Cloud Formation Questions

Jun 16, 2004
Minneapolis, MN
I wish I would have taken a picture of this the other night but I didn't have my camera with me so I'll have to describe it.

My view was obscured by trees and houses in a suburban setting, so from what I saw, it appeared to be a very well developed cell with an anvil and overshooting top. When I finally got into full view of it, I found that the base was VERY high, higher than I had ever seen. So it looked like it was a fully mature cell with the base squished up into the atmosphere. I couldn't even guess how high it was - take your average cell that we all chase and cut off the top third, that's where the base was. So my question is, what causes cloud development at such altitudes (this was over South Dakota)?

My next question I do actually have a picture...

[Broken External Image]:

How are the wispy clouds at the bottom formed? It looks as though it's fog, but they're maybe 20 feet in the air or so. They were moving fast too.

I know these are probably simple answers but I'm kind of new to all this and still learning.
The ragged wispy clouds underneath a cloud base are called scud. They form from condensation of moist air rising into the storm.
High-based convection occurs when the lifted parcels (from the near-ground air, or Planetary Boundary Layer, PBL) are lacking the moisture to condense nearer the surface. That is why chasers are so keenly interested in dewpoints (Td).

If there is a large Td depression.....the difference between the Td and temperature, then the air must rise higher before it condenses than if there is a smaller Td depression.

There are other considerations, but the lapse rate is 9.8C/1k meters.

If the PBL temperature is 78F, and the Td is 69F, then Td depression is 9F (5C), and the rising air will basically have to ascend 5/9.8x1000m (~1675 feet) before it has cooled enough to condense into your cloud base. At that point, (this is the LCL) the air becomes saturated, and the relative humidity is 100%.

If , however, the PBL temp is 78F, but the air is drier, having a Td of only 51, (and it may have been lower than that where you were "the other night"), then the Td depression is 27F or 15C, and thus PBL would have to be lifted 15/9.8x1km=~5020 feet.

So, dry air=high base. Damp air=low base.

As far as the low clouds.....

Probably had some cool air descending from the storm top and mixing with moist PBL there, causing some condensation (umm....clouds!). But others can probably give a better answer to that one.

I saw some really bizarre examples of that in NM last year:

Hope this helps!