Where is the coolest place to leave something in a car parked in full sun — boot, footwell, glovebox?

For question 3, what works and what does not:

Genuinely effective: a reflective screen on the windscreen. It is the biggest piece of glass and the one the sun goes through most directly; a screen can take several degrees off the whole cabin and a great deal off the dashboard.

Effective and underused: park facing the sun's path, not perpendicular to it. A car nose-on to the sun exposes far less glass area than one broadside. Thinking about where the sun will be in four hours, rather than where it is now, is worth more than any accessory.

Marginal: cracking the windows. It helps a little by letting the hottest layer escape, and the effect is much smaller than people believe because the heat is arriving by radiation, not by trapped air. Weigh it against rain and security.

Does nothing: parking in the shade of a tree at 10am when you are back at 4pm. Shade moves. This one catches everybody, including me.

The footwell wins, and consistently — specifically the rear footwell, on the shaded side of the car.

The reasons stack up:

• It is below the windows. Almost all the heat comes in as sunlight through glass, and the footwell is the largest volume in the car that direct sun never reaches.
• Hot air rises. The car stratifies dramatically: the layer near the roof can be twenty degrees hotter than the layer at the floor.
• It is against the ground. The floor pan is in contact with air under the car, which stays close to shade temperature, and the ground itself is a large heat sink.

The boot is the usual runner-up and it depends on the car. A saloon boot is enclosed and shaded, which is good, but it is also unventilated and often above the exhaust and the fuel tank. An estate or hatchback boot is under glass and is one of the hottest places in the vehicle.

The glovebox is a popular answer and a poor one: it is small, insulated, faces the windscreen and is above the floor.

Numbers, because they are more alarming than people expect.

On a 30°C day in full sun, a closed car's cabin reaches roughly 50-60°C within an hour, and the dashboard surface can exceed 80°C. Most of the rise happens in the first twenty to thirty minutes — the curve is steep early and then flattens as the car approaches equilibrium.

The stratification is the part that answers your question: floor to roof can differ by 15-20°C in a car standing in sun. So the footwell might be at 40°C while the parcel shelf is at 60°C.

That is a large difference and it is often the difference between "warm" and "ruined", but note what it is not: it is not cool. Nothing in a car parked in sun is cool, and nothing that must not exceed body temperature should be left in one.

Since you mentioned the gas canister: that one is not a temperature-preference question, it is a do-not.

Canisters have a stated maximum storage temperature, usually around 50°C, and a car in sun exceeds that comfortably. Same for lithium batteries and power banks, which degrade permanently well below the temperature at which anything visible happens, and aerosols of any kind.

Footwell is the right answer for a packed lunch and a bottle of water. For anything pressurised or with a battery in it, the right answer is that it comes with you or it stays at home.