Why are planes so loud today? Wind, cloud and temperature explained

On some days every plane seems to rattle the windows; on others you barely notice them. The answer comes in two parts. Most of the big differences come from where the planes are. The weather then adds or takes away a few decibels on top.

What makes planes sound louder on some days

A plane does not get louder because of the weather; the aircraft makes much the same noise every day. What changes is how much of that noise reaches you. In rough order of size:

Every 10 dB sounds about twice as loud to the ear, so a few decibels is a real but modest change. Being under the landing path instead of beside it is a far larger one.

Wind carrying the sound towards you

Wind is slower near the ground than higher up, and that difference bends sound. Downwind of a noise, the sound curves down towards the ground and levels go up; upwind, it curves away and levels drop. Studies of road traffic noise have measured differences of up to 7 dB(A) caused by the wind, within 100 m of a road.

Bending needs distance to add up, so for aircraft it matters most for sound that travels a long way close to the ground: the roar of planes starting their take-off roll, reverse thrust after landing, or planes a few kilometres to one side. For a plane passing straight overhead, the sound has a short, steep path to you and bends less. Southampton Airport notes that strong winds can carry sound a long way, and that in gusty, rainy weather the sound breaks up and planes seem quieter.

Still, cold air: temperature inversions

By day the air is usually warmest near the ground and cools with height, which bends sound upwards and away from you. At night the ground cools the air next to it, so for a while the air gets warmer with height. This is called a temperature inversion, and it bends sound back down towards the ground, raising levels.

Inversions form at night and can last into the early morning, which is when many airports see their first arrivals: at Heathrow, early flights may land from 04:30. Together with a quiet street and an open bedroom window, an inversion is one reason why the first planes of the day can sound louder than the same planes at noon.

Does cloud make planes louder?

Nobody seems sure. The Australian government's fact sheet for the new Western Sydney airport says cloud cover, like an inversion, bends sound waves and makes aircraft seem louder, and Perth's Jandakot Airport says low cloud reflects noise back to the ground. Southampton Airport says the opposite: that low cloud has an insulating effect and cloudless days seem louder.

We found no measurements showing how large any cloud effect is, so treat it as uncertain. If planes seem louder under low cloud, check the wind and the runway in use first: both are known to matter.

Humidity and the air itself

Air absorbs sound as it travels, and it absorbs high-pitched sound much more than low rumbles. Over more than about 30 m, this absorption becomes significant for high frequencies. That is why a distant plane is mostly a rumble and a close one has a sharper edge.

How much the air absorbs depends on its temperature and humidity. In a test for the FAA in 1975, the measured noise of a DC-9 flying over the same microphones varied by up to 4 EPNdB, a unit used for certifying aircraft noise, depending on how absorbent the atmosphere was. So humidity and temperature do change what you hear, but by a few decibels at most.

A quieter background

How loud a plane seems depends on what else you can hear. The same plane stands out more at 6 a.m. on a Sunday than in weekday traffic, and more in a quiet cul-de-sac than on a main road. The Western Sydney fact sheet lists the surrounding noise level among the things that change how people react to aircraft.

Summer adds to it. Jandakot Airport points out that aircraft noise is more noticeable in warm weather because windows are open and people spend more time outside. The planes may be no louder; you are simply more exposed to them.

Heavier aircraft and slow climbs on hot days

Aircraft are not all equal. Large long-haul aircraft climb more slowly than smaller ones, so they are heard at higher levels for longer, as Airservices Australia puts it. The UK's night flight rules give every aircraft type a quota count based on its certified noise, in bands 3 decibels wide, so that a noisy type uses up more of the airport's allowance than a quiet one. See the loudest and quietest passenger planes for how the common types compare.

Heat matters as well. Hot, humid air is thinner, and thinner air reduces an aircraft's performance, including its climb. On a hot afternoon, a heavy aircraft taking off may pass over the same house lower than it would on a cold morning. Landings are less affected, because they follow the same fixed approach slope whatever the weather.

A different runway or route

This is the change that turns a quiet day into a loud one. Planes take off and land into the wind, so when the wind changes, the airport turns round, and a street that had nothing, or high climbing take-offs, now has every landing low overhead.

Our sample report for Kew Green, 10 km from Heathrow, counts 92 loud landings a day with westerly winds and 4 loud flights a day with easterly winds. No weather effect on sound comes close to that. Runway alternation, delays and air traffic control can also move planes closer to you on a given day; why planes are flying over your house today goes through each.

How to tell what's making the difference

• Check the runway direction. If the planes are coming from the opposite direction to yesterday, the wind has turned the airport round, and that explains most of it.
• Check the heights. Heathrow's WebTrak shows the height of recent flights and readings from its noise monitors. If today's planes are lower or closer than usual, the cause is the aircraft or its route, not the air.
• Compare with a normal day. Our free flight map replays a whole day of landings and take-offs, so you can put today next to a quieter day and see what changed.

If the planes were in the usual place at the usual height and still sounded louder, it was probably the weather: a wind blowing from the airport towards you, or a still, cold morning. Neither lasts, and neither changes what the planes themselves are doing.