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Polar air
 Orographic effect – This picture of Hawaii shows the way in which clouds build up over the mountains while leaving the coast bathed in sunshine.
Nimbus
Meaning rain-bearing and referring to clouds: cumulonimbus is used for tall thunderclouds; nimbostratus for thick, rain-bearing layer clouds.
North Atlantic Drift
A warm ocean current that flows across the North Atlantic Ocean from the east coast of North America (where it is called the Gulf Stream) and warms the coasts of northwest Europe.
Northeaster, noreaster
A strong (often gale-force) wind that blows across New England from the northeast.
Norther
A cold winter wind that sweeps south from the Canadian prairies across the southern United States and then out over the Gulf of Mexico. It is often pulled south behind passing depressions. A norther may reach a speed of 60km/h, and it often makes the air very dusty.
O
Occluded front, occlusion
A kind of weather front that
exists in some depressions. An occlusion occurs when the cold air in a depression completely lifts the warm air off the ground. The result is the formation of one wide belt of rain-bearing cloud, instead of the two bands that normally separate cold fronts and warm fronts.
Overcast sky
A sky that is covered with grey clouds, usually layer, or stratus, clouds.
Overhead Sun
The position of the Sun at mid-day. Also means the place on Earth where the Sun is directly overhead at midday and therefore where
the Sun’s heating is greatest.
The global weather between the equator and the poles is closely related to the position of the overhead Sun through the year and is the reason why they vary in a predictable way with the seasons.
Ozone
A form of oxygen molecule consisting of three atoms of oxygen formed in the upper atmosphere. It absorbs incoming ultra-violet rays from the Sun. In places where ozone is thin, exposing the skin for long periods in sunny weather can be a health hazard.
P
Pacific high
The subtropical high that dominates the Pacific Ocean. Particularly strong in the North Pacific, off the west coast of North America. Stronger in summer than in winter.
Pampero
A cold southerly wind, similar to the norther, but occurring east of the Andes in Argentina.
Photochemical smog
(See: Smog.)
Polar air
A cold, dry air mass that forms over the centres of northerly continents in winter and also over Antarctica.
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Orographic effect, relief effect
This is the effect that mountains have on rainfall. Air flowing towards a mountain range is forced to rise to cross the mountains. This lifting makes the air cool, and often
clouds and rain are produced. This rain is in addition to the rain that might otherwise fall had there been no mountain range.