Chapter 24, Observations 24-45
TDWR has long- and short-range scans as well as monitor and hazardous weather modes. Update times
vary from around five minutes in monitor mode to one minute in hazardous weather mode. Select TDWR
products are available on NWS radar websites.
24.7 Satellite Observations
24.7.1 Description
Satellite is perhaps the single most important source of weather data wo rldwide, particularly over
data-sparse regions, such as countries without organized weather data collection and the oceans.
There are two kinds of weather satellite: geostationary and polar -orbiting. Geostationary satellites are
located about 22,000 mi above the equator and orbit the Earth at a very fast speed that effectively makes
them appear stationary. Polar-orbiting satellites orbit the Earth at a much lower altitude. Their track is such
that the Earth rotates underneath the satellite as it orbits from the North Pole to the South Pole , then back
to the North Pole.
The vast majority of weather satellite imagery for aviation comes from NOAA’s Geostationary Operational
Environmental Satellites (GOES).
24.7.2 Imagery Types
Four types of satellite imagery are commonly used: GeoColor, visible, infrared (IR), and water vapor.
Visible imagery is only useful during daylight hours. IR and water vapor imagery are useful day or night.
24.7.2.1 GeoColor Imagery
GeoColor imagery (see Figure 24-17) is a multispectral product composed of true color (using a simulated
green component) during the daytime, and an IR product at night. During the day, the imagery looks
approximately as it would appear when viewed with human eyes from space. At night, the b lue colors
represent liquid water clouds such as fog and stratus, while gray to white indicate higher ice clouds, and the
city lights come from a static database.
Chapter 24, Observations 24-46
Figure 24-17. GeoColor Satellite Image—U.S. Example
24.7.2.2 Visible Imagery
Visible imagery (see Figure 24-18) displays reflected sunlight from the Earth ’s surface, clouds, and
particulate matter in the atmosphere. Visible satellite images, which look like black and white photographs,
are derived from the satellite signals. Clouds usually appear white, while land and water surfaces appear in
shades of gray or black.
The visible channel senses reflected solar radiation. Clouds, the Earth’s atmosphere, and the Earth’s surface
all absorb and reflect incoming solar radiation. Since visible imagery is produced by reflected sunlight
(radiation), it is only useful during daylight.
Chapter 24, Observations 24-47
Figure 24-18. Visible Satellite Image—U.S. Example
24.7.2.2.1 Visible Image Data Legend
The data legend on a visible image displays albedo, or reflectance, expressed as a percentage (see Figure
24-19). For example, an albedo of 72 means 72 percent of the sunlight that struck a feature was reflected
back to space.
The gray shades (values) represent albedo, or reflectance, expressed as a percentage.
Figure 24-19. Visible Satellite Image Data Legend
24.7.2.3 Infrared (IR) Imagery
IR images (see Figure 24-20 and Figure 24-21) display temperatures of the Earth ’s surface, clouds, and
particulate matter. Generally speaking, the warmer an object, the more IR energy it emits. The satellite
sensor measures this energy and calibrates it to temperature using a very simple physical relationship.
Clouds that are very high in the atmosphere are generally quite cold (e.g., -50°C), whereas clouds very near
the Earth’s surface are generally quite warm (e.g., +5 °C). Likewise, land may be even warmer than the
lower clouds (e.g., +20°C). Those colder clouds emit much less IR energy than the warmer clouds, and the
land emits more than those warm clouds.
Chapter 24, Observations 24-48
The data measured by satellite is calibrated and colorized according to the temperature. If the temperature
of the atmosphere decreases with height (which is typical), cloud -top temperature can be used to roughly
determine which clouds are high-level and which are low-level.
When clouds are present, the temperature displayed on the IR images is that of the tops of clouds. When
clouds are not present, the temperature is that of the ground or the ocean. A major advantage of the
IR channel is that it can sense energy at night; therefore, this imagery is available 24 hours a day.
The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors
indicate warmer temperatures.
Figure 24-20. Infrared (Color) Satellite Image—U.S. Example
Chapter 24, Observations 24-49
The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades
indicate warmer temperatures.
Figure 24-21. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example
24.7.2.3.1 Infrared Image Data Legends
The data legend on an IR image is calibrated to temperature expressed in degrees Celsius (see Figure 24-22
and Figure 24-23). The legend may vary based on the satellite image provider.
The colors (values) represent temperature in degrees Celsius.
Figure 24-22. Infrared (Color) Satellite Image Data Legend
The gray shades (values) represent temperature in degrees Celsius.
Figure 24-23. Unenhanced Infrared (Black and White) Satellite Image Data Legend
Chapter 24, Observations 24-50
24.7.2.4 Water Vapor Imagery
Water vapor imagery (see Figure 24-24) displays the quantity of water vapor generally located in the middle
and upper troposphere within the layer between 700 mb (approximately 10,000 ft MSL) and 200 mb
(approximately FL390). The actual numbers displayed on the water vapor images correspond t o
temperature in degrees Celsius. No direct relationship exists between these values and the temperatures of
clouds, unlike IR imagery. Water vapor imagery does not really “see” clouds, but “sees” high-level water
vapor instead.
The most useful information to be gained from the water vapor images is the location and movement of
weather systems, jet streams, and thunderstorms. Another useful tidbit is aided by the color scale used on
the images. In general, regions displayed in sha des of red are very dry in the upper atmosphere and may
correlate to crisp, blue skies from a ground perspective. On the contrary, regions displayed in shades of
blue or green are indicative of a lot of high-level moisture and may also indicate cloudiness. This cloudiness
could simply be high -level cirrus types or thunderstorms. That determination cannot be ascertained from
this image by itself but could easily be determined when used in conjunction with corresponding visible
and IR satellite images. A major advantage of the water vapor channel is that it can sense energy at night,
so this imagery is available 24 hours a day.
The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper
troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere.
Figure 24-24. Water Vapor Satellite Image—U.S. Example
Chapter 24, Observations 24-51
24.7.2.4.1 Water Vapor Image Data Legend
The data legend on water vapor images is calibrated to temperature expressed in degrees Celsius (see Figure
24-25). The actual data values on the water vapor images are not particularly useful. Interpretation of the
patterns and how they change over time is more important. The legend may vary depending on the satellite
image provider.
The colors (values) represent temperature in degrees Celsius.
Figure 24-25. Water Vapor Satellite Image Data Legend
24.7.3 Polar Operational Environment Satellites (POES)
“POES” stands for the Polar Operational Environment Satellites. Polar satellites are not stationary. They
track along various orbits around the poles. Typically, they are somewhere between 124 and 1,240 mi above
the Earth’s surface. The satellites scan the Earth in swaths as they pass by on their tracks.
The NWS AAWU posts POES images on their website.
24.7.3.1 Benefits
Because polar satellites are so much closer to Earth, you can get very high resolution (i.e., better than 0.5 km
(or about 5/8 mi)). This allows for weather and surface features to be seen in much greater detail.
This is particularly useful over the poles and arctic areas. The quality of geostationary satellite data degrades
the closer you get to the poles, while polar satellite data provides high resolution in those areas.
24.7.3.2 Shortfalls
By far the most significant shortfall is the latency, or the time between the satellite scanning the area and
the time that the data is available to a user. Because polar satellites are moving, they cannot continuously
transmit to a single station. Instead, there is a series of stations around the globe through which the data is
collected. Data is then transmitted from those stations to other locations. At times, it can be several hours
old (or more) by the time it reaches operational users in the United States (in polar areas it is much quicker).
There are some direct ground stations closer to the United States that can cut the latency to about 45 minutes
when utilizing the newer polar satellites.
24.8 Upper Air Observations
24.8.1 Radiosonde Observations (Weather Balloon)
Since the late 1930s, the NWS has taken routine scheduled upper air observations with radiosondes attached
to weather balloons, usually referred to as soundings. Weather data from the radiosondes are foundational
to all computer model forecasts produced by the NWS and others.
The radiosonde is a small, expendable instrument package (weighing 100 to 500 g) that consists of radio
gear and sensing elements and is suspended below a large balloon inflated with hydrogen or helium gas
(see Figure 24-26). As the radiosonde rises at about 300 m per minute (about 1,000 ft per minute), sensors
on the radiosonde measure profiles of pressure, temperature, and moisture. These sensors are linked to a
battery-powered radio transmitter that sends the sensor measur ements to a ground tracking antenna. Wind
speed and direction aloft are also obtained by tracking the position of the radiosonde in flight using the
GPS. Most stations around the world take rawinsonde observations. However, meteorologists and other
data users frequently refer to a rawinsonde observation as a radiosonde observation.