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Weather
and diving.
 The
Highs and Lows of UK Weather
The
UK's mid latitude position between 50 - 60°N leaves it
between the warm south and the cold north. The atmosphere
is driven by a need to reduce this temperature gradient by
sending warm air north and cold air south using cyclones and
anticyclones, or the highs and lows which make up our 'synoptic-scale'
weather, and which we see on the pressure charts.
When low pressure systems form in the western Atlantic, they
pick up warm moist air which is then mixed with cooler air
from the north as they move east across the top of the Azores
high. The boundaries between these air masses are marked by
fronts.
A typical anticlockwise rotating low centre will reach western
Ireland and then curve north up the western Scottish coast.
The warm front moves across the UK first, with warm moist
air behind it bringing spells of rain and often reduced visibility
with south to south westerly winds. The cold front chasing
along behind is the boundary between this warm moist air,
and the cooler more unstable brighter and more showery weather
behind, with winds typically from the south west to north
west.
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The transition
of these fronts is something the diver has to take note of
as the passage of these fronts over head will bring strong
winds and occasionally heavy rain which can quickly turn a
pleasant day's diving into a dash for shore.
High pressure usually follows these low pressure systems.
These anticyclones are clockwise rotating sinking air masses
which we associate in the summer season with light winds and
sunny skies, and often building afternoon onshore or sea breezes.
The pressure
imbalance created between the high and low pressure areas
is what drives the wind as it tries to reduce this pressure
gradient. Wind which we see on the water is, in the northern
hemisphere, more backed (further left looking upwind) than
this 500m pressure gradient wind due to the effect of friction
or drag on the wind by the surface.
The rougher the surface the more drag on the wind, i.e. winds
are more backed over the land (20 - 40°) than they are
over the smoother sea (10 - 20°). This is something to
look for on the water when winds are blowing off the shore,
as they will veer (clockwise, or shift right looking upwind)
downwind of the shore.
Heat from
the sun has a noticeable effect of the surface winds. How
many mornings have you looked out the window to see calm conditions
but by lunchtime the winds have picked up nicely?
During the night the earth cools and so in turn it cools the
air directly above it, which reduces its 'energy' or mixing
and so reduces the wind speed.
As the sun heats the earth, the air next to the surface begins
to warm and so begins to rise and mix as cooler air sinks
to the surface to replace it, which results in an increased
wind speed.
Friction and heating are just two examples of the many factors
which we must take into account when on the water. The forecasts
we receive are often for a much wider area and there are small
scale processes which affect the local winds.
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