Weather Information — Page 104, Lesson 92

I want to walk you through the weather information section, starting with Volmet broadcasts. Volmet is a service that provides continuous or scheduled broadcasts of meteorological information for aviation. You'll find the frequencies for Volmet in the Aeronautical Information Publication, or AIP, or in en route booklets such as Aerad. Each Volmet transmission gives you meteorological information for a group of aerodromes, and these broadcasts are made at set times past the hour. Now, let's look at the standard contents of a Volmet broadcast. These broadcasts, along with other meteorological transmissions, follow a standard format. I want you to be familiar with the terms and units of measurement you'll encounter, which you can study in detail in the UK AIP MET section. Here's what a Volmet broadcast includes, item by item. First, aerodrome identification — that's the name or code of the airport the report is for. Next, surface wind. This is given in degrees and knots. For example, you might hear "250 degrees 10 knots," meaning the wind is coming from 250 degrees true at a speed of 10 knots. Then, visibility. This is reported in metres when visibility is below 5 kilometres, or in kilometres when it's 5 kilometres or more. After that, Runway Visual Range, or RVR. This is reported in metres, but only when the RVR is between 50 and 1500 metres. Outside that range, it's not reported in the Volmet. Next, weather. This is given without units — for example, "rain" — just the type of weather phenomenon present. Then, cloud. Cloud cover is reported in octas, where one octa equals one eighth of the sky covered. The standard terms are: Few, which means 1 to 2 octas; Scattered, which is 3 to 4 octas; Broken, which is 5 to 7 octas; and Overcast, which means the sky is completely covered. Temperature is given in degrees Celsius, and dew point is also in degrees Celsius. Then, QNH, which is the barometric pressure adjusted to sea level, reported in hectopascals or millibars — they're numerically the same. Finally, trend. This is a forecast of how the weather is expected to change, and it's given without units. Now, there's a special term you need to know: CAVOK. It's pronounced "CAV-O-KAY." CAVOK is a shorthand that tells you a combination of conditions are better than prescribed values. Specifically, it means three things are all true at the same time. First, visibility is 10 kilometres or more. Second, there is no cloud below 1500 metres, which is 5000 feet, or below the Minimum Sector Altitude, whichever is greater, and there are no cumulonimbus clouds, which we call CB. Third, there is no significant weather — that means no precipitation, no thunderstorm, no shallow fog, and no low drifting snow. Note carefully: surface wind is not included in the CAVOK condition. CAVOK only covers visibility, cloud, and weather — not the wind. So even if conditions are CAVOK, the wind could still be strong or gusty, and that would be reported separately in the surface wind part of the broadcast. That figure shows examples of pressure settings, which relates to the QNH you'll hear in a Volmet. But for now, you have the full breakdown of what a Volmet broadcast contains and what CAVOK means.