Fronts and Mid latitude cyclones Flashcards
What is a front
a boundary between two different air masses
how wide are fronts
100-200km wide
are transitions over fronts sharp and abrupt
yes
Where do fronts mainly occur
mid latitudes
what type of pressure are fronts associated with
low pressure systems
In a warm front what air is where
warm air flows up over dense air
how much is the inclination of this frontal surface
0.5-1
what (cloud) signals the approach of a warm front
high cirrus or cirrostratus clouds with the cloud base lowering as the front approaches
what rain occurs at the warm front
rain starts ahead of the surface front and is widespread and persistent
what happens after the passing of a warm front
skies clear quickly
explain what happens with a cold front (1)
- dense cold air pushes forward into the warmer air which is forced upwards
- this forms deep convective clouds above the surface front with heavy rain along this band
- behind the front the cloud base lifts, eventually clearing
- near the surface the cold air may surge forward, producing a very steep frontal zone
explain what happens with a cold front (2)
- Dense cold air pushes forward into warmer air, and over-runs it.
- Deep convective clouds form ahead of upper front, heavy rain in narrow band along upper front
- Between upper and surface fronts, there is shallower cloud.
Explain what happens at a stationary front
There is only a small difference between the air masses either side of the stationary front – the front is defined by the direction of motion only
When the boundary between air masses does not move it is called a stationary front
Note that the wind speed is not zero – the individual air masses still move, but the boundary between them does not
Explain what happens at an occluded front
In general cold fronts move faster than warm fronts, and may thus catch up with a warm front ahead – the result is an occluded front
There are two types of occluded fronts: warm and cold, depending on whether the air behind the cold front is warmer or cooler than the air ahead of the warm front
Warm occlusions are the more common type in the UK
Occlusion is part of the cycle of frontal development and decay within mid-latitude low pressure systems
explain what happens at a warm occlusion
In both warm and cold occlusions, the wedge of warm air is associated with layered clouds, and frequently with precipitation
Precipitation can be heavy if warm moist air is forced up rapidly by the occlusion
Explain a cold occlusion
Regarded to be very rare. Some believe that they do not exist
what is a mid-latitude cyclone
Low pressure systems (or mid-latitude cyclones) are a characteristic feature of mid-latitude temperate zones
What happens with a mid-latitude cyclone
Low pressure systems form in well defined zones associated with the polar front
Provide a strong temperature gradient and convergent flow resulting from the global circulation
Explain the Norwegian cyclone model
-Low pressure forms at surface over “polar front” due to divergence aloft
-As rotation around initial low starts, a ‘wave’ develops on the polar front
-Flow near surface is directed towards the low centre due to friction
-Mass balance: inward flow compensated by large-scale lifting cooling cloud formation
-Surface low is maintained (or deepens) due to divergence aloft exceeding convergence at surface
-Cold sector air pushes cold front forward; warm sector air flows up warm front – warm front moves slower than cold
-Cold front overtakes warm front to form an occlusion, which works out from centre
-Depression usually achieves maximum intensity 12-24 hours after the start of occlusion
-Low starts to weaken as inflowing air ‘fills up’ the low pressure
Low continues to weaken, clouds break up
what is a conveyor belt
belts of low wind levels identified ahead of cold and warm fronts
explain the warm conveyer belt at the cold front
- Main source of warm, moist air that feeds the cyclone
- Originates in the warm sector of the cyclone
- Air ascends as it approaches the cyclone
explain a cold conveyer belt at a warm front
- Originates on the poleward side of the warm front
- Air becomes saturated through ascent and rain falling from WCB above
Crossed-winds rule: if the upper wind from your LEFT
the weather is likely to deteriorate
Crossed-winds rule: if the upper wind from your RIGHT
the weather is likely to improve
crossed-winds rule: if the upper wind is BEHIND or AHEAD of you
there is likely to be a little change in the weather
