Learning hub · Meteorology · Lesson 5

Wind, fog and visibility

Wind decides which runway you use and how the aeroplane behaves near the ground. Fog and poor visibility decide whether you fly at all. Both follow a few simple rules.

About 12 minutes, plus the quiz. Part of Meteorology. It builds on Pressure, QNH and the altimeter.

1. Why the wind blows

Wind is air moving from high pressure to low pressure. The difference in pressure over a distance is the pressure gradient, and it is shown on a chart by how close together the isobars are. Close isobars mean a strong wind.

Air does not go straight from high to low, because the Earth turns. The Coriolis effect deflects moving air to the right in the northern hemisphere. Above the surface, the two forces balance, and the wind (the gradient wind, or geostrophic wind when the isobars are straight) blows nearly parallel to the isobars.

Buys Ballot’s law gives the direction: in the northern hemisphere, stand with your back to the wind and the low pressure is on your left. Winds circulate anticlockwise around a low and clockwise around a high.

2. Surface wind, backing and veering

Near the ground, friction slows the wind. With less speed, Coriolis becomes weaker, and the pressure gradient wins a little: the wind turns towards the low pressure, across the isobars. Over land the change is larger than over the sea. As a rough guide, the surface wind is turned by something in the order of 10° to 30° and is much weaker than the wind a few thousand feet up.

In the northern hemisphere this means the wind veers (turns clockwise) as you climb out of the friction layer, and backs (turns anticlockwise) as you descend toward the ground. The wind also usually becomes stronger with height. On a day with a 15 kt surface wind, the wind at 2,000 ft may be 25 kt and from a different direction, which affects the ground track of your flight and the drift you correct for. See The wind triangle.

On a warm sunny day, the surface wind is gustier, as thermals bring faster air down to the ground. A sea breeze is a local wind: the land warms faster than the sea, the warm air above the land rises, and cooler air from the sea flows in to replace it. It starts in the late morning, is strongest in the afternoon, and flows from sea to land. At night the land cools faster and a weaker land breeze may blow the other way. A sea breeze can change your runway in use during the day.

3. Fog: how it forms, and when it clears

Fog is cloud on the ground. It forms when air cools to its dew point, or when moisture is added to it. By international convention, fog means visibility below 1,000 m, and between 1,000 m and 5,000 m it is called mist.

• Radiation fog: on a clear, calm night the ground loses heat and cools the air above it to the dew point. It forms in valleys and low ground, and typically clears in the morning when the sun heats the ground, but in winter it can last all day.
• Advection fog: warm, moist air blows over a colder surface, such as the sea or snow. It needs wind rather than calm, and can form at any time of day. It only clears when the wind changes or the air mass moves on.
• Hill fog (upslope): moist air is forced to rise over higher ground, cools, and condenses. It is really cloud on the hill.
• Frontal fog: rain falling from a warm front into cooler air below adds moisture until the air saturates.
• Steam fog: very cold air blows over a warmer water surface.

Rule of thumb: when the temperature and dew point are within 2 to 3°C of each other, and the gap is closing on a calm, clear evening, fog is likely during the night. This is a guide, not a forecast. Use the TAF and the trend.

4. Visibility and RVR

In a METAR, visibility is given in metres up to 9,999, meaning 10 km or more. It is the distance at which an object can be seen and recognised, reported by the observer or an instrument. If it differs by direction, the lowest may also be given.

RVR (runway visual range) is different. It is the distance over which a pilot on the runway centreline could see the runway markings or lights, and it is measured by instruments at the runway. RVR matters for low-visibility take-offs and instrument approaches. It is added to a METAR as a group like R24/0800 (runway 24, 800 m) when visibility or RVR is low, normally below 1,500 m.

For a VFR student pilot, the numbers in the METAR are a starting point. The visibility you actually get in flight can be worse than at the airfield: looking into the sun, through haze, or in a rain shower, it may be much less.

The 2 to 3°C rule and the typical numbers here are guides. Real minimum visibility and cloud for VFR depend on airspace and altitude, so check SERA and your national AIP, and set personal minima that are stricter.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. In the northern hemisphere, you stand with your back to the wind and find the low pressure to your left. A friend says the wind at 3,000 ft is from the south-west and the surface wind is from the south. Is this the usual pattern?
   Show the answer

   Buys Ballot is satisfied for the gradient wind. Friction backs the wind near the ground, so the surface wind should be anticlockwise of the wind aloft: south is anticlockwise of south-west. Yes, this is the usual pattern.

   The wind veers (turns clockwise) from south to south-west as you climb. The surface wind should also be weaker.

2. At 1800 UTC the METAR shows 13/11, wind calm, clear sky, and the temperature is falling about 1°C every hour. What would you suspect for the night and the next morning? What might change that?
   Show the answer

   The spread is 2°C and shrinking, with clear sky and calm air: ideal for radiation fog. At that rate the temperature reaches the 11°C dew point in about 2 hours, around 2000 UTC, and keeps cooling after that, so fog is likely during the night (the dew point itself may also fall). The cooling rate will not stay steady.

   What could change it: cloud arriving (it stops the ground from cooling), or a wind picking up (it stirs the air). Check the TAF and the latest METARs before you decide.

3. Explain in two sentences why a sea breeze blows from sea to land in the afternoon, and what it does to your runway wind.
   Show the answer

   The land heats faster than the sea, so the air over land warms and rises, and cooler sea air flows inland to take its place. The breeze starts late morning and is strongest in the afternoon.

   If the gradient wind was blowing offshore, the sea breeze can oppose it or turn the surface wind. Expect the runway in use to change during the day, and check the latest METAR before returning.

6. Watch

The pressure gradient, the Coriolis effect and surface friction, which are the three ideas in the first sections. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A short explanation of the sea breeze from the UK national weather service.

Open on YouTube if the player does not load. Not made by VectorEFB.

The main fog types and how they form. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: Weather charts and the briefing. Back to the learning hub.