Download Wind direction is always given from where it is coming from. Surface

Winds
Winds affect boating in many ways, understanding the weather and predicting its effects makes for safer
boating.
Surface Wind
Wind direction is always given from where it is coming from. Surface wind differs cloud level
winds because surface friction prevents an exact balance between the pressure gradient and
Coriolis force. As a result the wind near the surface is slowed down and blows at an angle
across the isobars towards the low pressure. The greater the surface friction, the more it is
slowed and the larger the angle.
Over sea - 15 degrees backed from gradient wind 15% to 30% lighter
Over land 30 to 40 degrees backed from gradient wind 50% or more lighter
Wind traveling over the sea causes waves, the distance traveled over the sea influences the
size of the wave, this distance is called the Fetch.
Stability & Instability
Stability is the ease with which the air aloft can get down to the surface bringing its wind to
replace the air which has been slowed by friction. The words stable and unstable are used to
describe how buoyant the air is, that is how easily it rises. Air that is warmed when the land
surface is heated by the sun becomes buoyant (unstable) and rises. Air which is cooled, often at
night over cold ground, becomes stable and resists any attempt to make it turn over. Unstable
air is continually turning so that the wind aloft, the gradient wind, is brought down in gusts. In
stable air there is little interaction with the gradient wind / air higher up so the air near the
ground may stop moving altogether.
An air mass is only considered unstable when only a small amount of heating at the earth's
surface causes the air to rise to considerable heights.
Stability & Instability on a clear sunny day / night
Over land there is a marked 24hour (diurnal) variation in wind strength. The sun heats the land
and the air close to it causing it to become increasingly unstable as it rises and is replaced by
colder air from aloft. The wind increases because the air aloft is moving with the pressure
gradient wind.
As the sun goes down the temperature falls and the wind decreases. After dusk, if there is little
or no cloud, the ground cools rapidly and the cools the air near it making it stable, so the wind
quickly dies away.
A variation in wind speed may also occur at sea although this is usually masked by other
variations such as water temperature as the tides move and also at estuary mouths.
Stability & Instability on a cloudy day / night
If the low cloud persists day and night the sea temperature will stay constant as there is no sun
to warm it up and no clear sky to allow it to cool - yet the top of the cloud will warm up by day
and cool down by night. The colder the cloud top and the greater the temperature drop with
height, the more unstable the air and the stronger the wind. The warmer the cloud top and the
smaller the temperature drop with height, the more stable the air and the lighter the wind.
Hence under a cloudy sky, the wind will be strongest in the early hours and lightest in the
afternoon - so long as the pressure gradient is not changing.
Air Masses
An air mass is simply a large volume of air of fairly uniform character. Air masses are named
according to where they come from i.e. Arctic, Polar, Tropical etc. If they originate over a
continent they are Continental and if they originate over a sea they are Maritime. The names
may be linked to give a more precise location such as Polar Continental and Tropical Maritime.
Their origin determines their character - air from a continent is dry and air from the sea is
relatively wet, tropical is hot and arctic is cold.
The masses affecting UK are: Arctic (Siberia), Polar Continental (Scandinavia), Tropical
Continental (Mediterranean), Tropical Maritime (Atlantic Ocean) and Polar Maritime
(Greenland).
Wind States
Wind speed, described in terms of the Beaufort Scale. This scale is only a rough guide and
reflects average speeds, gusts up to the next force should be anticipated. Sea conditions are
generally more severe nearer to land.
Sea State
Description
Wave
Wave Height
Calm
Glassy
0m
Calm
Rippled
0m - 0.1m
Smooth
0.1m - 0.5m
Slight
0.5m - 1.25m
Moderate
1.25m - 2.5m
Rough
2.5m - 4m
Very Rough
4m - 6m
High
6m - 9m
Very High
9m - 14m
Phenomenal
> 14m
Force
Force 0
Calm. Wind less than 1 knot.
Force 1
Light. Winds1-3 knots, light airs,
ripples
Force 2
Light Breeze. Winds 4-6 knots,
small wavelets.
Sea Conditions
Force 3
Gentle Breeze. Winds 7-10 knots,
crests begin to break.
Force 4
Moderate breeze. winds 11-16
knots, waves becoming larger,
frequent white horses.
Force 5
Fresh breeze. Winds 17-21 knots,
moderate waves, many white
horses, some spray.
Force 6
Strong breeze. Winds 22-27
knots, large waves, extensive
white horses.
Force 7
Near gale. Winds 28-33 knots,
sea heaps up, waves breaking,
white foam blown in streaks.
Force 8
Gale. Winds 34-40 knots,
moderate high waves, crests
break into spindrift, white foam.
Force 9
Strong gale. Winds 41-47 knots,
high waves, crests topple spray
affects visibility.
Force 10
Storm. Winds 48-55 knots, very
high breaking waves, dense foam
streaks.