Air pressure is the force of the air pressing down on the Earth's
surface. As air is all around us, there is air pressure all around us
too. Near sea level there is more air above you than there would be on
the top of Mount Everest, therefore the air pressure is higher near the
sea than it is on the top of a mountain.
mass of low pressure is an area of air that is rising.
Similarly, if the temperature is lower, the air pressure will increase.
Warm air near the surface will tend to rise. A mass of low pressure is
an area of air that is rising. As is rises it expands and cools. Cooler
air cannot hold as much water as warmer air, so as the air rises the
water will condense and form clouds. This is why an area of low pressure
will often be accompanied by clouds and rain.
Conversely, an area of high pressure is a section of air which is
sinking. As the air sinks it warms, so is able to hold more water, and
therefore areas of high pressure are often accompanied by fair weather.
Points of the same pressure can be joined up to form lines called
'isobars'. These form rings around high and low pressure centre's.
Pressure charts show the strength
of high and low pressure areas (often marked with a H or L ) and plot the
lines of equal pressure. It's simplest to think of a pressure chart as
similar to a geographical map, with highs like mountains and lows like
valleys - The lines of equal pressure (isobars) on a pressure map are
equivalent to the lines of equal altitude (height contours) on a
The red semicircles indicate a warm front, which is the leading edge
of a low pressure (warm air is advancing to replace cold air). The blue
triangles indicate a cold front, which is the trailing edge (cold air is
advancing to replace warm air). You'll also see lines that have both
triangles and semicircles. These are "occluded" fronts, where the cold
front has combined with the warm front.
The pressure at the centre of a storm is not enough to tell us how
strong the winds and waves are around that storm. This is because wind
speed is determined by -
The pressure gradient - how closely spaced the isobars are.
The latitude of the storm.
This means that
- You can get strong winds and big waves from a weak low if it is
next to a powerful high.
- A 980bM storm off the coast of Portugal will produce bigger
winds & waves than a 980mB storm off the coast of Scotland
blow in an attempt to combat the differences in air pressure. They try
to flow directly from a high to a low pressure, but due to the spinning
of the earth and friction of the surface, actually flow around the
pressure centre's, following the isobars. The larger the difference in
pressure the stronger the winds will blow.
isobars on a weather chart are close together, it will be a blustery
and when this is accompanied by an area of low pressure, it will
also be wet. A wet and windy day is not exactly unusual for the British
Isles nor indeed are areas of low pressure and closely packed isobars.
British Isles Wind Chart today, courtesy of www.windfinder.com
Wind charts show the wind speed and direction for a
given altitude. Be careful to check the altitude your wind chart applies
to - for fishing, you want surface-level winds. To get
around surface friction problems, surface-level wind speeds are normally
given for 10 metres above ground level.
Wind speed and direction. Wind speed may be in knots
(20kts is 23mph or 37kmph), miles per hour or kilometres per hour. On
met charts, wind speed is often represented by feathers on a wind arrow,
where one big feather = 10kts and one small one = 5kts. Wind direction
is represented by the direction the arrow is pointing towards.
Wave charts describe waves across the oceans.
But if you look at the sea surface in open ocean you see an almost
random pattern. With hundreds of waves, each with different heights and
Wave charts cannot show every wave and every ripple. They usually
show only one type of wave at a given position. The three main
types of waves are
- Swell (long-travelled waves)
- Windsea (short-travelled
- Significant waves (a mathematical average of swell and windsea).
To simplify things further, wave charts usually only display one or
two of the following three characteristics that are needed to fully
describe a wave -
- Wave height
- Wave direction
- Wave period
The first two are fairly obvious.
The latest Mid Atlantic significant wave height & direction, from
Wave period is the time (in seconds) between successive waves. Wave
period charts show either Swell period (long-travelled waves), Windsea
(short-travelled waves), or Peak period (a mathematical average of swell
and windsea wave periods).
On Peak period charts, waves that have been marching along for days
(swell waves), may suddenly all but disappear. Remember - The long swell
waves are still there, but the chart has masked them with
locally-generated (windsea) waves.
Remember the saying
when it's windy and the fish are not biting,