Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
ENV 111: Introduction to Meteorology
Lecture 5
Air masses and fronts
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses and fronts
Air masses and sources
Air mass characterization
Principal air masses of eastern Africa
A frontal system and its occurrence
Frontal systems characterization
Fronts on weather maps
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses
Figure 20 : Air mass
An air mass is a huge body of air whose
physical properties, notably temperature
and humidity, are more or less uniform
horizontally
Air-mass source is a region where such air
masses exist
For a huge mass of air to develop uniform
characteristics, the source region must be:
⇒ generally flat and of uniform composition
⇒ with light surface winds
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses
Figure 21 : Air mass sources and
paths over North America
An air mass changes its physical
characteristics as it leaves the source region
Its life history depends on:
⇒ Nature of the source: Air mass may be dry or
moist, stable or unstable
⇒ Path: Whether it travels toward colder or
warmer, moister or drier regions
⇒ Age: How long the air mass stay in contact
with the surface
Uniform surface with anticyclonic winds is
effective for production of uniform airmass
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses
There are five basic air masses categories:
Table 1 : Air mass classification and characteristics
Source region Arctic Polar Tropical
Land cA cP cT
Continental extremely cold, cold, dry, hot, dry,
(c) dry, stable; stable stable air aloft;
ice- and snow- unstable surface
covered surface air
Water mA mP mT
Maritime cool, moist, warm, moist;
(m) unstable usually unstable
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
The Principal Air masses of eastern Africa
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses in eastern Africa
Annual cycle of circulation and climate across East Africa is largely
influenced by thermally direct Hadley circulation (i.e. the
Inter-Tropical Convergence Zone, ITCZ)
Timing, length, and number of rainy seasons in the region are different
What are the plausible factors?
⇒ complex nature of the ITCZ
⇒ ITCZ passage,
⇒ extensive areas of high topography,
⇒ the presence of the great lakes
So the complexity of the principal air masses influencing the region
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses in eastern Africa
Figure 22 : Predominant
near-surface airflow (arrows) and
convergence zones (dashed lines)
over southeastern Africa (after
Torrance 1972)
Three principal air masses (and source
regions) that influence precipitation
patterns over the East African region:
⇒ South east monsoon (SE) from the Indian
Ocean; predominant during June - August
period
⇒ North Eastern monsoon; predominance the
December - February period
⇒ Congo basin airmass, ultimately originating
from the Atlantic Ocean
The three air masses interact to form a
more localized convergence region as also
influenced by complex topography
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Air masses in eastern Africa
The south-east trade, originates over the Indian Ocean to the
southeast (Mascarene High)
The northeasterly flow, originates as far away as the Indian
subcontinent (Siberian High)
These air masses are thermally stable, hence associated with subsiding
dry air (Nicholson 1996)
A westerly flow originating over the South Atlantic (St. Helena High)
It causes rainfall as is advected eastward across the Congo basin to
the extensive high elevation areas of East Africa where it cools
adiabatically to saturation and converges with the two easterly flows
(Torrance 1972; Nicholson 1996)
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Frontal systems
A frontal system accompanied by a cyclone (wave cyclone)
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Frontal systems
. . . . . . . . . . . . . .
Figure 23 : Possible atmospheric motions with a tendency to bring different air masses into
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Frontal systems
Figure 24 : The main four types of
fronts
A front is a boundary between unlike
air masses
Fronts are classified based on:
⇒ the thermal and moisture characteristics
of the air masses,
⇒ the direction of the movement of the air
masses,
⇒ whether the boundary is in contact with
the ground (a surface front), or is aloft
(an upper level front)
So fronts are named after the drifting
air mass
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Cold fronts
Figure 25 : Weather across cold fronts
Cold fronts form when cold air displaces
warm air
It is associated with heavy precipitation
events, rainfall or snow, combined with
rapid temperature drops
Steep front slope, typically 1:100
Behind it, flow northwesterly winds and
southwesterly in ahead of the front
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Warm fronts
Figure 26 : Weather across warm fronts
Warm fronts occur when warm
air displaces colder air
Characterised by shallow
horizontal stratus clouds and
light precipitation
Frontal fogs may occur as falling
raindrops evaporate in the
colder air near the surface
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Warm fronts
Also sleet and freezing rain may occur
Gentle slope, nearly half the slope of cold fronts, typically (1:200)
Move slowly, about 20 km/hr (12 mph)
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Stationary fronts
Figure 27 : Weather across
stationary fronts
A stationary front exists when two unlike
air masses remain side by side without
encroaching upon each other
The cold air on the north side of the front is
moving parallel to the front, while the warm
air to the south is moving toward the front
and get lifted along the frontal boundary
A line of showers and thunderstorms may
develop in the warm side of the front if the
air is unstable, or if stable, rain falls on the
cold side of the front
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Occluded fronts
Figure 28 : Weather across warm
occluded fronts
Occlusion: the warm air is cut off from the
surface by the meeting of two fronts
Usually, a fast-moving cold front catches a
slow-moving warm front
Cold-type occlusion: a cold front associated
with cP air meets a warm front with mP air
ahead (eastern continent)
Warm-type occlusion: the cold front,
associated with mP air invades an area
where cold cP air is entrenched (western
continent)
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Fronts
Table 2 : Characteristics of cold and warm fronts
Characteristic Cold front Warm front
Air mass distribution cold air mass press warm air mass
the warm air mass ascends above the
cold air mass
Gradient of the frontal large (1/50) small (1/100)
Weather intense (storms, weak to moderate
winds, rain etc.) (rain, winds)
Band of weather 100 km ahead, 400 km (before)
50 km behind
Duration of weather small large
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Fronts on weather maps
Figure 29 : Symbols of different front types on weather maps
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Introduction Winds Air mass Fronts Clouds and Precipitation Systems
Clouds and Precipitation Systems
Assignment 3: Precipitation systems
Processes: collision-coalescence vs ice crystal (Bergeron)
Types:
⇒ Rain (drizzle, shower, virga, cloud burst);
⇒ Snow (snowflakes and snowfall, sleet and freezing rain, snow grains
and snow pellets)
⇒ Hail
Measurements: consider the different ways
Precipitation types on weather maps
Precipitation systems over tropics contributes the two-thirds of the
global rainfall and are a major source of latent heating for the
atmosphere.
ndettoel@2016 ENV 111: Introduction to Meteorology