Educational - Cloud formation

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Clouds form when air is cooled below its condensation temperature (dew point). The water vapor in the air condenses around tiny dust or salt particles suspended in the air, forming liquid water droplets, or if the temperature is cold enough, ice crystals (Dunlop and Wilson, 1982, p30). The key to forming clouds is to get air moving upwards. When air moves upwards, it expands and cools, and if the air is lifted high enough, it will cool to its dew point and form clouds. Clouds can be formed through three major mechanisms:

  1. Mechanical lifting by an object such as a mountain. When air moving horizontally hits a mountain, the air is forced upwards. The resulting cooling of the air creates clouds around the mountains.
  2. Radiant heating of the ground by the sun. On clear days, the sun heats the surface of the Earth, which warms the air near the surface. This warm air is less dense than the surrounding air, and rises. As the warm air rises, it expands and cools, forming puffy cumulus clouds (or cumulonimbus thunderstorm clouds) when the moisture in the air condenses.
  3. Moving air masses. When two masses of air of different temperatures and densities meet, the warmer, less dense air is forced upwards, resulting in cooling of the air and cloud formation. The boundary between these two air masses is called a front.


Contents

Appearance of clouds

Clouds appear white because water droplets in the clouds scatter all colors of light equally. Thick clouds can appear dark because not much light can penetrate through them. The color of a cloud depends on the position of the observer in relationship to the sun and cloud. If the observer and sun are on the same side of the cloud, it appears white. When the observer and sun are on opposite sides of a cloud, it appears dark (underneath a large storm cloud). Thin clouds appear bright from below because they are too thin to reflect much light.


Humidity

Water can exist in the air in vapor, liquid, and gas forms. Humidity is a measure of the amount of water vapor in the air. This amount of water vapor the air can hold is related to its temperature. The higher the temperature, the more water vapor can be stored in the air. Relative humidity is measured as a ratio of the amount of water vapor actually present in the air to the maximum amount it can hold at that temperature.


Other Information

Clouds are constantly changing. Water molecules constantly enter and leave a cloud. The molecules enter as vapor, condense into droplets, then evaporate. A good example is the cloud of steam coming out of a tea kettle. The cloud appears in the same place, but the individual water molecules in the cloud are continuously changing.


Above Ground Level

Clouds (Above Ground Level) means that the cloud elevations are above the ground. For example, Broken at 2500 ft means that there are partly cloudy conditions at 2500 ft above the ground.

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