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Predicting Weather Meteorologists are scientists who monitor weather conditions. They sample a wide network of weather stations and use satellite images to map out the positions of the large air masses circling the Earth. Since air masses interact in a relatively predictable way, meteorologists are able to predict weather patterns with some degree of accuracy. As explained above, Fronts are responsible for most changes in weather. They occur when a large mass of cold air meets a large mass of warm air. The front is the zone along which the masses come into contact with each other. Cold Fronts occur when a cold air mass, which has a high density, pushes under a warm air mass. The warm air is pushed upward at a sharp angle, causing moisture to condense rapidly. Heavy precipitation is often the result. Warm Fronts occur when a mass of warm air passes over a mass of cold air at a moderate angle. Geography - While fronts indicate the character of prevailing weather systems, local weather is greatly influenced by geography. Every weather prediction is based on the chance that the weather conditions will act together in a certain way. However, forecasts can be wrong. Some general patterns help meteorologists predict weather for a local area. Most weather conditions across the United States move from west to east. Moving weather conditions are due to wind patterns across the earth. In any given place, a clue to tomorrow's weather may be found by looking at the weather of the area to the west. However, geographic features in your area such as large bodies of water and mountains can affect local weather. For example, if you live near the Atlantic Ocean, conditions to the east often influence your weather. And whether you live near the east coast or west coast, temperatures near the ocean may be higher than they are even short distances inland. The heat-holding ability of large bodies of water causes warming of the air along the coast. In a city such as Chicago, which is located on Lake Michigan, forecasters often give two sets of information: one for people living within 5-10 miles of the lake, and the second for people in outlying areas. In the spring, for example, it's generally warmer in the outlying areas than it is closer to the lake. People who live on the eastern side of a mountain range such as the Sierra Nevadas have mostly dry weather. Even when moisture-laden air masses move into the area, their interaction with the mountains causes most of the moisture to fall on the western slopes. So keep in mind that you can't just look at a weather map and expect to make totally accurate predictions. Lots of factors affect what happens in an area on any given day. For geography, remember these five points: coastal areas have more moderate temperatures than inland areas, and are generally warmer in the winter and cooler in the summer. in hilly areas, hot air moves up the slopes during the day and down the slopes at night. high altitude areas are usually colder and receive more precipitation than low altitude areas. the air above cities is often warmer than the surrounding area. In certain situations, this can create an artificial low pressure system. in coastal areas, cool air usually blows inland during the day and out to sea at night. Tools We Use for Weather Prediction While the National Weather Service uses sophisticated equipment such as Doppler radar and high-altitude balloons to collect data, you can use many of the same tools they use. Barometer Barometers measure the air pressure, which is sometimes referred to as barometric pressure. The pressure of the air on the pool of mercury in the barometer causes the mercury to rise in a tube. We measure the height of mercury in the tube in inches. Therefore, air pressure is often stated in inches of mercury. More common are anaeroid barometers, which don't contain mercury but have a small box inside instead. The air pressure on this box causes it to change shape, moving a needle on a gauge that indicates the air pressure. Normal air pressure readings vary from 28 to 31. Quick changes in air pressure often mean a change in the weather is about to occur. That's why you'll often hear and read about barometric pressure during local weather reports. Anemometer Meteorologists use anemometers to measure wind speed, but you can estimate wind speed just by looking around. Watch how smoke rises in chimneys, how leaves move in trees, and how flags wave in the wind. Sailors and other people sometimes rate their observations of wind speed according to the Beaufort scale. If you don't have an anemometer to measure wind speeds, you can get a good idea of how fast the wind is blowing just by looking at objects around you. In 1805, the British Admiral Sir Francis Beaufort devised an observation scale for measuring winds at sea. The Beaufort Scale measures winds by observing their effects on sailing ships and waves. Beaufort's scale was later adapted for use on land and is still used today by many weather stations. Look at the diagrams below. Each represents one level on the Beaufort scale. Compare the two pictures below of winds at different speeds on water with the Beaufort scale. Wind Vane While anemometers measure how fast the wind is blowing, wind vanes tell you from which direction the wind is blowing. And knowing where the wind is coming from might give you clues to the temperature and the amount of water in the air moving into an area. For example, winds from the south are often warmer and carry more moisture than winds from the north. Psychrometer Psychrometers, or wet bulb thermometers, measure relative humidity. A psychrometer uses two thermometers, one bulb of which is covered with a wet cloth. As the cloth dries, the cooling effect of evaporation lowers the temperature on that thermometer. Then the temperatures on the two thermometers are compared on a special chart to find the relative humidity. Often, the relative humidity is the weather condition that makes people the most uncomfortable. Thermometer Thermometers measure the air temperature via the expansion or contraction of a liquid or a metal as the air temperature changes. Some thermometers contain red-colored alcohol, others contain mercury, while still others have a bimetal coil attached to a gauge. In the United States, temperature is measured using two scales, both of which are based on the state of water at sea level. Most people use the Fahrenheit scale, on which water freezes at 32 degrees and boils at 212 degrees. Scientists and other people who regularly use metric measurements measure temperature on the Celsius scale, on which water freezes at 0 degrees and boils at 100 degrees. Rain Gauge Rain gauges are very simple instruments used to measure the amount of liquid precipitation. Any open container with a flat bottom and straight sides will work just by adding a scale of inches to it. Other kinds of precipitation are usually recorded by collecting the precipitation in a similar instrument, then letting the precipitation melt to find out the liquid equivalent. But at home, you might just want to use a ruler or yardstick to measure a deep snowfall! Then you can use the average conversion of 10 inches of snow equals 1 inch of rain to find out how much water fell.