UKC

Can anyone explain why rain showers sometimes form showers? In the south west today there was obvious lines of very heavy showers going roughly SW to NE that were quite wide, but also a couple of very narrow bands of showers at right angles to these. Interested to know what causes this (especially the narrow band of showers).

In reply to Andy 976853:

Clouds, probly.

In reply to Andy 97685

... why rain showers form showers ??? Don't really understand the question

Sounds like you are seeing cloud streets http://en.wikipedia.org/wiki/Horizontal_convective_rolls These align along the prevailing wind direction, which fits with your SW to NE observation.

As for the shower lines at 90 degrees to the streets, I don't know. Could possibly be showers from clouds formed by gust fronts from the larger cells in the streets perhaps...

Rug

In reply to Andy 976853: Cross winds?

In reply to Andy 976853: God has OCD

In reply to rug:
Sorry should be showers forming lines not showers...

Cloud streets fit with the SW to NE observation and are something I've often observed, was really wondering what causes the lines at 90 degrees to these. If you have a look at the met-office rainfall radar for the SW at 17:30 today there are some clear very thin bands of rain at 90 degrees to other rainfall.

In reply to Andy 976853:

i know what you mean, was drving on M way nr glasgow last week and passed through the most horrendous rain for about 10 seconds and then it was dry again like a really narrow band of cloud,was well strange

In reply to Andy 976853:
I suspect that the SW-NE lines of heavy rain you saw were caused by sea breeze fronts:
http://en.wikipedia.org/wiki/Sea_breeze

The geography of the south-west England is such that sea breezes move inland towards each other from the north and the south. When they meet, they can force air up and form clouds. On days like today that are prone to thunderstorms, this can cause very heavy rain along the line where the breezes meet.

This is consistent with the SW-NE line being stronger in the mid-afternoon and dying out towards the evening as the sea breeze weakened.

Cloud streets do form along the prevailing wind direction but tend to be restricted to shallow convection (little fluffy cumulus clouds!) rather than deep convection (thunderstorms).

In reply to BethanWhite:

Thanks, was aware of causes of sea breezes but didn't realise they caused fronts. Not sure this was what happened today, although the rain under the SW-NE lines was very heavy but at times two fairly indistinct lines were present, could be N/S coast influence I suppose, coupled with effect of the moors. In contrast the 90 degree thin lines were fairly light rain and hardly moved throughout the day (the rain wasn't 100% consistent but whenever the 'rain line' returned it was very obvious - looking at rainfall radar - most obvious line was from Cardiff/Weston Super Mare/Shepton Mallet/Shaftesbury)

In reply to Andy 976853:
I've just looked at the surface pressure chart for noon on Monday and there is a trough line (of low pressure) marked roughly SW-NE along southwest England.

http://www.metoffice.gov.uk/public/weather/surface-pressure/#?tab=surfacePr...

Having seen this, I now think it's very likely that the main line of storms formed along this. Colliding sea breezes may have helped to intensity the showers along the SW-NE line, as could the higher ground on the moors.

I'd have to look in more detail and/or ask around to come up with an explanation for the bands at 90 degrees to this. Did you see them in real life or just on the rain radar? I've also just looked at the UKMO rain radar loop for today and the "90 degree" bands look quite stationary from 1545 all the way through to 2145 and also seem to radiate from a common origin - I wonder if it's something to do with the radar rather than a real observation...

In reply to Andy 976853:
There are sometimes 'spokes' in the rainfall radar images radiating out from the radar stations. These aren't rain, just data errors.

In reply to anonymouse:
Exactly - looking at the bands on the radar, there's a common origin near the Capel Iwan radar site in Carmarthenshire. I think it's a data problem rather than a reliable observation.

In reply to BethanWhite:
Didn't see the 90 degree bands just observed them on the radar (whilst trying to work out whether I could finish laying a patio). The bands were virtually stationary all day from fairly early morning and agree they appear to radiate from a common origin, note that the length of the bands cover a large area - more than I'd expect from a single radar. The bands were visible on both the MetOffice site and Netweather - not sure if they take information from same source or not.