The data gathered from surface and upper altitude observations form the basis of all weather forecasts, advisories, and briefings. There are four types of weather observations: surface, upper air, radar, and satellite.

12.1.1 Surface Aviation Weather Observations #

Surface aviation weather observations (METARs) are a compilation of elements of the current weather at individual ground stations across the United States. The network is made up of government and privately contracted facilities that provide continuous up-to-date weather information. Automated weather sources, such as the Automated Weather Observing Systems (AWOS), Automated Surface Observing Systems (ASOS), Air Route Traffic Control Center (ARTCC) facilities, as well as other automated facilities, also play a major role in the gathering of surface observations.

Surface observations provide local weather conditions and other relevant information for a radius of five miles of a specific airport. This information includes the type of report, station identifier, date and time, modifier (as required), wind, visibility, runway visual range (RVR), weather phenomena, sky condition, temperature/dew point, altimeter reading, and applicable remarks. The information gathered for the surface observation may be from a person, an automated station, or an automated station that is updated or enhanced by a weather observer. Although the reports cover only a small radius, the pilot can generate a good picture of the weather over a wide area when many reporting stations are looked at together.

12.1.2 Air Route Traffic Control Center (ARTCC) #

The ARTCC facilities are responsible for maintaining separation between flights conducted under instrument flight rules (IFR) in the en route structure. Center radars (Air Route Surveillance Radar (ARSR)) acquire and track transponder returns using the same basic technology as terminal radars. Earlier center radars displayed weather as an area of slashes (light precipitation) and Hs (moderate rainfall). Because the controller could not detect higher levels of precipitation, pilots had to be wary of areas showing moderate rainfall. Newer radar displays show weather as three shades of blue. Controllers can select the level of weather to be displayed. Weather displays of higher levels of intensity make it difficult for controllers to see aircraft data blocks, so pilots should not expect air traffic control (ATC) to keep weather displayed continuously.

12.1.3 Upper Air Observations #

Observations of upper air weather are more challenging than surface observations. There are only two methods by which upper air weather phenomena can be observed: radiosonde observations and pilot weather reports (PIREPs). A radiosonde is a small cubic instrumentation package which is suspended below a six foot hydrogen or helium filled balloon. Once released, the balloon rises at a rate of approximately 1,000 feet per minute (fpm). As it ascends, the instrumentation gathers various pieces of data such as air temperature and pressure, as well as wind speed and direction. Once the information is gathered, it is relayed to ground stations via a 300 milliwatt radio transmitter.

The balloon flight can last as long as 2 hours or more and can ascend to altitudes as high as 115,000 feet and drift as far as 125 miles.

Since the pressure decreases as the balloon rises in the atmosphere, the balloon expands until it reaches the limits of its elasticity, at which point the balloon pops and the radiosonde falls back to Earth, its descent slowed by a parachute. Each year over 75,000 balloons are launched; of that number, 20 percent are recovered and returned for reconditioning.

Pilots also provide vital information regarding upper air weather observations and remain the only real-time source of information regarding turbulence, icing, and cloud heights. This information is gathered and filed by pilots in flight. Together, PIREPs and radiosonde observations provide information on upper air conditions important for flight planning. Many domestic and international airlines have equipped their aircraft with instrumentation that automatically transmits inflight weather observations through the DataLink system to the airline dispatcher who disseminates the data to appropriate weather forecasting authorities.

12.1.4 Radar Observations #

Weather observers use four types of radar to provide information about precipitation, wind, and weather systems. (1)

The WSR-88D NEXRAD radar, commonly called Doppler radar, provides in-depth observations that inform surrounding communities of impending weather. Doppler radar has two operational modes: clear air and precipitation. In clear air mode, the radar is in its most sensitive operational mode because a slow antenna rotation allows the radar to sample the atmosphere longer; images are updated about every 10 minutes in this mode.

Precipitation targets provide stronger return signals, therefore the radar is operated in the precipitation mode when precipitation is present; a faster antenna rotation in this mode allows images to update approximately every 4 to 6 minutes. Intensity values in both modes are measured in dBZ (decibels of Z) and depicted in color on the radar image. [Figure 12-1] Intensities are correlated to intensity terminology (phraseology) for air traffic control purposes. [Figures 12-2 and 12-3]

(2)

FAA terminal doppler weather radar (TDWR), installed at some major airports around the country, also aids in providing severe weather alerts and warnings to ATC. Terminal radar ensures pilots are aware of wind shear, gust fronts, and heavy precipitation, all of which are dangerous to arriving and departing aircraft. (3) The FAA airport surveillance radar is used primarily to detect aircraft, but it also detects the location and intensity of precipitation which is used to route aircraft traffic around severe weather in an airport environment. (4) Airborne radar is equipment carried by aircraft to locate weather disturbances. The airborne radars generally operate in the C or X bands (around 6 GHz or around 10 GHz, respectively) permitting both penetration of heavy precipitation, required for determining the extent of thunderstorms, and sufficient reflection from less intense precipitation.

12.1.5 Satellite #

Advancement in satellite technologies has recently allowed for commercial use to include weather uplinks. Through the use of satellite subscription services, individuals are now able to receive satellite transmitted signals that provide near real-time weather information for the North American continent.

12.1.6 Satellite Weather #

Recently private enterprise and satellite technology have expanded the realm of weather services. Pilots now have the capability of receiving continuously updated weather across the entire country at any altitude. No longer are pilots restricted by radio range or geographic isolations such as mountains or valleys.

In addition, pilots no longer have to request specific information from weather briefing personnel directly. When the weather becomes questionable, radio congestion often increases, delaying the timely exchange of valuable inflight weather updates. Flight Service Station (FSS) personnel can communicate with only one pilot at a time, which leaves other pilots waiting and flying in uncertain weather conditions. Satellite weather provides the pilot with a powerful resource for enhanced situational awareness at any time. Pilots have a choice between FAA-certified devices or portable receivers as a source of weather data.

12.1.7 Satellite Weather Products #

Significant Meteorological Information (SIGMET): SIGMETs are weather advisories issued concerning weather significant to the safety of all aircraft. SIGMET advisories can cover an area of at least 3,000 square miles and provide data regarding severe and extreme turbulence, severe icing, and widespread dust or sandstorms that reduce visibility to less than three miles. [Figure 12-4]

Airmen’s Meteorological Information (AIRMET): AIRMETs are weather advisories issued only to amend the area forecast concerning weather phenomena which are of operational interest to all aircraft and potentially hazardous to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications. AIRMETs concern weather of less severity than that covered by SIGMETs or convective SIGMETs. AIRMETs cover moderate icing, moderate turbulence, sustained winds of 30 knots or more at the surface, widespread areas of ceilings less than 1,000 feet and/or visibility less than three miles, and extensive mountain obscurement. [Figure 12-5]