Monday, July 08, 2013

Things That Spin In The Sky Besides Tornadoes

Of all the things that spin in the sky (on the mesoscale, at least) there are two types: the tornado, and the not-actually-a-tornado-but-people-will-say-it-is-anyway.

Tornadoes are violently rotating columns of air in contact with the ground. And while they are both fascinating and destructive, they’ve received more than their fair share of attention lately. So today, I’m writing about their equally as interesting cousins, including things that are actually considered tornadoes, like landspouts and waterspouts, and things that are not, like cold air funnels, dust devils, and gustnadoes.

It is fair game to call landspouts and waterspouts tornadoes. In fact, the technical term for a landspout is a “non-mesocyclone tornado.”

Most tornadoes form when an area of mid-level rotation inside a thunderstorm, known as a mesocyclone, descends to the ground with the aid of the thunderstorm’s downdraft.

However, a landspout forms when pre-existing surface vorticity is stretched into the vertical by a vigorous updraft and tightened.

What I’ve just said is a lot of fancy meteorological speak, but in simple terms the difference is this: regular tornadoes stretch down from strong thunderstorms; landspouts stretch up from the ground in weaker showers.

Landspouts commonly occur in places where there is intense surface heating, but where there is also enough moisture for thunderstorms to form. This combination is typically found on the High Plains. One very common place for landspouts happens to be Denver Airport, where the local topography causes surface wind circulations to form all the time.

This photograph from Twitter user Mike Sellers of a landspout passing over Denver Airport just last month shows what a normal landspout looks like: a long dusty tube with a small funnel at the cloud base.

While landspouts are not as dangerous as mesocyclone tornadoes, they still often produce damage like that of an EF1 tornado, and they are treated as tornadoes with Tornado Warnings issued for them by the local National Weather Service office. You should react to a landspout the same way you would to a tornado.

Waterspouts are a common occurrence offshore of Florida, as well as on the Great Lakes in the fall. They can sometimes act just like landspouts (except for the obvious fact that they exist over water), but they can also form from supercells over water. The former case is much more common, and thankfully, relatively harmless. The second case, however, is much more serious. This type of waterspout – essentially a mesocyclone tornado that just happened to form over water – should be treated just like a regular tornado. NWS meteorologists will usually issue a Tornado Warning for this kind of waterspout, but not for the first kind.

These are the only spinning things that can legitimately be called tornadoes. However, they are not the only spinning things.

Gustandoes often form near severe thunderstorms, and are often mistaken for actual tornadoes. However, they are not connected to the cloud base. They form when outflow from that same thunderstorm, often referred to in this case as a gust front, interacts with the ambient air flow, creating a circulation. Gustandoes are brief and weak. They are only visible when some other material – often dust – is picked up into them and circulated around them. Otherwise, they are invisible.

Cold air funnels are a lot more like regular tornadoes than gustandoes, and yet they are not technically tornadoes because they lack one significant criterion: they are not in contact with the ground.

Cold air funnels look just like regular tornado funnels, except that they are usually much thinner and much higher off the ground. They form when significant mid-level vorticity, often associated with a cold pool aloft, is stretched vertically to an even higher level of the atmosphere.

Because they are so rare, they are nearly impossible to predict in advance. Even in real time, they are essentially invisible to anyone but an eyewitness, since they form in weak rain showers (rather than strong thunderstorms) that are often otherwise innocuous.

Several cold air funnels were spotted in northwestern Ohio last Friday. A news outlet in Toledo compiled a slideshow of the various user-submitted photos, which you can view here.

And while cold air funnels happen where it is cold (hopefully that doesn’t surprise you), dust devils only happen where it is very warm.

Dust devils form when a near-surface layer of air is considering super-adiabatic, meaning that its rate of temperature decrease from the ground up is greater than the Dry Adiabatic Lapse Rate of 9.8 degrees Celsius per kilometer.

In deserts and on the High Plains where it is dry, the ground heats up very quickly, and can get so much warmer than even the air immediately above it that the air immediately starts rising. It isn’t clear exactly what causes this rising air to start rotating, however. But since dust devils rarely harm anyone, there is also little research about them.

While the other types of spinning things we’ve looked at all happened around clouds, dust devils only happen when there is bright sunshine and very little wind. They only last for a few seconds to a minute, and are also almost always harmless.