|
Introduction
The storms of 28 - 30 October
The passage of a depression - the example of 30 October storm
Exercises
| The passage of a depression |
The example of the 30
October storm
The changes in weather features which took place
in southern England on 30 October, were a typical
example of the marked changes that can happen when
an intense and well-developed frontal system passes
over during the winter months. The table below shows
the typical changes that occur in weather features.
| Table 1.
The typical changes in weather features associated
with the passage of a mature depression across
the UK in the winter months |
| |
Ahead of the warm front |
The passage of the warm
front |
The warm sector |
The passage of the cold
front |
The cold sector |
Pressure |
starts to fall
steadily |
continues to
fall |
steadies |
starts to rise |
continues to
rise |
Temperature |
quite cold, starts to rise |
continues to rise |
quite mild |
sudden drop |
remains cold |
Cloud
cover |
cloud base drops
and thickens (cirrus and altostratus) |
cloud base is
low and thick (nimbostratus) |
cloud may thin
and break |
clouds thicken
(sometimes with large cumulonimbus) |
clouds thin
with some cumulus |
Wind
speed and direction |
speeds increase and direction
backs |
veers and becomes blustery with
strong gusts |
remain steady, backs slightly |
speeds increase, sometimes to
gale force, sharp veer |
winds are squally |
Precipitation |
none at first,
rain closer to front, sometimes snow on leading
edge |
continues, and
sometimes heavy rainfall |
rain turns to
drizzle or stops |
heavy rain,
sometimes with hail, thunder or sleet |
showers |
| The passage of the warm and cold front can be seen clearly on Figs
5, 6 and 7, which show the synoptic pressure
charts for 0000, 0600 and 1200 UTC on 30 October. |
 |
Figure
5. 0000 UTC, 30 October 2000 |
Figure
6. 0600 UTC, 30 October 2000 |
 |
Figure
7. 1200 UTC, 30 October 2000 |
 |
Table 2 shows
the changes in rainfall amounts and temperature that
took place every hour at the weather station at Heathrow
Airport between 1000 on 29 October and 1700 on 30
October. The changes can be easily linked with the
passage of this depression. Temperatures start to
rise as the warm front passes over in the early hours
of 30 October. They remain high until around 0600,
as the warm sector passes over. Further rain occurs
as the warm air is undercut by descending cold air
from the occlusion. A sharp decrease in temperatures
takes place as the cold front approaches around 0700.
A heavy period of rain then occurs as this front
passes over, before easing off in mid-morning as
the cold sector passes over behind the warm front.
| Table 2 .
Changes in rainfall/temperature, Heathrow Airport,
between 1000 on 29 October and 1700 on 30 October |
Date |
Time |
Rainfall
amount (mm) |
Dry
bulb temp (°C) |
29/10/00 |
1000 |
0.0 |
10.0 |
29/10/00 |
1100 |
0.0 |
10.8 |
29/10/00 |
1200 |
0.0 |
11.6 |
29/10/00 |
1300 |
0.0 |
11.6 |
29/10/00 |
1400 |
0.0 |
11.8 |
29/10/00 |
1500 |
0.0 |
11.6 |
29/10/00 |
1600 |
0.0 |
10.5 |
29/10/00 |
1700 |
0.2 |
10.8 |
29/10/00 |
1800 |
1.6 |
11.4 |
29/10/00 |
1900 |
2.0 |
12.1 |
29/10/00 |
2000 |
2.4 |
11.7 |
29/10/00 |
2100 |
2.4 |
12.7 |
29/10/00 |
2200 |
1.6 |
13.2 |
29/10/00 |
2300 |
0.8 |
11.4 |
30/10/00 |
0000 |
5.4 |
13.1 |
30/10/00 |
0100 |
2.4 |
13.3 |
30/10/00 |
0200 |
2.4 |
13.2 |
30/10/00 |
0300 |
1.6 |
13.1 |
30/10/00 |
0400 |
1.6 |
13.0 |
30/10/00 |
0500 |
2.0 |
13.1 |
30/10/00 |
0600 |
1.4 |
12.8 |
30/10/00 |
0700 |
2.8 |
9.8 |
30/10/00 |
0800 |
5.0 |
10.2 |
30/10/00 |
0900 |
1.0 |
8.9 |
30/10/00 |
1000 |
0.0 |
8.4 |
30/10/00 |
1100 |
0.0 |
6.6 |
30/10/00 |
1200 |
0.2 |
7.6 |
30/10/00 |
1300 |
0.4 |
9.9 |
30/10/00 |
1400 |
0.2 |
10.8 |
30/10/00 |
1500 |
0.0 |
11.3 |
30/10/00 |
1600 |
0.0 |
11.0 |
30/10/00 |
1700 |
0.0 |
10.5 |
next: Exercises |
