6 Lesser-Known Terms for Weather Phenomena

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With the summer storm season upon us in full force, you’re bound to hear emergency broadcasts, news reports, and videos of massive thunderstorms, with commentary and terminology you might not be familiar with. Knowing what those words mean can tell you a lot more about what’s going on than just what’s shown on the screen. Here are six lesser-known terms that are associated with the storm season.

1. Derecho

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Directly translated from Spanish, “derecho” means “straight,” and, fittingly, straight-line wind damage is a defining characteristic of this weather event. Derechos, like tornados, tend to accompany massive thunderstorms, and the storms that form them are frequently preceded by low, dark “shelf clouds” (arcus, as seen above). However, unlike tornadoes, the damage they cause is not from rotating wind or vortices. They’re formed by cold wind from thunderstorms being pushed downwards (a downburst) and rapidly spreading out in all directions once it hits the ground. This blast can lead to major damage over a large area.

Straight-line wind damage is common in thunderstorms, but a derecho is defined as a wind-damage swath that extends more than 240 miles (approximately 400 km), and which has wind gusts of at least 58 mph (93 km/h) throughout most of its length. Derechos are most common along the “corn belt” in the United States, but even where they’re most prevalent, there are rarely more than two in a year (compared to 10 to 15 tornadoes a year in the most vulnerable areas). Their rarity is due to the fact that, unlike tornadoes or hurricanes, they’re not a unified and singular event, but rather an uncommon phenomenon that was considered noteworthy enough to be given a name back in 1888. The damage and top wind speed of the derecho often varies along its length because of the fact that it isn’t a unified event, but rather a long line of individual downbursts, each with their own microbursts and microclimates.

2. Squall line

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Also known as multicell lines, squall lines develop from a common “lifting mechanism,” such as a cold front. Included in squall lines are multiple thunderstorm cells, all around the same stage in their lifecycle. They differ from other thunderstorm types, which are known as single cell (or pulse) storms, multicell clusters (where the storm cells are in different stages and don’t necessarily connect or move together), and supercell thunderstorms.

The greatest risk in squall lines tends to be the strong downdrafts, which can cause serious problems for aviation, and can cause major damage on the ground, such as in the case of derechos. Most derechos in North America develop from squall lines.

3. Virga

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From the Latin word meaning “rod, branch,” virgae often look like fuzzy rods or curtains hanging from clouds, and are a common meteorological phenomenon. These rods are shafts of precipitation that fall from clouds, but never reach the ground. They can be seen year-round, often over the desert or prairie, especially in temperate climates.

Precipitation often falls as ice crystals in the high atmosphere but melts as it falls. In the case of virga, this melted water eventually evaporates before hitting the ground. As one might expect, virga tends to develop from high-altitude clouds, when the atmosphere is somewhat warm and dry, allowing it to evaporate moisture easily. The evaporative cooling caused by virga can cause sometimes cause a dramatic temperature drop and strong convective surface winds or microbursts.

4. Crepuscular rays

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Like with virga, you’ve almost certainly seen crepuscular rays before, but might not have known the name. These rays are the “sunbeams” you see coming from the clouds, and the beams of light seen during the crepuscular (“twilight”—dawn or dusk) hours. They appear to converge at the sun, even though they’re actually parallel beams of light. The convergence is similar to how a train track appears to converge on the horizon, even though you know that it remains parallel.

These rays are formed due to the sunlight bouncing off of particulate matter and water vapor in the atmosphere. Since the sunlight passes through ten times the amount of atmosphere at dawn and dusk as compared to midday, there are many more particles for it to bounce off of before it reaches our eyes.

5. Haboob

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Despite its Arabic name (meaning “blasting”), haboobs are a worldwide phenomenon. In North America, “haboob” is occasionally used interchangeably with any dust storm, but it’s more frequently used in the context of a very intense dust storm wall that’s associated with the gust front of a severe thunderstorm. They can overtake a neighborhood or city in minutes, with wind speeds over 40 mph and dust so thick that there is zero visibility. As the haboobs can begin suddenly, be more intense than the average dust storm, and pick up any small particulate matter (such as infectious fungi and industrial metal waste) in their path, there is a serious risk posed to both transportation and public health when people don’t know how to react.

Protocol for haboobs is the same as other dust storms, but can be even more important, especially for those with chronic lung disease. If you’re outside, go inside if at all possible. If there are no indoor locations available nearby, cover your nose and mouth with fabric (such as a shirt). If you’re driving, pull over. Seriously, just wait out the dust; it won’t be that long. Despite the risk to lung health, nearly all deaths caused by haboobs are due to people continuing to drive through them, and getting into accidents.

Are you poetic? Do you understand the importance of waiting out a haboob? Maybe you can help out the Arizona Department of Transportation—this is the second year they’ve had a “Haboob Haiku” contest to promote dust storm safety.

6. Petrichor

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The storm has passed, and the scent in the air says as much. While the smell before rain may be simple (it’s ozone, created when the atmosphere is electrified), the smell after a storm is a bit more complex, and it has a name: petrichor. Coined in 1964 by Isabel Joy Bear and R. G. Thomas of Australia’s Commonwealth Scientific and Industrial Reasearch Organization, petrichor was originally defined as airborne molecules from decomposing plant and animal matter that have settled on mineral or clay surfaces. The molecules of decay recombine with the molecules naturally on the mineral surface during dry spells, and can be smelled after a storm because the addition of water allows the mixture of fatty acids, alcohols, and hydrocarbons to be released. The term petrichor now encompasses the entirety of the smell after rain, however, not just the sharp dusty-decay scent originally described.

One of the most abundant components of petrichor gives it a musty, earthy smell. This scent is the result of the molecule geosmin. It’s a metabolic by-product of blue-green algae in water, and of Actinomyces bacteria in the soil. While it may be a beckoning call to gardeners, it’s been known for almost a century due to the problems it’s caused in winemaking—geosmin contamination leaves a wine tasting “muddy” or “moldy.”

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July 15, 2013 - 2:00pm
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