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Air mass
 Adret – How the rising Sun warms one side of a valley faster than the other.
Sunny side of the valley. The sunshine warms the houses and fields on this side of the valley first, so it is a far more attractive place to live.
Shady side of the valley
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Acid rain
Rain that has been polluted with acid gases. This form of pollution is connected with the release
of sulphur and nitrogen gases when fossil fuels are burned, for example, in power stations or by vehicles. The gases combine with water droplets in the clouds to form dilute acids and then fall
as rain.
Adret
The (morning) sunny side of a mountain valley; the opposite slope is known as the ubac. The adret
is important in mountainous areas because the side with the morning Sun warms up and becomes frost- free soonest. It is the side of a valley where most houses are built.
Advection
When air flows horizontally from one area to another.
Advection fog
A fog produced when warm air flows across a cold ocean. The air is cooled until the moisture cannot all be held as vapour and then some of it condenses into droplets, creating fog. Advection fog makes banks of fog. It is not blown away by wind, but is often made more dense and widespread. Advection fogs are common in summer off the coast of California, where
they are sometimes called Tullee fogs, and off the Grand Banks (Newfoundland) through much of the year.
 Advection fog – Dense advection fog partly obscuring the Golden Gate Bridge, San Francisco.
Air mass
A part of the atmosphere with
the same kind of air that makes
a particular kind of weather.
The air mass is named after the place where the air took on its temperature and humidity. Some typical air masses are called arctic air, polar air, tropical maritime air and polar continental air. An air mass that forms over the tropical oceans will be moist and warm.
An airstream leaving this kind of air mass will cause hot, sultry and sometimes thundery weather (often called dog days in summer). In contrast, air streaming away from a polar continental air mass in winter will be very cold, but also very dry
and the sky will be cloudless.
Air masses are changed as they move from the places where they formed. For example, polar continental air is dry; but if the airstream leaving it flows over an ocean, it will pick up moisture and warmth and be likely to cause showers of sleet and snow.
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