Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 Horse Latitudes Trade Winds 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! Gliding Weather – Bronze & Beyond March 2008