Shower formation - rules of thumb

[jack.h...@gmail.com]

Hardly scientific but it seems to work. 

From a SkewT, convection must reach the level where it is minus 10C for showers to be possible.

Similarly convection needs to go as high as the minus 20C level for thunderstorms.

A forecast I saw today suggested hail. 
I do not have a rule of thumb for that.  Minus 30C perhaps?

Jack

[Freddie]

Minus 10 will pretty much guarantee you showers, but they can fall with cloud top temperatures less than that - especially on coasts (abundance of marine cloud condensation nuclei) and where there is a small difference between temperature and dewpoint through the boundary layer.

Probably more like minus 18 for thunder, but minus 20 is close enough.

Hail forms when there's a good separation between parcel temperature and environment temperature.  This leads to stronger updrafts which are generally necessary for hail to form at a large enough size to be solid on reaching the ground - especially during the summer when the hail falls through a deep layer of air with a positive temperature.  You can also get hail from a surprisingly shallow cloud when the freezing level is low.

[Freddie]

On Wednesday, 17 August 2022 at 20:41:48 UTC+1 Freddie wrote:

Minus 10 will pretty much guarantee you showers, but they can fall with cloud top temperatures less than that 

I meant "... with cloud top temperatures greater than that ..." (too tired to be responding really) 

[Graham Easterling]

Minus 10 will pretty much guarantee you showers, but they can fall with cloud top temperatures less than that - especially on coasts (abundance of marine cloud condensation nuclei) and where there is a small difference between temperature and dewpoint through the boundary layer.

Indeed, I've got very wet in drizzly showers when the vertical extent is fairly minimal, with faint shadow from the sun.

It often gets drizzly just as sea fog starts to clear.

Graham

Penzance

On Wednesday, 17 August 2022 at 20:41:48 UTC+1 Freddie wrote: