Archives For evaporative cooling

synoptic chart for heat spike

synoptic chart for heat spike

In the event: July 1st 2015 Heathrow Airport recorded highest ever July Tmax at 36.7C. More here

http://blog.metoffice.gov.uk/2015/07/07/on-the-record-observing-a-heatwave/

Reigate and the southern half of Britain could see some very high temperatures from mid-week next week.  Technically this will probably struggle to become a “heatwave” because it looks like too brief a hot spell, ending by or through the weekend, to exceed the 5 day threshold for an official designation (see below). Nevertheless, a significant HEAT SPIKE is certainly on the cards.

Some very high, possibly record breaking upper air temperatures are due to arrive aloft (forecast 23C 850hPa temperatures for South are quite unheard of in recent years) and, if it’s not too cloudy, 2m surface temperatures could soar to over 30C and possibly even nudge up to 35C.

This is not a forecast, and models will ebb and flow with the event intensity up to the wire, but it’s a review of some factors that will play a part in this heat spike episode.  The synoptic set-up, on charts below and top, shows a blocking HIGH developing over Scandinavia (an omega block) and a trough in the Atlantic with a heat LOW over Iberia. This set-up brings the well known “Spanish Plume”.  Typically this involves a warm, dry upper air flow from the South, drawn up by a perky northward limb of the jetstream and accompanied by an easterly or SE surface flow.  The combination is associated with heat and thunderstorms, though not necessarily extremes of either.

Whilst the Scandinavian Omega Block persists, the easterly continental wind and drier conditions should prevail over the UK but we are close to the edge of the anticyclone and in the line of fire from Atlantic fronts nibbling at the edges and thermal LOWS from Spain running north under the jetstream. The interaction of the advancing cool Atlantic air with the warm upper flow and the increasing surface heat can crank up convective instability as the plume migrates north across France to the UK.  The CAPE and skew-t charts below show the possibility of (elevated) thunderstorms by mid week.  Interestingly at times there is a strong cap near the surface, but great instability aloft, so storms likely to be elevated AcCast (altocumulus castellanus) with potentially strong lightning shows but maybe little rain getting to the surface, at least at first / Wednesday. CAPE is through the roof on some runs but too dry at lower levels and too strongly capped to yield widespread storms as shown below on the skew-t chart.

2015-06-27_08-35-10

upper level instability for potential elevated thunderstorms

Later in the week cool Atlantic air from the west is likely to interact with the plume, descending behind cold fronts and this process can cause CAPE values to increase bringing the chance of more organised thunderstorms that usually herald an imminent invasion of a cooler westerly regime.  Cool tropical maritime air behind fronts typically descends and causes increased lift as it runs into the unstable plume. Recent runs show the GFS wants to hang on to the heat longer while the ECMWF brings back westerlies more promptly by the weekend.  (update: now reversed!) This is not, therefore, likely to be a completely dry hot episode, because thunderstorms threaten especially after any really hot days.

The cross section below shows the flow of upper air clearly swinging round from a southerly direction.  Note the surface flow from the SE.  This combination, brief though it is, raises the risk of unstable conditions and thunderstorms indicated by the raised lifted index (LI) and Total Totals at the foot of the chart.  The average weekly 2m temperature anomaly charts below show how brief the heat might be… the second 5 day average returns back to normal.

Despite the likely short duration, it could be a notable period due to other factors playing a part.  Important ingredients that can be thrown into the mix of heat and thunderstorm potential are sea surface temperatures and soil moisture content. Despite a cool Atlantic, the seas immediately surrounding southern Britain are currently anomalously warm and ripe for transport of heat and thunderstorms across the Channel.

Similarly, dry soil enhances potential heat build up by reducing evaporative cooling and possibly kicking up temperatures higher as less heat is “used up” evaporating soil moisture.  Also, dry soil can enhance instability due to the rapid surface heat build increasing lapse rates.  I’m not quite sure how models handle these factors when producing their 2m temperature forecasts.

Locally, it’s worth noting the frequency of summer days exceeding 30C in Reigate, Surrey. Since 2009 only 29 days have reached or exceeded 30C and only 2 of these have been in June, most in July.  Also note the increase in hots days recently, though of course this is not a big enough sample to be significant.

2015-06-25_20-44-03

It’s still too early to be precise about when and how hot and how thundery it might get next week but the ingredients are at least in place for some very interesting weather!.  Further ahead this looks like a brief heat spike as westerlies resume promptly.  However, the overall pattern seems to favour a blocking HIGH over Europe which could continue to feed the UK with warm Southerly or at least SSW winds for a while.  A cool Atlantic also favours higher pressure so this ought to reduce the chances of very wet conditions, especially here in the South East. Phase 6 of the MJO is also correlated with blocking HIGH patterns over Europe.  So July ought to continue warm, dry and occasionally sultry but with possible thundery episodes.

Media references:

http://www.metoffice.gov.uk/news/releases/archive/2015/hot-july?utm_source=twitterfeed&utm_medium=twitter

http://www.theguardian.com/uk-news/2015/jun/25/mini-heatwave-forecast-for-uk-next-week-temperature-30c

http://www.dailymail.co.uk/news/article-3138816/Glastonbury-Wimbledon-UK-weather-Britain-set-hottest-temperatures-year-week.html

http://blog.metoffice.gov.uk/2015/06/25/hotter-weather-for-the-start-of-july/

http://www.theguardian.com/uk-news/shortcuts/2014/sep/17/continental-blow-torch-warm-weather-britain

http://www.telegraph.co.uk/comment/11733731/Met-Office-caught-out-over-its-hottest-July-day-ever-claim.html (twaddle)

http://www.express.co.uk/news/uk/587218/Heatwave-UK-weather-forecast-summer

https://notalotofpeopleknowthat.wordpress.com/2015/07/05/new-data-from-ruislip-casts-more-doubt-on-heathrow-record-temperature-claims/

http://www.carbonbrief.org/blog/2015/07/met-office-wind-data-dispels-doubt-about-cause-of-heathrow-high-temperatures/

A strong jetstream to the SW of the UK will manufacture some tricky weather over the next 48 hours and continues to make details for the weekend uncertain.  Here’s an outline of the likely scenario for Reigate and SE but the advice must be to watch the forecast if you are doing anything weather-related!

The jetstream is located to the SW of the UK and is blowing strongly from NW to SE.  This means the UK is sitting in that vulnerable zone to the cool north-side of the jetstream where active small scale depressions can run-up rapidly and deepen off the left-exit region of the jetstream where temperature and pressure gradients are greatest.  These depressions are small scale but can produce a lot of rain and their track is sometimes rather erratic: and tiny changes in track can make a big difference to the weather experienced!

Two LOWS are predicted to spring out of the jet from Fri-Sun: the first during Friday – will track directly over SE England and drop possibly 10mm of rain onto Reigate.  As they track broadly S and E of UK, these LOWS will, in turn, swing the winds into a much cooler northerly or NE direction.  Reigate is unlikely to get any strong wind because the LOW pressures are due to pass almost overhead… meaning we are likely get mostly light wind but potential for heavy showers as the cool polar air “warms through” during any sunny periods which will cause convective uplift and showers.  There are also occluding fronts wrapped round these systems that could bring more persistent rain at times.  The second LOW emerges on Saturday and is currently due to track off in a more southerly direction through France but could bring rain across S coasts.  This one needs watching carefully as any movement north will cause a much wetter and unsettled Saturday than is currently on forecasts: which currently show Sat largely dry but with some poss showers later. Showers are possible as the cool air warms through and sparks convective showers on either Sat or Sun. Higher pressure ridges between these lows will bring cold nights, possibly frosty in places if cloud clears.

The complicating factor is that pretty cool air is wrapped up in these depressions, almost cool enough for sleet and wintry precipitation at times later on Friday for Reigate and during almost any showers that rotate around the remnants of the LOWS during Saturday and even into Sunday.  Whilst wintry ppt is unlikely for Reigate, further north and over hills in Wales and N England there will be snow, probably patchy. short lived temporary wet snow at lower levels but accumulations possible over higher ground.

The only way Reigate will see any fleeting proper snow is if we get any HEAVY rain during the passage of fronts later on Friday and any showers left behind in low pressure Sat/Sun.  All rainfall during this period will start as snow aloft, in fact almost all our rain in the UK starts as snow up high and simply melts as it descends.  However, if rainfall is heavy enough it can actually COOL the air sufficiently so that further precipitation falls as snow.  This is called evaporative cooling: some rainfall always evaporates on descending to the ground, even in cold weather.  If enough rain evaporates then this will cool the air sufficient for freezing levels to fall to the surface and then, bingo, snow will descend to the surface as well!  So, whilst rain can NEVER “turn to snow” (completely impossible!), it is possible for heavy rainfall falling through a sufficiently cool air mass (around -5c at 850hPa), to cause snow to reach the surface.

How much colder is it 100 metres above you, right now? We tested this today by driving up the North Downs which stand 100 metres above Reigate, Surrey town centre.  A Kestrel 3500 was used to collect the data from the hill and temperatures were given a good while to stabilize.  The RGS weather station sits at the foot of the same hill at 96 metres. So, the (rather un-scientific!) results at 2.15pm were as follows:

Reigate 96m: Temp 3.1ºC; dew point 2.5ºC
Reigate Hill 200m: Temp 2.1ºC; dew point 1.5ºC

So Reigate Hill is a whole degree colder than the town, the wind chill was -1C. This was noticeable also in the heavy sleet on the hill.
The decrease in temperature with height is called “lapse rate” and it, usually, continues to drop for another 10,000 metres, the top of the troposhere (“weather atmosphere”). The 1C drop per 100 metres today is a steep lapse rate. We would normally expect around 0.6C drop per 100m, called the environmental lapse rate. The freezing level is currently only 300 metres above Reigate town centre and, with temperatures falling tonight the freezing level will almost certainly arrive at the surface.  The clouds were interesting: thick nimbostratus formed a solid cloud base at 300 metres; amazing shreds of fractus cloud formed on the stiff SE wind blowing up the hill.  Fractus or thicker Pannus clouds are those whispy shreds which appear under rain clouds and indicate deteriorating weather.

The impending LOW approaching from the NW this weekend is a real teaser!  It will track SE across the country and the centre of the LOW will pass to to the SW of Reigate sometime over Sunday night. It will then “stall” for a day or two and drag in cool winds from the East.  Fronts associated with this LOW will bring every type of precipitation known…probably!  A weakening warm front slowly drifting east across the country on Saturday is not likely to reach Reigate until the evening. Saturday will be cloudy, calm and cool but hopefully remain dry. The leading edge of the warm front could bring some light snow by Saturday evening.  Sunday will be the BIG ONE… wet, cold and miserable, all day. North of London there could be heavy snow.  Reigate is on the VERY EDGE of the forecasted snow area (see map). Temperatures will remain above freezing at up to 3-4C, too warm for snow. However, despite this, heavy rain could turn to snow any time.  From Sunday afternoon onwards a process called evaporative cooling could cause snowfall. Some raindrops always evaporate on their descent from clouds, even in really cold air. Evaporation (turning a liquid into a gas) uses energy / heat (think of how chilly you feel stepping out the shower; the cause = evaporation!). Now imagine millions of tiny raindrops evaporating, for every evaporating raindrop some energy (heat) is taken out of the atmosphere and lowers the air temperature. In marginal snow conditions this temperature drop can be sufficient to turn HEAVY RAIN (the more raindrops the more cooling) into SNOW, even if surface temperatures are above freezing.  So, the question for Reigate this weekend is not “will it snow on Sunday?” but “will it rain hard enough to snow on Sunday?”  Enjoy the precipitation anticipation!