Meteorology for Bronze

Transcription

1 Meteorology for Bronze By Sid Smith and Alison Mulder 1 Alison Mulder

2 Meteorology Part 1 The Atmosphere Air Temperature and Pressure Convectional Stability and Instability Types of Pressure Distribution 2

3 The Atmosphere Structure Divided into layers, two main layers Troposphere - lower layer, where almost all weather occurs Stratosphere - upper layer, steady conditions, strong horizontal winds, few vertical currents/clouds Separated by Tropopause Higher at equator 11km Lower at poles 4km ~7km over Britain Temp decreases from ground to Tropopause Temp increases upward in Stratosphere No distinct upper surface to atmosphere Air decreases in density with height 3

4 Composition Mixture of gases Nitrogen 75.5% Oxygen 23% Argon 1.25% Rest is mixture of CO 2 and rare gases and variable amounts of water vapour Oxygen essential for life Nitrogen and Argon inert gases however play important part in breathing 4

5 Air Temperature and Pressure Radiation Earth receives heat from Sun, transmitted as short wave lengths, pass through the atmosphere and heat Earth s surface Increase in surface temp depends on latitude, time of day, type of surface heated (light/dark, water/rock) and season of year Hot surfaces re-radiate heat waves of longer wavelengths Longer heat waves absorbed by air molecules and reflected back to Earth by clouds/water vapour Conduction Some heat conducted downwards into the Earth 5

6 Air Temperature and Pressure Convection When air heated by Earth s surface it expands Expansion causes an decrease in density Warm air now lighter than surrounding cold air and can begin to rise Carries heat from the Earth s surface to bulk of atmosphere Warm up currents, cold down currents and horizontal winds produce complex + changing convection system over surface of Earth 6

7 Air Temperature and Pressure Humidity The wetness of the air, how much water vapour it holds (%Humidity) Warm air can hold more water vapour than cold air History of an air mass important in estimating humidity Tropical warm Maritime air likely almost completely saturated Tropical warm Continental air will be fairly dry Polar Maritime air will be damp and cold Polar Continental air will be dry and cold 7

8 Air Temperature and Pressure Density Air is compressible, upper layers of atmosphere compress the lower Max density at surface of 1.36kg per cu Mtr. Decreases with height However increase in density caused by lower temps at height is completely masked by loss of density caused by lower pressure at altitude 8

9 Pressure Air Temperature and Pressure Pressure of air due to weight of air column above a certain place Average SLP is 72kg per sq. Mtr. Barometric pressure - height of a column of mercury supported by the air pressure At SL av. height of mercury barometer in UK is 76 cm. of mercury (1013.2mb) Changes from day to day hour to hour Pressure and Temp variations plays vital role in weather forecasting Measure pressure in units millibars 1000mb = 1 bar = 760 mm of mercury In lower atmosphere pressure decreases with height at 1mb for 30ft 9

10 Convectional Stability and Instability Lapse Rate Rate of decrease in temperature with height Average rate decrease is C per 1000ft Known as Normal or Environmental lapse rate However Normal Lapse rates may vary enormously day to day from this 10

11 Convectional Stability and Instability Dew Point Temperature If mass of warm moist air is cooled eventually it reaches 100% Humidity Air is said to be saturated If cooled further water vapour condenses as water droplets Forms a cloud Temp which air becomes saturated is called Dew Point Temp (Dew Point) 11

12 Convectional Stability and Instability Latent Heat Heat needs to be supplied to evaporate water Heat overcomes the forces of attraction between molecules No change in Temp as changes from liquid to vapour state When condensation occurs, the heat is released Heat necessary for evaporation, Latent Heat of Vaporization 12

13 Convectional Stability and Instability Adiabatic Cooling If air rises expansion takes place and air said to cool adiabatically at a pressure equal to its surrounding atmosphere Descending air is compressed and is heated adiabatically (bicycle pump) If air contains water vapour expansion cooling causes air to become saturated Cools to Dew Point, cloud of water droplets form 13

14 Convectional Stability and Instability Dry Adiabatic Lapse Rate (DALR) As long as air rising remains unsaturated Rate of cooling is 3 0 C per 1,000ft increase in altitude Wet Adiabatic Lapse Rate (WALR) When dew point reached condensation begins Release of latent heat causes decrease in lapse rate C per 1,000ft in lower atmosphere As cooling continues less and less water condenses Latent heat release is diminished WALR rises towards the DALR at temps below dew point C per 1,000ft applies to saturated air at C 14

15 Convectional Stability and Instability Stable Atmosphere Atmospheric condition is stable when lapse rate for the day is < DALR Lapse rate on day is C air heated to 20 0 C rises Expands cools at DALR 3 0 C per 1,000ft Air stops rising at ~5,000ft 15

16 Convectional Stability and Instability Unstable Atmosphere If in previous example the lapse rate for the day was C per 1,000ft With increased altitude Temp difference between rising and surrounding air becomes greater Air will continue to rise rapidly This state of affairs is called instability 16

17 Convectional Stability and Instability Tephigram Local atmospheric conditions Normally represented by a Tephigram (not required in any detail for Bronze) 17

18 Convectional Stability and Instability Convection Cloud In practice air contains some moisture Previously if dew point is C, attained at 4,000ft a cumulus cloud forms Air rising in cloud cools at WALR In stable atmosphere clouds not very large, useable lift underneath In unstable atmosphere with high lapse rates get strong vertical currents These clouds grow to great height Cumulonimbus, hail, ice formation, lightening encountered Clouds best avoided 18

19 Convectional Stability and Instability Inversions In some circumstances Temperature can increase with altitude Has number of causes 1. Passage of warm air over a cold sea, sea fog if cooled below dew point 2. Ascent of warm moist air over denser cold air (warm front) 19

20 Convectional Stability and Instability Inversions (cont) 3. Above a continuous layer of cloud when pressure is high - Top of cloud cools by radiation, air layers above top of cloud warmer. - Acts a lid on lower atmosphere, - Rising warm air below cannot pass through warm inversion layer, - Causes fogs smog 4. On still clear night earth cools rapidly, radiates heat to space - Air cools at surface, warmer layer above. - Causes mist/fogs if temp drops below dew point near surface. - Warmth of following day may not heat air enough to disperse fog - or heat air enough to pass through warm inversion layer 20

21 Types of Pressure Distribution Isobars Lines drawn though places with same barometric pressure on weather map Form characteristic patterns around high/low pressure Intervals can be 2, 4 or 6mb depending on map scale Closer intervals = Stronger winds usually stronger around low pressure 21

22 Types of Pressure Distribution Wind Direction Wind flows from areas of high to low pressure Winds follow roughly lines of isobars at 1,500ft However at ground level wind blows at ~30 0 to isobars, surface friction Air subjected to 3 forces 1. Pressure Gradient 2. Geostrophic Force due to Earth s rotation - deflects moving air to right in N Hemisphere - deflects moving air to left in S Hemisphere 3. Cyclostrophic (Centrifugal) force tries to push air blowing in a curve away from the centre 22

23 Buys Ballots Law Types of Pressure Distribution If you stand with your back to the wind in the northern Hemisphere, the Low pressure is on your left 23

24 Veering and Backing Types of Pressure Distribution Wind blows parallel to isobars at around 1500 At lower altitudes wind is slowed up by ground friction and turbulence around obstacles Cyclostrophic force is reduced so wind blows more in towards the low it is Backed At higher altitudes the wind velocity is (usually) higher Cyclostrophic force is increased and it blows further out away from the low it is Veered Wind said to Veer when changes direction clockwise Wind said to Back when changes direction anti clockwise 24

25 Types of Pressure Distribution Diurnal Variation During the day, thermal mixing occurs from thermal currents (or other turbulence) mixing slower, lower airmass with that above wind velocity at lower levels increased and so veers wind velocity at higher levels reduced and so backs effect is reversed in the evening as thermal activity decays - important to note when ridge soaring!!! 25

26 Bad Weather Systems Depression or Low Types of Pressure Distribution Rotation of wind is anticlockwise in N Hemisphere Created when large upper air mass is rising and cooling adiabatically Can be deep very low barometric pressure at centre 960mb, strong winds Can be shallow small pressure decrease at centre, light winds Is an area of rising air currents, much precipitation and cloud Normally travel west to east across UK ~20mph 26

27 Bad Weather Systems (cont) Secondary Depression Types of Pressure Distribution Can form and grow within Primary Depression Weather usually worse with gales and strong winds Moves with primary and rotates around it in anticlockwise direction (N.Hem.) Sometimes can usurp role of primary 27

28 Types of Pressure Distribution Bad Weather Systems (cont) Trough of Low Pressure Bad weather particularly along the trough If isobars along trough form a sharp bend, may be a front present Maybe severe squalls and heavy showers associated Wind Veers on passage of trough Pressure rise behind trough 28

29 Types of Pressure Distribution Fine Weather Systems - Anticyclone or High Rotation of wind is clockwise in N Hemisphere Surface wind blows outwards angle of with isobars Created when large upper air mass is sinking and heating adiabatically Slow moving/stationary can last weeks Longer it lasts, stronger inversion form near the ground, low cloud anticyclonic gloom in winter In summer sun strong enough to break up cloud, break through inversion, early morning mist 29

30 Fine Weather Systems - Wedge (Ridge) of High Pressure Sticks out to the N between two Lows (N Hemisphere) Moves with the Lows at ~20mph Short lived fine weather hours On approach wind veers, strong from NW Pressure rises at centre, winds light and backs westerly On passing wind increases and backs further from S.W Types of Pressure Distribution 30

31 Fine Weather Systems - Col Types of Pressure Distribution Occurs between a pair of Highs and a pair of Lows Winds are light Weather depends on previous history of air mass Winter fogs, land and sea breezes, anabatic and katabatic winds, summer thunderstorms Lasts hours Moves with the two Lows 31

32 Meteorology Part 2 Pressure Effects on Altimeters Local Effects Types of Clouds Anatomy of Fronts Useful Reading 32

33 Pressure Effects on Altimeters Accuracy of altimeters is affected by pressure variations Aircraft trying to fly at a constant altitude towards an area of low pressure will in fact descend Altimeter reads pressure altitude and will read higher if moved to an area of lower pressure 33

34 Local effects Several topographical effects on the wind and weather. 34

35 The Anabatic Effect Local effects Air in a valley is heated by the sun warming the slopes during the day and so rises up the slopes (fairly gently against gravity) and draws air in along the valley floor 35

36 The Katabatic Effect Local effects At night the the hill tops radiate heat more quickly than the valley air in contact cools more rapidly and descends down the slopes forcing air out of the valley warmer moister air in valley floor forced to rise and cool can cause mist or fog 36

37 Sea-breezes Local effects During the day air over the land warms more quickly than over the sea and rises (thermals!) Colder moister air drawn in from the coasts over the land, creating an onshore sea-breeze 37

38 Local effects Sea-breezes Cold, denser maritime air drives a wedge under the warm air over the land forcing it to rise more rapidly and cool A line of cumulus cloud is formed bigger and stronger than that inland Sea-breeze fronts can move several miles inland even against the prevailing wind Seaward side of front is usually unsoarable with poor visibility due to cold maritime air requiring more heat to start convection 38

39 Local effects Land breezes At night sea retains heat better than the land so the air above it is heated and rises Draws cooler descending air over the land out towards the coast An Offshore wind 39

40 Fohn Effect Wind blows against a line of hills or mountains and forced up into wind slope Air expands and cools causing any moisture to condense as cloud or even rain Downwind of the slope the now dry air is forced to descend and heats up Local effects Wales Severn Valley 40

41 Cloud Types Cloud will form whenever warm, relatively moist air is forced to cool either by contact with a cold surface or by being forced to rise and therefore expand Generally: Cirro- means ice, Strato- means layer, Alto- means at medium level (10000 to ft) and Nimbo- means rain bearing Cloud amounts expressed in Oktas or eighths of cloud cover 4/8 cloud cover is 50% and 8/8 is total 41

42 Cloud Types Cirrus (Ci) High wispy cloud at around to ft formed by ice crystals Normally on the leading edge of a warm front 42

43 Cloud Types Cirrostratus (Cs) More solid uniform ice layer cloud Is the second cloud type of an approaching warm front 43

44 Cloud Types Altostratus (As) Solid uniform layer at medium altitudes The third cloud of a warm front 44

45 Altocumulus (Ac) Cloud Types Convective cloud formed off the top of a medium level inversion with an unstable layer above is often an indication of approaching thundery weather 45

46 Stratus (St) Cloud Types Low level layer cloud that can produce light precipitation Nimbostratus (Ns) Heavier low level cloud producing significant rain 46

47 Stratocumulus (Sc) Cloud Types Formed by low-level turbulence forcing warm moist air to rise and cool Alternatively cumulus clouds that overdevelop 47

48 Cumulus (Cu) Convective cloud Cloud Types 48

49 Cumulonimbus (Cb) Cloud Types Cumulus clouds that form in an unstable atmosphere Can rise to the Tropopause where inversion layer causes upper ice crystals in the cloud to spread out creating classic anvil shape Classic thunderclouds causing rain, thunder and hail Can cause strong gusts and changes in local wind direction due to violent upcurrents 49

50 Cloud Types Fog Cloud where locally reduced surface visibility to less than 1000metres (Mist is visibility between 1000 and 2000m) Can be formed where ground radiates heat overnight cooling warm moist air in contact with it ( Radiation Fog ) Orographic cloud Formed where warm moist air blowing up a hillside is forced to rise and cool Often hiding the hilltops (then sometimes referred to as Cumulus Granitus ) 50

51 Spreadout Cloud Types Cumulus cloud with strong vertical movement hits a stable but nearly saturated inversion layer Water vapour is trapped and forced to Spreadout Spreadout layer can shut off convection If convection is shut off, spreadout layer may dissipate allowing convection to start again and cycle 51

52 Cloud Types Overdevelopment Stratocumulus cloud caused by convection where condensation occurs at a level where air is already almost saturated Cloud amounts build up and can shut off sun reducing or stopping convection Conditions sometimes cycle, but rarely 52

53 Fronts Formed where one type of airmass rotating around a low pressure overtakes another A succession of such fronts is known as a frontal system 53

54 Fronts Warm Front Shown on Synoptic chart as a red line of semicircles Formed where warm saturated maritime air (typically) overtakes colder denser air - Forms a wedge forcing the warm air upwards and cooling it. As it is almost saturated, rapidly condenses at relatively low levels Characterised by steadily increasing and lowering cloud often with steady rain but without extreme effects Wind will tend to decrease behind the front 54

55 Fronts Warm Front 55

56 Fronts Warm Sector Area of warm relatively humid and thus stable air behind a Warm Front In summer will tend to produce blue hazy conditions Cloudy drizzly conditions in winter 56

57 Fronts Cold Front Shown on Synoptic chart as a Blue line of triangles Often follows Warm Sector Wedge of cold dense relatively fast moving air overtakes the warm sector pushing it rapidly upwards Air cools adiabatically forming large cumulus or cumulonimbus clouds often with heavy showers Relatively narrow compared to Warm Front Sometimes referred to as a Squall-line Behind the cold front is a region of cold unstable air that may remain showery Wind usually increases and veers and visibility improves 57

58 Fronts Cold Front 58

59 Fronts Occluded Fronts Where one type of front overtakes another causing a mixture of frontal characteristics. Warm Front occlusion where warm front overtakes cold front is rare Cold Front occlusions are most common type of front in the UK 59

60 Useful Reading Meteorology Simplified covers all that is needed for Bronze without any complexity Meteorology for Glider Pilots by Wallington For more in-depth study, anything by Tom Bradbury (several publications) 60