"Sunshine and Showers"

[Summer of 95]

Today we seem to be having yet another midwinter "sunshine and showers" day here, the latest of many this "winter".

It's my understanding that this phenomenon is caused by the sun heating the air at ground level, which then rises, condenses and precipitates to the ground in short sharp bursts; and that it's associated with low pressure and "unstable" (favourable to convection) air masses.

It's also stuck in my mind that March and April, when the sun is relatively strong yet the air still often cold and unstable, is the commonest time to expect this phenomenon, although in March especially the showers often fall as snow or hail. It also seems quite common (and annoying) in summer when a pleasant sunny day is interrupted by sudden bursts of rain but with no thunder- I often think of summertime convective showers as "failed thunderstorms".

However to me it is not something I associate with December, January or early February (except in the form of those "east coast snow showers" in a cold easterly); surely the sun is not strong enough to create any serious convection? It seems to me that it is becoming more and more common to get April-type "sunshine and showers" days in January in particular in recent years, usually in a cyclonic zonal/westerly airflow, and that this is the main reason for the dramatic increase in winter sunshine, as well as the recent tendency towards winter months that are both sunnier and wetter than average. January 1999 was another good example as well as last month and the start of this one.

Is it just my imagination or is "sunshine and showers" in winter traditionally a rare phenomenon; and if not how can such a week sun generate sufficient convection?

[Delta76]

Although with very limited weather knowledge i cannot expand on what you have just posted , it was only the other day when i was thinking the same thing. If i did'nt know as much i would have thought it was March or April .

[Wynn D Woo]

Just seems to be showers with out the sun down here,

[Nick F]

However to me it is not something I associate with December, January or early February (except in the form of those "east coast snow showers" in a cold easterly); surely the sun is not strong enough to create any serious convection? It seems to me that it is becoming more and more common to get April-type "sunshine and showers" days in January in particular in recent years, usually in a cyclonic zonal/westerly airflow, and that this is the main reason for the dramatic increase in winter sunshine, as well as the recent tendency towards winter months that are both sunnier and wetter than average. January 1999 was another good example as well as last month and the start of this one.

Is it just my imagination or is "sunshine and showers" in winter traditionally a rare phenomenon; and if not how can such a week sun generate sufficient convection?

From November to March, Sea Surface Temperatures (SSTs) are the main source of warmth for convection, when the seas are warmer than the air crossing it, so when cold dry air of Polar maritime/Polar arctic/Polar continental airmasses cross the sea -warm air from the surface rises upwards to form showers when pressure's low. However, in the colder months, despite coastal areas being mainly favoured areas for showers, troughs or fronts can move across inland areas and give showers well away from coasts - all is needed is a frontal boundary for cold air to force mild/moist air upwards or a cold pool behind a trough to force mild/moist air upwards - for showers to get well inland in winter.

I don't think "sunshine and showers" is becoming more common in January, all it take is some contrasting colder air aloft in the form of a trough, frontal activity, or polar air with it's drier colder clearer air crossing the warmer seas and you have the perfect recipe for "sunshine and showers" during any month in winter. Polar air brings similar "sunshine and showers" in April, but more so over inland areas due to the stronger sun warming the ground beneath the cold air and allowing sharp showers to form well inalnd as a result.

I should add that the sunshine between showers is often the result of the air which rises up then descends out of cumulonimbus clouds (downdraft) then dries as it descends evaporating any cloud and creates the familiar dry slots with sunshine betwwen each shower, more particualrly in polar airstreams.

[Thundery wintry showers]

I think Nick F has provided a good enough description- generally if troughs can get inland you will get "sunshine and showers" widely in winter. The weather type tends to dominate across much of the country between November and February when polar maritime airmasses dominate (note that such months aren't always cold zonal; the aforementioned case of January 1999 had the polar maritime air regularly "return" a large distance over the Atlantic, so the month as a whole was mild).

I'm not sure that the frequency of "sunshine and showers" in winter has increased. Certainly, this winter has seen much more of it than the past few winters, but that's largely because the last few winters were quite blocked, and when polar maritime air came in, secondary lows kept cutting off the showery flow; I think this winter has merely shown a return to the sort of frequency that we saw during the winters of the 1990s.

The increase in westerlies during the 1990s probably led to more sunshine-and-showers days than was the case during the winters of earlier decades, though there will have been exceptions; months like December 1982 and January 1984 look like they had a lot of bright showery weather. The weather can also be quite showery when an unstable east-to-north-east flow persists, or northerlies from the Arctic come accompanied by troughs and polar lows.

I also came across an article on "sunshine and showers" frequency across the country, which showed that western areas generally have a much higher frequency than eastern areas, and that the lowest-frequency areas are inland SE England (especially around Bedford) and around the Kingdom of Fife.