Weather for Kids
Subject: Clouds
A cloud is a visible accumulation of minute particles of water or ice, or of both, in the air. This gathering of particles may also include larger solid particles, such as those present in fumes, smoke or dust. The appearance of a cloud may be described by its dimensions, shape, structure, texture, opaqueness and color. Clouds come in many sizes and a seemingly endless variety of shapes. They may display a very threatening appearance or be exceptionally beautiful and peaceful.
Clouds are usually formed when moist air is pushed upward and cooled. This is because cool air holds less water vapor than warm air, so the vapor condenses into either liquid or ice. These condensed particles are what we see as the cloud. The cloud is, however, mostly air - the water droplets and ice particles often make up as little as one millionth of the volume of the cloud! When a cloud forms at relatively warm temperatures, the particles usually consist of tiny liquid drops. At very low temperatures, the particles are usually ice crystals.
The most common way to form a cloud is to force air to rise and cool. What pushes moist air upward in the first place? A difference in temperatures can be a mechanism to force air to rise. As sunlight warms the surface of the Earth, the air closest to the ground warms as well. Warmer air is less dense and more bouyant so it tends to rise. This is also called an updraft. Updrafts are associated with lower pressures: you can think of a high pressure system acting as a 'lid' which holds air down, while low pressure allows air to flow upward.
Another method to form a cloud is when air encounters a mountain, forcing the moisture in the air to lift upward. The wind becomes redirected up the mountain side. A third way to develop a cloud is when a large region of moving air (what meteorologists call an air-mass) runs into a cooler air-mass. The cooler air is denser, and tends to stay near the ground. The warmer, lighter air rides up and over the cooler air-mass forming clouds.