Let’s begin with gusts — Page 151, Lesson 130

I want to walk you through the topic of winds, starting with some important definitions for special wind phenomena that you’ll need to recognise as a professional pilot. Let’s begin with gusts. A gust is a sudden increase in wind speed, often accompanied by a change in direction, and it lasts for less than one minute. It’s a local effect — meaning it happens in a small area, not over a wide region. Now, here’s the operational threshold you need to remember: a gust will only be reported or forecast if it is 10 knots or more above the mean wind speed. So if the average wind is 15 knots and a sudden burst hits 24 knots, that’s only 9 knots above — it wouldn’t qualify as a reportable gust. But if it hits 25 knots, that’s 10 above, and it gets reported. The opposite of a gust is a lull. A lull is a sudden decrease in wind speed. There’s no specific threshold given for reporting a lull in this excerpt, but the concept is straightforward — it’s the calm counterpart to a gust. Next, we have squalls. A squall is also a sudden increase in wind speed, often with a change in direction, but the key difference from a gust is duration and scale. A squall lasts for one minute or more, and it can cover a wide area — not just a local spot. Squalls are often associated with cumulonimbus cloud and cold fronts. So if you see a line of thunderstorms moving through, expect squally conditions. Now, gale. A gale exists when the sustained wind speed exceeds 33 knots, or when gusts exceed 42 knots. Notice the word "sustained" — that means the average wind over a period, not a momentary peak. So if the sustained wind is 34 knots, that’s a gale. Alternatively, even if the sustained wind is lower, if gusts are hitting 43 knots, that also qualifies as a gale. Finally, hurricane force wind. This exists when the sustained wind speed exceeds 63 knots. That’s a very high threshold — well beyond what you’ll normally encounter in routine operations, but it’s the definition used for the most extreme conditions. Now let’s move on to how we actually measure wind. Surface wind is measured using two instruments working together. First, a wind vane — this aligns itself with the wind direction, so it tells you where the wind is coming from. Second, an anemometer — this measures the wind speed. The standard anemometer is a set of three hemispherical cups mounted on a shaft. As the wind blows, it pushes the cups, causing the shaft to rotate. The speed of rotation of the shaft is directly proportional to the wind speed — so twice the rotation speed means twice the wind speed. That rotation drives a small generator, and the electrical output from that generator is displayed on a gauge that is calibrated in knots. There’s an important ICAO requirement for where these instruments must be placed. The wind vane and anemometer should be positioned 10 metres (33 feet) above aerodrome level, and they must be located clear of buildings and obstructions that could affect the airflow and therefore the accuracy of the readings. So you won’t find them tucked next to a hangar — they need open exposure. An anemograph is a device that records both wind speed and direction over time, giving you a continuous trace rather than just a live reading. For upper winds — the winds at altitude — we measure them differently. The two main methods are GPS tracking of a radiosonde and aircraft reports. A radiosonde is a weather balloon-borne instrument package; as it rises, its position is tracked by GPS, and from that we can calculate the wind speed and direction at various levels. Aircraft reports, which you as a pilot will contribute to, provide actual in-flight wind data. That covers the key definitions for gusts, lulls, squalls, gales, and hurricane force winds, as well as the instruments and methods used to measure surface and upper winds.