Archives For windy

2015-02-08_22-03-51

HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.

2015-02-08_23-38-08

The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?

2015-02-08_22-09-10

Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.

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Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.

subg2015-02-08_22-18-44

Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.

superg2015-02-08_22-18-44

Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

useful reference

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/grad.rxml

http://www.metoffice.gov.uk/learning/wind/what-causes-wind

http://education-portal.com/academy/lesson/factors-that-affect-wind-pressure-gradient-forces-coriolis-effect-friction.html

 

Forecast models are building a significant autumnal stormy picture across the UK next week. Depressions sweeping in from the west on a lively jetstream are looking likely to swing south and bring high winds, cooler temperatures and plenty of rain, especially to the west, but significantly also to the south.  Gale force winds will be significant in coastal and mountain areas if these models come right.  It’s early days but more model agreement means more confidence is building in this scenario with only the ECMWF finding potential calmer conditions with high pressure.  Watch the temps fall away from recent mild conditions as these LOWS also drag in cooler NW winds… more normal for the time of year.  Tmax 9 or 10c compared with current 17c. Tmin 5 or 6c compared with low teens now. In the wind it’ll feel much cooler than the balmy conditions we have got right now.

end of october cool off
This situation is extremely mobile and one to watch carefully, but increasing agreement between models (GEM, GFS, UKMO) shows increasing confidence in something wicked this way coming! The situation brews over the weekend and unleashes early in the week with another storm later. Check back for details as they emerge. 

Reigate weekend weather will be windy and wet first thing Saturday with showers and bright intervals following after a well defined front moves away early. Typically, no deluge is forecast for Reigate and the East but early rain on Saturday could be heavy for a time around breakfast. Most rain will fall further west. The most significant feature of weekend weather for Reigate will be wind: gusting from the west over 35mph at times on exposed hills on Saturday, less on Sunday.  

A summer depression with a LOW central pressure for the summer (994mb) is passing straight across Scotland over the weekend. This has winds spiralling anti-clockwise around the centre of the LOW, for Reigate this means gusts over 30mph on the hills and generally widespread 20mph winds. Quite a breezy day for mid-summer. As the cloud breaks in the afternoon it will be good kite flying weather. More rain and cloud could be widespread on Sunday, some of it heavy and showery in the afternoon.
For next week things look much better as a large HIGH pressure builds across the entire Atlantic and pushes any more depressions well to the North over Iceland. This HIGH will settle over the west of the UK and build across the East as the weekend LOW moves out into the North Sea. We can look forward to some pleasant dry summer weather well into next week, reaching over 20C. But always with the threat of a shower in the east where winds will be from the NW or west.
So…total rainfall for the next week in Reigate is forecast to be a mere 10mm or so, hardly very much. At least 8mm of this is forecast to fall this weekend, making next week very dry indeed!

During the recent last spell of warm humid sticky weather with the warm southerly plume of potentially thundery downpours, Reigate got no showers at all! The capped inversion layer prevented any significant convection.  Despite immensely high forecast CAPES and lifted index, each forecast would predict heavy showers and potential thunderstorms but none would arrive as the last model runs would lower capes and LI.  Below the inversion, with no significant convection to break the cap, cloud simply spread out into dull strato-cumulus formations covering the sky and reducing the warming by the sun and the instability of the lower layer of the atmosphere was therefore reduced.  No cap was busted, no spectacular cumulonimbus were seen round Reigate.  Some occured nearby in Kent and the Channel and running up the N Sea.  Several models (NMM, GFS, NAE) consistently predicted showery rain almost everyday, some of it heavy, yet none arrived.  Other models were nearer the mark and picked up better on the lack of powerful convection beneath the inversion (UKMO, HIRLAM).  So, in Reigate, being so sheltered and pleasant, it never rains but it pause!

Thursday morning will see winds in excess of 100mph for Reigate… fortunately well above the town at 20,000 feet or so! This is the jetstream. These winds are pivotal in producing strong surface winds which will “touch-down” later in the day in Reigate. Surface gusts of over 40mph are possible with +30mph average winds. These winds are due to a deepening area of LOW pressure crossing the north of the UK over the next few days. The Irish Sea, English Channel, south coast and hilly areas like the South and North Downs will see the highest gusts. With trees in leaf there may be the odd branch falling. Batten down the hatches and secure loose bins! (Video: you can see the LOW crossing Northern England.  Reds 60mph; yellows 40mph; greens 30mph).

Quick update for NEXT week: LOW pressure dominates as a cyclone sinks down from the NW and an upper TROUGH sits over the UK while HIGH pressure builds to the NORTH: this will bring a distinctly AUTUMNAL feel to Reigate next week: temperatures struggling to get into the mid-teens, with periods of showery and breezy weather: so, NOT hot at all: cool, unsettled and rather wet for the time being.  More details later! Check @RGSweather for regular updates.

A deep depression centered over Iceland is crossing to the North of the UK bringing windy conditions to south east through Thursday / Friday:

Wind gusts could reach a whopping 50mph overnight thursday 22 November to friday here in the South East.  Average wind speeds will be more like 20mph+.  The cold front moves through overnight and this will herald a taste of that cold wind brought down from the North – more of this next week!