Fact Sheet United States Air Force Reserve

WC-130 Weather Equipment

The weather equipment aboard the Air Force Reserve’s fleet of WC-130s, collectively called the Improved Weather Reconnaissance System (IWRS), marks a tremendous advance in hurricane surveillance. IWRS was born from a Statement of Need following Hurricane Camille in 1969, and became operational in 1987.

Studies have shown that the weather data sent directly from the aircraft to the National Hurricane Center in Miami improves the hurricane forecasts by 25%.

Flight-level data

The flight meteorologist (officially called the Aerial Reconnaissance Weather Officer) analyzes flight-level data, measured as the airplane slices through the atmosphere at specific altitudes. Meteorologists around the world analyze weather at these standard levels: surface (we'll fly at 500-1500 feet to approximate the surface weather), 925 millibars (2500 feet), 850 mb (4780 feet), 700 mb (9880 feet), 500 mb (18,290 ft), 400 mb (23,570 ft), or 300 mb (30,070 ft). Storm missions are flown at low-level (500-1500 feet) when the storm is developing, changing to higher altitudes as the storm strengthens, but no higher than 10,000 feet.

Special weather sensors mounted on the WC-130 are tied directly into the IWRS computers, and measure temperature, dewpoint (humidity), barometric pressure, actual (radar-measured) altitude of the aircraft, and winds to calculate a complete weather observation every 30 seconds. The computers are also connected to the aircraft navigation system, which adds position to each observation.

The on-board computers display the data to the weather officer every second, and also can be used to quickly graph any parameter. The speed and flexibility with which data can be viewed aid the ARWO’s analysis and decisions, dramatically improving his/her ability to maintain control of the mission in the rapidly changing environment of a hurricane. Prior to IWRS, a flight meteorologist working at top speed could manually produce a weather observation about every five minutes.

Data are archived every 10 seconds, and there is an option of saving data every second if needed for research. Observations are saved every 30 seconds, and are sent directly to the computers at the National Hurricane Center in Miami for an unprecedented level of information to initialize their forecast models.

The Hurricane Hunters fly patterns through the tropical cyclones to determine the extent of damaging winds in each sector, the sea-level pressure in the eye (lower pressure means a more powerful storm), the temperature difference inside and outside the eye (hotter eye usually means a stronger storm), and then repeat the pattern to determine current storm movement.

The weather officer also sends other observations which are disseminated to the world-wide weather network, including RECCO observations periodically along the route, and VORTEX messages from the eye. The weather officer adds additional details to these observations, such as clouds, turbulence, radar depictions, and sea-surface winds, which the computer cannot get from the sensors. Sea-surface winds are determined by looking at the condition of the water: white caps, foam, patches of green foam, and various types of streaks all point to wind direction and speed to the trained observer.

Dropsonde windfinding system

While the weather officer is developing a picture of the storm at flight level, the Dropsonde System Operator launches an instrument to get a profile of the atmosphere from the aircraft all the way down to the surface. The dropwindsonde does the same thing as a weather balloon, except it goes down instead of up.

The sonde deploys a drogue parachute 10 seconds after ejection and then falls with a vertical velocity of about 2500 feet per minute. During descent the sonde senses pressure, temperature, humidity, wind speed and direction, and radios these data back to the plane. The aircraft receives these data real-time from up to four sondes simultaneously.

The sonde measures winds by a rather elegant method. The sonde triangulates its position relative to several different Global Positioning System (GPS) satellites in orbit. If there were no wind, the sonde would fall straight down, and its position over the earth would not change. However, as the sonde drifts, it calculates the wind speed and wind direction needed to push the sonde by that amount.

The sonde is a meteorological instrument 16 inches long by 2.25 inches in diameter, weighing about one pound. It is comprised of : sensors, a receiver, a transmitter, plus a battery pack. The irretrievable sondes may be dropped from altitudes between 10,000 to 40,000 feet, and tracked for a distance of 200 miles.

After drop termination, the dropsonde systems operator manually edits the collected data and formats the data to a standard code. The edited and formatted message is then ready for immediate transmission to the ground station via SATCOM.

Satellite communications

A key part of the IWRS is its ability to send data instantaneously via satellite directly to the forecasters at the National Hurricane Center in Miami, or to another transceiver at Keesler Air Force Base, or a portable station which moves with the Hurricane Hunters wherever they go to chase the storms. The weather officer can also send written messages to the forecasters and staff on the ground, providing details of the storm and coordinating the mission.

Specifics about the sensors

Temperature is measured by a Rosemount thermistor, which uses the principle that the resistance to conductivity changes with temperature in metals. The temperature is corrected for frictional heating of the probe, and is accurate to 0.25C +0.5%C.

The Edgetech Model 137-C3 Dewpoint Hygrometer measures humidity (technically "dewpoint") by cooling the air inside a chamber until dew forms. It is accurate to +0.5C for dewpoints above freezing, and +1C for dewpoints below freezing.

The Combined Altitude Radar Altimeter measures the actual altitude of the aircraft by bouncing radar waves off the ground. As we fly into low pressure (such as a hurricane), the plane descends, but the radar altimeter will tell us if we're getting too low! It is accurate to within +2% up through 5000 feet, +100 feet at 5000-10,000 feet, and +1% above 10,000 feet.

The AirResearch Pressure Altimeter measures the altitude of the aircraft with an accuracy of +10 feet, using a reference datum of 29.92 inches of Mercury. This value, contrasted against the Radar Altitude, results in a height of standard surface or sea-level pressure measurement, after correction factors are added for humidity, latitude, and aircraft- specific adjustments.

Winds are computed from a variety of sensors. There are two probes measuring changes in pressure across the openings in each probe to compute true airspeed and side-slip, while the navigator's Self-Contained Navigation System provides ground speed and heading information to complete the wind calculation.

Position is provided with pin-point accuracy from the with a hand-held Global Positioning System (GPS), within a fraction of a mile.

The Dropwindsonde contains a microprocessor to control and digitize the sensor data.

(CURRENT AS OF OCTOBER 1999)