It's a late autumn afternoon and the sky is clear. You decide to go for a short sunset flight. You check the nearest METAR and it looks good. What could possibly go wrong?
EGGW 041450Z VRB02KT 9999 CAVOK 06/05 Q1026
Can you identify the possible threat in this METAR?
During the autumn and winter after sunset, the temperature begins to drop quickly. As the sun sets, the ground no longer receives solar radiation and starts to cool. If the sky is clear, it will cool quickly.
As the earth cools, it will begin to cool the air just above the surface too. Notice that the temperature and dew point are very close together. This means that the relative humidity is high. As the air cools, it becomes more saturated (remember, cold air can't hold as much water vapour). After it cools to the dew point, any further cooling will result in condensation causing fog. This type of fog is called radiation fog.
Ingredients for radiation fog:
clear skies;
calm winds;
high relative humidity;
a stable atmosphere.
Knowing this, you can look at the weather report and identify whether such conditions exist and are likely to cause a problem. The METAR above indicates light winds (VRB02), clear skies (CAVOK), high relative humidity (06/05) and high pressure (Q1026) which is indicative of a stable atmosphere. It is also worth checking the Low Level Weather Forecast (F215) chart which may explicitly state that fog is expected.
Such a situation could result in the pilot taking-off in clear conditions only to find themselves trapped above a layer of radiation fog that has formed whilst they are flying.
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