Why balloons fly at dawn and almost nowhere else
For most of the day the air near the ground is boiling. Balloons fly in the two short windows when it is not.
If you have only ever seen balloons in photographs, you have mostly seen them at sunrise. That is not a stylistic preference. It is very close to the only time they can fly.
The problem is the sun
Through the day the sun heats the ground, the ground heats the air touching it, and that warm air breaks away in bubbles and rises — thermals. By late morning the lowest kilometre of atmosphere is full of them: columns of rising air with sinking air between, all of it moving, all of it invisible.
For a glider that is fuel. For a balloon it is a nuisance and, near the ground, a hazard:
- Vertical currents overwhelm the burner. A balloon that is climbing when you wanted it level is not something you can correct quickly — see how a balloon rises for why every input arrives late.
- Thermals make the surface wind gusty and variable in direction, which is exactly what you do not want in the final thirty seconds of a landing.
The two windows
After sunrise. Overnight the ground cools and the air near it cools with it, often leaving colder air lying under warmer — an inversion. That layer is stable: nothing wants to rise through it, so the air is smooth and the wind runs in clean layers. It lasts from first light until the sun has had an hour or two to undo it.
Before sunset. The reverse, as heating shuts down and the thermals die out. Shorter, less reliable, and it ends in darkness — which is why the morning window is the one people plan around.
Stable air keeps its layers separate, so the difference in direction between one height and another is at its sharpest just after sunrise. The calmest air of the day is also the most steerable. That is not a coincidence — both come from the same stability.
The other constraints
Wind speed is the hard limit. Above roughly 15 knots at the surface most operations stop, because a balloon lands at whatever speed the wind is doing and a basket arriving at 30 km/h is a bad day.
Then rain, which soaks the envelope, adds weight and cools it. Then thunderstorms, which can produce violent up and down currents from a long way away — the one weather feature everyone gives an enormous margin to.
Which leaves: clear, calm mornings. Ballooning is a hobby with a very small window, and most of the skill is in reading whether today has one.