Weather satellites take pictures of a wide range of atmospheric and surface parameters in the visible, infrared and near infrared spectrum. They are used for meteorological forecasting, impact studies, climate modeling and many other applications.
They come in two basic types: geostationary and polar orbiting.
Geostationary Satellites
Geostationary satellites orbit at a fixed altitude, usually above the equator. The speed at which they travel in this orbit matches the Earth’s rotation, enabling the satellite to appear motionless and stationary above one point on the surface. This makes geostationary satellites useful for communications, television broadcasting and weather forecasting.
The United States, Europe and Japan all operate satellites in this orbit, allowing meteorologists to monitor the development and track severe weather systems across the world. In the US, NOAA operates a two-satellite constellation in this orbit – known as GOES – that covers the eastern and western hemispheres.
During the 1960s and 1970s, a series of satellites were launched into the geostationary orbit that revolutionized the science of atmospheric monitoring and forecasting. Initially these satellites were only capable of transmitting analog data, but later satellites such as the first ATS (Advanced Technology Satellite) and three-axes stabilized GOES used new technology to transmit digital information.
Another advantage of geostationary satellites is that they can be observed from most parts of the Earth. Since they are in an unchanging area of visibility, receiving and transmitting antennas on the ground do not need to be tracked, saving money and space.
There are several hundred communication satellites in this orbit and a few meteorological satellites as well. Despite the number of satellites in this orbit, however, it is important to note that this particular orbit is finite and will eventually become saturated, much like the polar orbits.
This limitation means that satellites in this orbit have to be kept away from other satellites in order not to interfere with each other during uplink and downlink communications. This is accomplished by stationkeeping, where the satellite is repositioned so that it stays in the same position within the geostationary belt as when it was launched.
In addition, the satellite must be positioned to avoid interference from other communication and military satellites in the same orbit. This is often accomplished by adjusting the inclination of the satellite so that it keeps its assigned position.
This method of correction is used for most of the meteorological satellites in this orbit. It also allows satellites to correct for any slight perturbations of the satellite that could affect its performance. These corrections are called “stationkeeping” maneuvers and are performed over a scheduled period of time to ensure that the satellite remains in its assigned position.
Polar Satellites
Polar satellites circle the Earth from the North Pole to the South Pole 14 times a day, capturing the whole planet in images twice a day. These are more detailed than GOES images and provide critical weather data that support the development of 0-7 day forecasts.
They can also see snow and ice covering the North and South Poles, which is important for weather observations and warnings. The polar satellites in the Joint Polar Satellite System (JPSS) are responsible for providing global weather measurements and atmospheric and cloud images.
These satellites are low-altitude satellites that orbit the Earth in a polar or near-polar orbit, passing within 20 to 30 degrees of the North and South Poles each time they pass overhead. These satellites have a narrow “strip” that they monitor on each orbit, with each strip being viewed from different angles and compared together to form a picture of a larger area.
Because a polar or near-polar satellite is moving, it can be a great deal harder to get good pictures than with geostationary satellites. They can also suffer from time delays and distortions, making them less useful for weather observation purposes.
In contrast, a geostationary satellite is higher up on the Earth’s surface, so it can view a greater area of the globe. This helps scientists gain a better understanding of the earth’s resources and its climate.
However, these satellites are much slower to orbit than polar ones. This is because they have to slow down and change their speed to keep up with the rotation of the Earth. They are also less effective at tracking the sun because of this, so they are less useful for solar resource monitoring than polar satellites.
These are the satellites in NOAA’s Joint Polar Satellite System, which collect data on atmospheric, oceanic and land conditions around the world. They help develop short-term and long-range weather forecasts.
The National Oceanic and Atmospheric Administration estimates that 85% of the global weather data flowing into American weather forecast models comes from these polar-orbiting satellites. This is an extremely important source of data because more weather data allows for smarter forecasting. With the upcoming launch of NOAA-20, which will join Suomi-NPP in a Sun-synchronous polar orbit that crosses the Earth’s surface twice daily, more data will be available to improve forecasting.
Visible Satellites
Weather satellites record visible images of the Earth that show clouds, snow, and terrain. These data sets can be useful to forecasters who use them to monitor the progress of storms. In addition, these images are used for fire monitoring and prevention.
Clouds and snow appear white on visible satellite imagery due to their high albedo/reflectance (the percentage of incoming sunlight that is reflected back by an object). The ocean and trees appear dimmer as they have low albedo/reflectance.
Unlike the IR channel, which measures different spectral wavelengths to reveal more features, visible images are recorded 24 hours a day as they depend on the reflection of sunlight. This means that they are most useful at sunrise and sunset when the sun is above the horizon, when they can see more of the sky.
Visible satellites are usually polar orbiting, meaning that they circle around the equator and keep pace with the Earth’s rotation. These satellites are the most common for monitoring weather.
In addition, there are many geostationary satellites, which orbit the Earth at an altitude of 22,000 miles above the equator once every 24 hours. These satellites are called geostationary operational environmental satellites, or GOES.
A number of these satellites are used to observe the movement of storms as they pass over the United States and its territories. These weather satellites often take multiple photographs of a storm, which can be displayed in sequence to form a movie that shows the movement of a storm system over time.
The satellites that are used to observe the movement of storms are referred to as weather radars, because they measure weather by detecting electromagnetic radiation in the atmosphere. These instruments are not always able to detect the exact position of storms because of their varying heights and distance from the ground.
They also vary in their sensitivity to atmospheric conditions and the degradation of their sensors. This is a critical issue for weather forecasters as this can mean the difference between a severe thunderstorm and no storm at all.
The sensitivity of the weather radars to atmospheric conditions is a key factor in determining whether or not the satellites can detect storms. This is especially important when it comes to predicting severe weather such as tornadoes and hurricanes.
Infrared Satellites
Infrared (IR) satellite imagery is useful for weather forecasters because it can detect energy even when the sun is not shining. Unlike visible satellites, which use sunlight that is reflected up to the satellite, infrared satellites can sense the re-emitted energy of the earth’s surface and clouds. This allows them to construct loops that extend for 24 hours.
The amount of infrared radiation that reaches the satellite depends on the temperature of the object emitting that radiation. Generally speaking, warm objects emit more radiation than cold ones.
As the earth absorbs some of the incoming solar energy, it re-emits this absorbed heat in the form of electromagnetic radiation (the wavelength of which is determined by the physics behind the laws of energy and mass). This IR signal can be interpreted using mathematical equations that correlate with temperature.
This information can then be converted to a color or grayscale, which is how an infrared satellite image looks to the human eye. Some infrared images, like those on weatherTAP, are color enhanced to enhance features that meteorologists want to see, such as low cloud tops.
During the day, the Earth’s surface and clouds absorb about half of the incoming IR energy. This IR energy is then re-emitted by the atmosphere and land as heat. The resulting IR image is brighter when the Earth is hotter and darker when it is colder, as shown in Figure 2.
On this map, the darkest colors correspond to areas where the ground and ocean are very warm, while the whitest colors represent those where the surface and clouds are very cold. The temperature of the air above the clouds, in particular, is the most important factor when interpreting an infrared image.
The infrared signal also reflects back to the satellite what is happening on the ground, which helps with fog and low cloud detection. In addition, the IR image can be enhanced with other wavelengths, such as UV, to help identify dust in the air or volcanic ash.