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Satellites defined: The driver of forecast models

Weather Glossary

Satellites have quickly become the main driver of weather information that flows into our computer models that forecast. The ability to scan an entire area for water vapor, temperature, and other current conditions has become critical to accurate modeling and forecasting. Here we go into the many aspects of satellites in the meteorological world that make them so important.

Electromagnetic waves are used by satellites to detect and scan the earth from space. They sound complicated, but this is the same way your microwave works, or what you see when you watch TV and listen to the radio. Both electricity and magnetism combined can make waves that travel through different material, including a vacuum such as space. This makes it the ideal way to scan an area.

Image Courtesy the University of Oregon.

Different electromagnetic radiation waves have different wavelengths. Some of those we can see, like the light entering your eye. Most we cannot see, but know how long the wavelength is based on the strength of the electromagnetic radiation. As certain waves move through certain material, they can either completely pass through them, partially pass through, or get totally absorbed. Some waves cannot pass through certain material, like light waves through a brick wall. Other waves can get through a brick wall, like the X-ray. That is why if you get X-rays done, they use a lead cover that prevents from X-rays entering the body as it absorbs the harmful radiation.

As different wavelengths are absorbed, we can use that to help identify temperature differences, clouds, moisture, and more. This is done through the atmospheric window. Simply put, as most solar radiation is absorbed by the earth and its atmosphere while a small sliver of outgoing infrared radiation is emitted back up into space. That can be measured and we can discern what is happening in the atmosphere based on the image provided. More on the atmospheric window here.

POLAR ORBITING SATELLITES:

Some of the original satellites that were launched moved around the earth revolving over the north and south poles. The satellite runs about 520 miles above earth’s surface and sends in about half a dozen images a day and never is over the same space in a day, so while the images may not be easily compared, it can give a good view as to what is occurring at the poles. Polar satellite imagery can be found here. Most of the images are overlayed with Geostationary satellite imagery, which is much more useful.

GEOSTATIONARY SATELLITES:

The first geostationary satellite (GOES) was launched in 1966 and they are much higher up at around 22,236 miles. As alluded to from their name, these satellites sit in a fixed spot over the earth’s surface and orbit the earth once in 24 hours, providing the same image of earth again and again. That makes these satellites the preferred option for meteorologists to use for analysis of water vapor, temperature, cloud coverage, and much more. These satellites are far enough away to view the entire earth at the same time and can record images every minute. They can also be adjusted ever so slightly to get a different perspective if necessary.

In 2016, the GOES R series was launched to bring a significant boost to quality and resolution to satellite images. It was placed of 70°W longitude and once put officially into operation, it is now GOES-16 or GOES EAST. In 2018, the GOES R series for the west coast was launched and placed over 137°W. This was given the name GOES-17 or GOES WEST.

The two main scans we get are visible and infrared, both from only one of the six instruments on the satellite. This instrument is the Advanced Baseline Imager (ABI). Remember that there are certain wavelengths in which we use to get the images needed. These are called channels and each one is noted below.

The yellow section is visible light while the blue is infrared.

GOES-16 EAST

This image is a blend of visible channels 1, 2, and 3.

The visible wavelengths will often be viewed in black and white to allow for easier distinction between what is ground and what is cloud, but there are ways to get a more real picture of the atmosphere, like seen above. Visible channels start with red and blue, named based on their location on the electromagnetic spectrum. Red is often used for ‘now-casting’ weather like severe storms, lake-effect snow, fog, and much more as the resolution is down to about .5km. Remember that these images cannot be captured at night. Visible channels are 1-6 and all serve a different purpose.

Middle-level water vapor helps identify the location of the jet stream, how hurricanes are tracking, forecasting for mid-latitude cyclones, and more.

Above is channel 9 infrared image. Think of this as water vapor. That is also what these are commonly referred to by meteorologists, but it is important to remember that these are not actually measuring water vapor, but the temperature of clouds.

The satellite interprets a low temperature as high water vapor content. When much radiation is received by the satellite, it senses a high temperature and consequently a low amount of water vapor. – NWS

Based on the wavelength, certain channels view different levels of water vapor. High, middle, and low-level are measured (the above image is middle-level). These three are the main tools for a forecaster. Channels 11-16 all serve a different purpose and are often used within computer models to assist in model output.

GOES-16 WEST

A look at the visible channel 2 (Red band)

The GOES-WEST is very similar to GOES-EAST in that it uses all 16 channels to scan with very high precision. If you want up-to-the-minute high quality images of these satellites, one of the most user friendly sites is from the College of DuPage here.

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