It’s usually the flight home, still wearing your shorts and flip flops, you notice how remarkably cold planes can be. No, it’s not a considerate attempt by the airline to help passengers reacclimatise to the awaiting chill of Blighty’s drizzle, but instead a regulated effort to ensure maximum comfort throughout the cabin.

Aircraft temperatures are generally kept at between 22°C and 24°C, about the same air temperature maintained in most office environments, with the extremes of the range reaching 18°C and 27°C. Such temperatures might seem reasonable, but it is important to bear in mind that passengers rarely move when flying so work up little body heat of their own, hence the overwhelming sense of coldness.

Why is it kept so cold?

Broadly speaking, it’s all about keeping the cabin pleasant. In a report commissioned by the House of Lords, Airbus explained “the heat given off by passengers in a fully occupied cabin is considerable. Incoming air needs to be at or below the required cabin temperature if that temperature is to be maintained”.

The French plane manufacturer added that, though the levels of individual seats cannot be controlled - apart from from the tiny, air conditioning nozzle above seats - the cabin crew can adjust temperatures by “zone”.

'I'll be having that blanket, thank you' Credit: Izabela Habur/izusek

Boeing, Airbus's American rivals, told the House of Lords committee that flight crew normally change cabin temperatures in response to passengers representations, however, adding that “because passengers are normally in repose and cabin crew are working, their perceptions of thermal comfort are likely to be different”.

The report also made note of a number of complaints from passengers accusing cabin crew of “inappropriate settings” to encourage passengers to sleep after meals.

The coolness of mid-flight is normally marked in contrast with conditions before take-off. This is because the air conditioning units on the aircraft are often turned off to save fuel before the plane is in the air, so the body heat of scores of people heats the cabin quickly.

Larger, heavier planes sometimes need the power drawn away by the air conditioning units for take-off, which is why they are not turned on until after the aircraft has ascended to a suitable altitude.

A more scientific reason for why aircraft are kept so cool comes from a study conducted by the American Society for Testing and Materials, which found that more people are likely to faint in a warm cabin, owing to hypoxia, a medical condition common in airline passengers that occurs when body tissue does not receive enough oxygen.

Indeed, there was an occasion in Dallas when an American Airlines flight attendant reportedly fainted in an overheated plane.

Is there a maximum temperature?

The industry standard maximum temperature is 90˚F or 32.2˚C, however, that temperature would rarely be experienced during flight, as it is really very warm.

Just remember that it’s easy to add more layers than it is to take off more layers, without being arrested for public indecency.

What about humidity levels?

Patrick Smith, author of Cockpit Confidential and pilot, writes: “If passengers have one very legitimate gripe, it’s about dryness. Indeed, the typical cabin is exceptionally dry and dehydrating. At around 12 per cent humidity, it is drier than you will find in most deserts.

“This is chiefly a by-product of cruising at high-altitudes, where moisture content is somewhere between low and nonexistent.”

Smith goes on to argue that humidifying the cabin might seem like the solution, but such equipment is expensive, and the resulting increase in moisture is both marginal and potentially corrosive to the guts of the airframe.

What about in the hold?

The hold, where the luggage is kept, along with your darling pets, is pressurised just like the cabin. Doug Morris, a pilot with Air Canada on an Airbus A320 explains that the same air used to condition the cabin is used for the hold.

“Many are surprised to hear that the cargo hold in the belly of an airliner is pressurised,” he said. “Conditioned air is directed from the cabin, so the air tends to be a little cooler by the time it reaches the cargo areas, which are also less insulated than the cabin. Cargo temperatures vary in our fleet. The Boeing 767 maintains its baggage hold above 7˚C, but the bulk area (where animals are carried) can be heated above 18˚C.

“Controlled temperature cargo bins are also available when temperature-sensitive goods are being shipped.”

Why do stowaways freeze to death?

As people who run onto an aircraft at the last-minute and hide in the wheel arches are not inside the pressurised cabin, they are exposed to the elements. The higher you get, the colder it gets, up until 40,000 feet. If the temperature at ground level was 20˚C, at 40,000 feet it would be -57˚C. At 35,000 feet the air temperature is about -54˚C. Very few would survive such temperatures.

Do you have any other burning aviation questions? Get in touch: travelonline@telegraph.co.uk





