6.1: Introduction
- Page ID
- 46231
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Cumulus clouds (Photo by Shintaro Russell).
What is a cloud and how does it form?
A cloud is a collection of suspended particles of water droplets and/or ice crystals in the atmosphere. The cloud droplets or crystals are so small that their terminal velocity, the highest velocity possible as an object falls through a fluid, is negligible. They are falling but they fall so slowly that they appear to be suspended in the air.
Recall from Chapter 4, the Clausius-Clapeyron diagram. The line represents the vapor pressure at saturation for a given temperature. The region to the right of the line represents air that is sub-saturated, and the region to the left of the line represents air that is super-saturated. Sub-saturated air has a relative humidity below 100% and super-saturated air has a relative humidity above 100%.
Imagine a point on the right side of the line, at \(T=75 { }^{\circ} C\), and \(e_0=12.5 kPa\). To become saturated, the air either needs to cool with the same moisture content (move left on a constant e0 line), or increase the moisture content (move upward on a constant T line) until the air meets the saturation curve. To put this another way, we know that clouds form when the atmosphere is saturated. The list below describes the three ways air parcels can become saturated.
Clouds can form from
- Adding Moisture (or mixing with cool moist air)
- Sea smoke
- Contrails
- Cooling towers
- Exhaling
- Cooling by Removing Heat
- Radiation fog: clear nights on land
- Advection fog: air moving over cold ocean currents
- Cooling by Adiabatic Expansion
- Upward air motion
- Vortices, like wing tip vortices on aircraft, or tornados
- Supersonic flight
Saturation is typically achieved by either adding moisture until the dew point temperature is equivalent to the temperature or cooling until the temperature is lowered to the dew point temperature. In some cases, both moisturizing and cooling can happen at the same time. The mixture of two unsaturated air parcels can even cause saturation in the resulting mixture. For example, a person’s breath on a cold day or jet contrails can both form clouds because of this mixing process.