Aviation Weather: Satellites Thirty years ago it was amazing to think a handful of manmade satellites could be circling Earth. Today, hundreds of satellites clutter the space above this planet. Those satellites address four major fields: telecommunications, national defense, scientific study and research, and weather. Considering it was weather monitoring _ with defense implications _ which kick-started the race to put satellites in space, amazingly, they make up by far the smallest section of the celestial pie. For the United States, there are less than a dozen satellites providing terrestrial weather information. Weather satellites basically come in two flavors, geostationary or polar-orbiting. The Geostationary Satellites sit around 25,000 miles in space traveling at a speed equal to that of Earth. This allows the satellite to remain pointed at a single area of the globe day after day. The Polar-Orbiting Satellites fly about 500 miles above the Earth passing over the north and south poles each orbit, thus their name. They cover a 1,800 mile-wide swath at the Equator and can cover the entire planet in 12 hours. That means every point in the United States is flown over only twice daily. That limits them polor-orbiting satellites to mostly research work, though their higher resolution can at times help with forecasting. In theUnited States, we normally have at least two geostationary satellites looking down on the country each day. The GOES-10 geostationary satellite currently covers the US west coast and eastern Pacific. The GOES-8 geostationary satellite covers the eastern US and Caribbean. National Oceanic and Atmospheric Administration (NOAA) _ which oversees the National Weather Service, Tropical Prediction Center and a host of other US national weather research centers _ prefers to always have three GOES satellites in orbit, just in case of a failure. That has happened before, requiring the remaining satellite to move west during the winter months and east during the tropical season. That reduces coverage in vital areas at times and impacts the effective life of the satellite as they have limited propellant. Currently being put through its first paces is GOES-11, the planned replacement for GOES-8. After the initial checkout, the plan was to mothball the satellite in August. However, because of the predictions for a major 2000 Atlantic Hurricane Season, NOAA may wait until later in the year to mothball the satellite, allowing the use of some new instruments. Okay, enough reading. Here is an example of a full disk shot from GOES-10. It allows forecasters to see storms days before they impact the US west coast. Want to see the current view from the eastern based GOES-8? Just click on the image below. 
Now let's take a look at the three most common types of images you will find being downlinked from a GOES satellite. These sample images shown here are centered over the 48 contiguous states and were taken at the same time. As seen with the full-disk shot above, the GOES satellites can provide a host of image sizes. We will start with theVisible Image, below, which basically sees the Earth just as we would from 25,000 miles up. You will note that the image is not as bright as those normally seen on television. That is because by 2315Z in June, the Sun is beginning to set over the east coast. Just as our eyes can't see that well at night, neither can the visible view. To view the current visible image, just click anywhere on the image below. 
So what do we use the visible image for? The visible image is the best view of low-level clouds and fog. It is also easiest to pick out a forming surface low pressure system. Low clouds seen in the early morning, especially along large bodies of water, can be a prime indicator of fog. But despite the attributes visual images bring to the table, there is no way we can adequately predict weather if we can only get views during daylight. So what do we do? Well we turn to heat, or actually the lack of heat. Astronomers have used infrared images for years. These telescopes and imagers are designed to pick out heat instead of visible light. Since cloud tops cool as they push up through the Troposphere, contrasting themselves to the warmer areas of clouds or even the Earth's surface below, the infrared spectrum provides weather forecasters with the ability to view the Earth's surface 24 hours per day. Infrared Images allow GOES satellites to literally see in the dark. Take a look at the current U.S. infrared image by clicking anywhere on the image below. 
Well the visual images give us a peek at the low-level cloud structure, so what does the infrared mage offer other than 24-hour coverage? Basically the opposite where clouds are concerned. Infrared images allow us to basically measure how high cloud tops are. Those 45,000-foot cloud tops bursting higher and higher, and standing out brighter and brighter in the infrared image, are not fog or puffy whites drifting across the sky. They are the trademark of thunderstorms and even those massive meso-convective complexes _ thunderstorm complexes to you and me. In addition, we can also look at loops of infrared images. If low level clouds are moving one way and upper level clouds another way, that also can set up an unstable atmosphere. Well, if we can image low-level clouds with the visual images and upper-level clouds with infrared images, what else could we want? How about the ability to follow those rivers of air and moisture we talked about in the Jet Stream Tutorial. The answer is called Water Vapor Imagery, which is basically a look at those rivers of air. The lighter the spot, the more moist air present, like over southern Texas as thunderstorms pop up in the moist hot environment.The darker the area, the drier the air. Take a look at the comma shaped complex stretching from the Minnesota and Ontario borders to the Oklahoma Panhandle. The coma shape denotes a low pressure area. The light areas represent warm moist air ahead of a cold front stretching south from the low. The dark area behind the front is cooler drier air. The bright areas in Wisconsin are thunderstorms popping up as the juicy warm air and dry air clash. To see the current water vapor image, just click anywhere on the image below. 
On the DataJump page you will find these and more links. In the Fog, Clouds, and other Visibility Issues tutorial, we will look more closely at the different clouds imaged by these satellites. (For Flight Simulation Use Only) |
