Weather Flashcards
What is a depression?
A depression is a free-moving cell of air whose central pressure is lower than that of the surrounding atmosphere. Depressions are a primary cause of high winds and of all forms of precipitation, including fog.
Which holds more water vapour, cold or warm air?
Warm air. A mass of air moving northwards up the Atlantic from the tropics will be rich in moisture by the time it reaches the temperate zones. This type of air is referred to as tropical maritime.
Air originating in polar and subpolar zones is called?
Polar maritime. It is inclined to be drier and denser. In the North Atlantic the enormous mass of Greenland’s two-mile-thick ice cap together with the Canadian Arctic supplies a powerful source of so-called polar maritime air. ‘Polar’ for obvious reasons; ‘maritime’ because it has come across enough water to take the edge off its dryness.
What is the convergence line of the tropical maritime and polar maritime air masses called?
The convergence of these two global air masses is known as the “polar front”. It is nowhere near the actual Pole, but meanders seasonally between the 35th and the 60th parallels (north and south).
Describe a northern hemisphere depression
A northern hemisphere depression moves in an anti-clockwise fashion, with a warm front at the leading edge and a cold front toward the rear. Wind circles anti-clockwise around a low pressure centre.
What is the first signifier of a warm front arriving?
The first visible evidence of a warm front arriving is the presence of high ‘mare’s tail’ clouds known as cirrus.
Describe cirrus clouds
White fluffy mare’s tails. 20,000 to 40,000 feet with no precipitation.
Describe the process of a weather front arriving and passing overhead
- Cirrus clouds appear, followed by cirrostratus clouds in a thin layer which can create a halo.
2 Altostratus clouds appear, providing a blanket of clouds. - Low-level clouds (stratus, nimbostratus) arrive accompanied by a falling barometer, and the wind backing (shift anti-clockwise).
- The breeze will stiffen and veer as the front goes through. through. The barometer will probably stop falling. Rain is now intermittent and visibility remains moderate or poor with fog a possibility.
- The boat is now in the warm sector, in which you might remain for 12 hours. Drizzle and low visibility.
- The cold front approaches, the wind may back a little and pressure often drops then rises suddenly and then classically, the wind veers sharply into the north-west with a ripper of a squall to accompany it. Rain will fall more heavily and conditions generally deteriorate. You don’t get a lot of warning, but you can’t miss it when it comes. The barometer climbs rapidly and visibility suddenly opens up as the cold, clear air streams down. Huge cumulonimbus clouds tower up to 30,000ft
Describe a sea breeze
Sea breezes These develop in coastal areas when convection over the land on a warm sunny day causes strong upward currents of air. The rising air is replaced by an inflow of air from over the sea which creates an onshore wind.
Describe a land breeze
Land breeze At night the process is reversed. The Earth cools quickly after sunset while the sea retains its temperature and so air is drawn off the land to replace the warm air rising from over the sea.
Describe cirrostratus clouds
Thicker more joined up cirrus clouds that don’t quite fill the sky.
Describe altostratus clouds
Thick mid level clouds that mainly blanket the sky.
Describe katabatic winds
Katabatic winds In areas where the coastline is more dramatic and cloudless skies at night result in radiation cooling of the land, a strong, down-slope wind can develop as air in contact with the ground becomes chilled and flows rapidly down the hillside.
What is advection fog?
Advection (sea fog) This occurs when warm moist air flows over a relatively cold sea surface and the temperature of the air in contact with this surface is lowered. If the sea temperature is below the dew point temperature of the air and cooling continues until the air is saturated, condensation will take place and form mist or fog.
What is land fog?
Radiation (land fog) A clear night sky results in rapid cooling of the land. Should the surface temperature fall below the dew point temperature of the air, saturation will occur and condensation will take place with the formation of fog, which may drift out to sea. Typically spring and autumn. This will typically burn off and at any rate won’t push far out to sea.
Give rough windspeeds up to force 10
How many millibars an hour is a sure sign of gales
Two millibars drop per hour for three hours. The windspeed around a ‘low’ is dependent upon how tightly packed are the isobars that indicate its depth. At sea, as soon as a depression is approaching, you should plot the barometer readings every hour. If the barometer drops at a rate of two millibars per hour for at least three hours, stand by for gale-force winds.
What is the an “isobar squeeze”?
In summer months you can find yourself in an area of strong winds, with no particular weather system in the offing. This may be caused by a squeeze of isobars. It’s most likely when you are situated between high- and low-pressure centres, neither of which is going anywhere particularly quickly. The winds generated can be strong and stable, giving near-gale conditions.
