A Weather Forecaster’s Eye in the Sky By John Jensenius
Weather systems are constantly moving around the globe. Storms develop, intensify and eventually dissipate as they move from one area to another. While these storms are over land, meteorologists rely on a variety of weather observations. However, while over the oceans, meteorologists rely on satellites to observe these weather systems. In particular, satellites are extremely important for tracking tropical storms and hurricanes as they develop and move across the ocean. There are two main types of weather satellites: polar-orbiting and geosynchronous.
The polar-orbiting satellites orbit around the north and south poles as the earth rotates underneath their orbit. Each pass of the polar-orbiting satellite can be stitched together to form a complete picture of the weather around the globe.
The second type of satellite is the geosynchronous (sometimes called geostationary). These satellites orbit the globe around the equator and revolve around the earth at the same speed as the earth rotates. Because these satellites move at the same speed as the earth’s rotation, we say that their orbit is synchronous with the earth. This synchronous movement allows them to appear as if they are stationary — that is, they always stay over the same location on the earth. To maintain a synchronous orbit, they remain above the equator at an altitude of about 23,000 miles.
Weather satellites are equipped with a variety of sensors that measure different weather variables. One sensor measures the amount of sunlight reflected back to space by objects on the ground and in the atmosphere. This data creates a visible satellite picture. Clouds show up white on the visible image because they reflect much of the sun’s light. Like clouds, snow on the ground also shows up very light. In contrast, the oceans, which absorb most of the sunlight, appear dark on the visible image, as do heavily forested areas. One of the limitations of the visible image, is that it can only be used during the sunlit hours. At night, the visible image is dark.
Another type of sensor is the infrared sensor. This sensor measures the temperatures of the objects it sees. Although temperatures can be shaded any color, on most black-and-white images, the warmer objects are colored darker while the colder objects are lighter. For example, warm tropical waters and hot land areas appear dark, whereas clouds, especially high clouds in the atmosphere where temperatures are very cold, appear white. On sunny days, you can watch land areas on the infrared image turn darker as the sun heats them. One advantage of the infrared sensors is that they can produce images both day and night since objects always have a temperature.
Other sensors onboard weather satellites provide meteorologists with information on moisture in the air, the temperature structure of the atmosphere, whether fog is forming and a variety of other information about the atmosphere. In the next few years, satellites will start providing information on lightning flashes across the United States.
So, whether it’s day or night, weather satellites are keeping an eye on the weather.
