Precipitation and types of rainfall

Precipitation. Continuous condensation makes droplets grow until air resistance can no longer hold them against gravity, and they fall. Release of moisture after condensation is precipitation, liquid or solid.

• Rainfall: precipitation as water.
• Snowfall: when the temperature is below 0°C, moisture falls as fine flakes made of hexagonal ice crystals.
• Sleet: frozen raindrops and refrozen melted snow. It happens when a layer of air above freezing lies over a sub-freezing layer near the ground: raindrops leaving the warm layer freeze in the cold layer and reach the ground as small pellets of ice, no bigger than the raindrops.
• Hail: raindrops released by clouds that freeze into small, rounded pieces of ice as they pass through colder layers. Hailstones have several concentric layers of ice.
• Sleet and hail are limited and occur sporadically.

Types of rainfall by origin (in each, air must rise and cool):

• Convectional rain: heated air becomes light and rises in convection currents, expands, cools and condenses into cumulus clouds. It brings heavy rain with thunder and lightning, but not for long. It is common in summer or the hot part of the day, and very common in equatorial regions and continental interiors, especially in the northern hemisphere.
• Orographic (relief) rain: saturated air meets a mountain, is forced up, expands, cools and condenses. The windward slopes get the most rain. Descending the other side, the air warms and can hold more moisture, so the leeward slopes stay dry: the rain-shadow area.
• Cyclonic (frontal) rain: along fronts in extra-tropical cyclones (see the chapter on atmospheric circulation).

Example (new): Mumbai, on the windward side of the Western Ghats, gets heavy monsoon rain, while Pune, just across the Ghats on the leeward side, gets far less: a rain-shadow town only about 150 km away.