The daily, temporary state of the atmosphere over any place on the Earth's surface is known as the weather there. The average of weather conditions that have prevailed over a given area during a long time period is called its climate. The scientific study of weather is known as meteorology. ElementsThe basic conditions in the atmosphere that make up weather include wind, pressure, temperature, humidity, clouds, and precipitation. These weather elements can be studied scientifically in order to predict, or forecast, the weather. WindWind is the movement of air parallel to the Earth's surface. Winds are named according to the direction from which they are blowing. The direction of a wind is significant in weather forecasting. In the Northern Hemisphere, for example, northwest winds usually indicate colder, drier weather. Easterly winds mean cloudiness and precipitation, and southerly winds mean warm, humid weather. Atmospheric pressureAtmospheric pressure is the weight of air above a given area on the Earth's surface or within its atmosphere. Winds blow from areas of high pressure toward areas of low pressure. Falling pressure generally indicates an approaching storm, with winds rushing in from high-pressure areas. Rising pressure indicates the approach or continuation of fair weather. HumidityClouds, fog, rain, and snow are made from water vapor (water in its gaseous state) in the air. The amount of moisture in the air is known as humidity. Warm air can hold more moisture than cold air can. Relative humidity is the amount of water vapor present in the air at a given temperature as compared with the maximum amount that the air could hold at that temperature. It may vary from almost none over deserts to as much as 100 percent in thick fog or rain. CloudsClouds form when cooled water vapor forms tiny but visible water droplets or ice crystals. Dew point is the temperature at which water vapor cools enough to form liquid water. This process is called condensation. Fog is a cloud formed on the ground when moist air cools below its dew point. Dew forms when moist air meets a surface such as grass that has been cooled below the air's dew point. When the temperature is below freezing, frost forms instead of dew. PrecipitationWhen moist air rises to cooler heights, it condenses and forms water droplets. Depending on other conditions, the water falls as rain, hail, sleet, or snow. This process is called precipitation. Rain is precipitation of liquid water drops. StormsA storm is a disturbed state of the atmosphere. It is marked by strong winds, rain, sleet, hail, and lightning. Thunderstorms, windstorms, and cyclones are different types of storms. ThunderstormsWhen hot, moist air is carried to great heights by the strong updraft in a rain-bearing cloud, the water vapor condenses. Then heavy rain, thunder, and lightning occur along with strong wind, and sometimes hail. This is a thunderstorm. WindstormsMeteorologists and mariners consider winds to be at storm strength when they reach a speed of 73 miles (117 kilometers) per hour. Windstorms, or gales, have little or no precipitation. SandstormsSandstorms and dust storms are dry windstorms. Sandstorms are associated with desert areas. They occur when high winds lift particles of sand into the air and drive them with great force. Cyclones, hurricanes, and tornadoesThe word cyclone or hurricane is frequently used for any wind with a speed more than 75 miles (121 kilometers) per hour. A cyclone is a storm or system of winds that blows toward an area of low air pressure, over which they rotate in an inward spiral. Cyclones often cover thousands of square miles. The swift winds often bring heavy rain. In the tropics, cyclones are very destructive. Tropical cyclones always start over an ocean and usually move across areas of warm water, absorbing energy and moisture along the way. They sweep over islands and peninsulas and frequently skirt along coastlines. The Pacific Ocean, the Gulf of Mexico, and the Atlantic Ocean are the main sources of moisture for tropical cyclones that hit the United States. In Atlantic waters these cyclones are called hurricanes. In the western Pacific, they are called typhoons. A tropical cyclone has a relatively quiet, cloudless center known as the eye. While the storm itself may be 60 to more than 1,200 miles (100 to 2,000 kilometers) in diameter, the eye is usually 12 to 62 miles (20 to 100 kilometers) across. Violent winds and heavy rains mark tropical cyclones. The storm's first winds and rain are suddenly followed by calm, clear skies. This is when the eye is passing overhead. However, within an hour or two, the opposite side of the storm hits, bringing winds and rains again. A tornado is a violent, inland cyclonic storm. The tornado has a funnel-shaped trunk that reaches down from a dark thundercloud and whirls at speeds up to 300 miles (482 kilometers) per hour. Tornadoes can tear the roofs from houses. AnticyclonesThe anticyclone is the reverse of a cyclone. It is a system of winds that rotates around a center of high atmospheric pressure. The winds spiral outward. Anticyclones usually originate in high latitudes and are usually associated with dry, cool weather. In North America, anticyclones have carried cold air as far south as the Gulf of Mexico and into Florida. Weather instrumentsThere are different instruments to measure weather conditions. Surface wind speeds are usually measured with an anemometer. A vane, which is a pointer that swings with the wind, shows wind direction. An aneroid barometer measures atmospheric pressure. This is a box that expands or contracts with changes in pressure. A mercury barometer is a glass tube in which the height of a column of mercury varies with pressure changes. It also measures atmospheric pressure. A thermometer measures temperature. It is a glass tube in which the height of a column of mercury or alcohol varies with changes in temperature. Observation and predictionThrough the ages, weather forecasts were based on local observations made by the human senses. Accurate measurements of temperature and atmospheric pressure were possible only after the thermometer and the barometer were created in the 17th century. The invention of the telegraph in the 19th century made possible the rapid collection and broadcasting of weather observations. In modern times, rockets and satellites collect weather information. Weather satellites have made it possible to detect storms from the time they begin. The first of these weather satellites was the Television and Infra Red Observation Satellite (Tiros) in 1960. The preparation and study of weather maps is one of the most common methods of weather forecasting. A weather map is a chart that shows the weather elements at a given time over a large area. Until the 1960s weather maps were drawn by hand and analyzed at each local weather office. Computer-drawn maps now predict wind, temperature, and humidity patterns. The weather observations used for weather maps are made at thousands of weather stations around the world. |