Air Masses and Fronts Flashcards
What is an air mass?
A large body of air whose physical properties, (temperatue, moisture content and lapse rate) are more or less uniform for hundreds of kilometers
What two primary factors are airmasses classified by?
- Temperature giving arctic, polar or tropical air. 2. Surface type of region of origin giving maritime and continental catagories
How is air transferred into an ‘air mass’
By radiative and turbulent transfers of energy and moisture between the land or ocean surface and the atmosphere
How long must an airmass remain at souce to come into equilibrium with surface conditions?
around 3-7 days
Where are the principle sources of cold air masses?
Northern hemisphere
Where do cP air masses come from
Anticyclones of Siberia and N.Canada (continental Polar)
Where do cA air masses form
The Arctic Basin, (continental Arctic)
Characteristics of cold continental airmasses
Dryness, little cloud and low temps
Where are Tropical air mass sources?
mT - oceanic subtropical high-pressure cells. cT originating from subtropical cells continental land mass
How is mT air mass characterised?
High temps, high humidity of lower layers over oceans, and stable stratification
What clouds associated with mT air masses
Warm and moist surface air produces Stratiform clouds - move poleward from source
How is cT air mass characterised?
Steep lapse rate from land warming lower layers; instability and low moisture content preventing development of clouds
What us air mass modification?
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.
What is an isosteric surface
A surface of equal specific volume
What is an isobaric surface
A surface of constant pressure
What are thermodynamic changes to an airmass?
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.
What effects do heating and cooling of air mass from below have
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
What are dynamic changes to an air mass?
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.
What is difference between diabatic and adiabatic changes?
Diabatic is the radiative and advective exchanges. Adiabatic is the ascent or descent of air cause temp change.
Give example of resultant effect of cold air mass modification
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.
What physical process is caused by turbulence associated with convective instability?
Gusty conditions
Give example of resultant effect of warm air mass modification
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
What is Frontogenesis
The discovery of day to day changes that are associated with the formation and movement of boundaries or fronts between different airmasses
Explain frontal waves
A typical geometry of an airmass interface or front resembles a wave form - they are very unstable.
Describe a depression
Cyclone - low pressure - circular isobaric pattern - area of 1500-3000km - lifespan of 4-7 days
Give the main characteristics of a front
Zone of mixing - convergence (frontogenisis) - density discontinuity - dynamic zone - baroclinic zone - uplift (forced convection) - Ana or kata form depending on air motion aloft
What does the character of frontal weather depend upon?
The vertical motion of the airmasses
Describe an Ana front
If the air in the warm sector is rising relative to the frontal zone the fronts are usually very active.
Describe a Kata front
Sinking of warm air relative to the cold airmasses produces a less intense front
Describe ana warm front characteristics
Rising warm air - multi layered cloud - first wispy cirrus followed by sheeted cirrus and cirrostratus and altostratus
Describe kata warm front characteristics
Descending warm air restricts medium and high level cloud development - mainly stratocumulus - light precipitation
What is the issue with forecasting the extent of rain belts
Most fronts are not ana or kata fronts thoughout their length or even at all levels in the troposphere.
Describe the ana cold front
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.
Describe the kata cold front
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
Describe the occulsion
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
Describe the main processes in a cold front
Cold dry air replaces warm moist - Pressure increases - Td increases sharply - steep Td gradient - Heavy band of rain
Name the four main kinds of circulation system
Mid latitude depression - anticyclone - sea breeze system - tropical system
What is Cyclogenesis?
Depression formation - upper air divergence removes rising air more quickly than convergence at lower levels replaces it.
What weather implications do depressions have
Rising air - low pressure - elevation of tide above predicted and strong winds leads to storm surges.
What are non-frontal depressions?
Not all depressions originate as frontal waves - tropical depression mainly non frontal. Lee cyclone, thermal low, Polar air depressions, the cold low.
What is a Lee Cyclone
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
What is a Thermal Low?
Occur in summer - intense daytime continental heating - usually hot and dry but if enough moisture the instability produced by heating may lead to showers.
What is a polar air depression?
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
What is the Cold low?
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
