Physical Geography 101
Tropical Weather Systems

Tropical and Equatorial Weather Systems

Tropical and Equatorial Weather differs from Mid-latitude and High Latitude weather in the following ways:

Easterly Wave

Trough of low pressure.
Moves westward due to easterly Trade Winds.
Form between 2-30° Lat.
Showers form on the eastern side of the trough; converging winds forced to rise.

Equatorial Lows

Occur along the Intertropical Convergence Zone.
Shifts with the seasons.
Small weak storms are often associated with these low pressure centers.

Tropical Cyclones



Characteristics include:
– very low pressure center
– high winds.
– central eye – calm center area storm.
– clear sky conditions common.
– highest wind speeds in eye wall
– intense precipitation
– Widespread wind and water damage

Stages of Hurricane Development

1) Tropical Disturbance - wind speeds <37 kph (23 mph)
2) Tropical Depression - wind speeds 37-60 kph (23-37 mph)
3) Tropical Storm - wind speeds 61-118 kph (37-73 mph
4) Hurricane - wind speeds >119 kph (>64 knots, >74 mph)

Hurricane Formation

Always begin forming over warm, evaporating equatorial waters.
Release of latent heat is driving force (condensation).
Warm rising air creates low pressure.
Convergence and rising sustains this process.

No Coriolis effect at equator.
Weak Coriolis effect where most hurricanes begin development.
Coriolis effect increases as storm system moves poleward.
Faster spin rate develops.
Intensify low pressure.

Hurricane Classification

Saffir-Simpson Hurricane Damage Potential Scale

Scale Number Central Pressure (mb) Winds (mph) Storm Surge (ft) Damage
1 > 980 74-95 4 - 5 damage mainly to trees, shrubbery and unanchored mobile homes
2 965 - 979 96-110 6 - 8 some trees blown down; major damage to exposed mobile homes; some damage to building roofs
3 945 - 964 111-130 9 - 12 foliage removed; large trees blown down; mobile homes destroyed; some structural damage to small buildings
4 920 - 944 131-155 13 - 18 all signs blown down; extensive damage to roofs, window and doors; complete destruction of mobile homes; flooding inland
5 < 920 >155 > 18 severe damage to windows and doors; extensive damage to roofs; small buildings overturned and blown away; major damage to lower floors of all structures less than 15 feet above sea level within 500 meters of shore.

Hurricane Damage

1) Storm Surge
Dome or wave of water located at eye and radially away from eye.
Forms due to:
- Friction from wind pushing wave of water.
- Low pressure.

2) Wind Damage
High speed winds damage plants and buildings.
Tornadoes: embedded within thunderstorms within hurricane.

3) Inland Flooding
Heavy precipitation
Hurricane prone areas: low coastal plains.
– typically have poor drainage.
Overwhelms inland area drainage networks.

Hurricane Decay

Decrease in intensity and organization due to:
1) Movement over cold water – Cuts off supply of warm, evaporating water.
2) Movement onto land – Cuts off supply of warm, evaporating water.
3) Disruption of inward air flow.

Hurricane Locations

Hurricanes form in Indian Ocean, parts of Pacific, and North Atlantic.
Hurricanes do not normally develop in South Atlantic or southeastern portion of Pacific Ocean.
Why Not?