Temperature: Temperature is a measure of the heat in the atmosphere . It can be hot or cold and is measured in Fahrenheit (the main system used in the United States) or Celsius (used by most other parts of the world and by scientists). Temperature is relative and ever changing. Weather is warmest near the earth’s equator and coldest at the North and South Poles. Atmospheric pressure: Atmospheric pressure refers to the weight that the earth’s atmosphere is exerting. When you tune into the weather forecasters on television, they often talk about high-pressure and low-pressure systems. High-pressure systems usually clear out the skies and bring cooler temperatures, while low pressure systems generally bring rainfall and warmer conditions. Atmospheric pressure is measured in units called “millibars of mercury.” At sea level, atmospheric pressure is standardized to one “atmosphere” or about 1,013 millibars of mercury. A low-pressure systemmeasures 995 millibars, while a high-pressure one measures about 1,030 millibars. Atmospheric pressure and altitude have an inverse relationship; at high altitudes, atmospheric pressure is much lower. For example, at the top of Mount Kilimanjaro, the atmospheric pressure is just 40 percent of the pressure at sea level. Mount Kilimanjaro is a dormant volcano in Tanzania and the highest mountain in Africa at 19,341 feet (5,895 meters) high. Wind: What is wind? Certainly, it is the movement of air, but how does wind form? When there are differences in temperature and atmospheric pressure, wind forms. Wind develops in areas of high atmospheric pressure and blows to areas of low pressure. The upper atmosphere contains strong winds called “jet streams,” bands of very strong air that encircle the earth several miles above. These strong, fast winds occur five to nine miles above the earth and can reach 275 miles per hour. Jet streams push weather systems around the earth. Humidity and precipitation: Humidity is a measure of the amount of water vapor in the air. What is water vapor? Water becomes vapor in the atmosphere. In its gaseous state, vapor helps


The Fundamentals of Weather

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