Air Masses and Fronts Flashcards
(44 cards)
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.
