Air Masses and Fronts

Question 1

What is an air mass?

Answer 1

A large body of air whose physical properties, (temperatue, moisture content and lapse rate) are more or less uniform for hundreds of kilometers

Question 2

What two primary factors are airmasses classified by?

Answer 2

1. Temperature giving arctic, polar or tropical air. 2. Surface type of region of origin giving maritime and continental catagories

Question 3

How is air transferred into an ‘air mass’

Answer 3

By radiative and turbulent transfers of energy and moisture between the land or ocean surface and the atmosphere

Question 4

How long must an airmass remain at souce to come into equilibrium with surface conditions?

Answer 4

around 3-7 days

Question 5

Where are the principle sources of cold air masses?

Answer 5

Northern hemisphere

Question 6

Where do cP air masses come from

Answer 6

Anticyclones of Siberia and N.Canada (continental Polar)

Question 7

Where do cA air masses form

Answer 7

The Arctic Basin, (continental Arctic)

Question 8

Characteristics of cold continental airmasses

Answer 8

Dryness, little cloud and low temps

Question 9

Where are Tropical air mass sources?

Answer 9

mT - oceanic subtropical high-pressure cells. cT originating from subtropical cells continental land mass

Question 10

How is mT air mass characterised?

Answer 10

High temps, high humidity of lower layers over oceans, and stable stratification

Question 11

What clouds associated with mT air masses

Answer 11

Warm and moist surface air produces Stratiform clouds - move poleward from source

Question 12

How is cT air mass characterised?

Answer 12

Steep lapse rate from land warming lower layers; instability and low moisture content preventing development of clouds

Question 13

What us air mass modification?

Answer 13

As an airmass moves away from its source region it is affected by different heat and moisture exchanges with the ground surface and by dynamic processes in the atmosphere. Therefore a barotropic airmass is moderately changed into a baroclinic airstream in which isosteric ans isobaric surfaces intercest one another.

Question 14

What is an isosteric surface

Answer 14

A surface of equal specific volume

Question 15

What is an isobaric surface

Answer 15

A surface of constant pressure

Question 16

What are thermodynamic changes to an airmass?

Answer 16

Airmass heated from below passing from cold to warm surface or solar heating of ground. Air may also be cooled from below. Changes also occur from increased evaporation or abstraction of moisture - addition or loss of heat from associated moisture.

Question 17

What effects do heating and cooling of air mass from below have

Answer 17

Heating increases airmass instability - rapid spread of effect through thick air Surface cooling produces temp inversion to limit the vertical extent of the cooling - cooling more gradual through radiative heat loss by the air

Question 18

What are dynamic changes to an air mass?

Answer 18

These are mechanical and involve mising or pressure changed associated with movement. For instance sfc friction lead to turbulence and upward transfer of heat and moisture.

Question 19

What is difference between diabatic and adiabatic changes?

Answer 19

Diabatic is the radiative and advective exchanges. Adiabatic is the ascent or descent of air cause temp change.

Question 20

Give example of resultant effect of cold air mass modification

Answer 20

cP air from Canada over western Atlantic in winter heating over gulf stream - lead to lower layer instability and evaporation lead to sharp increase of moisture content ans cloud formation. When it reaches central Atlantic become cool, moist mP airmass.

Question 21

What physical process is caused by turbulence associated with convective instability?

Answer 21

Gusty conditions

Question 22

Give example of resultant effect of warm air mass modification

Answer 22

Slow process - air moving poleward over cool surfaces becomes increasingly stable in the lower layers. mT air with high moisture - surface cooling produce advection fog.
Dry cT air from arid subtropics has unstable lower levels - dust storms - move over Mediterranean and rapidly get moisture - release potential instability as thunderstorms

Question 23

What is Frontogenesis

Answer 23

The discovery of day to day changes that are associated with the formation and movement of boundaries or fronts between different airmasses

Question 24

Explain frontal waves

Answer 24

A typical geometry of an airmass interface or front resembles a wave form - they are very unstable.

Question 25

Describe a depression

Answer 25

Cyclone - low pressure - circular isobaric pattern - area of 1500-3000km - lifespan of 4-7 days

Question 26

Give the main characteristics of a front

Answer 26

Zone of mixing - convergence (frontogenisis) - density discontinuity - dynamic zone - baroclinic zone - uplift (forced convection) - Ana or kata form depending on air motion aloft

Question 27

What does the character of frontal weather depend upon?

Answer 27

The vertical motion of the airmasses

Question 28

Describe an Ana front

Answer 28

If the air in the warm sector is rising relative to the frontal zone the fronts are usually very active.

Question 29

Describe a Kata front

Answer 29

Sinking of warm air relative to the cold airmasses produces a less intense front

Question 30

Describe ana warm front characteristics

Answer 30

Rising warm air - multi layered cloud - first wispy cirrus followed by sheeted cirrus and cirrostratus and altostratus

Question 31

Describe kata warm front characteristics

Answer 31

Descending warm air restricts medium and high level cloud development - mainly stratocumulus - light precipitation

Question 32

What is the issue with forecasting the extent of rain belts

Answer 32

Most fronts are not ana or kata fronts thoughout their length or even at all levels in the troposphere.

Question 33

Describe the ana cold front

Answer 33

Weather conditions variable depending on the stability of the warm sector air and relative vertical motion. Classical cold front model is ana-type with cumulonimbus cloud. Brief heavy downpours and thunder.

Question 34

Describe the kata cold front

Answer 34

Weather conditions variable depending on the stability of the warm sector air and relative vertical motion. Kata type cold front cloud is generally stratocumulus with light precipitation

Question 35

Describe the occulsion

Answer 35

Cold front move faster relative to warm and catches up. Can be cold or warm occlusion depending on relative states of the airmasses lying in front and the rear of the warm sector

Question 36

Describe the main processes in a cold front

Answer 36

Cold dry air replaces warm moist - Pressure increases - Td increases sharply - steep Td gradient - Heavy band of rain

Question 37

Name the four main kinds of circulation system

Answer 37

Mid latitude depression - anticyclone - sea breeze system - tropical system

Question 38

What is Cyclogenesis?

Answer 38

Depression formation - upper air divergence removes rising air more quickly than convergence at lower levels replaces it.

Question 39

What weather implications do depressions have

Answer 39

Rising air - low pressure - elevation of tide above predicted and strong winds leads to storm surges.

Question 40

What are non-frontal depressions?

Answer 40

Not all depressions originate as frontal waves - tropical depression mainly non frontal. Lee cyclone, thermal low, Polar air depressions, the cold low.

Question 41

What is a Lee Cyclone

Answer 41

Westerly airflow over N-S mountain barrier - vertical contraction over ridge - expansion on the lee side. Tendancy for divergence and anticyclonic curvature over crest and convergence and cyclonic curvature on the lee side of the barrier. Such depression frequent in winter south of the Alps

Question 42

What is a Thermal Low?

Answer 42

Occur in summer - intense daytime continental heating - usually hot and dry but if enough moisture the instability produced by heating may lead to showers.

Question 43

What is a polar air depression?

Answer 43

Develop in winter months - unstable mP or mA air currents stream equatorward eastern side of a N-S high pressure ridge. Key feature is presence of an ascending, moist southwesterly flow and heat input from the sea to produce instability and cause coastal showers

Question 44

What is the Cold low?

Answer 44

Evident in the circulation and temperature fields of the middle troposphere. Bring lots of medium and high cloud in Arctic winter. May be linked to low or high pressure sfc cells