Definition of a shower

lukegriffen

Do Met Eireann have a reasonable definition for a shower ? eg. in a 4 hour period should there be x% dryness and y% rain before something can be considered a shower ?

This afternoon's forecast : "Heavy, thundery showers over the eastern half of the country, with some torrential downpours and local flooding"

There was about a 40 to 60 minute period when it didn't rain, but since 2 it's been belting down, grrrr ! So basically there were showers of dryness. I wouldn't have cycled to work if they'd said 'heavy rain lasting 3 hours over a 4 hour period' instead of bloody 'showers'

Snowbie

Its very hard to predict where showers are going to fall on a location in general,you can use an area like the east,south etc.Its really a hit and miss affair.They can be so localised,with down the road bone dry.

You could use the radar loop by back tracking a bit and see if one is going to come near your location before setting out but as today event and the right conditions,a shower can spring up in a short space of time.They got the eastern side of the country right today as its easy to predict where a convergence of this type will form but not a location (town,city) where a shower will fall in.That be a best guestimate.

Supercell

lukegriffen wrote:

Do Met Eireann have a reasonable definition for a shower ? eg. in a 4 hour period should there be x% dryness and y% rain before something can be considered a shower ?

This afternoon's forecast : "Heavy, thundery showers over the eastern half of the country, with some torrential downpours and local flooding"

There was about a 40 to 60 minute period when it didn't rain, but since 2 it's been belting down, grrrr ! So basically there were showers of dryness. I wouldn't have cycled to work if they'd said 'heavy rain lasting 3 hours over a 4 hour period' instead of bloody 'showers'

Well I live in the eastern half of the country and there was only a couple of showers and in the evening nothing at all, so I'd rate the forecast pretty accurate - some areas did have torrential downpours with local flooding(D6 seems to be the main area affected), and others like here just had an average showery day with dry spells in between.

lukegriffen

Longfield wrote:

Well I live in the eastern half of the country and there was only a couple of showers and in the evening nothing at all, so I'd rate the forecast pretty accurate - some areas did have torrential downpours with local flooding(D6 seems to be the main area affected), and others like here just had an average showery day with dry spells in between.

Oh that's good to hear that not all places got washed out. Funnily enough my cycle journey home took me straight through D6. One of those times where you don't know whether to laugh or cry. I just feel sorry for any tourists here.

lukegriffen

The answer to my original question was in the Irish Times today. I'm gonna summarise, i hope someone's still interested in the answer, now that we haven't had rain in over 6 hours !

The difference between rain & showers has nothing to do with the duration, or the amount of wetting, it's to do with the type of cloud it comes from.
Showers fall from individual convective clouds, like cumulonimbus, while rain falls from a 'layer' cloud, like altostratus, which is spread uniformly over a large area.

Convective clouds have a vertical structure, and can go 1000s of feet into the atmosphere, whereas rain forms in the flat layers of altostratus.
So while usually a breeze would push along an individual convective cloud relatively quickly, if it's calm, one area could be drenched for hours, yet it's still considered a shower.
Whereas with rain, it may stop at intervals, so you may think it's a shower, but it's actually rain, becuase it's falling from a layer cloud.

Snowbie

Altostratus is a mid altitude layered cloud and yes rain can fall from it albeit light and most evaporates before hitting the ground like altocumulus.
Nimbostratus is the continuous rain cloud usually associated with a warm front and is low altitude cloud formation.

You can also get rain from all low leveled convective cumuli clouds.
Now there is a difference between a shower and frontal rain,but both can be continuous and both can be weak.The difference is frontal rain covers a wider area while a shower can be local, both can last same length of time too,depending on winds on a specific location.

Another type of scenario is a squall line.These are cumulonimbus(CB) forming on a cold front which can range from only a few miles across but several hundred miles long.Same type of cloud that produces showers but in a line.Also depending on direction its moving some places could have a downpour lasting moments while other places could be deluged for hours if line is moving up through this area.
Now according to the papers would this be classed as a shower or rain.?