What is an area of above-average air pressure called?
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high
In a high the air sinks, dissolves clouds and usually brings calm weather.
💡 Did you know? That is why „high pressure“ also means a stable situation figuratively.
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Which way does the wind turn around a low in the northern hemisphere?
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anticlockwise
The Coriolis force caused by the Earth's rotation is responsible.
💡 Did you know? In the southern hemisphere it is exactly the other way round.
What instrument measures air pressure?
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barometer
A falling barometer signals bad weather, a rising one good weather.
💡 Did you know? Evangelista Torricelli built the first barometer in 1643 from a tube of mercury.
In which unit is air pressure given today?
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hectopascal
One hectopascal is exactly the same as the older unit millibar.
💡 Did you know? The switch from millibar to hectopascal was purely a change of name.
What is normal air pressure at sea level, roughly?
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1013 hPa
Higher values count as high pressure, lower ones as low pressure.
💡 Did you know? On the summit of Mount Everest the pressure is only about a third of that.
What is the air-mass boundary where polar air pushes under mild air and lifts it called?
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cold front
The steeply rising warm air builds towering cumulus clouds.
💡 Did you know? Cold fronts bring short, violent showers: warm fronts long steady rain.
What instrument measures humidity?
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hygrometer
Early hygrometers used a human hair, which lengthens in damp air.
💡 Did you know? The little weather house with two figures is essentially a hair hygrometer.
What is the temperature at which air can no longer hold its water vapour called?
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dew point
Below the dew point the vapour condenses into dew, fog or clouds.
💡 Did you know? Glasses fogging up when you enter a warm room is dew point physics in action.
What causes wind?
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differences in air pressure
Air always flows from high to low pressure: the bigger the gap, the stronger.
💡 Did you know? The technical term for that gap is the pressure gradient.
What is the scale for wind strength called?
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Beaufort scale
It describes wind by its visible effects on land and at sea.
💡 Did you know? British admiral Francis Beaufort devised it in 1806, at first only for the navy.
How far does the Beaufort scale go?
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12
Force 0 is calm, force 12 a hurricane from 118 kilometres per hour upwards.
💡 Did you know? It is not enough for tropical cyclones: those get their own scales.
In which layer of the atmosphere does the weather happen?
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troposphere
The troposphere reaches from the ground to about 8 to 15 kilometres up.
💡 Did you know? It holds roughly three quarters of the entire mass of the atmosphere.
By how many degrees does air cool on average per 100 metres of altitude?
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0.65 degrees
Meteorologists call this the temperature lapse rate.
💡 Did you know? That is why snow lies on mountains while rain falls in the valley below.
What is the value called that tells how much water vapour the air holds as a percentage of its maximum?
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relative humidity
Warm air can hold considerably more water vapour than cold air.
💡 Did you know? That is why heated air feels so dry in winter: warming it lowers the relative humidity.
What is the band of extremely strong winds at high altitude called?
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jet stream
The jet stream blows at around 10 kilometres up at speeds to 500 kilometres per hour.
💡 Did you know? Flights from America to Europe are often a whole hour shorter than the other way.
From which direction does weather usually arrive in central Europe?
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west
The mid-latitude westerlies drive the lows in from the Atlantic.
💡 Did you know? That is why Ireland often knows what weather Germany will get two days later.
What is the long-term average of the weather at a place called?
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climate
Climate is usually averaged over 30 years of weather.
💡 Did you know? A popular saying: climate is what you expect, weather is what you get.
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