The inter-tropical convergence zone
The ITCZ occurs along a line where the NE trade winds blowing away from the subtropical high pressure in the N hemisphere meet the SE trade winds blowing away from the subtropical high in the S hemisphere. Combined with intense heating in the low latitude this causes air to rise.
The ITCZ marks and area of low pressure + precipitation that moves N + S of the equator throughout the year following the movement of the sun.
Spring- The ITCZ is over the equator and the topic of cancer and capricorn are dry
Summer- The ITCZ is over the tropic of cancer so the tropic of capricorn is dry
Autumn- The ITCZ is over the equator and the tropic of cancer and capricorn are dry
Winter- The ITCZ is over the tropic of capricorn so the tropic of cancer is dry
Monsoon climates found in coastal region between 5 and 20 degrees N + S of the equator (between the ITCZ + the zone of subtropical high pressure.
- Hot temperature all year round- varying 10 degrees between summer and winter
- High rainfall (3000mm a year)- mostly in the wet season, little in the dry season
- Winds change direction with the seasons- in the west season the winds blow in from the sea and in the dry season the winds blow in from the land
- The climate is like this due to the sun being always directly overhead and the shift of the ITCZ
Seasonal shift of heat asnd pressure zones are relatively small over the oceans as water heats and cools slowly. While large land masses experience large temperature variations. During the summer the land is heated intensly and because solids have a greater capacity for heat transfer the temperatures are much higher than over the oceans
Cool, dry season
- ITCZ moves south over the ocean so India lies under the high pressure zone
- ITCZ is over the ocean which forces wind to move from land to sea taking any moisture away
- Land cooler than the sea so the wind cools the area of low pressure over the sea
Warm, wet season
- ITCZ moves north bringing low pressure and heavy rainfall to tropic monsoon N areas
- ITCZ over land so winds are forced from high pressure over seas to the land
- The ocean is warmer than the land so the SE winds are warm
Tropical revolving storms
- Begin with area of low pressure caused by surface heating (disturbance near sea surfac triggers)
- Sea warmer than 26.5 degrees to at least 50m below the surface (so lots of water can evaporate)
- Convergence of air in the lower atmostphere (either within the ITCZ or along the boundary between cold and warm air masses) forced air to rise
- Located at least 5 degrees from the equator (any less than this and they dont form due to the lack of coriolis effect that causes it to spin)
- Rapid out flow of air in the upper atmosphere
- Lose strength over lad because their supply of warm, moist air is cut off
- Initially move westwards due to easterly winds in the tropics
- Move away from the equator due to the coriolis effect
- Exists while there is a supply of laten heat (energy released from water condensating) and moisture to provide energy and low frictional drag on the ocean surface
- Once maturity is reached a central eye develops (10-15km in diameter, calm conditions, clear skies, high temperatures and descending air.
Magnitude and frequency
Tropical revolving storms are measured on the SAFFIR-SIMPSON SCALE which has 5 levels based on central pressure, wind speed, storm surge and damage potential.
- Central pressure 920 mb of less
- Wind speed at 2050 kmh or more
- Storm surges at 5.5m or more
- Damage potential 'complete root failure'
Average life span of a storm is 7-14 days. Around 50 tropical revolving storms occur every year
Carribean sea= Hurricanes
Bay of Bengal= Cyclones
China sea= Typhoons
Northern Austrailia= Willy Willies
Factors effecting impacts
Vulverability of people depends on...
- Intensity of storm
- Speed of movement (length of time in one area)
- Distance travelled of sea
- Physical geography of coastal areas
Imapcts usually higher in LEDCS...
- Less money to spend on responses
- Buildings of a poorer quality
- Evacuation hard to organise (poor communication to rural areas)
- Health care is poor
- Many people rely on agriculture which is destroyed by the winds and flooding
- Infrastructure poor making distributing aid difficult
- Weather bureaus such as the National Hurricane Centre in Florida collect data from geostationary satellites and from land+ sea based recording centres
- USA also maintains round the clock surveillance by weather aircraft of storms
- Such info is compared to computer models to predict a path for the storm and to warn people
- High economic cost associated with evacutation + false alarms can cause people to become complacent and refuse future advice. (estimated evacuation cost of coastal areas in USA $1 million per km)
- The storms follow irregular paths so its not always possible to give more than 12-18 hours warning