CLIMATE AND WEATHER
Mid-latitude Cyclones
• general characteristics
• areas where mid-latitude cyclones form
• conditions necessary for their formation
• stages of development and related weather conditions
• weather patterns associated with cold, warm and occluded fronts
• rea...
Climatology
● Mid latitude cyclone start at the polar front and occur in the Westerly wind belt
● Tropical cyclones start at the ITCZ outside 5º North and South over oceans warmer than 28º C
Origin and Development of Mid-latitude Cyclones
Origin of Mid-latitude Cyclones
● Form in mid-latitudes: 400 to 600 North and South
● Polar Front: warm, moist subtropical air meets cold polar air
Development of Mid-latitude Cyclones
Stage 1: Initial/Early Stage
● A wave forms in the polar front (caused by jet stream, mt. range or coastline)
● Low pressure cell develops: winds deflect (Coriolis force) as air blows into low pressure cell along
pressure gradient
Mature Stage
Stage 2: Open stage
● Wave deepens and a stronger pressure gradient develops
● Polar front forms cold front (ahead of cold sector) and warm front (ahead of warm sector)
● Air blows in a clockwise direction into the low pressure
● Cold front – thunderstorms (cumulonimbus clouds); warm front – light, widespread rain
(nimbostratus clouds)
Stage 3: Occluded Stage
● Cold, dense air sinks and wedges underneath less dense, warmer air – warmer air is lifted off the
ground
● Warm sector appears to narrow (on plan view) as cold front overtakes warm front
● Nimbostratus clouds with rain
Stage 4: Degeneration Stage
● Only cold air at ground level, rain and clouds clear up
● Isobars even out
, 2
Origin and Development of Tropical Cyclones
Tropical cyclones develop at the ITCZ, but outside 5º north and south, because a Coriolis Force is absent on
the equator. It is an intense low pressure storm which causes a lot of wind and flood damage. Tropical
cyclones only form under very specific conditions. They are named alphabetically in the season they occur.
Tropical cyclones develop over the sea in the tropical areas. The water must be warmer than 28ºC which
leads to a lot of evaporation and very hot, humid, unstable air. The hot air starts rising and forms an intense
low pressure on the surface. The tropical jet stream in the upper air causes an upper air low pressure and
this intensifies the low pressure on the surface. Air is sucked into the low pressure. If this happens outside
5ºN and S, Coriolis force will cause the winds to spiral towards the low pressure. This intensifies the low
pressure to such an extent that a clear funnel, called the eye of the cyclone, develops.
The large scale condensation releases latent heat in the atmosphere which makes the air more unstable and
contributes to more rising. This causes large cumulonimbus clouds to form, and torrential rain. The air
pressure is so low that some air is sucked down in the middle to form this clear, calm, cloudless eye where
the air is warmer due to adiabatic heating. Tropical cyclones are named alphabetically in the season as they
develop. Tropical cyclones move from east to west
Origin of Tropical Cyclones
● Form in the tropics: 5° to 30° North and South
● Form over warm oceans: must be approx. 27 °C and higher.
● Oceans provide source of moisture for evaporation
● Latent heat is released during the process of condensation – helps to sustain the tropical cyclone
● Coriolis force: necessary for the deflection of winds into the cyclone (Coriolis force is zero at
equator, therefore tropical cyclones can not form at the equator)
● Tropical easterlies help to propagate tropical cyclone in a westward direction
, 3
Development of Tropical Cyclones
Stage 1: Formative Stage
● Strong updraughts of warm, moist air
● Low pressure intensifies
● Pressure gradient steepens
● Winds deflect and spiral into the low pressure
● Gale force winds
● Cumulus clouds developing
Stage 2: Immature Stage
● Bands of cumulonimbus clouds develop further
● Pressure drops below 1000hPa
● Heavy rain
● Hurricane strength winds 50km from eye
● No clear eye
Stage 3: Mature Stage
● Wind speeds reach hurricane strength
● Hurricane strength winds up to 300km from eye
● Torrential, widespread rain
● Clear eye has formed
● Pressure has dropped below 950 hPa
Stage 4: Decaying Stage
● Pressure starts to rise
● Surface friction (on land) and lack of moisture cause cyclone to dissipate
● Heavy rains continue but start to clear up
● Winds decrease in speed
Phenomena Associated With Tropical Cyclones:
Storm Surge:
Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the
storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can
increase the average water level 4.5 m or more.
Inland Flooding
In addition to the storm surge and high winds, tropical cyclones bring torrential rains and flooding in the
area. Even after the wind has diminished, the flooding potential of these storms remains for several days.
Floods may destroy agricultural land and cattle, infrastructure and cause most deaths as a result of
drowning
Flash flooding
Flash floods are rapid occurring events. This type of flood can begin within a few minutes or hours of
excessive rainfall. The rapidly rising water can reach heights of (10 m) or more and can roll boulders, rip trees
from the ground, and destroy buildings and bridges.
Urban/Area floods
Urban/Area floods are also rapid events although not quite as severe as a flash flood. Still, streets can
become swift-moving rivers and basements can become death traps as they fill with water.
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