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My introduction flight was canceled because of "the turbulence due to heat". They called half an hour before the time of the flight (Cessna 172).
I'm okay to reschedule it but I'm a bit confused:
If it was "due to heat" why did they call right before the flight? We have had this heat the whole week, or even greater (≈ +31°C). Is it possible they didn't know there was turbulence? How do they even know?
How can the heat cause turbulence?
Thanks!
The weather creating the conditions is atmospheric convection from a combination of the sun heating the ground (unevenly) and instability of the atmosphere in between the ground and 5-10,000 feet.
The instability occurs when the lapse rate (temperature drop with altitude) is steep, so that if a package of air is warmed at the ground and something gets it moving up, it wants to go higher faster, the higher it goes. More or less like a hot air balloon.
So you end up with a vertical circulation in the atmosphere between the ground and cloud base. Air rising, and beside it, air falling.
The vertically rising columns of air are called Thermals. If the humidity of the air is high enough, cumulus clouds form at the top of the thermal as the air cools to the dew point at 5, 7, or 10000 feet or wherever.
Gliders exploit this vertical circulation to remain airborne. For power pilots, it causes an unpleasant ride as you constantly fly through air that is rising, then air that is falling to fill in for the air that was rising.
Anyway, they called you before because the weather conditions were rough, due to that kind of thermal activity, and they knew there was a high risk you'd get airsick, which is bad for intro flights, as you can imagine. They were playing it safe for a potential long term customer.
It'll be worst between 11 am and about 4 pm and if your flight was say, scheduled for 1 pm, and the required conditions are present, the chances are really high it'll be too rough. If the winds are high, it breaks up the thermals and makes the ride even choppier, so worse still. I would never take anybody new flying on an afternoon with lots of convection and especially convection + high winds. They'd be barfing within 20 minutes.
Next time you book your flight, try to book it for early morning, say 8 or 9 am, when the air will be silky smooth even if it's going to be a convective day. Or, evening, say after 5 pm.
At some point later you'll find out if you have a problem with air sickness in the bumps. Generally even if you do get sick, repeated exposure desensitizes you and the airsick problem goes away. If it is a problem, spending some time on a park swing can help with the desensitization. It's basically a variation on seasickness.
Weather forecasting is not a perfect science, forecasters can predict with reasonable accuracy how the conditions will be over a large area, but there will often be significant variations within that area. There's no way to predict a week ahead of time how conditions will be over an airport with enough accuracy to cancel an introductory flight.
The sun heats the ground, which heats air, causing it to rise and expand. Cooler air falls and contracts, then gets heated and rises again. This cycling is called convection. Air moving up next to air moving down causes the air in the boundary between to rotate. This rotation happens all the time in air, the hotter it is the more energetic the rotation becomes and eventually it causes chop or turbulence which is unpleasant. Every day is different, variations in weather patterns can mean that two days with the same air temperature can have different air conditions, for instance an overcast day has less sunlight hitting the ground, so less convective weather.
Conditions can change throughout the day as well, mornings are usually smoother than afternoons, some days are flyable all day, other times conditions may deteriorate relatively quickly. What most likely happened was that flights came back reporting unpleasant flying conditions and they made the decision to cancel introductory flights as you wouldn't enjoy getting tossed around like a shoe in a dryer, and you may even throw up.
As for why they didn't tell you sooner, they wanted to fly, that's why they are there. This isn't uncommon, when I was in training I would sometimes get called in the morning telling me flying was off. Even today with a full instrument rating I'll still occasionally get to the airfield and then make the decision not to fly because of conditions. It's just part of flying.
As other answers have mentioned, differential heating, from different levels of light absorption of areas of the ground near each other, can cause turbulent air in sunlight, and how turbulent that ends up being can be unpredictable.
I have also flown near farmland which in the summer can massively change light absorptivity overnight - freshly-plowed fields are very dark and can be right next to lighter-colored crop or fallow grass fields. (FAA Glider Handbook, chapter 10, page 10-4 mentions dark ground and especially "large tilled black soil field" "heat quicker than grass covered fields". )
