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School Weather News 1999
From: "Malcolm Walker"
Subject: MetLinkInternational
Day 1 Date: Mon, 25 Jan 1999 20:55:23 -0000
Dear MetLink participants
Well done! Day 1 of MetLink's active phase has gone better than I
dared hope. I am using a whirling psychrometer at the RMS to measure
air temperature and wet-bulb temperature and a sensitive hand-held
anemometer to measure wind speed. I have a wind-direction indicator
close by in the form of a flag on the top of a nearby shopping
centre. I have no rain gauge and I have no means of measuring maximum
and minimum temperature. As I said in an earlier e-mail, the Society
is close to the centre of a big town, so that must be borne in mind
when interpreting my observations.
I had one query about correcting pressure to sea level: to do this,
you should take your station pressure and add 1 mb for every 10
metres you are above sea level. Thus, for a station pressure of 1005
mb and a height of 120 metres above sea level, add 120/10 = 12 mb,
making a sea-level pressure of 1017 mb.
So ..... what have we found on this first day? Well .... what a
contrast of temperature! For those with a temperature of -20 deg C,
it must have seemed quite a good idea to jump on an aeroplane and fly
south to the warmth and sunshine of southern Africa. But what of the
weather situations?
Firstly, in western Europe:- As you will have seen if you have been
looking at the weather charts prepared by the British Meteorological
Office (for their Web address, see Contact Message No.4), we have
today had a belt of low pressure all the way from southern Greenland
to Finland, with a slow-moving area of low pressure just off
south-east Greenland. This filled from about 980 mb at 0000 GMT to
about 985 mb at 1200. A small area of low pressure moved quickly
across northern Britain towards Scandinavia. It was near Larne at
0000 GMT and over Denmark by 1200 GMT. On its southern flank, there
has been quite a wide warm sector containing a moist airflow from the
west-south-west. An eastward-moving cold front oriented from
north-east to south-west gave a short burst of heavy rain as it
passed (over Reading just after 10 am). The stations in Finland were
east of the belt of low pressure and have therefore been affected by
a south-easterly flow which carried bitterly cold air over them from
western Russia. The reason for the reported wind directions at the
schools in Finland is that they were, at the time, in the circulation
of a small low which passed quite quickly over them.
MetLink friends in southern Norway and southern Sweden: I hope we
didn't send you too much snow or rain when the low which passed over
northern Britain during the morning of 25th reached you.
And what of the schools in Spain and Malta? According to the weather
charts, there has been a centre of high pressure of 1035 mb over
Spain today and a flow from the east or north-east over Malta. The
weather report from Tarragona gave the impression of a nice day. By
the way, Tarragona: please will you tell us something about the
tramontana. I remember one March day about ten years ago being at
an oil terminal near Tarragona in a wind averaging 45 kts with gusts
to 60 kts. It all seemed rather odd to me, accustomed as I was to
winds that strong bringing cloud and rain and fallen trees. That day
near Tarragona, there wasn't a cloud in the sky and the wind was
warm.
And now to Madagascar, Zambia and Zimbabwe: Peterhouse - please will
you tell us what caused the 31 mm of rain. Was it the thunder-
showers mentioned in the Zimbabwe Met Service's weather bulletin? And
Antananarivo had 26 mm of rain. Thundershowers? At both places,
what time of day did the rain fall, and how usual or unusual was the
rain? How near normal are the temperatures you had on 25th? The
Meteosat image D6 (infra-red) images give
the impression of tropical convection. Is that so?
Here are some additional Web addresses for meteorological information
from southern Africa:
http://www.intellicast.com/weather/africa
http://cirrus.sawb.gov.za/
the latter being the home page of the South African Weather
Bureau.
For a weather chart of the South Atlantic, you should visit:
http://cirrus.sawb.gov.za/www/ship/ship.gif
Today's weather chart shows that Tristan da Cunha has been enjoying
high pressure (1025 mb), centred just to the north-west of the
island. When interpreting weather maps from the southern hemisphere,
please remember that winds blow clockwise around low pressure centres
in the southern hemisphere, anti-clockwise around them in the
northern hemisphere.
What will tomorrow bring? The British Meteorological Office forecast
chart for 1200 GMT on 26th suggests: bitterly cold weather with
easterly winds in Finland; cloudy with some snow or rain in southern
Sweden and southern Norway; west-north-west winds, with showers (?),
at Edinburgh and Larne; moist westerly winds across southern Britain;
a cold front close to Bilbao; and light west to north-west winds at
Tarragona and Malta.
Something similar to today seems likely for Zambia, Zimbabwe and
Madagascar. I'm afraid I haven't found a forecast for Tristan da
Cunha.
That's all for today. What will tomorrow bring?!
