Fronts and Mid latitude cyclones

Question 1

What is a front

Answer 1

a boundary between two different air masses

Question 2

how wide are fronts

Answer 2

100-200km wide

Question 3

are transitions over fronts sharp and abrupt

Answer 3

yes

Question 4

Where do fronts mainly occur

Answer 4

mid latitudes

Question 5

what type of pressure are fronts associated with

Answer 5

low pressure systems

Question 6

In a warm front what air is where

Answer 6

warm air flows up over dense air

Question 7

how much is the inclination of this frontal surface

Answer 7

0.5-1

Question 8

what (cloud) signals the approach of a warm front

Answer 8

high cirrus or cirrostratus clouds with the cloud base lowering as the front approaches

Question 9

what rain occurs at the warm front

Answer 9

rain starts ahead of the surface front and is widespread and persistent

Question 10

what happens after the passing of a warm front

Answer 10

skies clear quickly

Question 11

explain what happens with a cold front (1)

Answer 11

• 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

Question 12

explain what happens with a cold front (2)

Answer 12

• 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.

Question 13

Explain what happens at a stationary front

Answer 13

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

Question 14

Explain what happens at an occluded front

Answer 14

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

Question 15

explain what happens at a warm occlusion

Answer 15

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

Question 16

Explain a cold occlusion

Answer 16

Regarded to be very rare. Some believe that they do not exist

Question 17

what is a mid-latitude cyclone

Answer 17

Low pressure systems (or mid-latitude cyclones) are a characteristic feature of mid-latitude temperate zones

Question 18

What happens with a mid-latitude cyclone

Answer 18

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

Question 19

Explain the Norwegian cyclone model

Answer 19

-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

Question 20

what is a conveyor belt

Answer 20

belts of low wind levels identified ahead of cold and warm fronts

Question 21

explain the warm conveyer belt at the cold front

Answer 21

• 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

Question 22

explain a cold conveyer belt at a warm front

Answer 22

• Originates on the poleward side of the warm front

- Air becomes saturated through ascent and rain falling from WCB above

Question 23

Crossed-winds rule: if the upper wind from your LEFT

Answer 23

the weather is likely to deteriorate

Question 24

Crossed-winds rule: if the upper wind from your RIGHT

Answer 24

the weather is likely to improve

Question 25

crossed-winds rule: if the upper wind is BEHIND or AHEAD of you

Answer 25

there is likely to be a little change in the weather