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Flying in the tropics


Tropical weather is usually ideal for flight operations. Here’s what you need to know if you’re flying your bizjet to the tropics.

By Karsten Shein
Comm-Inst. Climate Scientist

De Havilland Canada DHC-6-300 Twin Otter lands at SXM (Princess Juliana Intl, St Maarten). Although weather in many parts of the tropics is usually ideal, turbulence from water/land transitions, short island runways, heat, and complex terrain, can add challenges.

Approaching by GPS in late afternoon, neither pilot could see any visual reference to the Caribbean island ahead. The runway was a narrow strip nestled into a steep valley, which at the moment was obscured with clouds and rain-generated fog.

To make matters worse, the airstrip was on the east side of the island, and the sun had already dropped behind the high ridge running across the island, throwing the valley into shadow.

Just as they were about to call the missed approach, visibility improved enough to spot headlights from an airport vehicle moving along the dark runway scar in the verdant jungle canopy.

With a 10-kt tailwind, the crew had to work fast to slow the aircraft. When they stopped, they had just a few feet of asphalt to spare. As their passengers disembarked, a heavy shower opened up, soaking them all. Welcome to the tropics, the crew thought.

Whether for business or pleasure, many flights operate in the tropics – the equatorial zone that lies between 23.5° S (Tropic of Capricorn) and 23.5° N (Tropic of Cancer). These latitudes are not random. Rather, they represent the latitudes at which the noon sun is directly overhead at the June (23.5° N) and December (23.5° S) solstices.

As a result of this Earth-Sun geometry, it is a region where the sun is high in the sky year-round. This means a constant surplus of solar radiation reaching Earth’s surface and heating the overlying air.

Tradewinds and doldrums

This constant solar heating of the tropics is what fuels the circulation of Earth’s atmosphere and drives the weather across the planet. As air is heated beneath the sun, it rises, drawing cooler air from the subtropics.

Simultaneously, the heated tropical air that rose to the top of the tropical tropopause (50,000–60,000 ft above the surface) can’t keep rising into the stratosphere due to a temperature inversion, so it spreads out, moving poleward in both the northern and southern hemispheres. Now cool, it warms as it descends over the subtropics.

Towering cumuli over rainforest near FRS (Mundo Maya Intl, Flores, Guatemala). Ample heat and humidity mean long wet seasons during which pilots must be prepared to dodge widespread convection.

These circulations are called Hadley cells. The air descending over the subtropics creates a global belt of surface high pressure around 30° latitude, resulting in a region of limited rainfall where most of the world’s hot deserts can be found.

Rising air beneath the latitude of the sun’s zenith does the opposite, producing the surface low-pressure belt known as the Intertropical Convergence Zone (ITCZ). Both the subtropical highs and ITCZ are areas of limited surface wind, known respectively as the horse latitudes and the doldrums.

While subtropical highs can be a danger to sailing vessels, their calm winds and clear skies make for normally smooth aviation operations. The only time these regions become dicey is near and after the solstice, when the ITCZ may migrate into these latitudes, particularly over land areas, bringing low ceilings and copious, often heavy, monsoon rains that can flood airports and routinely delay flights.

Between the ITCZ low and the subtropical highs, the Earth’s rotation results in airflow that spirals clockwise out of the highs in the northern hemisphere (counterclockwise in the south), producing a more or less constant easterly wind known as the tradewinds.

While most runways in the tropics are oriented east–west, rugged terrain or a lack of suitable sites on small islands may preclude that, meaning that some airports in these latitudes will always have a crosswind.

Towering cumuli

Counter to the clear and dry conditions across the subtropics, the ITCZ is a region of converging warm and humid air, ample solar heating, and high humidity – the latter due to the predominance of ocean in the tropics.

As air in the ITCZ heats and humidifies, it becomes buoyant and rises into widespread cumuli. The ITCZ cloud belt is easily seen on satellite imagery. Many of these clouds will grow into towering cumuli (TCu) that, in the tropics, may extend well above the service ceilings of many business and commercial aircraft.

While it is usually not an issue to plot a course that avoids the most congested TCu regions, many tropical routings will cross the ITCZ and may penetrate these TCu, particularly at night, when seeing and avoiding the clouds is difficult.

High-level significant weather prognostic chart for the tropical Atlantic. In addition to areas of the ITCZ indicated for convection to FL500 over Central America, northern South America, and Central Africa, features such as tropical cyclones and active volcanoes also suggest an active tropical weather environment.

Pilots should recognize that, while these convective clouds are generally not thunderstorms and don’t contain the same energetic dangers posed by midlatitude storms, they can still produce moderate and occasionally severe turbulence, heavy rain, and even waterspouts.

Critically, these clouds grow far above the freezing level, and strong updrafts with unlimited water supplies may produce regions of moderate high-altitude supercooled, mixed, or frozen precipitation. Such conditions can generate cruise-level icing that can clog pitot-static ports, causing erroneous airspeed and altimeter readings and autoflight system disengagement – potentially creating a loss-of-control situation.

Given the near-saturated air even in the flight levels, pitot heat, situational awareness, and good crew resource management (CRM) are important when cruising across the ITCZ.

Tropical cyclones

Tropical cyclones are intense and usually compact low-pressure storm systems that develop as waves in the tropical easterly jet aloft and interact with the edges of the ITCZ, lowering the surface pressure and initiating a system that, if conditions are right, can sustain itself as a heat pump as it separates from the ITCZ and moves poleward.

When tropical cyclones strengthen enough (winds exceeding 65 kts), they are called hurricanes in the Atlantic, Caribbean, and eastern Pacific; typhoons in the western Pacific; and cyclones in the south Pacific and Indian Ocean.

Tropical cyclones are relatively rare, so, even if the specific conditions necessary to their formation are met, they still may not form. Those factors include being at least 5° from the equator (the Coriolis effect that supports the spinning storm is zero at the equator), high humidity and low windshear throughout the lower atmosphere, hot ocean water, and a trigger such as a passing jet trough. Once formed, conditions must remain favorable for them to maintain or increase intensity as they move.

Fortunately, near-global weather satellite coverage ensures that tropical cyclones can be tracked from formation to dissipation. When active, sigmet charts will indicate tropical cyclone locations, and computer models will provide guidance on its projected strength and path.

Beyond the outer bands of a tropical cyclone, flying weather is often very good, but any aircraft and personnel in the possible path of an approaching cyclone should be moved well in advance – if possible. Winds associated with strong cyclones can exceed 150 kts sustained, and tornadoes are frequently observed from the cyclone’s individual thunderstorm cells.

While some basins, such as the western Pacific and northern Indian Ocean, experience tropical cyclones year-round, most have a distinct storm period that coincides with that region’s warm season.

The Atlantic/Caribbean and eastern Pacific period is roughly June through November, and the southern Pacific and southern Indian region period runs from November through April.

The US National Oceanic and Atmospheric Administration (NOAA) tracks tropical cyclones in both the Atlantic and Pacific oceans, and each spring it issues a probabilistic forecast for tropical cyclone activity in the Atlantic basin.

For 2022, NOAA has predicted a strong chance of above-normal activity, with 14–21 named storms, 6–10 hurricanes, and 3–6 of these becoming Category 3 or stronger. This forecast suggests that flight operators in the Caribbean, Gulf of Mexico, and the US East Coast should consider reviewing their emergency evacuation and relocation plans.

Airport weather considerations

Outer rainband of Hurricane Isidore (Sep 2002) just south of Cuba. Tropical cyclones are storm systems that are most widely associated with tropical weather, but adverse weather in the tropics is more closely related to the position of the ITCZ.

With often limited and frequently rugged real estate, tropical island runways are usually short, and may be embedded in narrow valleys or on the sides of steep slopes. Often, this configuration can disrupt the otherwise consistent trade winds, producing lee turbulence.

Humid air cooling and descending the valley at night can create morning fog, particularly where the airport is surrounded by dense vegetation. Daytime heating and rising terrain on the island create sea breeze circulations that may produce daily rain showers moving inland to the afternoon and shifting back offshore at night.

Lastly, some airports at tropical locations, such as UIO (Quito, Ecuador) and LPB (La Paz, Bolivia), at 7900 ft MSL and 13,325 ft MSL, respectively, are both tropical and high-altitude. While the altitude can keep temperatures cool, these airports are often in rugged terrain and may experience low visibility in cloud and fog, high and gusty winds, mountain wave turbulence, and other factors more often encountered in mountain flying than what would be expected in the tropics.

A final consideration in tropical flying is that the tropics are home to many continental plate margins and associated volcanic activity. Many Caribbean and Pacific islands are volcanic, and ash from even minor eruptions can pose an aviation hazard.

Frequently, ash is entrained at middle altitudes in concentrations too thin to see, but sufficient to produce engine surging and potentially clog pitot-static ports. A check of volcanic ash sigmets is advised whenever flying in the tropics.

Closing remarks

Overall, tropical weather flying is normally a pleasure, with consistent winds and little chance for icing or doing a walk-around in blizzard conditions. Other than monsoons that may bring rain and low ceilings for several months, daily weather in the tropics is fairly uniform – warm, occasional rain, and no need to divert hundreds of miles to avoid a solid squall line.

However, the tropical atmosphere is still one that should not be ignored. Pilots can face weather as severe and subject to sudden change as anywhere else on Earth. A thorough weather briefing is important, even if it’s just for island hopping. As always, if you encounter weather that isn’t as you expected, please let your fellow pilots know with a pirep.

SheinKarsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and an NOAA and NASA climatologist. Shein holds a comm-inst pilot license.