To get a good understanding of aviation weather, you must know three things: global rules, local rules, and exceptions. An excellent weather teacher used to tell that all meteo rules but one have exceptions.

Global rules can be learned from the books. Click here if you look for a good aviation weather book. One can learn there that air tends to move from high to low pressure, and Coriolis force make wind blog along isobars, how thunderstorms develop, and what advection fog is.

Most ot this takes place in the very formal International Standard Atmoshpere, a.k.a. ISA, which is a model of the Earth atmosphere. ISA describes pressure, temperature, density, and their relations. It’s a good learning tool, but never matches reality. Some call it “Hollywood”.

Local rules almost all derive from the principles described in the ISA, and result from local specificities, like mountains, desert, lakes, shores, cities, forests, glaciers, or even absence of such things. Thunderstorms are a good example how local environment influence weather. When atmosphere gets unstable enough, any parcel of ascending air can trigger a thunderstorm. Wind blowing towards a mountain is perfect for that.

When the Geneva (LSGG) METAR mentions FEW / SCT CB or TSRA, approaches from the East are usually not an issue, as most CBs start over the Alps or the Jura. Here in Frankfurt, the Taunus hills are a possible trigger, but it’s also frequent to see CBs build out of “nothing” but stronger instability. I also shown pure thunderstorm – the model with no rain at all, which are rather seldom in Switzerland, 500 kilometers away only.

Because such CB’s triggers are smaller, they require more instability to start, resulting in much quicker and stronger developments, and theses can start anywhere. Their lower predictability makes them somehow more dangerous, particularly for light aircrafts flying without radar (no, stormscope is no replacement).

Another typical very local phenomenon is sea-haze. I read about it in books (global rule), and experienced it in Cannes (local rule). When overflying clouds coming from the sea, I overlooked them as they looked really benign. Few minutes after landing, these clouds did reach the airport, and within less than 15 minutes, weather was below minimums (which are rather high due to terrain).

It’s hard to write about exceptions, which are unexpected by nature. High pressure haze usually involves weak winds in the lower levels. I flew once in typical summer high pressure haze, visibility was below 6 kilometers at 2′000 feet AGL… with nearly 30 kts wind !

The only way to prevent any negative consequences is good flight planing, with sufficient fuel to fly to an alternate not susceptible to be affected by the same kind local weather. Being prepared but also ready to divert will help to keep you on the safe side.

If you have personal experience with cases where Mother Nature seemed to behave in an unruly way, I’d be happy if you share them in a comment.

PS: The rule which suffers not exception is that atmospheric pressure decreases when altitude increases.