Will you need your snow boots tomorrow? Should you bring an umbrella? Accurate weather predictions are important for planning our day-to-day activities. Farmers need information to help them plan for the planting and harvesting of their crops. Airlines need to know about local weather conditions in order to schedule flights. Weather forecasting helps us to make more informed daily decisions, and may even help keep us out of danger.
Modern weather forecasting involves a combination of computer models, observation, and a knowledge of trends and patterns. Using these methods, reasonably accurate forecasts can be made up to about five days in advance. Beyond that, detailed forecasts are less useful, since atmospheric conditions such as temperature and wind direction are very complex.
Most of the computer models used for forecasting are run by the National Weather Service, which creates forecast models based on complex formulas. These models are used by many different weather and news services in preparing daily forecasts. Local weather observers, balloons, satellites, and weather stations also help provide data for forecasts.
You don’t need to have a supercomputer or weather balloon to try your hand at forecasting, though. The most basic weather forecasting consists of simple observation. For example, you can look up at the clouds and try to recognize telltale patterns as people did in the past. High, wispy clouds usually presage good weather. An overcast sky means rain or snow is on the way. Certain weather features seem to be associated with certain types of weather, at least most of the time.
Weather wisdom and weather lore
Forecasting has a long history. Most early forecasting was based on observation of weather patterns. Over the years, the observation of weather patterns has resulted in folk wisdom about the weather, a good deal of which is inaccurate, but some of which is supported by science. You’ve probably heard the expression, “Red skies in morning, sailor take warning; red skies at night, sailor’s delight.” A red Sun can presage rain, since it occurs when the air is full of dust and water vapor. Though the rhyme itself may not be 100 percent accurate, the observation is based on weather patterns.
You may have been raised to believe that groundhogs can predict the length and severity of winter weather. Groundhog Day, which is celebrated on February 2 in the United States, is based on ancient Celtic beliefs about winter weather patterns. Celtic people thought that if winter’s midpoint was sunny and clear, there would be “two winters in the year”: in other words, winter would be long and cold. They passed their beliefs down to Romans, Germans, and Americans. Nowadays, in the U.S., a groundhog named Punxsutawney Phil is the keystone of the celebration. If he sees his shadow, it is thought that this presages a long winter. (Unfortunately, Phil’s “predictions” have been accurate only about one-third of the time.)
Meteorologists use a variety of tools to help them gather information about weather and climate. Some more familiar ones are thermometers which measure air temperature, anemometers which gauge wind speeds, and barometers which provide information on air pressure. These instruments allow meteorologists to gather data about what is happening near Earth’s surface. Collecting data from other sources—and other parts of the atmosphere—helps to create a more descriptive picture of weather.
For viewing large weather systems on a worldwide scale, weather satellites are invaluable. Satellites show cloud formations, large weather events such as hurricanes, and other global weather systems. With satellites, forecasters can see weather across the whole globe: the oceans, continents, and poles. Recent satellite data is very detailed, even to the point of showing states and counties.
On each satellite there are two types of sensors. One is a visible light sensor called the “imager,” which works like a camera in space and helps gather information on cloud movements and patterns. This sensor can only be used during daylight hours, since it works by capturing reflected light to create images. Since different surface features reflect light in distinctive ways, they can be distinguished from each other in the images. Water reflects very little light, making it appear black on the satellite image. Land masses tend to appear as shades of gray, depending on their temperature and moisture.
The second sensor is called the “sounder.” It’s an infrared sensor that reads temperatures. The higher the temperature of the object, the more energy it emits. This sensor allows satellites to measure the amount of energy radiated by Earth’s surface, clouds, oceans, air, and so on. Infrared sensors can be used at night—a helpful feature for forecasters, considering that the imager can only pick up data during daylight hours.
Doppler radar is another essential meteorological tool. Radar works a little differently from satellite sensors. Instead of reading reflected light or energy, radar measures reflected sound waves.
When sound waves are broadcast from a radar antennae, they may come into contact with objects in their path, such as dust particles or ice crystals. If they come into contact with an object that is moving away from the radar, the sound waves will be reflected back at a decreased frequency (that is, fewer sound waves will be reflected back within a certain time period). If the object they come in contact with is moving toward the radar, the sound waves will be reflected back at an increased frequency. This effect was discovered in 1842 by Christian Doppler. Later, scientists applied Doppler’s principle to weather radar. Using Doppler radar, meteorologists can get a picture of precipitation that allows them to track a storm’s progress over time.
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