Equinoctial gales
1
Weather – April 2007, Vol. 62, No. 4 111 A halo on Mars – a reply I was interested in the article ‘A Halo on Mars’ in the June 2006 issue of Weather, but I do not think the explanation is correct. In my view the optical phenomenon depicted and subsuns in general are due to the reflec- tion of the Sun from the spacecraft’s surface or, as in the case of those observed from the Earth, from an aircraft’s window. If you could somehow view the scene without the pre- sence of a spacecraft or aircraft, I suggest that you would not see any phenomenon. I am prompted to offer this explanation by what I saw during a flight from Edinburgh to London on 20 March 1988 at a flight level of 37 000 feet and at about 1530 UTC. When flying over the Carlisle area I noticed a coloured subsun to the west, apparently just above the tops of some cumulus mediocris. As the aircraft progressed southwards, it entered the top of a very active front. The cloud was quite dense at and below the flight level, but not enough to prevent the Sun’s disc being reflected from the aircraft’s window and projected onto the cloud below. The subsun was brilliant and slightly off-white, apparently quite close, with a vividly coloured halo around it, complete with areas of contact – the top one inter- secting where the halo from the real Sun would have been. This was not something which was passed by, but which followed or moved with the aircraft. Surely this phe- nomenon must have been observed by pas- sengers and aircrew flying more frequently? I suggest the odd shape of the ‘halo’ on Mars was due to the shape of the spacecraft, the surface of which is probably as highly reflec- tive as an aircraft window. George Watt Nr Auchenflower, Ballantrae, Girvan, South Ayrshire doi: 10.1002/wea.69 Persistent frosts In more than half a century of enjoying and studying the weather in the Guisborough area of North Yorkshire, I have never come across this before: for two days (20 and 21 December 2006) there has been frost on the ground – on pavements, side roads and grass – despite the temperature having been well above freezing for more than 24 hours. Even at the current (22 December) 6 °C there is still frost on the ground – though none on roofs, cars etc. This is remarkable, especially as the lowest overnight minima for the last few days have been about 1 °C. Last night the minimum temperature was around 3 °C! More remark- ably, last night there was frost on the sandy beach close to Redcar. Why hasn't the ground frost thawed by now – indeed why did it not thaw several days ago? Any explanations would be welcomed. Ken Mitchell Guisborough, Cleveland doi: 10.1002/wea.64 Equinoctial gales After reading the recent article in Weather (October 2006) regarding the Bristol Channel floods of 1607, my thoughts turned to the Equinox and the equinoctial gales. Based on the experience of many observers over 50 years of the Ocean Weather Service (OWS) there was a fairly widely held percep- tion that the tracks of equinoctial gales were migrating south. Unscientific conjecture – yes, but at the time the OWS started up (1946) there was a lot of forecasting based on observation and hypothesis. Computer models and charts give, however, a very bland indication of what an equinoctial gale can really be like. To illustrate this point these extracts are quoted from the meteoro- logical log of OWS Weather Surveyor on sta- tion ‘Lima’ on 20 March 1976, during a gale that originated as a shallow wave depres- sion over the mid-Atlantic. Over two hours of winds of hurricane (force 12) and above were included in this list. The majority of the data were logged by myself as duty observer. An equinoctial gale of similar magnitude in the Bristol Channel could easily prove to be as devastating in 2007 as in 1607. Mike Hatch Dyserth, Rhyl, Denbighshire, doi: 10.1002/wea.68 Letters Wind Time direction Wind speed Pressure (GMT) (deg) (knots) (mbs) 1200 170 38 965.6 1400 120 34 958.4 1530 130 12 1630 Calm (wetbulb wicks changed) 1640 310 02 1700 260 15 950.9 1710 280 40 1730 Gust 81 1800 260 66 956.6 1822 Gust 90 1900 290 70 965.3 1928 Gust 95 (estimated) 2000 290 72 973.3
-
Upload
mike-hatch -
Category
Documents
-
view
214 -
download
0
Transcript of Equinoctial gales
Weather – A
pril 2007, Vol. 62, No. 4
111
A halo on Mars – a reply
I was interested in the article ‘A Halo onMars’ in the June 2006 issue of Weather, butI do not think the explanation is correct. Inmy view the optical phenomenon depictedand subsuns in general are due to the reflec-tion of the Sun from the spacecraft’s surfaceor, as in the case of those observed from theEarth, from an aircraft’s window. If you couldsomehow view the scene without the pre-sence of a spacecraft or aircraft, I suggestthat you would not see any phenomenon. Iam prompted to offer this explanation bywhat I saw during a flight from Edinburgh toLondon on 20 March 1988 at a flight level of37 000 feet and at about 1530 UTC. Whenflying over the Carlisle area I noticed acoloured subsun to the west, apparently justabove the tops of some cumulus mediocris.As the aircraft progressed southwards, itentered the top of a very active front. Thecloud was quite dense at and below theflight level, but not enough to prevent theSun’s disc being reflected from the aircraft’swindow and projected onto the cloudbelow. The subsun was brilliant and slightlyoff-white, apparently quite close, with avividly coloured halo around it, completewith areas of contact – the top one inter-secting where the halo from the real Sunwould have been. This was not somethingwhich was passed by, but which followed ormoved with the aircraft. Surely this phe-nomenon must have been observed by pas-sengers and aircrew flying more frequently?I suggest the odd shape of the ‘halo’ on Marswas due to the shape of the spacecraft, thesurface of which is probably as highly reflec-tive as an aircraft window.
George WattNr Auchenflower,
Ballantrae, Girvan,South Ayrshire
doi: 10.1002/wea.69
Persistent frosts
In more than half a century of enjoying andstudying the weather in the Guisborougharea of North Yorkshire, I have never comeacross this before: for two days (20 and 21December 2006) there has been frost on theground – on pavements, side roads andgrass – despite the temperature havingbeen well above freezing for more than 24hours. Even at the current (22 December)6 °C there is still frost on the ground –though none on roofs, cars etc. This isremarkable, especially as the lowestovernight minima for the last few days havebeen about 1 °C. Last night the minimumtemperature was around 3 °C! More remark-ably, last night there was frost on the sandybeach close to Redcar.
Why hasn't the ground frost thawed bynow – indeed why did it not thaw severaldays ago?
Any explanations would be welcomed.
Ken MitchellGuisborough,
Cleveland
doi: 10.1002/wea.64
Equinoctial gales
After reading the recent article in Weather(October 2006) regarding the BristolChannel floods of 1607, my thoughts turnedto the Equinox and the equinoctial gales.Based on the experience of many observersover 50 years of the Ocean Weather Service(OWS) there was a fairly widely held percep-tion that the tracks of equinoctial gales weremigrating south. Unscientific conjecture –yes, but at the time the OWS started up(1946) there was a lot of forecasting basedon observation and hypothesis. Computermodels and charts give, however, a very
bland indication of what an equinoctial galecan really be like. To illustrate this pointthese extracts are quoted from the meteoro-logical log of OWS Weather Surveyor on sta-tion ‘Lima’ on 20 March 1976, during a galethat originated as a shallow wave depres-sion over the mid-Atlantic.
Over two hours of winds of hurricane(force 12) and above were included in thislist. The majority of the data were logged bymyself as duty observer. An equinoctial galeof similar magnitude in the Bristol Channelcould easily prove to be as devastating in2007 as in 1607.
Mike HatchDyserth, Rhyl, Denbighshire,
doi: 10.1002/wea.68
Letters
WindTime direction Wind speed Pressure
(GMT) (deg) (knots) (mbs)1200 170 38 965.61400 120 34 958.41530 130 121630 Calm
(wetbulb wicks
changed)1640 310 021700 260 15 950.91710 280 401730 Gust 811800 260 66 956.61822 Gust 901900 290 70 965.31928 Gust 95
(estimated)2000 290 72 973.3
