Rain occurs in the atmosphere when the air cools below its dew point and some of the moisture vapour in the air condenses into tiny droplets. Ordinarily, these droplets are so small they actually form clouds, not actual rain. However, if the air temperature drops a lot, and there is a lot of moisture vapour present, these tiny floating droplets will eventually aggregate together as they are buffeted about, forming larger and larger droplets until they begin to fall, or precipitate. If they form high in the atmosphere and fall a long way through similarly condensed cloud, they will aggregate more droplets as they travel, resulting in heavier rain.

Figure 1 - The after effects of rainfall.

The cooling of air in the atmosphere can occur under a range of different circumstances. One of these, called Convergence, occurs when two air masses moving in different directions meet. As the air in each front can only go up (the Earth gets in the way if it wants to go down), it rises, loses heat and some of its moisture vapour condenses. A similar situation occurs in Orographic rainfall, where an air mass is forced to rise in order to pass over a large hill or mountain range. In most situations, there are valleys between the mountains that funnel the moisture laden air, however in the case where the mountain ranges run parallel to the ocean, for example, the entire air mass has to rise. Rain can also occur due to thermal currents created by differential heating at the surface of the Earth (one side of a hill in direct sun, the other in shade). This causes a plume of warm air to rise, usually creating large billowing cumulonimbus cloud formations.

Air at the top of the plume soon loses its heat energy to the surrounding cold air and precipitation results.

Snow and Ice

If the condensation into moisture droplets occurs when the dew point of the atmosphere is below the freezing point of water, they will form into tiny small ice crystals. As these ice crystals are buffeted around, they too aggregate until they are too heavy to remain afloat and fall to the ground as snow.

Figure 2 - Snow loads can sometimes be an important structural consideration.

Hail

If, however, a cloud of moisture droplets forms and is then further cooled, the tiny droplets do not normally freeze, but become super-cooled at a temperature below freezing but still a liquid. When they contact an airborne solid, such as a dust particle of an already crystallised droplet, they will attach themselves and instantly freeze. This is how hail forms.

Hail is frozen rain drops which are bigger than 5 mm in diameter. Very large hail stones can form if they stay in the clouds for a long time. When they fall they collide with water droplets which freeze and make another layer, they can circulate up and down in the clouds for a long time until they are large enough and heavy enough to form a down-draught. Hail usually forms in cumulonimbus clouds and is more common in areas with warm summers where there is enough heat to trigger the original uplift.

Sleet

Sleet is a mixture of ice and snow formed when the upper air temperature is below freezing, allowing snowflakes to form, and the lower air temperature near the ground is just above freezing, which allows them to partially melt.

Glazed Frost

This is the opposite of sleet and occurs when water droplets form in the upper air which then fall and turn into ice when they come into contact with the ground, forming a layer of ice. This is also known as black ice on roads.

Dew, Hoar Frost and Fog

Dew, hoar frost and radiation fog all form under calm, clear conditions when the Earth is losing a lot of its heat at night. The ground cools the air and this causes the moisture present to condense. If the dew point is above freezing then dew will form. If it is below freezing then a frost will be found. Dew and hoar frost usually occur within 1m of the ground.

If the air is relatively warm and moist, and the ground cools quickly, then a fog can form. It is called a mist if visibility is more than 1km or a fog if it is less. For a fog to develop, a gentle breeze is needed to stir the air and allow the cooling to affect more of the air. These fogs usually occur in valleys and are thickest around sunrise. The water droplets formed in them are small enough to remain floating in the air, but is likely to thicken if a temperature inversion has taken placed and the air is trapped by overlying warmer air.

Measuring Rainfall

Rain is measured using a rain gauge, or more technically, a Liquid Precipitation Accumulation Sensor. This is simply a cup or tube with a vertical scale in mm. The actual diameter of the tube doesn't really matter, as long as it is not too small such that half the raindrops miss it. It is very important, however, that the scale accurately reflect the collection area. There is no point having a huge funnel flow into a tiny tube and finding out that over 1200mm of rain fell in the last light shower.

Figure 3 - An example of a clear plastic rain gauge.

It is also important when taking rainfall readings to take them soon after the rain has fallen in order to avoid losses due to evaporation. There are some more sophisticated rain gauges that use a tipping bucket system to automatically record data as the rain is falling to avoid this problem.