Rain

Rain is water that falls from clouds and reaches the ground. It is the most common form of precipitation, along with snow, sleet, and hail. Rain connects the atmosphere and terrestrial water, and thus forms a crucial part of the water cycle.

Rain determines the life of plants and animals, agriculture, and freshwater availability. In areas where rain is scarce, drought results; in areas of abundant rain, flooding occurs. The distribution of rain across the year is one of the major factors defining the climate of regions.

How rain forms

Water vapor in air cools as it rises. When it reaches the dew point temperature, the vapor condenses on tiny particles suspended in the air (called nuclei), forming minute water droplets. These droplets merge and become heavy enough to fall from the air. Rising air can originate from ground heating (convective rain), from mountains lifting air, or from collision of warm and cold air masses (weather fronts).

Measurement

Rain is measured by the depth of water in millimeters, using a rain gauge or radar and satellite. The millimeter figure indicates the depth of water that would cover level ground if no water seeped or evaporated. Rain intensity, the amount of millimeters falling per hour, is also measured.

Rain in Somalia and the Horn of Africa

Most of Somalia has two rainy seasons linked to the monsoon wind patterns. Gu' is the first rainy season in the middle of the year; Dayr is the second in the latter part. The intervening seasons are called Jiilaal and Xagaa. The distribution of these rains determines pastoral livestock movement and planting seasons; scarce rain brings drought and lower pasture quality.