• Bryan Norcross

DUST DOMINATES THE TROPICS AND WHY IS MIAMI SO HOT?

A tropical disturbance is approaching the southern Caribbean islands, but it has run into hostile upper-level winds and Saharan Dust – and it’s falling apart. The disturbance will move through the islands tomorrow as a slight moisture surge before dissipating completely in the Caribbean.

The disturbance represents a moist gap between two clouds of dry Saharan Dust. To the west, dusty air a mile or two above the ground is spread across the Caribbean, the Gulf of Mexico, Florida, the southeastern U.S, and north into the Upper Midwest. To the east, another dust cloud is in the pipeline.

Fortunately, the dust is not as concentrated as the intense plume that covered the Caribbean last week, but there’s enough dust to make the skies milky, lower the chances of thunderstorms, and keep the tropics calm.


As everybody in South Florida knows, it’s been extraordinarily hot. The feels-like temperature – the combination of the heat and humidity - has been 90 or above night and day for a week. There has been a lot of talk on TV and online from and among meteorologists about the cause of the excess heat. A number of factors seem to be coming together, each of which leads to warmer than normal weather.

The fact is, however, I don’t think anybody knows exactly what’s going on. The factors we’re seeing are not that rare or extraordinary. Could it be they are all ganging up? Maybe. But most of them gang up with some regularity in the summer, yet there is no persistent stretch of weather like this in the record book.

The Saharan Dust is, no doubt, playing a part. The dusty, dry air limits the number of thunderstorms that develop. A strong thunderstorm pulls cool, dry air from high up in the atmosphere down to the ground and generally stirs up the air around us. That’s how we get pleasant evenings after stormy afternoons in the summer. Obviously, that hasn't been happening lately.


In addition, the Saharan Dust acts like a blanket, trapping heat near the earth at night. But that always happens when we get a dust outbreak, yet the heat at night this year is the warmest on record.

The fact that this is happening in late June makes a difference, because this is the point in the year when the sun is highest in the sky. That means we receive more energy than at any other time. If we don’t have a lot of cloudiness or storminess, more energy reaches the earth. Of all of the factors, this is the only one that can’t be replicated during hot spells later in the summer.

There is an area of high-pressure aloft, which is positioned in a way to contribute to hotter than normal temperatures. The air subsides over us, which warms the atmosphere much like a bicycle pump gets hot as you fill a tire. Still, it’s not what we would consider a super-hot weather pattern, like we had the one time it reached 100 degrees on July 21, 1942. With this current pattern we’d expect hot weather, but not crazy hot.

And the ocean water is a bit warmer than normal near shore, above the reef. In the early morning hours, it’s running about 83 or 84 degrees, and in the mid to upper 80s in the heat of the day. But again, that’s not extreme.

Miami International, where the official readings are taken, tends to run a bit hotter than the surrounding locations, which is probably related to the tremendous amount of development around the airport over recent years. There are bigger highways and more buildings, all of which retain heat.

Miami/Opa-locka airport is just 8 miles north, and it tends to be cooler, especially in the early morning. Yesterday, when Miami set a daily record and tied the warmest-on-record low temperature at 84 degrees, Opa-locka was 79, which is close to the long-term normal.


This morning, Miami was 82 at 7:00 AM while Opa-locka was 77. There has been a lot less large-scale development in the northern part of Miami-Dade County. (Ironically, the one 100-degree reading in the official Miami record book was measured very close to today Opa-locka airport at the old Miami Municipal Airport, which was just to the south.)

On top of everything, of course, the world is getting warmer. Climate change, however, is probably not the direct driver of the current dramatic heatwave – but instead, a contributor. Some studies have shown that weather systems move more slowly in a warmer world. Slow-moving systems can produce extreme results, but it’s not clear that’s what’s happening here.

So if you hear some hemming and hawing from meteorologists, it’s because the exact reason it’s so amazingly hot – if there is one reason – is not obvious. Whatever it is, it probably involves these factors, and maybe something else we haven’t thought of as well.

Elsewhere in the Atlantic, the potential system the National Hurricane Center has been mentioning off the Carolina coast is not expected to develop. No systems of concern are forecast into the weekend.

© 2019 by Bryan Norcross Corporation

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