Chapter 17, Tropical Weather 17-1
17 Tropical Weather
17.1 Introduction
Technically, the Tropics lie between latitudes 23½° N and 23½° S. However, weather typical of this region
sometimes extends as much as 45° from the Equator. One may think of the Tropics as uniformly rainy,
warm, and humid. The facts are, however, that the Tropics contain both the wettest a nd driest regions of
the world.
This chapter describes the basic circulation over the Tropics, terrain influences that determine arid and wet
regions, and transitory systems that invade or disturb the basic tropical circulation.
Chapter 17, Tropical Weather 17-2
17.2 Circulation
Chapter 7, Earth -Atmosphere Heat Imbalances, stated that wind blowing out of the subtropical
high-pressure belts toward the Equator form the northeast and southeast trade winds of the two hemispheres.
These trade winds converge in the vicinity of the Equator where air rises. This convergence zone is referred
to as the Intertropical Convergence Zone (ITCZ). In some areas of the world, seasonal temperature
differences between land and water areas generate rather large circulation patterns that overpower the trade
wind circulation; these areas are monsoon regions. Tropical weather discussed here includes the subtropical
high-pressure belts, the trade wind belts, the ITCZ, and monsoon regions.
17.2.1 Subtropical High-Pressure Belts
If the surface s under the subtropical high -pressure belts were all water of uniform temperature, the
high-pressure belts would be continuous highs around the globe. The belts would be areas of descending or
subsiding air and would be characterized by strong temperature inversions and very little precipitation.
However, land surfaces at the latitudes of the high-pressure belts are generally warmer throughout the year
than are water surfaces. Thus, the high -pressure belts are broken into semipermanent high -pressure areas
over oceans with troughs or lows over continents, as shown in Figure 17-1 and Figure 17-2. The subtropical
highs shift southward during the Northern Hemisphere winter and northward during summer. The seasonal
shift, the height and strength of the inversion, and terrain features determine the weather in the subtropical
high-pressure belts.
In the warm Northern Hemisphere, warm land areas tend to have low pressure and cool oceanic areas tend
to have high pressure. In the cool Southern Hemisphere, the pattern is reversed: cool land areas tend to have
high pressure and water surfaces have low pressure. However, the relationship is not so evident in the
Southern Hemisphere because of relatively small amounts of land. The subtropical high -pressure belts are
clearly evident at about 30° latitude in both hemispheres. The red dashed line shows the ITCZ.
Figure 17-1. Mean Worldwide Surface Pressure Distribution and Prevailing Winds Throughout the World in July
Chapter 17, Tropical Weather 17-3
In this season, the pattern from Figure 17-1 is reversed. In the cool Northern Hemisphere, cold continental
areas are predominantly areas of high pressure, while warm oceans tend to be low -pressure areas. In the
warm Southern Hemisphere, land areas tend to have low pressure and oceans have high pressure. The
subtropical high-pressure belts are evident in both hemispheres. Note that the pressure belts shift southward
in January and northward in July, with the shift in the zone of maximum heating. The red dashed line shows
the ITCZ.
Figure 17-2. Mean Worldwide Surface Pressure Distribution and Prevailing Winds Throughout the World in January
17.2.1.1 Continental Weather
Along the west coasts of continents under a subtropical high, the air is stable. The inversion is strongest
and lowest where the east side of the subtropical high -pressure area overlies the west side of a continent.
Moisture is trapped under the inversion; fog and low stratus occur frequently. However, precipitation is
rare, since the moist layer is shallow and the air is stable. Heavily populated areas also add contaminants to
the air which, when trapped under the inversion, add to the visibility problem.
The extreme southwestern United States, for example, is dominated in summer by a subtropical high. Most
people are familiar with the semiarid summer climate of Southern California. Rainfall is infrequent, but fog
is common along the coast.
In winter, the subtropical high-pressure belts shift southward. Consider Southern California as an example.
In winter, the area comes under the influence of mid-latitude circulation, which increases the frequency of
rain. Also, an occasional wintertime outbreak of polar air brings clear skies with excellent visibility.
The situation on eastern continental coasts is just the opposite. The inversion is weakest and highest where
the west side of the subtropical high-pressure area overlies the eastern coast of a continent. Convection can
penetrate the inversion, and showers and thunderstorms often develop. Precipitation is generally sufficient
to support considerable vegetation. For example, in the United States, Atlantic coastal areas at the same
latitude as Southern California are far from arid in summer.
Low ceiling and fog often prevent landing at a West Coast destination, but a suitable alternate generally is
available a few miles inland. Alternate selection may be more critical for an East Coast destination because
of widespread instability and associated hazards.
Chapter 17, Tropical Weather 17-4
17.2.1.2 Weather Over Open Sea
Under a subtropical high over the open sea, cloudiness is scant. The few clouds that do develop have tops
from 3,000 to 6,000 ft, depending on height of the inversion. Ceiling and visibility are generally sufficient
for VFR flight.
17.2.1.3 Island Weather
An island under a subtropical high receives very little rainfall because of the persistent temperature
inversion. Surface heating over some larger islands causes light convective showers. Cloud tops are only
slightly higher than those over open water. Temperatures are mild, showing small seasonal and diurnal
changes. A good example is the pleasant, balmy climate of Bermuda.
17.2.2 Trade Wind Belts
Figure 17-1 and Figure 17-2 show prevailing winds throughout the Tropics for July and January. Note that
trade winds blowing out of the subtropical highs over ocean areas are predominantly northeasterly in the
Northern Hemisphere and southeasterly in the Southern Hemisphere. The inv ersion from the subtropical
highs is carried into the trade winds and is known as the trade wind inversion. As in a subtropical high, the
inversion is strongest where the trade winds blow away from the west coast of a continent and weakest
where they blow onto an eastern continental shore. Daily variations from these prevailing directions are
small, except during tropical storms. As a result, weather at any specific location in a trade wind belt varies
little from day to day.
17.2.2.1 Weather Over Open Sea
In the trade wind belt, on average, about one -half of skies over open water are covered by clouds. Tops
range from 3,000 to 8,000 ft, depending on the height of the inversion. Showers, although more common
than under a subtropical high, are still light with comparatively little rainfall. Flying weather is generally
quite good.
17.2.3 Continental Weather
Where trade winds blow offshore along the west coasts of continents, skies are generally clear and the area
is quite arid. The Baja Peninsula of Baja California is a well -known example. Where trade winds blow
onshore on the east sides of continents, rainfall is generally abundant in showers and occasional
thunderstorms. The east ern coast of Mexico is a good example. Rainfall may be carried a considerable
distance inland where the winds are not blocked by a mountain barrier. Inland areas blocked by a mountain
barrier are deserts; examples include the Sahara Desert and the arid regions of the southwestern
United States. Afternoon convective currents are common over arid regions due to strong surface heating.
Cumulus and cumulonimbus clouds can develop, but cloud bases are high, and rainfall is scant because of
the low moisture content.
Flying weather along eastern coasts and mountains is subject to the usual hazards of showers and
thunderstorms. Flying over arid regions is good most of the time but can be turbulent in afternoon
convective currents; be especially aware of dust devils. Blowing sand or dust sometimes restricts visibility.
17.2.3.1 Island Weather
Mountainous islands have the most dramatic effect on trade wind weather. Since trade winds are
consistently from approximately the same direction, they always strike the same side of the island; this side
is the windward side. The opposite side is the leeward side. Winds blowing up the windward side produce
copious and frequent rainfall, although cloud tops rarely exceed 10,000 ft. Thunderstorms are rare.
Downslope winds on the leeward slopes dry the air, leaving relatively clear skies and far less rainfall. Many
Chapter 17, Tropical Weather 17-5
islands in the trade wind belt have lush vegetation and even rain forests on the windward side , while the
leeward is semiarid. For example, the island of Oahu, HI, is about 24 mi wide in the direction of the trade
winds. Annual rainfall averages from about 60 in on the windward coast to 200 in at the mountaintops,
decreasing to 10 in on the leeward shore.
The greatest flying hazard near these islands is obscured mountaintops. Ceiling and visibility occasionally
restrict VFR flight on the windward side in showers. IFR weather is virtually nonexistent on leeward slopes.
Islands without mountains have little effect on cloudiness and rainfall. Afternoon surface heating increases
convective cloudiness slightly, but shower activity is light. However, any island in either the subtropical
high-pressure belt or trade wind belt enhances cumulus development , even though tops do not reach great
heights. Therefore, a cumulus top higher than the average top of surrounding cumulus usually marks the
approximate location of an island. If it becomes necessary for a pilot to ditch in the ocean, the pilot should
look for, and head toward, a tall cumulus. It probably marks a land surface, increasing chances of survival.
17.2.4 Intertropical Convergence Zone (ITCZ)
Converging winds in the ITCZ force air upward. The ITCZ appears as a band of clouds consisting of
showers (with occasional thunderstorms) that encircles the globe near the Equator. The solid band of clouds
may extend for many hundreds of miles and is sometimes broken into smaller line segments. It exists
because of the convergence of the trade winds. In the Northern Hemisphere, the trade winds move in a
southwesterly direction, while in the Southern Hemisphere , they move northwesterly. The tendency for
convective storms in the tropics is to be short in duration, usually on a small scale, but they can produce
intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed one in/hour. Greatest
rainfall typically occurs during midday. On the Equator, this occurs twice a year in March and September,
and consequently there are two wet and two dry seasons.
Figure 17-1 and Figure 17-2 show the ITCZ and its seasonal shift. The ITCZ is well-marked over tropical
oceans but is weak and ill-defined over large continental areas.
Convection in the ITCZ carries huge quantities of moisture to great heights. Showers and thunderstorms
frequent the ITCZ, and tops to 40,000 ft or higher are common, as shown in Figure 17-1. Precipitation is
copious. Since convection dominates the ITCZ, there is little difference in weather over islands and open
sea under the ITCZ.
Flying through the ITCZ usually presents no great problem if one follows the usual practice of avoiding
cumulonimbus clouds and any thunderstorms.
Since the ITCZ is ill-defined over continents, this handbook will not attempt to describe ITCZ continental
weather as such. Continental weather ranges from arid to rain forests and is more closely related to the
monsoon than to the ITCZ.
17.2.5 Monsoon
As shown in Figure 17-1 and Figure 17-2, over the large land mass of Asia, the subtropical high pressure
breaks down completely. Asia is covered by an intense high during the winter and a well -developed low
during the summer. The same occurs over Australia and central Africa, although the seasons are reversed
in the Southern Hemisphere.
The cold, high pressures in winter cause wind to blow from the deep interior outward and offshore. In
summer, wind direction reverses, and warm moist air is carried far inland into the low -pressure area. This
large-scale seasonal wind shift is the monsoon. The most notable monsoon is that of southern and
southeastern Asia.
Chapter 17, Tropical Weather 17-6
17.2.5.1 Summer or Wet Monsoon Weather
During the summer, the low over central Asia draws warm, moist, and unstable maritime air from the
southwest over the continent. Strong surface heating, coupled with rising of air flowing up the higher
terrain, produces extensive cloudiness, copious rain, and numerous thunderstorms. Rainfall at some stations
in India exceeds 400 in per year with highest amounts between June and October.
The monsoon is so pronounced that it influences circulation many miles out over the ocean. Note in Figure
17-1 that in summer, prevailing winds from the Equator to the South Asian coast are southerly and
southeasterly; without the monsoon influence, these areas would be dominated by northeasterly trade winds.
Islands within the monsoon influence receive frequent showers.
17.2.5.2 Winter Monsoon Weather
Note in Figure 17-2 how the winter flow has reversed from that shown in Figure 17-1. Cold, dry air from
the high plateau deep in the interior warms adiabatically as it flows down the southern slopes of the
Himalayan Mountains. Virtually no rain falls in the interior in the dry winter monsoon. As the dry air moves
offshore over warmer wat er, it rapidly takes in more moisture, becomes warmer in low levels, and is,
therefore, unstable. Rain is frequent over offshore islands and even along coastal areas after the air has had
a significant overwater trajectory.
The Philippine Islands are in an area of special interest. During the summer, they are definitely in southerly
monsoon flow and are subjected to abundant rainfall. In the winter, wind over the Philippines is
northeasterly—in the transition zone between the northeasterly trade winds and the monsoon flow. It is
academic whether the phenomenon is called the trade winds or monsoon; in either case, it produces
abundant rainfall. The Philippines has a year-round humid, tropical climate.
17.2.5.3 Other Monsoon Areas
Australia in July (Southern Hemisphere winter) is an area of high pressure with predominantly offshore
winds, as shown in Figure 17-1. Most of the continent is dry during the winter. In January, winds are
onshore into the continental low pressure, as shown in Figure 17-2. However, most of Australia is rimmed
by mountains and coastal regions that are wet and have onshore winds blow ing up the mountain slopes.
The interior is arid and has downslope winds that are warm and dried.
Central Africa is known for its humid climate and jungles. Note in Figure 17-1 and Figure 17-2 that
prevailing wind is onshore much of the year over these regions. Some regions are wet year -round; others
have the seasonal monsoon shift and have a summer wet season and a winter dry season. The climate of
Africa is so varied that only a detailed area-by-area study can explain the climate typical of each area.
In the Amazon Valley of South America during the Southern Hemisphere winter (July) , southeast trade
winds, as shown in Figure 17-1, penetrate deep into the valley, bringing abundant rainfall, which contributes
to the jungle climate. In January, the ITCZ moves south of the valley, as shown in Figure 17-2. The
northeast trade winds are caught up in the monsoon, cross the Equator, and penetrate the Amazon Valley.
The jungles of the Amazon result largely from monsoon winds.
17.2.5.4 Flying Weather in Monsoons
During the winter monsoon, excellent flying weather prevails over dry interior regions. Over water, pilots
should pick their way around showers and thunderstorms. In the summer monsoon, low ceilings and heavy
rain often restrict VFR flight. IFR flight copes with the hazards of thunderstorms. The freezing level in the
Tropics is quite high (14,000 ft or higher), so icing is restricted to high levels.
Chapter 17, Tropical Weather 17-7
17.3 Transitory Systems
Prevailing circulations are not the only consideration in analyzing weather. Just as important, are migrating
tropical weather producers —the shear line, the Tropical Upper Tropospheric Trough (TUTT), tropical
waves, areas of converging northeast and southeast trade winds along the ITCZ, and tropical cyclones.
17.3.1 Remnants of Polar Fronts and Shear Lines
Remnants of a polar front can become lines of convection and occasionally generate a tropical cyclone. By
the time a cold air mass originating in high latitudes reaches the Tropics, temperature and moisture are the
same on both sides of the front. A shear line, or wind shift, is all that remains (see Figure 17-3). These
influence storms in the Atlantic Ocean, Gulf of America, or Caribbean Sea early or late in the hurricane
season.
Figure 17-3. A Shear Line and an Induced Trough Caused by a Polar High Pushing into the Subtropics
A shear line, also shown in Figure 17-3, results when a semipermanent high splits into two cells, inducing
a trough. These shear lines are zones of convergence creating forced upward motion. Consequently,
considerable thunderstorm and rain shower activity occurs along a shear line.
17.3.2 Tropical Upper Tropospheric Trough (TUTT)
Troughs above the surface, generally at or above 10,000 ft, move through the Tropics, especially along the
poleward fringes. These are known as TUTTs. Figure 17-4 shows such a trough. As a TUTT moves to the
southeast or east, it spreads middle and high cloudiness over extensive areas to the east of the trough line.
Occasionally, a well-developed trough will extend deep into the Tropics, and a closed low forms at the
equatorial end of the trough. The low then may separate from the trough and move westward, producing a
large amount of cloudiness and precipitation. If this occurs in the vicinity of a strong subtropical jet stream,
extensive, and sometimes dense cirrus and some convective turbulence and CAT, often develop.
TUTTs and lows aloft produce considerable amounts of rainfall in the Tropics, especially over land areas
where mountains and surface heating lift air to saturation. Low -pressure systems aloft contribute