Download Weather Systems - CIC Geography Form 6

Weather Systems
Unit 2: Module 1
objectives
 Overview of the development and associated weather
conditions of high and low pressure systems:
1. Anticyclones
2. Depressions
3. Hurricanes
4. Inter Tropical Convergence Zone (ITCZ)
 Overview of weather conditions associated with air masses
and fronts
Air Mass & Fronts
Air Masses
 An air mass is a large volume of air which travels from one area to
another.
 The weather an air mass brings is determined by the region it has
come from and the type of surface it has moved over.
 The temperature of an air mass is also determined by seasonal
changes.
naming the air mass and describing its characteristics
Source area
Characteristics
Land
Dry
Oceans
Wet
Tropics
Warm
Arctic
Cold
Warm Fronts
 Warm fronts are formed when warm air rises over a mass of cold
air.
 As the air lifts into regions of lower pressure, it expands, cools and
condenses the water vapour as wide, flat sheets of cloud.
 Warm fronts are shown on synoptic charts by a solid line with
semicircles pointing towards the colder air and in the direction of
movement.
 On coloured weather maps, a warm front is drawn with a solid red
line with red semicircles.
Cold Fronts
 Cold fronts are usually associated with depressions.
 A cold front is the transition zone where a cold air mass is replacing
the warmer air mass.
 The cold air is following the warm air and gradually moves
underneath the warmer air.
 When the warm air is pushed upwards it will rain heavily.
 Often more rain will fall in the few minutes the cold front passes
than it will during the whole passage of a warm front.
Cold Fronts
 As the cold front passes, the clouds roll by and the air temperature
is cooler.
 Cold fronts are shown on synoptic charts by a solid line with
triangles along the front pointing towards the warmer air and in the
direction of movement.
 On coloured weather maps, a cold front is drawn with a solid blue
line with blue triangles.
Occluded Fronts
 Occluded fronts occur at the point where a cold front takes over a
warm front or the other way around.
 If a cold front undercuts a warm front it is known as a cold
occlusion and if the cold front rises over the warm front it is called
a warm occlusion.
 Occluded fronts bring changeable weather conditions.
 On a synoptic chart occluded fronts are represented by semicircles
and triangles positioned next to each other.
 The triangles are in blue and the semicircles are in red, or both are
purple (mixing both red and blue colours together).
Depressions
Depressions
 Depressions are areas of low atmospheric pressure which produce cloudy, rainy
and windy weather.
 At the warm front, lighter, warmer air from the south (tropical maritime air)
meets cooler air from the north (polar maritime air) and rises gradually over it.
 As the warm air slowly rises it cools, its water content condenses and clouds
form (nimbostratus then altostratus).
 The result is steady rain, later giving way to drizzle and finally clearer skies
with high cirrus clouds.
 Behind the warm front is an area of warm, rising air and low pressure - the
centre of the low-pressure system.
 As this part of the depression passes over, there may be a short period of clear,
dry weather.
Depressions
 However, at the trailing cold front, heavier, cooler air meets the
warm air at the centre of the depression, undercutting it and
forcing it steeply upwards.
 Quickly moving air masses produce high winds and cooler
temperatures.
 As the rapidly rising warm air cools, its water condenses and
clouds form (cumulonimbus, then cumulus).
 The result is heavy rain or thunderstorms, giving way to showers
and finally to clear skies as the cold front moves away eastwards.
Origin and Infancy
Maturity
Occlusion
Depressions
 A low pressure system, also known as a depression occurs when the weather is
dominated by unstable conditions.
 Under a depression air is rising, forming an area of low pressure at the surface.
 This rising air cools and condenses and helps encourage cloud formation, so the
weather is often cloudy and wet.
 In the Northern Hemisphere winds blow in anticlockwise direction around a
depression.
 Isobars are normally closely spaced around a depressions leading to strong winds.

Depressions can be identified on weather charts as an area of closely spaced isobars,
often in a roughly circular shape, where pressure is lower than surrounding areas.
 They are often accompanied by fronts
Anticyclones
Anticyclones
 Anticyclones are the opposite of depressions - they are an area
of high atmospheric pressure (typically above 1020mb) where the air
is sinking.
 In contrast to depressions, anticyclones only involve one type of air mass
which usually cover large areas and do not have any fronts.
 They are high pressure systems in which the air moves downwards
towards the earth's surface.
 As the air descends, the molecules become compressed, the pressure
increases and it warms.
 When air is warming, any moisture in the atmosphere is evaporated so
no clouds can form.
 The sky is clear, with settled, dry and bright weather conditions..
Anticyclones
 In summer (temperate areas), anticyclones bring dry, hot
weather. In winter, clear skies may bring cold nights and frost.
 In cold conditions, anticyclones may also bring fog and mist. This
is because the cold forces moisture in the air to condense at low
altitudes.
 Anticyclones can be very large, typically at least 3,000 km wide
which is much larger than depressions.
Anticyclones
 Because of these stable conditions, cloud formation is inhibited, so
the weather is usually settled with only small amounts of cloud
cover.
 In the Northern Hemisphere winds blow in a clockwise direction
around an anticyclone.
 As isobars are normally widely spaced around an anticyclone, winds
are often quite light.
 Anticyclones can be identified on weather charts as an often large
area of widely spaced isobars, where pressure is higher than
surrounding areas.
Inter-Tropical Convergence Zone (ITCZ)
Intertropical Convergence Zone (ITCZ)
 The ITCZ is a zone of convergence at the thermal equator where the
trade winds meet.
 It is a low pressure belt and migrates with the changing position of the
thermal equator (lag of 4-6 weeks behind the moving sun).
 The thermal equator receives the most intense heat from the Sun.
 Around 20 June each year the Sun is overhead at 23½º North, the Tropic
of Cancer.
 Around 20 December the Sun is overhead at 23½º South, the Tropic of
Capricorn.
 The movement of the thermal equator shifts the belts of planetary winds
and pressure systems to the north and to the south annually.
Intertropical Convergence Zone (ITCZ)
 The ITCZ is an area of low pressure that forms due to the action of the
Hadley Cell, where the Northeast Trade Winds meet the
Southeast Trade Winds near the Earth’s equator.
 As these light winds (called doldrums) converge, moist air is forced
upward. This causes water vapour to condense, or be “squeezed” out, as
the air cools and rises, resulting in a band of heavy precipitation around
the globe.
 This band moves seasonally, always being drawn toward the area of most
intense solar heating, or warmest surface temperatures.
 This variation in the location of the ITCZ dramatically affects rainfall in
many equatorial nations, resulting in the wet and dry seasons of the
tropics.
Tropical Cyclones
Tropical Cyclones
 These are classified differently around the world:
1. Cyclones hit Southern Asia (between June and November)
2. Willy Willies hit Australia (between January and March)
3. Typhoons hit the western Pacific (between July and October)
4. Hurricanes hit the Atlantic (between June and November)
 Typically these generate winds of between 40 and 119 km/hour.
 Small low pressure systems are called tropical storms (63-
118km/hr) and tropical depressions (0-62km/hr).
 On average 20,000 people/year lose their lives to tropical
cyclones.
Hurricanes
Hurricanes- Conditions for formation
 Generally these intense low-pressure systems (less than 920 Mb is common) form
between 30° N and S, in an area called the Intertropical Convergence Zone.
 With Coriolis Effect at its greatest in the tropics (between 5° North and South of
the Equator), the systems’ rotation can be easily initiated (winds in the order of
119 to 300 km/hr are common).
 The ITCZ is where convergent air meets near the equator. This area is intermittent
and can vary its position.
 The sea in this area is well heated, and exceeds the minimum 26° to 27°C
necessary (between August and October) for latent heat to be released (because of
condensation of water vapour) to strengthen the hurricane (relative humidity of
60% is common). A deep layer of water of around 60m.
 The ocean then provides the initiating and sustaining energy for hurricanes. They
quickly ‘die’ once they move onto the land. The high levels of moisture held also
point to a sea origin (typically in the order of 100 to 250 mm/day).
Hurricanes- Conditions for formation
 Towering (15 km is common) cumulonimbus clouds form around the
central eye in highly unstable conditions. These clouds further fuel the
hurricane as latent heat energy exchanges moisture from gas to liquid.
Spinning weather sub-systems develop all around the main hurricane
mass.
 Wind spins around an eye of 25 to 50 km. This decreases in size as
hurricanes speed up.
 Relatively stable and uniform atmospheric conditions of temperature,
humidity and pressure. In the upper troposphere air drawn in at lower
altitudes must be able to escape.
 Little change of horizontal wind with height.
 Existing cyclonic spinning of winds in the lower troposphere.
Structure of hurricane
 The eye is 20-60km across, existing in centre of the hurricane
where conditions are calm and clear.
 Hurricanes can turn into massive systems up to 800km wide, but
the intense storm around the eye rarely exceeds 300km.
 Intense rainfall due to rapidly cooling and rising air. Amount can
be up to 250mm a day.
 Strong winds. In order to be classified as a hurricane they must
average 119km per hour.
THE END