Naked Science Forum
Life Sciences => The Environment => Topic started by: neilep on 02/07/2008 12:35:05
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Dearest Huggable Peeps Who Know Klevur Stuff,
See this ?
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It's a piccy of what is called a dry storm !!
It's dry because ....well..it's not wet !!...in other words no rain reaches the ground !
Why's that then ?
Thanks
Neil
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There are two ways this can come about.
Lightning is a discharge of static electicity, built up largely by friction between air molecules. The air molecules have to be moving relative to each other, so the key ingredient here is turbulant air, which may or may not extend to ground level. While the moisture content of the air is a factor in the build up of static, it doesn't have to be high enough to condense and fall as rain, so if it isn't, it won't.
The other way that a rainless storm can occur is when the rain that falls from the storm evaporates before it reaches the ground. This is more likely when there's a layer of very dry air below the rain layer.
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Again Lee is correct. The below text is from the weather channel.
Dry lightning is lightning that occurs where rain is not falling at the strike location or soon thereafter. This allows the lightning strike to trigger a wildfire and allow it to begin to grow.
Dry lightning comes in two types. One type is lightning that comes down from a thunderstorm anvil into an area where it is not raining. The raindrops aloft may be evaporating before they reach the ground, with lots of low-relative-humidity air present between the relatively high anvil cloud and the ground. We call this virga -- raindrops (or snowflakes) not reaching the ground.
The other type of dry lightning comes from a dry thunderstorm -- one that is producing basically no rain at all that reaches the ground. This type of thunderstorm has a very high cloud base even within its updraft. Again, any rain generated aloft within the storm evaporates before reaching the ground (virga).
High-based thunderstorms form when it becomes very warm, but is relatively dry, so that the relative humidity at the ground is low. The figure below shows the temperature (red) and dew point (green) profiles taken by a weather balloon launched into such an environment on Tuesday evening from Salt Lake City, UT. Temperatures are warmest on the right side of the diagram and cool toward the left side. Upward on the diagram indicates increasing height (lowering air pressure), with altitudes above ground and air pressures indicated on the left side of the diagram.
On this occasion the surface temperature was 98F, while the surface dew point (a measure of the moisture in the air) was just 35F, at the bottom of the profiles above. The temperature decreases rapidly upward, while the dew point does not decrease as much, and the two nearly come together at the 500 mb level. This location marks the likely cloud base of the thunderstorm on this occasion, where a heated bubble from the surface first becomes a cloud. This is at about 15,000 feet above ground here.
I've marked on the above diagram the layer where the thunderstorm cloud is likely to be found, with its top at about 40,000 feet above ground. This diagram, by the way, is called a Skew-T-log-P diagram, and is used by meteorologists to plot and analyze the data from weather balloons.
When hot weather develops over the West, the increasing temperature can be enough to spawn thunderstorms even when the relative humidity at the surface is low, as in the sounding above, especially in hilly terrain where upslope flow can give the air an initial upward boost. That's what happened recently.
Besides the dry lightning, there is a second bad aspect of dry thunderstorms. They can spawn dry microbursts. These are narrow, intense downdrafts and outflow winds generated as the raindrops evaporate and the downdraft reaches the ground not accompanied by rain. The air - cooled by evaporating rain - crashes downward almost like a rock in a pond. The cooler, denser air accelerates downward and then spreads rapidly outward in strong gusty winds near the surface.
The dry microburst winds can spread the incipient wildfire or an existing wildfire rapidly and uncontrollably. There was a dry microburst associated with the high-based thunderstorms predicted by the sounding above. It gave a measured wind gust of 64 mph in Emery County, northern Utah. Sometimes dry microbursts cause damage -- downing trees or even causing building damage.
So you could say that dry thunderstorms pose a double whammy -- dry lightning and dry microbursts. For firefighters they pose a triple threat, because the microburst is an extreme hazard for any aircraft being used to try to fight wildfires. They'll have to suspend aircraft operations until the microburst threat is gone.
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Again , I thank ewe both (LeeE and Paul) for your wonderful information and explanation.
Gosh, you too ought to be meteorologists, climatologists or Weatherologists...(as long as it has 'ologist ' at the end of it !)
Yes...yes...reading the above reminds where I heard all this....it was on Radio 5 Live (very very a.m.).....they were discussing about somewhere in the world (country name forgotten) that the storms were causing havoc for firefighters because they were dry !..and the lightning was causing lots of fires !!
THANKS Chaps
Hugs The Chaps (ooooh....sore !)
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How about Skyologist? I've just always been fascinated and curious about anything to do with the sky. It's rare for nothing to be happening to or in the sky, so even when nothing appears to be happening, it's interesting for that very reason.