Article

Rain

  • Grades: 3–5, 6–8

Q: How does rain form?
A: Water droplets form from warm air as the warm air rises in the sky and cools. Water vapor (invisible water in the air) always exists in our air. Warm air holds quite a bit of water you may have heard people complain about high humidity. When enough of these droplets collect together, we see them as clouds. If the clouds are big enough and have enough water droplets, the droplets bang together and stick like bubble gum, forming even bigger drops. When the drops get heavy, they fall because of gravity, and you see and feel rain. (Al Peterlin)

Q: Why do some areas get more rain then others? Why do some areas get almost no rain?
A: Before it can rain, you must have moisture or humidity in the air. In order for there to be moisture in the air, water must evaporate. The best source for the air's moisture is the oceans and to a lesser extent large lakes like the Great Lakes. Air flows over these large bodies of water picking up moisture as it evaporates off the surface. The air then flows over the land and we feel it in the form of humidity. When air rises, such as up a mountain slope, or when encountering a cold front or warm front, the air cools and the moisture condenses into clouds and rain. Therefore, areas near oceans and lakes, and areas where the air flows off an ocean and up a mountain, are likely to get a lot of rain. If you live in a part of the country that gets a lot of weather fronts coming through, you will have more rain than areas that rarely see weather fronts, such as Arizona. If you live on the downwind side of the mountain, you will not see much rain. We call this a "rain shadow." This occurs because most of the moisture falls out as rain as the air rises up the one side of the mountain. Then, the air dries as it descends down the other side. This is another reason why Arizona and much of the Rocky Mountain region is much drier than other places in the country. (Barbara McNaught Watson)

Q: Why do rainbows form an arch?
A: A rainbow is formed when raindrops reflect (bend) sunlight toward you, breaking the light into colors. The rainbow would actually be a complete circle, but we see only part of it (an arch) because the surface of the earth is interrupting our line of vision. You can do an experiment at school with a prism to show how light can be broken into colors. (Al Peterlin)

Q: We tested the rain in our Lower East Side, New York City, neighborhood last week and were surprised when it tested neutral. We are in a very industrial section of the city too. Why wouldn't we test positive for acid in this part of the country?
A: It is always difficult to explain why something does not happen, but I will at least offer some thoughts and questions.

  • 1. How many samples were taken? Was the equipment calibrated? Were all procedures for proper observation followed?X What history of readings was available?
  • 2. Assuming the answer to #1 is that all observational steps were accurate and verified, there may be some circumstances to explain nonpositive readings.
  • a. What were the low- and mid-level winds, direction, and speed?
    If there was exceptional mixing, there could be minimal particles of acid.
  • b. When in the shower (rain) process was the sample taken? Early in the rain, acidity may be greater. If the sample was taken later in the rain, the rain could have flushed out the pollution early, leaving a neutral sample late.
  • c. Is it possible the factories have closed shop, improved their output (cleaned it up), or reduced their output of pollution?
  • d. It is possible you picked a nonrepresentative period?
  • e. Finally, the pollution problem in your area may not relate to acidity but to some other characteristic. (Al Peterlin)

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