Weather Satellites: Forecasting Earth's Weather (From Space!)

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The Earth's Selfie

A satellite view of planet Earth (and North America). NASA

There's no mistaking a satellite image of clouds or hurricanes. But other than recognizing weather satellite imagery, how much do you know about weather satellites?

In this slideshow, we'll explore the basics, from how weather satellites work to how the imagery produced from them is used for forecasting certain weather events.

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What is a Weather Satellite?

There are 2 types of weather satellites: polar orbiting and geostationary. iLexx/E+/Getty Images

Like ordinary space satellites, weather satellites are man-made objects that are launched into space and left to circle, or orbit, the Earth. Except instead of transmitting data back to Earth that powers your television, XM radio, or GPS navigation system on the ground, they transmit weather and climate data that they "see" back to us in pictures. (We'll talk more about how weather satellites do this in slide 5.) 

What's the advantage of weather satellites? Just like rooftop or mountaintop views offer a wider view of your surroundings, a weather satellite's position several hundred to thousands of miles above Earth's surface allows for weather in a neighboring part of the U.S. or that hasn't even entered the West or East Coast borders yet, to be observed. This extended view also helps meteorologists spot weather systems and patterns hours to days before being detected by surface observing instruments, like weather radar.

Since clouds are weather phenomena that "live" highest in the atmosphere, weather satellites are notorious for monitoring clouds and cloud systems (such as hurricanes), but clouds aren't the only thing they see. Weather satellites are also used to monitor environmental events that interact with the atmosphere and have broad areal coverage, such as wildfires, dust storms, snow cover, sea ice, and ocean temperatures.  

Now that we know what weather satellites are, let's take a look at the two kinds of weather satellites that exist -- geostationary and polar orbiting -- and the weather events each is best at viewing.

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Polar Orbiting Weather Satellites

The COMET Program (UCAR)

The United States currently operates two polar orbiting satellites. Called POES (short for Polar Operating Environmental Satellite), one operates during the morning and one during the evening. Both are collectively known as TIROS-N.

TIROS 1, the first weather satellite in existence, was polar orbiting -- meaning it passed over the North and South Poles each time it revolved around the Earth.

Polar orbiting satellites circle the Earth at a relatively close distance to it (roughly 500 miles above Earth's surface). As you might think, this makes them good at capturing high resolution images, but a drawback of being so close is they can only "see" a narrow swath of area at one time. However, because the Earth rotates west-to-east underneath a polar orbiting satellite's path, the satellite essentially drifts westward with each Earth revolution (the satellite doesn't physically move, but its path moves underneath it).

Polar orbiting satellites never pass over the same location more than once daily. This is good for providing a complete picture of what's happening weather-wise across the globe, and for this reason, polar orbiting satellites are best for long-range weather forecasting and monitoring conditions like El Niño and the ozone hole. However, this is not-so-good for tracking the development of individual storms. For that, we depend on geostationary satellites.

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Geostationary Weather Satellites

The COMET Program (UCAR)

The United States currently operates two geostationary satellites. Nicknamed GOES for "Geostationary Operational Environmental Satellites," one keeps watch over the East Coast (GOES-East) and the other, over the West Coast (GOES-West).

Six years after the first polar-orbiting satellite was launched, geostationary satellites were put into orbit. These satellites "sit" along the equator and move at the same speed as the Earth rotates. This gives them the appearance of staying still at the same point above Earth. It also allows them to continuously view the same region (the Northern and Western Hemispheres) throughout the course of a day, which is ideal for monitoring real-time weather for use in short term weather forecasting, like severe weather warnings.

What's one thing geostationary satellites don't do so well? Take sharp images or "see" the poles as well as it's polar orbiting brother. In order for geostationary satellites to keep pace with Earth, they must orbit at a greater distance from it (an altitude of 22,236 miles (35,786 km) to be exact). And at this increased distance, both image detail and views of the poles (due to Earth’s curvature) are lost.

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How Weather Satellites Work

(A) Sun acts as an energy source. (B) Energy interacts with the atmosphere and (C) with an object. (D) A remote sensor records the energy and (E) it is transmitted to a ground-based receiving/processing station. (F,G) Data is processed into an image. Canada Centre for Remote Sensing

Delicate sensors within the satellite, called radiometers, measure radiation (i.e., energy) given off by the Earth's surface, most of which is invisible to the naked eye. The types of energy weather satellites measure fall into three categories of the electromagnetic spectrum of light: visible, infrared, and the infrared to terahertz.

The intensity of radiation emitted in all three of these bands, or "channels," is measured simultaneously, then stored. A computer assigns a numeric value to each measurement within each channel and then converts these into a gray-scale pixel. Once all of the pixels are displayed, the end result is a set of three images, each showing where these three different kinds of energy "live."

The next three slides show the same view of the U.S., but taken from the visible, infrared, and water vapor. Can you notice the differences between each?

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Visible (VIS) Satellite Images

GOES-East satellite view of cloud distribution around 8 a.m. on May 27, 2012. NOAA

Images from the visible light channel resemble black-and-white photographs. That's because similar to a digital or 35mm camera, satellites sensitive to visible wavelengths record beams of sunlight reflected off of an object. The more sunlight an object (like our land and ocean) absorbs, the less light it reflects back out into space, and the darker these areas appear in the visible wavelength. Conversely, objects with high reflectivities, or albedos, (like the tops of clouds) appear brightest white because they bounce large amounts of light off of their surfaces.

Meteorologists use visible satellite images to forecast/view:

  • Convective activity (i.e., thunderstorms)
  • Precipitation (Because cloud type can be determined, precipitating clouds can be seen before rain showers appear on radar.)
  • Smoke plumes from fires
  • Ash from volcanoes

Since sunlight is required to capture visible satellite images, they are not available during the evening and overnight hours.

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Infrared (IR) Satellite Images

GOES-East infrared satellite view of cloud distribution around 8 a.m. on May 27, 2012. NOAA

Infrared channels sense heat energy given off by surfaces. As in visible imagery, warmest objects (such as land and low-level clouds) that soak up heat appear darkest, while colder objects (high clouds) appear brighter.

Meteorologists use IR images to forecast/view:

  • Cloud features at day and night
  • Cloud altitude (Because altitude is linked to temperature)
  • Snow cover (Shows up as a fixed grayish-white region)
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Water Vapor (WV) Satellite Images

GOES-East water vapor satellite view of cloud and moisture distribution around 8 a.m. on May 27, 2012. NOAA

Water vapor is detected for its energy emitted in the infrared to terahertz range of the spectrum. Like visible and IR, its images depict clouds, but an added advantage is that they also show water in its gaseous state. Moist tongues of air appear a foggy gray or white, while dry air is represented by dark regions.

Water vapor images are sometimes color-enhanced for better viewing. For enhanced images, blues and greens mean high moisture, and browns, low moisture.

Meteorologists use water vapor images to forecast things like how much moisture will be associated with an upcoming rain or snow event. They can also be used to find the jet stream (it's located along the boundary of dry and moist air).