Radar is the tool used by meteorologists for locating precipitation. Weather radar displays are seen on TV, Internet and on smart phones. The word radar is an acronym for Radio Detection and Ranging. A typical weather radar is a rotating dish, sitting on a tower, that points outward just above the ground. The radar dish is protected from high wind and hail by a radar dome. The radar sends a radio signal out, as a pulse. When the radio signal hits an object, the part that bounces back to the radar receiver tells how big the object is. The time it takes for the signal to return is used to calculate the range- that's the distance. A network of radars is used in many countries like the United States to get full coverage of precipitation. Colors on the radar show how heavy the precipitation is. Typically, blue and green are lighter, yellow and orange are moderate, and red and purple are heavy. Doppler radar can determine if there is rotation within a cumulonimbus cloud that may eventually lead to a tornado. In some cases, radar detects debris in the air. That tells a meteorologist that things that are a lot bigger than raindrops are in the air, likely picked up by a tornado.
Satellite cloud images show weather patterns over the oceans, long before they affect land. Satellite images present the complexity and beauty of clouds. Day and night, a constellation of weather satellites watches Earth. Geostationary satellites orbit the planet at the same rate of Earth's rotation so that they stay over the same spot, with the same view. In the United States, they are called Geostationary Operational Environmental Satellites; GOES. From a distance of over 22,000 miles away, these satellites freguently send cloud images back to Earth. Another type of weather satellite orbits at a much lower altitude. Polar orbiting satellites orbit north and south, crossing the poles as Earth rotates beneath them. They create a swath of images that can then be stitched together. At a typical altitude of 500 miles, these satellites display details within clouds and also on land. They easily show flooding, dust storms, large smoke plumes from fires and volcanic ash, thick fog areas, ice on rivers, icebergs in the ocean, and snow cover on the ground. Weather satellites have sensors to measure the heat radiated from Earth and from clouds. The infra-red sensor allows us to sense clouds at night because clouds at different heights are different temperatures. Low clouds are warmer and high clouds are colder. The difference in temperature is displayed as shades of gray which looks similar to a regular satellite picture of clouds using sunlight. Sometimes, the satellite cloud image is enhanced with color to make tracking cloud and storm features easier. Some satellites also carry instruments to detect lightning, and other environmental elements like ozone, dust, and gases. Some satellites carry sensors that point away from Earth to monitor solar activity. Many satellites carry search and rescue transponders to aid airplane and ocean incidents.
Weather instruments are launched by balloon twice a day, at the same time, at hundreds of locations on our planet. Through international cooperation, this upper-air data is collected and shared worldwide so that all meteorologists can know temperature, pressure, humidity, and wind, at multiple heights, around the globe. Computer models use the data to make forecasts. A small package containing weather sensors and a transmitter is attached to a large balloon filled with helium or hydrogen. After release, the balloon ascends. Wind carries it dozens and sometimes hundreds of miles away. Data is transmitted back to Earth in real-time to a radio receiver that tracks the balloon. This data results in a vertical set of weather data. The instruments are called radiosondes and the newer ones use GPS for precise tracking. A weather balloon easily reaches heights between 10 and 15 miles. Some rise as high as 20 miles. At higher altitudes, the air pressure is so low that the balloon swells and eventually pops. A parachute carries the instrument package down to the ground at a safe speed. Many radiosondes land in forests, lakes, and oceans, and are never recovered. Those that are recovered can be mailed back to the National Weather Service to be refurbished and reused.
A dropsonde is like a radiosonde except rather than being carried by a balloon upward, it is dropped from an airplane. A dropsonde has a parachute so that it slowly falls, collecting temperature, humidity, pressure, and wind data. These are used often by hurricane hunters, and only dropped over oceans.
The most basic tool used by meteorologists is a thermometer. Thermometers measure heat. They tell us how warm or cold the air is. You may see a big thermometer display in front of stores and other buildings. You may have a thermometer with a liquid like mercury or alcohol inside a sealed glass tube. The liquid expands when it heats up, and contracts as it cools down, to give different temperature readings. Other thermometers have a coil that is made of two different metals sandwiched together and connected to a pointer. This type of thermometer works on the principle that different metals expand and contract at different rates with temperature change, so when the temperature changes, the coil tightens or loosens to show the value.
A hygrometer is used to measure humidity. Did you know that even when the weather is dry there is invisible water in the air? It's called humidity. Humidity is a way to tell how much water vapor, or moisture, is floating in air. A low number on a hygrometer means the air is dry. That's what you find in a desert. We also find low humidity indoors, in winter, when our heaters are on. When hygrometers show high numbers, we say the air is humid and that means that clouds might form to make rain. Professional hygrometers use animal hair or human hair connected with a mechanism to a pointer to give relative humidity. As water vapor increases in the air, hair lengthens. This is why on a very humid day it's harder to keep your hair in control. You may have noticed that some wooden doors or drawers in your home stick or creak when humidity rises. Any object that can absorb moisture will react to humidity changes. Basic hygrometers use an absorbent material bonded to a coil of metal. As humidity changes, the material changes length to stretch or squeeze the coil and make the pointer move.
A rain gauge is one of the simplest instruments. It is a tube or cylinder, left in the open, to catch and measure rain. Basic rain gauges have markings on the side to show how much rain has fallen. Rain gauges that report data in real-time often use a miniature bucket that tips and empties each time it captures a tiny amount of water. Every time it tips, it sends a signal to a computer to indicate an additional amount of rain has fallen.
The instrument that measures air pressure is a barometer. Air pressure is controlled by how much air is above a certain location. Air pressure tells us how strong a high-pressure system is or how deep a low-pressure storm system is. Barometers, like those you would find in a home, use a sealed metal chamber that has only a little air inside. When outside air pressure increases, it squeezes the chamber. Springs and levers make the needle on the display move to a higher value. When outside air pressure decreases, the chamber expands and moves the display needle to a lower value.
Wind direction is easy to observe using a wind vane. It's an instrument, often seen on farms, older buildings and on boats, that points in the direction from which the wind is blowing. Some wind vanes have ornate and even whimsical designs with an arrow to show where the wind originates. Wind is named for the direction that it comes from. The wind vane used by a meteorologist is usually connected to a dial or meter to record and display the wind direction. Wind direction is also displayed at airports using a windsock. It's a brightly colored tube of fabric connected to a pole that helps pilots see the wind direction. Winds come from four main directions, but you need a compass to know which way is north, south, east, and west. Once a meteorologist knows where the wind is coming from, he or she can know if it will be a wet wind, dry wind, cold wind, or warm wind.
Wind speed is measured by an anemometer. That is an instrument with spinning cups or propellers. The stronger the wind, the faster the spin. Anemometers are connected to computers to keep track of the changing wind speed. The amount of a windsock that stretches outward also gives an idea of how strong the wind is.