the oceans.
Given the Earth's surface area, that means the globally averaged annual precipitation is 990 mm.

3. Precipitation - Rainfall
Precipitation - rain, snow, sleet and hail - is associated with areas of rising air and low pressure.
When air rises it cools, and the moisture it contains condenses out as clouds, which eventually produce precipitation.
In regions of high pressure, air is descending, the atmosphere is stable, the skies are usually clear, and precipitation is rare.

4. Precipitation – Formation of Rainfall
Air rises
Air expands and cools
Air condenses to form water droplets
Water droplets grow to form raindrops
Air can rise in different ways

5. Precipitation – Formation of Rainfall
For rain to fall, the air must cooled to its dew point temperature such that it becomes saturated (100% relative humidity).
Above this temperature, water vapour in the atmosphere begins to condense around condensation nuclei such as dust, ice, salt or soot to form water droplets.
Water droplets group together to form clouds (a group of visible and tiny water and ice particles suspended above the Earth's surface)
The fall rate of very small droplets is negligible, hence clouds do not fall out of the sky.
When air turbulence occurs, water droplets collide, producing larger droplets. This is called coalescence. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain.
For air to be cooled to its dew point, it must be cooled. There are many mechanisms which can cause this. The three most commonly associated with rainfall cause the air to cool by forcing it to rise.

6. Precipitation – Formation of Rainfall
There are three ways in which air may rise and each one forms a different type of rainfall:
Convectional lifting forms convectional rainfall.
Orographic uplift causes orographic or relief rainfall.
Convergence or frontal lifting causes cyclonic or frontal rainfall.

7. Rainfall Patterns High rainfall: > 1500mm
Moderate rainfall: 251mm – 1499mm
Low rainfall: < 250mm (technically a desert)

8. Precipitation - Hail
Hail falls as hailstones, which are like small balls of ice.
Their diameter usually ranges from <1cm to 5cm.
Larger hailstones can cause damage to crops and gardens, greenhouses and glass roofs.

9. Precipitation - Snow
During the winter months when temperatures are cold, (at or close to 0ºC) snow will fall from clouds instead of rain.
Snow starts off in the same way as rain, with tiny droplets of water joining together in clouds until they are too heavy.
However, when they fall it is important that the air is freezing for there to be snow. Otherwise the snowflakes will melt and become sleet or rain before they reach the ground.
Every snowflake that falls from clouds is different, made from ice crystals and with 6 sides.