gliding weather – bronze & beyond march 2008 the atmosphere atmosphere is 100km thick ...
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Gliding Weather – Bronze & Beyond
March 2008
The Atmosphere
Atmosphere is 100km thick Troposphere is about 10km Contains 80% of atmosphere.
Air above us has weight. Weight reduces with
increased height. Creates a pressure of about
15psi.
High pressure always moves towards low pressure.
Gliding Weather – Bronze & Beyond
March 2008
Pressure
ICAO standard altimeter setting is 1013.2mb Regardless of local conditions
Pressure reduces by 1mb per 30 feet Scale set to QNH represents altitude
i.e. height above mean sea level
Scale set to QFE represents height i.e. height above ground
Altimeter reads HIGH when flying towards a LOW.
Gliding Weather – Bronze & Beyond
March 2008
Coriolis Effect
Named after Gustave Coriolis (who had absolutely nothing to do with it)
Air would tend to move in straight line.
Coriolis effect causes veering. To the right in northern hemisphere To the left in southern hemisphere
Force increases towards the poles. Zero at the equator.
Gliding Weather – Bronze & Beyond
March 2008
Real Life Example
Gliding Weather – Bronze & Beyond
March 2008
Wind
Wind is pressure differences trying to reach equilibrium.
Air movements deflected to the right due to Coriolis effect. Anticlockwise rotation around a LOW. Clockwise rotation around a HIGH.
Gliding Weather – Bronze & Beyond
March 2008
Why is the prevailing wind in the British Isles South Westerly?
Gliding Weather – Bronze & Beyond
March 2008
Atmospheric Circulation
Antitrades
Trade Winds
Horse Latitudes
Doldrums
Gliding Weather – Bronze & Beyond
March 2008
Surface Wind
Surface wind is measured at 10m. Near the surface, the wind will:
Slow due to surface friction Turn inward across isobars
With height, the wind will: Increase Veer
Gliding Weather – Bronze & Beyond
March 2008
Solar Heating
During the day heat from the sun causes convection.
During the night the Earth gives off its heat.
Gliding Weather – Bronze & Beyond
March 2008
Wind & Hills
Sun shines on slope during day Air warms and becomes less dense. Rises up the slope Known as Anabatic
Slope cools at night Air cools and becomes more
dense. Flows down the slope Known as Katabatic
Gliding Weather – Bronze & Beyond
March 2008
Föhn Wind
Prevailing wind pushed up slope Pressure decreases Air expands and cools Water vapour falls out
Wind descends on other side Pressure increases Wind speed increases Temperature increases
Gliding Weather – Bronze & Beyond
March 2008
Humidity
Most air contains some water vapour. Percentage is called Relative
Humidity. Compared with maximum amount that
could exist at a given temperature.
Cooler air cannot hold as much water. Temperature at which air becomes 100%
saturated is the DEW POINT.
Gliding Weather – Bronze & Beyond
March 2008
Clouds
Coverage reported in Oktas. Orographic cloud
Moist air flowing over a hill
Rotor cloud Over hill tops In the lee of hills
Thermals can still develop under an extensive layer of strato-cumulus if there is sufficient instability in the atmosphere.
Gliding Weather – Bronze & Beyond
March 2008
Visibility
Less than 1000m is FOG Air being cooled to below dew point
Radiation fog – moist air cooling to below dew point overnight.
Hill fog – moist air being forced uphill and temperature cooled to below dew point.
Advection fog – warm moist air being cooled from below
If cooled below freezing creates Hoar Frost
1000m to 2000m is MIST
Gliding Weather – Bronze & Beyond
March 2008
What are Air Masses?
Large volumes of air. Characteristics of:
Temperature Atmospheric pressure Water content
Cover many hundreds of square miles
Most will be moving
Gliding Weather – Bronze & Beyond
March 2008
Origins
Designated by their origin Tropical Maritime Tropical Continental Polar Continental Arctic Maritime Polar Maritime Returning Polar Maritime
Gliding Weather – Bronze & Beyond
March 2008
Air Mass Characteristics
Gliding Weather – Bronze & Beyond
March 2008
Synoptic Charts
Gliding Weather – Bronze & Beyond
March 2008
Buys-Ballot’s Law
Wind blows at right angles to the atmospheric pressure gradient.
Observed by Dutch meteorologist Christopherus Buys-Ballot in 1857.
In the northern hemisphere, if you stand with your back to the wind, the LOW will always be on your left.
Gliding Weather – Bronze & Beyond
March 2008
Fronts
Boundary between two air masses. Principal cause of significant weather. Polar & Tropical air masses typically
clash in the middle latitudes. Produce changeable climates
Such as those experienced in the UK
Gliding Weather – Bronze & Beyond
March 2008
Warm Fronts Identified by a line of semi-circles on chart. Warm air advancing over cold. Usually preceded by precipitation and fog. Pressure falls then rises.
Gliding Weather – Bronze & Beyond
March 2008
Cross Section through a Warm Front
Gliding Weather – Bronze & Beyond
March 2008
Signs of a Warm Front
In the summer: High layer cloud slowly approaching. Weakening soaring conditions.
Often bring spells of prolonged and sometimes heavy rainfall, with strong winds.
After the front passes: Weather usually clears quickly. Wind shifts.
Gliding Weather – Bronze & Beyond
March 2008
Cold Fronts Cold air pushing underneath warmer air at the surface. Identified on weather charts as triangles. Bring short spells of heavy rainfall & squally winds.
Lots of cloud in the warmer air ahead of the cold front Pressure rises throughout the approach and passage.
Gliding Weather – Bronze & Beyond
March 2008
Cross Section through a Cold Front
Gliding Weather – Bronze & Beyond
March 2008
Occluded Fronts
Cold fronts move faster than warm fronts.
Can catch and overlap a warm front causing an occluded front.
Curve naturally poleward into the point of occlusion.
Similar characteristics to a cold front – but less intense.
Gliding Weather – Bronze & Beyond
March 2008
Ridges
Elongated areas of high pressure. Bring similar weather to that associated with
anticyclones. Good soaring likely - reduces the instability behind the
cold front preventing over convection.
Gliding Weather – Bronze & Beyond
March 2008
Troughs Elongated areas of low pressure. Bring similar weather to that associated with
depressions. Deep trough:
Particularly violent weather at the passage of the trough, including strong winds, heavy rain and Cb activity.
Gliding Weather – Bronze & Beyond
March 2008
Cols
Area of slack pressure bounded by two high pressure systems (anticyclones) and two low pressure systems.
Gliding Weather – Bronze & Beyond
March 2008
Sea Breeze Front Land heats more quickly during the day Air over land rises causing advection. Wind blows in from the sea. Meets warmer land air & creates shallow cold
front.
Gliding Weather – Bronze & Beyond
March 2008
Environmental Lapse Rate
As air rises, it expands due to the reduced pressure.
If no heat transfer occurs into or out of the parcel, the process is adiabatic.
A rising air mass cools at a given rate. Known as the Environmental Lapse
Rate. The ELR varies from day to day.
Gliding Weather – Bronze & Beyond
March 2008
Dry Adiabatic Lapse Rate
Dry or unsaturated air cools at 3°C per 1000 ft. Known as the Dry Adiabatic Lapse Rate (DALR). This is a theoretical rate and can be calculated. Most air masses contain a proportion of water
vapour. DALR only applies if RH is less than 100%.
Gliding Weather – Bronze & Beyond
March 2008
Saturated Adiabatic Lapse Rate
Cooler air cannot hold as much water vapour as warmer air
Rising air eventually becomes 100% saturated. This point is called the dew point. Marks the start of cloud vertical development. Saturated air cools at a different rate. Known as the Saturated Adiabatic Lapse Rate. SALR is 1.5°C per 1000 ft.
Gliding Weather – Bronze & Beyond
March 2008
The temperature drop of a rising parcel of air
Gliding Weather – Bronze & Beyond
March 2008
Stable Atmosphere If the ELR is less than the DALR the air mass is stable. Smoke, haze and dust may result in poor visibility. Cloud formation in stable air unlikely.
Gliding Weather – Bronze & Beyond
March 2008
Unstable Atmosphere If ELR greater than DALR, atmosphere is unstable. Often happens in the afternoon over land masses. Likelihood of cumulus & good soaring conditions.
Gliding Weather – Bronze & Beyond
March 2008
Rule of Thumb
Cloudbase = (Surface Temperature - Dew point) x 400
If:
Dew point = 16º
and Surface Temperature = 26º
Then:
Cloudbase = (26 – 16) x 400 = 4000 feet
Gliding Weather – Bronze & Beyond
March 2008
Inversions
Increase of temperature with height. Two main causes are:
Descending air warming due to compression and resting on the cooler air mass beneath.
Cooling of the surface on a clear night, where the air in contact becomes colder than the air above.
Once inversion is below the dew point, cumulus will not form.
Gliding Weather – Bronze & Beyond
March 2008
Adverse Conditions
Gliding Weather – Bronze & Beyond
March 2008
Thunderstorms
Three conditions necessary: Plenty of moisture. A mass of warm unstable air. A source of energy to lift the warm,
moist air mass rapidly upward.
Three main stages called in the life cycle are: Cumulus stage Mature stage Dissipating stage
Gliding Weather – Bronze & Beyond
March 2008
Icing
Causes weight to be increased. May alter the C of G position. Pitot tubes & statics may become
blocked. Radio communications degraded.
Hail is water molecules freezing in the up draughts of a Cb, growing with each cycle until too heavy to be sustained by the rising air.
Gliding Weather – Bronze & Beyond
March 2008
Ideal Conditions
Unstable air mass Surface heating Cloud amount Cloud base Wind strength
Gliding Weather – Bronze & Beyond
March 2008
What to Look For
Air fairly close to centre of a high or the axis of a ridge.
Pressure 1017 to 1028mb. Isobars with strong anti-cyclonic
curvature. Northerly component to the wind. Air coming from well to the north of UK. Geostrophic wind less than 16kts. Inland surface winds less than 12kts. Temperature spread between 12º – 15ºC.
Gliding Weather – Bronze & Beyond
March 2008
The Perfect Day!