Each one serves a specific purpose, and we'll go through them one by one — Page 512, Lesson 492

I want to walk you through the warning messages used in aviation meteorology. These are the alerts that keep us informed about hazardous weather conditions, both while we're en route and when we're operating at or near an aerodrome. Let's start with the introduction. There are five main types of warning messages used to alert pilots to meteorological hazards. These are: Aerodrome Warnings, Windshear Warnings, SIGMET, Volcanic Ash Advisory Messages, and Tropical Cyclone Warning Messages. Each one serves a specific purpose, and we'll go through them one by one. First, let's look at Aerodrome Warnings. These are issued by the competent meteorological authority — that's the official weather service responsible for that region. They cover a specific set of hazards. The list includes: gales; strong wind warnings, which are triggered when the mean wind speed exceeds 20 knots with gusts of 28 knots, or 25 knots with gusts of 37 knots — and the exact threshold depends on the individual aerodrome's requirements; thunderstorms, hail, or squalls; snow; frost warnings, which include hoar frost, glaze ice, and rime ice; fog, when the visibility is expected to fall below 600 metres; and freezing precipitation. There's also an additional warning for a marked temperature inversion at selected aerodromes. This is issued if a temperature increase of 10 degrees Celsius or more exists between the surface and 1000 feet above the surface. That's a significant inversion, and it can affect aircraft performance and engine operations. Now let's move to Windshear Warnings. Windshear warnings for aerodromes may be appended to METARs — though not in the UK — or they may be passed by Air Traffic Control. The format of these warnings is variable. They could be given as a loss or gain in airspeed, as crosswind variations, or as updraughts or downdraughts, depending on what has been experienced or what is likely to occur. These warnings are issued for conditions on approach or departure paths up to 1600 feet above aerodrome level, unless a greater height is deemed prudent. The Met Office will issue windshear warnings when conditions indicate that windshear is probable, and also when windshear has been reported by pilots. So to summarise what we've covered so far: we have five categories of warning messages. Aerodrome Warnings cover specific surface and low-level hazards like gales, strong winds, thunderstorms, snow, frost, fog, freezing precipitation, and marked temperature inversions. Windshear Warnings alert us to windshear on approach or departure paths, and they can come in various formats depending on what's being experienced.