• The equatorial trough is a permanent series of thermal lows in a more or less continuous belt around the earth.
• On both sides of the equatorial low pressure belt, troughs sometimes extend out towards the sub-tropical high pressure belts.
• These troughs, known as easterly waves , move westwards under the influence of the easterly upper winds.
• African easterly waves, have their origins over North Africa
• The waves are generated by an instability of the African easterly equatorial jet.
• The western flank of the trough is an area of subsiding and diverging air with fine weather.
• The eastern side is an area of convergence, with a north/south line of cloud, rain and possibly squally showers and thunderstorms.
• In West Africa the squally trough is known locally as a tornado but is not to be confused with a tornado as described in Section 8.

• The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km, typically one should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year.
• While only about 60% of the Atlantic tropical storms and minor hurricanes originate from easterly waves, nearly 85% of the intense
(or major) hurricanes have their origins as easterly waves
• It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa.

• The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone".
• The tropical cyclone is a low-pressure system which derives its energy primarily from evaporation from the sea in the presence of high winds and lowered surface pressure and the associated condensation in convective clouds concentrated near its center
• Warm ocean waters of at least 26.5°C throughout a sufficient depth, are necessary to fuel the heat engine of the tropical cyclone with latent heat.
• The storms tend to affect certain areas :
– They occur mainly on the western edge of oceans after the winds have had a long sea track and the air has acquired a large amount of moisture.
– They occur in the summer hemisphere in areas where the ITCZ has reached a latitude of 10 °N or S or greater and where there is sufficient geostrophic force to produce cyclonic rotation.
– They do not originate over land.

HURRICANES
(July - Oct)
CYCLONES
(May - Oct)
TYPHOONS
(May - Nov)
CYCLONES
(Dec - April)
CYCLONES
(Nov - May)

• A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub- tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation
• Tropical cyclones with maximum sustained surface winds of less than
17 m/s (34 kt) are called "tropical depressions".
• Typical tropical cyclones have an “eye” in the centre. The "eye" is a roughly circular area of comparatively light winds and fair weather found at the centre of a severe tropical cyclone. The winds are calm at the axis of rotation, but strong winds may extend well into the eye.
There is little or no precipitation and sometimes blue sky or stars can be seen. Eyes vary in size but most are approximately 30-60 km in diameter. The eye is composed of air that is slowly sinking.

• Once the tropical cyclone reaches sustained winds of at least 17 m/s
(34 kt) they are typically called a "tropical storm" and assigned a name.
• If winds reach 33 m/s (64 kt), then they are called:
– a "hurricane" (the North Atlantic Ocean, the Northeast Pacific
Ocean east of the dateline, or the South Pacific Ocean east of
160E);
– a "typhoon" (the Northwest Pacific Ocean west of the dateline);
– a "severe tropical cyclone" (the Southwest Pacific Ocean west of
160E or Southeast Indian Ocean east of 90E);
– a "severe cyclonic storm" (the North Indian Ocean);
– and a "tropical cyclone" (the Southwest Indian Ocean near
Madagascar)

– Generally tropical cyclones tend to follow the circulation around the subtropical highs existing over the oceans.

• Tropical Storm/Cyclone averages for the various areas are:
– N.W.Pacific (South China Sea) about 25 times per season,
– Atlantic about 9 -10 times per season
– N.E. Pacific (Coast of Central America) about 16 -17 times per season
– N. Indian (Gulf of Arabia and Bay of Bengal); 5 - 6 times per season.
– S.W. Indian Ocean (Madagascar) about 10 - 11 times per season.
– S.E. Indian Ocean (W. Australia) about 7 times per season.
– S.W.Pacific (E. Australia) 9 times per season.
– Global Average 84 -85 per annum.
– From 1979 individual storms were allocated alternate male and female first names alphabetically starting with A at the beginning of each storm season (e.g. Alfred, Barbara, Charles, etc.)
– N. Indian Ocean storms are only allocated numbers.
–
This aids indentification when there are several storms active at any one time.

• Which is the most intense tropical cyclone on record?
– Typhoon Tip in the Northwest Pacific Ocean on 12 October 1979 was measured to have a central pressure of 870 mb
• Which tropical cyclone intensified the fastest?
– Typhoon Forrest in September 1983 in the Northwest Pacific Ocean deepened by 100 mb
(976 to 876 mb) in just under 24 hr. Estimated surface sustained winds increased a maximum of 30 kt in 6 hr and 85 kt in one day (from 65 to 150 kt).
• Which tropical cyclone has produced the highest storm surge?
– The Bathurst Bay Hurricane produced a 13 m (about 42 ft) surge in Bathurst Bay, Australia in 1899
• What are the largest rainfalls associated with tropical cyclones?
– 12 hr 1144 mm (45.0") Foc-Foc, La Reunion Island Tropical Cyclone Denise 7-8 January,
1966
– 24 hr 1825 mm (71.8") Foc-Foc, La Reunion Island Tropical Cyclone Denise 7-8 January,
1966
• Which tropical cyclone lasted the longest?
– Hurricane/Typhoon John lasted 31 days as it traveled both the Northeast and Northwest
Pacific basins during August and September 1994. (It formed in the Northeast Pacific, reached hurricane force there, moved across the dateline and was renamed Typhoon
John, and then finally recurved back across the dateline and renamed Hurricane John again.)