21-4 over ocean areas are predominantly northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere. The inversion 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 trades 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.
21.1.2.1 Weather over Open Sea. In the trade wind belt, skies over open water are about one-half covered by clouds on the average. Tops range from 3,000 to 8,000 feet, depending on height of the inversion. Showers, although more common than under a subtropical high, are still light with comparatively little rainfall. Flying weather generally is quite good.
21.1.2.2 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 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.
21.1.2.3 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 feet. Thunderstorms are rare. Downslope winds on the leeward slopes dry the air leaving relatively clear skies and much less rainfall.
Many 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, Hawaii, is about 24 miles wide in the direction of the trade winds.
Annual rainfall averages from about 60 inches on the windward coast to 200 inches at the mountaintops, decreasing to 10 inches on the leeward shore.
21-5 The greatest flying hazard near these islands is obscured mountaintops.
Ceiling and visibility occasionally restrict VFR flight on the windward side in showers. Instrument flight rules (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 tops of surrounding cumulus usually marks the approximate location of an island. If it becomes necessary for a pilot to ditch in the ocean, he or she should look for, and head toward, a tall cumulus. It probably marks a land surface, increasing chances of survival.
21.1.3 The 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 their duration, usually on a small scale, but they can produce intense rainfall.
It is estimated that 40 percent of all tropical rainfall rates exceed 1 inch per 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 21-1 and Figure 21-2 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 feet or higher are common, as shown in Figure 21-1. 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, we 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.
21.1.4 Monsoon. As shown in Figure 21-1 and Figure 21-2, 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
21-6 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.
21.1.4.1 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 inches 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 21-1 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 trades.
Islands within the monsoon influence receive frequent showers.
21.1.4.2 Winter Monsoon Weather. Note in Figure 21-2 how the winter flow has reversed from that shown in Figure 21-1. 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 water, it rapidly takes in more moisture, becomes warmer in low levels, and, 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 trades and the monsoon flow.
It is academic whether we call the phenomenon the trade winds or monsoon;
in either case, it produces abundant rainfall. The Philippines have a year-round humid, tropical climate.
21.1.4.3 Other Monsoon Areas. Australia in July (Southern Hemisphere winter) is an area of high pressure with predominantly offshore winds, as shown in Figure 21-1. Most of the continent is dry during the winter. In January, winds are onshore into the continental low pressure, as shown in Figure 21-2.
However, most of Australia is rimmed by mountains, and coastal regions are wet where the onshore winds blow up the mountain slopes. The interior is arid where downslope winds are warmed and dried.
21-7 Central Africa is known for its humid climate and jungles. Note in Figure 21-1 and Figure 21-2 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. 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 trades, as shown in Figure 21-1, 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 21-2.
The northeast trades are caught up in the monsoon, cross the Equator, and also penetrate the Amazon Valley. The jungles of the Amazon result largely from monsoon winds.
21.1.4.4 Flying Weather in Monsoons. During the winter monsoon, excellent flying weather prevails over dry interior regions. Over water, pilots must pick their way around showers and thunderstorms. In the summer monsoon, VFR flight over land is often restricted by low ceilings and heavy rain. IFR flight must cope with the hazards of thunderstorms. The freezing level in the Tropics is quite high (14,000 feet or higher), so icing is restricted to high levels.
21.2 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.
21.2.1 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 virtually the same on both sides of the front. A shear line, or wind shift, is all that remains (see Figure 21-3). These mainly influence storms in the Atlantic Ocean, Gulf of Mexico, or Caribbean Sea early or late in the hurricane season.
21-8 Figure 21-3. A Shear Line and an Induced Trough Caused by a Polar High Pushing into the Subtropics
A shear line, also shown in Figure 21-3, results when a semi-permanent 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.
21.2.2 Tropical Upper Tropospheric Trough (TUTT). Troughs above the surface, generally at or above 10,000 feet, move through the Tropics, especially along the poleward fringes.
These are known as TUTTs. Figure 21-4 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 and clear air turbulence, often develop.
21-9 Figure 21-4. A TUTT Moves Eastward Across the Hawaiian Islands. Extensive Cloudiness Develops East of the Trough
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 significantly to the 300+ inches of annual rainfall over the higher terrain of Maui and the big island of Hawaii. Other mountainous areas of the Tropics are also among the wettest spots on Earth.
21.2.3 Tropical Wave. Tropical waves (also called easterly waves) are common tropical weather disturbances, normally occurring in the trade wind belt. In the Northern Hemisphere, they usually develop in the southeastern perimeter of the subtropical high pressure systems.
They travel from east to west around the southern fringes of these highs in the prevailing easterly circulation of the Tropics. Surface winds in advance of a wave are somewhat more northerly than the usual trade wind direction. As shown in Figure 21-5, as the wave approaches, pressure falls; as it passes, surface wind shifts to the east-southeast or southeast. The typical wave is preceded by very good weather, but followed by extensive cloudiness (see Figure 21-6), and often by rain and thunderstorms.
The weather activity is roughly in a north-south line.
21-10 Figure 21-5. A Northern Hemisphere Easterly Wave Progressing from A–B
Note that winds shift generally from northeasterly to southeasterly. The wave moves toward the west, and is often preceded by good weather and followed by extensive cloudiness and precipitation.
Figure 21-6. Vertical Cross Section along Line A–B in Figure 21-5
Tropical waves occur in all seasons, but are more frequent and stronger during summer and early fall. Pacific waves frequently affect Hawaii; Atlantic waves occasionally move into the Gulf of Mexico, reaching the U.S. coast.
21.2.4 West African Disturbance Line (WADL). On occasion, a line of convection similar to a squall line moves westward off the continent at tropical latitudes into the oceanic trade winds. In the North Atlantic this is known as the West African Disturbance Line (WADL). A WADL can move faster than easterly waves at 20-40 mph. Some WADLs eventually develop into tropical storms or hurricanes.
21-11
21.2.5 Tropical Cyclones. Tropical cyclone is a general term for any low that originates over tropical oceans. Tropical cyclones are classified according to their intensity based on the average 10-minute wind speeds. Wind gusts in these storms may be as much as 50 percent higher than the average 10-minute wind speeds. Over the north Atlantic and northeast Pacific Oceans, tropical cyclone classifications are:
1. Tropical Depression—sustained winds up to 34 knots (64 kilometers per hour).
2. Tropical Storm—sustained winds of 35 to 64 knots (65 to 119 kilometers per hour).
3. Hurricane—sustained winds of at least 65 knots (120 kilometers per hour), or more.
In other regions of the world, different terms are used for tropical cyclones meeting hurricane strength, such as typhoon in the northwest Pacific, severe tropical cyclone in the southwest Pacific and southeast Indian Oceans (e.g., near Australia), severe cyclonic storm in the north Indian Ocean, and just tropical cyclone in the southwest Indian Ocean. The term super typhoon is used if the maximum sustained winds are at least 130 knots (241 kilometers per hour).
21.2.5.1 Development. The prerequisites to tropical cyclone development are optimum sea surface temperature under low-level convergence and cyclonic wind shear.
Favored breeding grounds are: shear lines, TUTTs, tropical waves, and lines of convection in low latitudes moving from the continent to the tropical ocean
(e.g., WADL).
The low-level convergence associated with these systems by itself will not support development of a tropical cyclone. The system must also have horizontal outflow (divergence) at high tropospheric levels. This combination creates a chimney, in which air is forced upward causing clouds and precipitation. Condensation releases large quantities of latent heat which raises the temperature of the system and accelerates the upward motion.
The rise in temperature lowers the surface pressure, which increases low-level convergence. This draws more moisture-laden air into the system. When these chain-reaction events continue, a huge vortex is generated which may culminate in hurricane force winds.
Figure 21-7 shows regions of the world where tropical cyclones frequently develop. Notice that they usually originate between latitudes 5° and 20°.
Tropical cyclones are unlikely within 5° of the Equator because the Coriolis force is so small near the Equator that it will not turn the winds enough for them to flow around a low pressure area. Winds flow directly into an equatorial low and rapidly fill it.
21-12 Figure 21-7. The Tracks of Nearly 150 Years of Tropical Cyclones and their Strength Weave Across the Globe
The map is based on all storm tracks available from the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) through September 2006. The accumulation of tracks reveals several details of hurricane climatology, such as where the most severe storms form and the large-scale atmospheric patterns that influence the track of hurricanes (from National Aeronautics and Space Administration (NASA)).
Note: See Table 21-1 for wind strength associated with each scale on the Saffir-Simpson Scale).
21.2.5.2 Movement. Tropical cyclones in the Northern Hemisphere usually move in a direction between west and northwest while in low latitudes. As these storms move toward the mid-latitudes, they come under the influence of the prevailing westerlies. At this time the storms are under the influence of two wind systems: the trade winds at low levels and prevailing westerlies aloft. Thus, a storm may move very erratically, and may even reverse course or circle. Finally, the prevailing westerlies gain control, and the storm recurves toward the north, then to the northeast, and finally to the east-northeast. By this time, the storm is well into mid-latitudes.
21.2.5.3 Decay. As the storm curves toward the north or east (Northern Hemisphere), it usually begins to lose its tropical characteristics and acquires characteristics of lows in middle latitudes. Cooler air flowing into the storm gradually weakens it. If the storm tracks along a coastline or over the open sea, it gives up slowly, carrying its fury to areas far removed from the Tropics. However, if the storm moves well inland, it loses its moisture source and weakens from
21-13 starvation and increased surface friction, usually after leaving a trail of destruction and flooding.
When a storm takes on middle latitude characteristics, it is said to be extratropical, meaning “outside the Tropics.” Tropical cyclones produce weather conditions that differ somewhat from those produced by their higher latitude cousins and invite our investigation.
21.2.5.4 Weather in a Tropical Depression. While in its initial developing stage, the cyclone is characterized by a circular area of broken to overcast clouds in multiple layers. Embedded in these clouds are numerous showers and thunderstorms. Rain shower and thunderstorm coverage varies from scattered to almost solid. Diameter of the cloud pattern varies from less than 100 miles in small systems to well over 200 miles in large ones.
21.2.5.5 Weather in Tropical Storms and Hurricanes. As cyclonic flow increases, the thunderstorms and rain showers form into broken or solid lines, paralleling the wind flow that is spiraling into the center of the storm. These lines are the spiral rain bands frequently seen on radar. These rain bands continually change as they rotate around the storm. Rainfall in the rain bands is very heavy, reducing ceiling and visibility to near zero. Winds are usually very strong and gusty and, consequently, generate violent turbulence. Between the rain bands, ceilings and visibilities are somewhat better, and turbulence generally is less intense.
Most tropical cyclones that form eyes do so within 48 hours of the cyclone reaching tropical storm strength. In the eye, skies are free of turbulent cloudiness, and wind is comparatively light. The average diameter of the eye is between 15 and 20 miles, but sometimes is as small as 7 miles and rarely is more than 30 miles in diameter. Surrounding the eye is a wall of cloud that may extend above 50,000 feet. This wall cloud contains deluging rain and the strongest winds of the storm. Maximum wind speeds of 175 knots have been recorded in some storms. See Figure 21-8 and Figure 21-9 below, which contain a radar display and satellite photograph of a mature hurricane, respectively. Note the spiral rain bands and the circular eye. Notice the similarity between these two figures.
Table 21-1 identifies the wind speed and characteristic house damage for each level on the Saffir-Simpson Hurricane Wind Scale.
Figure 21-8. Radar Image of Hurricane Katrina Observed at New Orleans, Louisiana, on August 29, 2005
BASE REFLECTIVITY
KLIX - NEW ORLEANS, LA 08/29/2005 13:54:29 GMT LAT: 30/20/13 N LON: 89/49/30 W ELEV: 138.0 FT Mississippi MODE/VCP: A / 121 ELEV ANGLE: 0.50 ° MAX: 59 dBZ RANGE 248 NM Legend: (Category) dBZ
(15) 75
(14) 70
(13) 65
(12) 60
(11) 55
(10) 50
(9) 45
(8) 40
(7) 35
(6) 30
(9) 20
(3) 15 ПОАА (2) 10
(1) 5 Figure 21-9. Hurricane Andrew Observed by Satellite in 1992 Hurricane Andrew NORA NOAA-12 AVHRR HRPT Multi-spectral False Color Image August 23, 1992 @ 12:31 UTC 21-14
21-15 Table 21-1. Wind Speed and Characteristic House Damage for the Saffir-Simpson Hurricane Wind Scale Saffir-Simpson Hurricane Wind Scale Wind speed Characteristic House Damage 5 ≥157 mph ≥137 knots ≥252 km/h Almost complete destruction of all mobile homes will occur, regardless of age or construction A high percentage of frame homes will be destroyed, with total roof failure and wall collapse.
Extensive damage to roof covers, windows, and doors will occur.
Large amounts of windborne debris will be lofted into the air.
Windborne debris damage will occur to nearly all unprotected windows and many protected windows.
4 130-156 mph 113-136 knots 209-251 km/h Nearly all older (pre-1994) mobile homes will be destroyed. A high percentage of newer mobile homes also will be destroyed. Poorly constructed frame homes can sustain complete collapse of all walls as well as the loss of the roof structure. Well-built homes also can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Extensive damage to roof coverings, windows, and doors will occur. Large amounts of windborne debris will be lofted into the air. Windborne debris damage will break most unprotected windows and penetrate some protected windows.
3 111-129 mph 96-112 knots 178-208 km/h Nearly all older (pre-1994) mobile homes will be destroyed. Most newer mobile homes will sustain severe damage with potential for complete roof failure and wall collapse. Poorly constructed frame homes can be destroyed by the removal of the roof and exterior walls. Unprotected windows will be broken by flying debris.
Well-built frame homes can experience major damage involving the removal of roof decking and gable ends.
2 96-110 mph 83-95 knots 154-177 km/h Older (mainly pre-1994 construction) mobile homes have a very high chance of being destroyed, and the flying debris generated can shred nearby mobile homes. Newer mobile homes can also be destroyed. Poorly constructed frame homes have a high chance of having their roof structures removed, especially if they are not anchored properly. Unprotected windows will have a high probability of being broken by flying debris. Well-constructed frame homes could sustain major roof and siding damage. Failure of aluminum, screened-in, swimming pool enclosures will be common.
1 74-95 mph 64-82 knots 119-153 km/h Older (mainly pre-1994 construction) mobile homes could be destroyed, especially if they are not anchored properly as they tend to shift or roll off their foundations. Newer mobile homes that are anchored properly can sustain damage involving the removal of shingle or metal roof coverings, and loss of vinyl siding, as well as damage to carports, sunrooms, or lanais. Some poorly constructed frame homes can experience major damage, involving loss of the roof covering and damage to gable ends, as well as the removal of porch coverings and awnings. Unprotected windows may break if struck by flying debris. Masonry chimneys can be toppled. Well- -constructed frame homes could have damage to roof shingles, vinyl siding, soffit panels, and gutters. Failure of aluminum, screened-in, swimming pool enclosures can occur.