The following questions were answered by meteorologists Barbara McNaught Watson and Al Peterlin.

Q: Why do different clouds form at different heights in the atmosphere?
A: The characteristics of clouds are dictated by the elements available, including the amount of water vapor, the temperature at that height, the wind, the interplay of other air masses, mountains, etc. (Al Peterlin)

Q: Why do clouds float?
A: A cloud forms when air heated by the sun rises like a balloon. As it rises, it slowly cools until it reaches the saturation point and water condenses, forming a cloud. As long as the cloud (and air its made of) is warmer than the outside air around it, it "floats."

There are some other ways to make air rise until a cloud forms.
  1. A mountain will force upward the wind blowing against it.
  2. A weather front will act the same way: Cold air is heavy and it forces warmer air upward.
  3. Winds colliding at the surface, like two freight trains, have no where to go but upward. We call this convergence.
All these mechanisms help lift air upward until a cloud condenses. (Al Peterlin)

Q: Why do clouds turn gray before it rains? What makes clouds form rain?
A: Clouds are made up of tiny water drops or ice crystals — usually a mixture of both. The water and ice scatter all light, making clouds appear white. If the cloud gets thick enough (high enough) all the light above does not make it through, hence the gray or dark look. Also, if there are lots of other clouds around, their shadow can add to the gray or multicolor gray appearance.

Cloud drops form when warm air cools and the water in the air condenses. With lots of drops and movement (winds and currents) the drops bump and merge until heavy enough for gravity to pull out the drops as rain. (Al Peterlin)

Q: Why do clouds turn black?
A: All clouds are white, meaning the water droplets that make up a cloud scatter light in all wavelengths so they combine to produce white light. Clouds appear dark if they are in the shadow of other clouds or, sometimes, if the top of the same cloud produces a shadow on itself. There is also a darker look to some clouds if the background color is bright sunlight, making a great contrast. (Al Peterlin)

Q: How does a nimbostratus cloud form and what kind of weather does it bring?
A: A stratus cloud is one that is layered or stratified — looks like a sheet. Nimbus before or after the word means a cloud that is precipitating or raining. So, nimbostratus designates a low, layered cloud producing rain. (Al Peterlin)

Q: What would it look like if an altocumulus and altostratus cloud mixed? What type of weather would that bring?
A: Alto is a descriptor meaning "middle," and indeed altocumulus and altostratus clouds appear in the atmosphere between 6,000 and 20,000 feet. Stratus are layered clouds, generally depicting some stability or calming in the atmosphere. Cumulus are the dynamic, fluffy, and turbulent clouds generally thought of as part of a thundershower atmosphere. Indeed, in a dying thunderstorm there can be a mixture of clouds including these two. There can also be a mixture such as this with an occluded front. (See the question about occluded fronts.) (Al Peterlin)

Q: I'm just starting a weather unit in which I am asking students to record data. One of the recordings is to state the cloud cover. What is your opinion of how I should ask them to respond — clear, partly cloudy, etc.? I eventually would like to be able to identify various cloud formations, but am presently not very informed myself. Any suggestions on learning the formations and how to teach sixth and seventh graders this material?
A: Cloud descriptions can become a fun project. There are several aspects to consider. Students can use their observational perspective to break cloud coverage into several groups:

  • Clear or Sunny: lots of sun and stars, 3/4 or more open sky, 1/4 cloud coverage or less
  • Partly cloudy, partly sunny: 1/4 to 3/4 clouds
  • Mostly cloudy: 1/4 or less open sky
This can lead to teaching perspective, including the flat earth compared to round, high clouds and low clouds. Most people can fit observations into the three groups noted. Later you can have them judge the height of clouds low (close to the ground), middle, or high. A discussion of fog (cloud on the ground) and whether fog should be considered a cloud might follow. Finally, you can name the clouds by considering shape, height, and storminess. You bring in the Latin names here for fun. This is a great observational project. (Al Peterlin)