Context: What do temperature, clouds, winds, and rain have in common? They are all part of weather. Weather refers to the conditions of the atmosphere at a given time and place. Weather occurs because of unequal heating of the atmosphere. The source of heat is the Sun. The general principles behind weather can be stated simply: The Sun heats Earths surface more in some places than others. Where it is warm, heat from the Sun warms the air close to the surface. If there is water at the surface, it may cause some of the water to evaporate. Warm air is less dense, so it rises. When this happens, more dense air flows in to take its place. The flowing surface air is wind. The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall. The water cycle plays an important role in weather. When liquid water evaporates, it causes humidity. When water vapor condenses, it forms clouds and precipitation. Humidity, clouds, and precipitation are all important weather factors. Humidity is the amount of water vapor in the air. High humidity increases the chances of clouds and precipitation. Humidity usually refers to relative humidity. This is the percent of water vapor in the air relative to the total amount the air can hold. How much water vapor can the air hold? That depends on temperature. Warm air can hold more water vapor than cool air. You can see this in Figure 16.1. People often say, its not the heat but the humidity. Humidity can make a hot day feel even hotter. When sweat evaporates, it cools your body. But sweat cant evaporate when the air already contains as much water vapor as it can hold. The heat index is a measure of what the temperature feels like because of the humidity. You can see the heat index in Figure 16.2. Youve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools over night. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the dew point. If this temperature is below freezing, ice crystals of frost form instead of dew. As you can see in Figure 16.1, the dew point occurs at 100 percent relative humidity. Can you explain why? Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere. Clouds that form on the ground are called fog. Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals form instead of liquid water. Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and cumulus. Figure 16.3 shows these and other types of clouds. Cirrus clouds form high in the troposphere. Because it is so cold they are made of ice crystals. They are thin and wispy. Cirrus clouds dont usually produce precipitation, but they may be a sign that wet weather is coming. Stratus clouds occur low in the troposphere. They form in layers that spread horizontally and may cover the entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix nimbo- means rain. Cumulus clouds are white and puffy. Convection currents make them grow upward and they may grow very tall. When they produce rain, they are called cumulonimbus. Clouds can affect the temperature on Earths surface. During the day, thick clouds block some of the Suns rays. This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent 
 Question: type of cloud that forms in low, horizontal layers
Output: stratus