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20th March 2015 - Eclipse Weather

PostPosted: 2015 Mar 20, 15:25
by Nick Gardner
The cloud did not spoil the view of the partial eclipse as it was thin and allowed the sun to be viewed for most of the time.

It went surprisingly gloomy after a bright start to the day. The birds started singing at around 09:10 and then stopped at maximum coverage. Then at around 09:40 they started singing their 2nd dawn chorus of the day!

Below is a graph of the temperature, solar radiation and mean wind speed before, during and after the eclipse. The temperature rise was halted and dropped back slightly before continuing. The most dramatic effect was on the solar radiation that declined to a very gloomy 19 W/m²; a reading you would normally expect at about 10 minutes after sunrise. The wind speed also reduced noticeably.
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A photo I took at around the time of maximum coverage (85%).
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Re: 20th March 2015 - Eclipse Weather

PostPosted: 2015 Mar 20, 18:20
by Martin Rowley
... Here in West Moors (as for most of this end of Dorset & further afield to the east and north), dawn welcomed us with 8 oktas Stratus! And throughout the event, no change occurred in that cloud structure - base circa 1000 ft (~300m) hardly varying - and thick enough to allow only a slight rise in temperature from 5.6 at 0800Z to 5.8degC by 0900Z.

Then from 0910Z, distinctly gloomier - and screen temperature eased back to 5.5degC by 0925Z, which was the lowest through the event - being a 'flat-line' at 5.5 from 0922 to just before 1000Z, when the temperature started to pick up as the St thinned: up to 7.0degC by 1100Z. Over the grass, the lowest on the m-i-g thermometer was 5.3, having been 5.7 at 0900Z.

The 'prevailing' wind speed was Bft 2, then the wind dropped briefly to calm (or near calm) between 0925 & 0930Z (straddling neatly the eclipse maximum of ~0927Z). There was a brief, quite noticeable increase of wind (no more than Bft3) around 0935Z: presumably a 'recovery' isallobaric pressure wave.

After 0900Z, bird song became increasingly of the 'twilight' variety, and Blackbirds were perched (without singing) for 10 minutes approx. between 0925 & 0935Z: noticeable though that Great Tits took no notice of goings-on and called throughout!

After 0935Z, as skies started to lighten, resumption of 'general' birdsong and other activity.

Of course, the cloud broke up nicely when it was all over - well-broken low-base Sc by 1130Z.

Martin.

Re: 20th March 2015 - Eclipse Weather

PostPosted: 2015 Mar 22, 19:53
by Eddy Graham
Eclipse data & report from Stornoway:

There were more breaks in the cloud layer than predicted by various models at T+24 (which had indicated a uniform 8 okta low layer). From 06h30 to 09h30 it was predominately 4-7 okta stratocumulus /stratus undulatus with some wave activity apparent. Just after eclipse maximum ~9h30, more general showery 7-8 okta stratocumulus praecip. conditions rolled in from the west bringing showers (just 'smirs' of drizzle / a high density of very fine raindrops giving the broad rainbows, total precip to 9am on 21st was only 0.8mm). There was a noticeable rise in wind from WF3-4 (before eclipse maximum) to WNW/NW F4-5 (afterwards). From 11-12h onwards, the sun came out again in an open cell cloud structure, with the light showers dying out in the afternoon. With 98% occultation of the Sun in Stornoway, it grew quite dark (though perhaps not as dark as one might expect given a 98% loss of solar radiation, and the darkest period lasted only 2-3 minutes); cars had to put their headlights on, and it certainly felt chiller.

It was quite encouraging to see most people outdoors; most workers stopped their jobs and came outside to line the streets; school-children were also given free time to go outside.

I had three separate dataloggers running in Stornoway, at two different locations. Firstly, here at home in Stornoway town (I am on the summit of a slight hill, SW aspect):

(a) High resolution PRT record every 10 secs: The key thing noticeable here is the delay of the development of the convective boundary layer until 11h00! (identifiable by the sudden increase in high-frequency temp fluctuations).
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(b) The same Stornoway town site , Conrad Electronics Temp/RH logger every 1-min
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(c) A frost hollow site in a small glen near Lews Castle to the west of Stornoway (Gleann nam Brog), also Conrad Electronics Temp/RH logger every 1-min:
tn_Eclipse-allotment-TEMP-RH-DEWPOINT.jpg
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Summary:
Stornoway Town (PRT): -1.0degC drop in temp, 10-15min lag behind eclipse peak
Stornoway Town (Conrad logger): -0.7degC drop, 10-20 min lag
Gleann na Brog (Conrad logger): -1.6degC drop, 10-20min lag

I was expecting a larger drop at the frost hollow site (which was confirmed). Analysis of the satellite animation for British Isles didn't show any obvious or apparent change in the cloud structure, it just looked like a fairly typical WNW flow in early spring.

Eddie Graham