AWH 3-2

Chapter 3, Overview of Aviation Weather Information 3-2 3.2 Use of Aviation Weather Information 3.2.1 Understanding Weather Forecasts To forecast means to make a prediction about the future state of something. In weather forecasting, the meteorologist is trying to predict the future state of the atmosphere. There are many different types of weather forecasts that exist, and we want to cover a basic distinction between deterministic and probabilistic forecasts. • A deterministic forecast is one in which forecasters provide only a single solution. For example, “tonight’s low will be 31 degrees Fahrenheit (°F),” “0.30 inches of rain will fall tomorrow,” or “thunderstorms will happen at 3 pm tomorrow.” • A probabilistic forecast is one in which forecasters convey uncertainties by expressing forecasts as probabilities of various outcomes. For example, “the probability that tonight’s low will be below 32 °F is 40 percent,” “the probability of receiving at least 0.25 inches of rain tomorrow is 60 percent,” or “the probability of thunderstorms for tomorrow afternoon is 70 percent.” Probabilistic forecasts allow meteorologists to acknowledge and express the fact that weather forecasts contain uncertainty, especially as you go out further in time. However, it is important that a probabilistic forecast has reliability (e.g., if the forecast of thunderstorms for a particular day, time, and location is 70 percent, then thunderstorms do in fact occur seven times out of ten). 3.2.2 Product Latency With few exceptions, all weather information and products have latency. Latency is the element of data age. The total latency of weather information and products includes the total time between the actual occurrence of the phenomenon, the data collection, processing, transmittal, and the display or application of the information in the cockpit, on the pilot’s EFB, or other publication for use. It is important to be aware of the product time or “valid until” time on the particular data link information displayed in the cockpit or EFB. The amount of latency may limit the use or application of the information or product. An example of weather information without latency is the wind direction when looking at the windsock along the runway. However, the wind reported in the Automated Weather Observing System (AWOS) or Automated Surface Observing System (ASOS) broadcast has a latency of up to 3 minutes. Why? While the AWOS and ASOS wind (direction and speed) is continuously being recorded by the AWOS/ASOS system processor, the reported wind is the most recent average of the direction and speed over the past two minutes. That 2-minute average is then updated once a minute for the radio or telephone broadcast. Onboard aircraft radar has minimal latency, while NEXRAD data has a latency of 5 to 15 minutes or more with weather apps and data uplink services. This is why NEXRAD data is used for broad strategic avoidance of thunderstorms and never used to navigate through thunderstorms. 3.2.3 Additional Use Information Details on the use of both government and commercial aviation weather information are discussed in the AIM, Chapter 7, Section 1, Paragraph 7-1-3, Use of Aviation Weather Products. Items discussed include: • Approved sources for aviation weather information, • The development of new products through the FAA’s Next Generation Air Transportation System (NextGen) Aviation Weather Research Program (AWRP),