Download Air Masses

Air Masses
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Air Mass is a huge air body having
homogeneity in all horizontal directions
(temperature, humidity, stability, and
transparency).
Horizontal size of an air mass can be as large
as a few thousands of km. Vertical size is 5 – 8
km (in some cases up to the tropopause).
Between two air masses there is a transition
zone.
Air mass formation goes on over so called
source regions.
Air Mass Formation
1.
2.
3.
The source regions are the regions having homogeneous
surface (ocean, plane land, forest etc).
Circulating over a source region for a long time (a week or
so), the air mass takes in the properties of the region.
Eddy exchange plays a major role in the air mass formation
process.
Z
Z
dT
∂ 2T
=k 2
dt
∂t
dS
∂2S
=k 2
dt
∂t
4
∂ 2T
>0
∂t 2
T
∂ 2T
<0
∂t 2
T
This process goes on until the temperature (humidity) comes
to the equilibrium for the region temperature (humidity)
Tair = Teq
Air Mass Formation.
Deviation from the theoretical scheme
Reasons for the deviation
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2.
3.
4
The circulation pattern allowing the forming Air Mass to remain
within the same geographical region for a long time (a week or so)
can occur very rarely.
N
E
Air current may vary with height.
W
Different thermal conditions do not
L
allow the Air Mass to take in homogeneous
S
W
thermal properties.
The Air Mass may start moving from the source region not being
finally formed.
Conclusion: Air Masses are constantly forming and transforming and it
takes place not within some special souce region only.
Air Mass Properties
Main properties
1.
2.
3.
4.
Temperature
Humidity
Stability
Transparency (amount of suspended aerosol particles).
Secondary properties
1.
Cloudiness (forms and amount of clouds)
2.
Precipitation (rain, showers, snow)
3.
Other typical phenomena (fogs, haze, snowstorm, thunderstorm
etc…).
4.
Character of winds: eddy wind, gusty (fitful) wind, shifting
wind, squally wind)
Atmospheric pressure, wind velocity, vorticity and divergence are
not properties of the Air Masses.
Air Mass Synoptic Analysis
•
To reveal the Air Mass properties ( to determine the Air Mass
type).
•
To relate the properties to the current weather
conditions and foresee the conditions that may
arise.
To predict the values of the meteorological
parameters and phenomena resulting from:
1.
processes within the Air Mass
2.
advection
3.
transformation of the Air Mass
•
Diagram of the Air Mass analyses
Observed weather
conditions
Air Mass properties
External factors
The Air Mass
transformation
Weather
conditions
expected to
arise
Conservative parameters of the Air Mass
• Potential temperature
• Equivalent potential temperature
• Dew point temperature
• Specific humidity
These parameters do not vary as the air parcels
undergo vertical displacement, and their diurnal
variation is very small.
Change of these conservative parameters indicates the
Air Mass transformation.
Thermodynamic Air Mass Classification
The classification is based on the following three main properties.
Warm Air Mass Ta.m. > Teq
Cold Air Mass
Relatively warm Air Mass Teq
Relatively cold Air Mass
Humid Air Mass
Dry Air Mass
S a.m. > S N
Ta.m. < Teq
≥ Ta.m. > Tn.a.m.
Teq ≤ Ta.m. < Tn.a.m.
S a . m. < S N
Unstable Air Mass
Stable Air Mass
γ < γ m.a.
γ > γ m.a.
Thermodynamic Air Mass Classification
(Continuation)
Air Masses can be warm or cold (relatively warm or
relatively cold).
Air Masses can be Humid or Dry.
Air Masses can be Stable or Unstable.
Combination of these properties creates the types of
the Air Mass and associated weather conditions
Types of the Air Masses and typical
weather conditions
Stable Air Masses
Type of the Air Mass
Warm, humid Air
Mass
Weather in Summer
Hot, moist, Ac, As
Sometimes Ns and
rain
Weather in Winter
Low clouds, drizzle,
fogs. Thaw
Warm, dry Air Mass
Fair, hot weather.
Cold, humid Air
Mass
Sc, chilly weather
Moderate cold
weather, glaze,
sleet, low clouds
Dry, cold Air Mass
Cold, dry weather.
Gusty winds possible.
Very cold, fine
weather.
Types of the Air Masses and typical
weather conditions
Unstable Air Masses
Type of the Air Mass
Weather in Summer
Weather in Winter
Warm, humid Air Mass
Convective phenomena
Thunderstorms, squalls,
hailstorms etc.
Snow storms, gusty
wind, snow charges.
Warm, dry Air Mass
Gusty wind, dust
storms.
Thaw, snow melting,
gusty wind.
Cold, humid Air Mass
Shallow convection,
occasional showers
Snowstorms, gusty
winds.
Cold, dry Air Mass
Separate Cu hum,
chilly, gusty wind.
Deep frosts, gusty
very cold wind.
The weather indicated above can deviate form the typical one under impacts
of the local condition and external factors
Geographical Air Mass Classification
This classification is based on the source region the Air Mass has
been formed at.
Tropical air
Maritime
Polar air
Continental
Arctic air
Maritime
Maritime
Continental
Continental
Weather conditions in Air Masses of
different geographical types
The geographical type air mass weather is cited with respect to a definite
region (Here it is NW of Russia). For some other region it can be
different.
Air mass type
Tropical continental
Weather in summer
Weather in winter
Hot, dry, unstable with
occasional showers.
Relatively warm and moist
air, usually stable, St, Sc.
Tropical maritime
Unstable air, convective
clouds, thunderstorms,
squalls etc.
Warm moist stable air,
drizzle, fogs, low clouds,
sleet.
Polar continental
Relatively cold unstable air,
Convective phenomena.
Cold, dry air, clear sky.
Polar maritime
Unstable air, convective
phenomena
Not very cold moist air,
generally stable, St, Sc.
Arctic continental
Very cold, relatively dry
and unstable air, gusty
wind
Pair, very cold weather,
generally no phenomena.
Arctic maritime
Cold, unstable weather with
convective phenomena
Cold, moist stable ai. St, Sc.