Precipitation Formation: Part One

2011-11-16 17:06:58.000 – Rick Giard,  Weather Observer / Education Specialist

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Last week the general theme for these remarks was cloud types. This week we shall focus on the most important element of weather produced by clouds, namely precipitation. This element puts the ‘meteor’ in meteorology. The technical term given to precipitation is the ‘hydrometeor’ – particles of water in various forms that fall from the sky. If you have visited Mount Washington at any time of year, you know that the summit receives a great deal of precipitation in every conceivable variety. Have you contemplated how precipitation forms in the first place? To understand how water can be transformed into clouds and precipitation, we first need to know more about the physical characteristics of moist air. For, as with most atmospheric phenomena, there is much more than meets the eye.

Of the ingredients creating weather, water is the most essential. Water, combined with energy from the Sun, produces all the weather we experience. All air in the troposphere, that layer of the atmosphere closest to the surface where weather occurs, contains a certain amount of water. Even the driest desert air holds a substantial quantity of water in the form of invisible gaseous water vapor. The exact amount varies from approximately 1% to 4% of air by volume. Evaporation into the air from various surface sources increases water vapor content, and the amount present tends to decrease with altitude.

Importantly, warm air has a greater capacity to hold water than cold air, and as the temperature decreases the air comes closer to the maximum amount. Relative humidity (RH) is the ratio of water content to capacity expressed as a percentage. RH can be increased by either adding more water vapor to the air, or by cooling the air to reduce capacity. When content equals capacity of the air at a given temperature the RH is 100%, and we refer to this as saturated. The temperature at which saturation occurs is the dew point. At this stage the invisible water vapor begins condensing onto microscopic hygroscopic (water-attracting) particles known as cloud condensation nuclei, forming the first visible liquid droplets and creating a cloud.

If clouds are made of water, then why does precipitation not fall constantly from all clouds? Actually, cloud droplets are far too small to fall, and tend to remain suspended. The typical radius of cloud droplets is only about 20 micrometers (1 micrometer = one-millionth meter). This is approximately one million times less volume than a typical raindrop! The gravitational force acting to pull droplets downward increases with mass or size [F = m x g, where m = mass, g = gravitational acceleration]. As a droplet grows the gravitational force increases. The primary factor that determines whether a droplet will precipitate to the surface is size. It must be large enough to travel the distance from cloud base to ground before evaporating. Smaller drops tend to evaporate before reaching the ground, a phenomenon known as virga. Larger drops may survive to hit the ground.

Next time we will talk about some specifics of precipitation formation processes. (Clue: It’s much more than cloud droplets bumping into each other!)

Rick Giard,  Weather Observer / Education Specialist