Prognastic charts are prepared for pressure levels 200 mb, 300 mb therefore flying at FL310 you will pick chart for 300 mb.
Rate of fall of pressure with height is more in cold areas as compared to warm area.
Therefore, pressure aloft warm area is higher than cold area.
Pressure at 10,000 at Equator is more than at 10,000 over Poles.
Pressure over Equator is more than over Pole.
Pressure at a place changes during day.
Pressure is maximum two times a day
Maximum Pressure 10 A.M. and 10 P.M.
Pressure is minimum two times a day
Minimum Pressure 4 A.M. and 4 P.M.
Diurnal Variation is minimum and maximum value of pressure during day. Minimum pressure in day is at 4 A.M. and 4 P.M.
Maximum pressure in day 10 a.m. and 10 p.m.
Isobars= lines joining places of equal pressure.
Isallobar= lines joining places of equal pressure change.
On surface weather charts at low altitude charts Isobars are drawn at 2 mb intervals( only even mbs are drawn).
At high altitude charts Isobars are drawn at 4 mb intervals (i.e. only mbs ÷4 are drawn).
Low- area of low pressure enclosed between closed Isobars with a low pressure inside.
Secondary low– Low pressure area induced by another low area with in it’s periphery. The pressure gradient is steeper on the side away from the primary low therefore winds are stronger on the side away from primary low.
Press gradient- Rate of fall of pressure horizontally.
Unit taken to calculate is generally 60 n.m.
When Isobars are closer together the pressure gradient is steeper and therefore winds are stronger. When Isobars are farther the pressure gradient is stack and winds are calm.
Pressure Tendency– Tendency of pressure to change with time. It is found out by comparing present pressure with pressure taken 3 hours earlier.
Isallobars – lines joining places of equal press change.
Humidity is the amount of H2o vapor present in the Air. Air can hold H2o vapor in the dissolved form. Amount of H2o vapor held by the O2 in the dissolved form depends upon temperature of the air.
If temperature increases air can hold more water vapor.
Relative humidity – Ratio between amount of H2o vapor held by the air to the maximum amount of H2o vapor it can hold at that temperature and pressure. It is expressed in percentage.
Temperature decreases Relative humidity increases.
In a day humidity is lowest in the afternoon.
Dew Point temp is that temperature up to which the air must be cooed to become saturated.
When temperature of air is reduced further (below dew point temperature) Fog and cloud will form. If existing temperature and dew point temperature are with in 40 then there are chances of formation of FOG.
Frost point temperature is the lowest temperature to which when air is cooled it reaches its saturation point with reference to ice surface. If dew point in below 0° the dew will not form but frost will form.
Wet bulb temperature – is the lowest temperature to which air can be cooled by evaporation of H2 o into it.
Dry bulb temperature – is the Normal temperature.
Wet and dry bulb thermometers – Instrument to measure humidity.
When temp is less than 40 then there are chances of FOG formation.
Instruments to measure Relative Humidity:-
(i) Wet and dry bulb thermometer.
(ii) Psychrometer
(iii) Hygrograph
All 3 give Relative humidity.
Air density is the mass per unit volume.
Air density depends upon temperature and pressure.
Temperature increases air density decreases
Increasing of temp by 30c changes the air density by 1%.
Press decreases air density decreases.
Altitude increases air density decreases.
1000 feet increases in altitude cause a decrease in air density by 3%
Rate of fall of air density with increasing altitude is not constant.
At 20,000 feet- Air density is half the value of sea level value.
At 40,000 feet- Air density is ¼th value of the sea level value.
Air density is important because it affects performance of the A/c
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Atmosphere
Troposphere- Ht of Tropopause (upper layer of Troposphere) is different at equator and poles.
At Poles Tropopause is at 28,000 feet.
At Equator Tropopause is at 54,000 feet.
All weather exists only in Troposphere.
Lapse rate exists only in Troposphere.
Entire moisture is present in the Troposphere.
Stratosphere is above Troposphere, it extends to a height of 50 km. above Earth’s surface.
Temp is nearly constant in the Stratosphere.
Upper Stratosphere has a concentration of ozone layer (O3) saving Earth surface from warming (Height of Ozone above 50 Km.)
Around the mid Latitude (40-450) the Tropopause (thin layer between Troposphere and Stratosphere) is broken. The speed of Jet Stream is maximum where the Tropopause is broken.
Ozone layer (O3) is a special type of oxygen molecule that absorbs harmful solar radiation and accounts for the increase in temperature with increasing altitude in that part of the atmosphere.
The main concentration of ozone layer is approx 80,000 feet M.S.L.
Mesosphere is above Stratosphere, In the Mesosphere the temperature again starts decreasing.
Height of Mesosphere is approx 90 Km. above Earth’s surface.
Thermosphere is above Mesosphere, where temperature increases with increase in altitude.
Ht of thermosphere is 700 km above earth’s surface.
Exosphere is above Thermosphere. The exosphere is also termed is space.
Adiabatic process – When pressure of a gas is changed, it,s temperature changes and if in the process there is no exchange of heat with the surrounding air the process is called adiabatic.
Adiabatic lapse Rate – When air rise it cools, if no heat is exchanged with the surrounding the rate of cooling is called A.L.R.
Dry air – Air which is not saturated, but it may have moisture.
D.A.L.R – Rate of which dry air will cool down, if made to rise adiabatically.
DALR – cools at 30c/1000 feet or 100c/k.m.
S.A.L.R – Rate at which saturated air will cool down, if made to rise adiabatically.
SALR – cools at 1.50c/1000 feet or 50c/k.m.
S.A.L.R. is half of D.A.L.R.
S.A.L.R. is not fixed.
D.A.L.R. is more than S.A.L.R. because air is saturated, latent heat of condensation is given out which causes the temperature to decrease at a lower rate then D.A.L.R.
When it cools condensation takes place.
If H 2o content of air decreases then S.A.L.R. approaches the value of D.A.L.R.
If dew point temperature is decreasing, the moisture content is decreasing and therefore S.A.L.R. is close to D.A.L.R.
Stability/Instability of atmosphere is related to E.L.R.
E.L.R – Environmental Lapse Rate.
Rising air gives Instability
Sinking air contributes to Stability.
DLAR>ELR>SALR
Conditional stability /instability- Atmosphere is stable for Dry air and is unstable for Saturated air.
D.A.L.R is more than E.L.R. therefore D.A.L.R. is stable.
S.A.L.R. is less then E.L.R. therefore S.A.L.R. is instable.
If air is displaced upwards it will tend to sink down to its original level if dry.
Dry air will tend to sink down if displaced upwards.
Saturated air will tend to rise further up if displaced upwards.
When ELR is less than SALR (1.50c/1000 feet)
Absolute Stability- Total Stability
Because ELR is less than D.A.L.R and S.A.L.R.
Dry or Saturated air will tend to sink down to its original level when displaced upwards.
When ELR is more than D.A.L.R (30c/1000 feet)
Total Instability/Absolute Instability – Because ELR is more than D.A.L.R. and S.A.L.R.
Dry or Saturated air will continue to rise in its own when displaced upwards, even when displacing force is removed.
Neutral stability/Equilibrium that stability of atmosphere in which when air is displaced upwards, it will remain at its new level even when the disturbing force is removed.
Value of ELR will be different for Dry and Saturated Air.
If ELR= SALR (1.50c/1000feet)
Neutrally stable for Saturated air, but stable for Dry air.
If ELR= D.A.L.R (30c/1000 feet)
Neutrally stable for Dry air, because value of ELR is equal to D.A.L.R but instable for saturated air because value of ELR is above S.A.L.R.
Latent Instability – Latent instability is that state of atmosphere in which there is a small layer, stable layer at lower level with a large. unstable layer above it. So that if air is forced to rise through the stable layer, instability phenomena will take place.
Stable layer tends to come down. Instability layer air will be unstable. Therefore, stability is different in different layers.
Convective/Potential Instability – Convective/Potential Instability is that state of atmosphere in which there is high relative humidity at lower levels and low relative humidity at higher levels. Such that if the lower layer is bodily lifted, the ELR value through the air will gradually increasing to attain on unstable layer.
Super Adiabatic Lapse Rate – When ELR value exceeds value of D.A.L.R. (3°c /1000 ft.)
Super Adiabatic Lapse Rate takes place on hot summer afternoon in the layer of air very close to the surface of the earth.
Thunderstorm is a phenomena of instability.
Thunderstorm consists of more than one Cumulonimbus, which travels together. Each cloud is called//termed as a cell.
A Thunderstorm has many cells.
Condition necessary for Thunderstorm-
(i) High lapse rate/steep lapse rate
(ii) Adequate Moisture
(iii) Trigger Action- It gives initial upward push to the air.
Trigger action can be caused by following ways-
(a) Solar heating- Earth heats and air is lifted
(b) Low level convergence
(c) Orographic Lifting- Air rising along the slope of a mounntain.
(d) Frontal lighting- Air rising along slope of cold front.
Air rises, it cools down to its dew point, clouds will form and now cloud starts rising.
(i) Formation Stage – Formation stage has updrafts rising air cools at D.A.L.R.( 3c/1000 ft) It has cumulus clouds information stage.
(ii) Mature stage – Rain start falling to the ground, As updrafts increases they produce hail and hail grows bigger until they are big enough to be supported by updrafts, the hail start falling.
Mature stage has updrafts and down drafts. Snow pellets starts falling and when they come down they warm @ SALR (1.5°c/1000ft) because air is Saturated, and therefore it tend to come down. At every stage the air is colder then surrounding air Aa/c experiences updrafts and downdrafts causing Turbulence. Downdrafts when comes to the bottom spreads out in the form of Squall.
(iii) Dissipating Stage – Updrafts are decreasing and down drafts are increasing because moisture is decreasing cloud cannot keep itself erect and therefore cloud bends with the wind, Anvil cloud forms, In the direction of winds as cloud movement
Hazards of Thunderstorms
(i) Severe turbulence due to updrafts and downdrafts.
(ii) Poor Visibility.
(iii) Squall affects a/c, landing or operating close to ground at low levels.
(iv) Shower
(v) Lightning in the Thunderstorm causes temporary blindness
To correct
(a) Put instruments lights to maximum brightness.
(b) Wear dark glasses.
(vi) A/c icing.
(vii) Hail- Above freezing level hail size increases, below freezing level it melts and size decreases.
(viii) Static Interference.
(ix) Wind shear
A/c should be at a safe dist from Cumulonimbus/Thunderstorm because hail can hit a/c because hail moves with wind, safe dist is approx 10 km.
After cloud forms at the speed and direction of Geostrophic wind between 10,000-15,000 feet.
If Relative humidity of surrounding air is less, clouds height is less.
Down drafts are weaker than Up drafts.
Updrafts are more stronger.
Squalls bring about high pressure and drop in temperature.
First Gust/Gust Front – Leading edge of squall is in the direction of movement of cloud.
Roll Clouds – Leading edge of squall air gets into rolling or circular motion giving rise to circular clouds.
Line Squall – is narrow band of active Thunderstorms which contain active cells with severe weather and are seen 50-200 nm ahead of cold fronts.
Funnel clouds/Tornadoes – Spinning columns of air which descend from base of cumulonimbus cloud, Wind speed with in them may exceed 200 kts. This phenomenon is small in dimension, but it causes lot of destruction.
Down burst – Strong down drafts which induces an out burst of very strong winds. They are small scale phenomena in space and time and can only be detected by modern instruments like Doppler and large Radar.
Size of down burst varies between less than a km to the 10’s of km.
It is ÷ into 2
(i) Micro burst
(ii) Macro burst
(i) Micro burst- Intense localized down drafts, which spread out in all directions on reaching the surface. It creates very strong vertical and horizontal wind shear and poses a hazard to A/c operating close to the surface of the earth. It normally doesn’t cover more than 4 km on surface and peak winds lasts for around 2-5 minutes.
(ii) Macro burst- Large down drafts with outburst winds exceeding in excess of 4 km in horizontal dimension. An intense macro burst causes wind spread tornado like damage with strong winds lasting for 5-30 minutes with wind speeds as high as 120 kts.
Self generating type of Thunderstorm is a Thunderstorm which gives rise to the trigger action for the formation of another Thunderstorm during its own life cycle.
Example- Squall coming down of the clouds causes surrounding air to rise.
Norwester- Severe Thunderstorms of Eastern India, they form in the Chhota Nagpur region in Bihar plateaus.
They prevail in summer afternoons, and they are self-generating type of Thunderstorms.
After forming they move towards South Easterly direction towards Bengal.
At any given time 3-4 Thunderstorms move together giving rise to a severe line squall, causing a lot of destruction in Bengal.
Its local name is Kal Baisakhi because it occurs near the 13th April, which is a baisakhi day, a festival celebrated by Punjabi community.
It occurs in March to May in Pre-monsoon season.
Winds are horizontal movement of the air.
Turbulence- Vertical movement of air (Turbulence is wind for climbing straight up and descending straight down by an helicopter)
(i) Coriolis force is due to the rotation of the earth. It deflects all moving bodies to the right of their track in N.H and left of track in S.H.
Coriolis force is 0 at equator and is max at poles. It’s effect increasing as speed of moving body increases and vice versa.
(ii) Pressure Gradient acts from high pressure to low pressure at right angle of 900 to the Isobars. Pressure gradient causes wind to blow from high pressure to low pressure. If pressure gradient is greater, the wind speeds are greater/high. Pressure grand is move if Isobars are close together.
But coriolis force deflects wind to the right of track causing it to flow parallel with the isobars.
(iii) Cyclostrophic- Centripetal force acting on the wind moves along curved isobars.
(iv) Geostrophic wind- Wind balanced between Pressure Gradient and the Coriolis Force.
Wind blowing parallel to straight isobars is balanced between Pressure Gradient and the Coriolis force. But isobars are curved therefore Cyclostrophic wind acts on it.
(v) Gradient Wind- Wind blowing parallel to the curved isobars balanced between pressure Gradient force, the Cyclostrophic force and the Coriolis force.
Buys Ballot’s low states that if on observes stands with his back to the wind in Northern hemisphere the lower press is to his left and vice versa in Southern Hemisphere.
Wind given by the Meteorological office is an estimation of the actual winds and it is Gradient wind.
Geostrophic wind scale gives the wind speed.
The direction of the isobars gives the wind direction.
Geostrophic wind scale has different graduations on scale for different Latitudes because Coriolis force is different at different Latitudes and hence the wind will be different at different Latitudes.
Therefore some distance between isobars lines will give different wind speeds at different Latitudes.
Actual wind differs slightly from the Gradient wind.
In the N.H. or S.H. the Geotropic wind around low pressure is an over estimate of the actual winds, actual wind speed is less than the forecast winds.
In the N.H. or S.H. the Geostrophic wind around high pressure is an under estimate of the actual winds, actual wind speed is more than the forecast winds.
Around Low Pressure area Geostrophic wind forecast is more than actual. Therefore actual winds are less than forecast in L.P.
Around High Pressure Geostrophic wind forecast is less than actual. Therefore actual winds are more than forecast in H.P.
Effect of surface friction on winds.
Surface friction decreases wind speed and also wind direction decreases.
Because as wind speed decreases, Coriolis force decreases and wind tends to blow into the Low Pressure.
Ground friction can change wind direction by 300 over land and 150 over sea.
In N/H due to surface friction winds will back.
Back- Anti clockwise change in wind direction.
In S/H due to surface friction winds will veer.
Veer- Clockwise change in wind direction.
Thermal wind- Upper wind that blows under the influence of Temperature Gradient keeping low temperature area to the left in N.H. Therefore, because of Temperature gradient a pressure gradient is set up at an altitude and wind starts blowing from H.P. to L.P. but Coriolis force causes it to curve right of the track and it starts blowing parallel to the Isotherms keeping low temp to the left (out of the sheet).
Thermal wind is the wind added to Geostrophic wind at lower levels to get Geo-strophic wind at higher level.
Upper wind is the resultant of Geostrophic wind at 2000 (Because no surface friction) and the Thermal wind at that altitude.
As altitude increases the temperature differential increases and the thermal wind continues to increase.
Circulation of wind over the globe is westerly (because of upper winds) except in equatorial belt. If depends upon time of the year. During day the height of frictional layer is about 2000 feet from ground.
During Night the height of frictional layer decreases to about 1000 feet from ground.
This causes the Diurnal change of wind. From day to night for a given Isobar pattern, the surface wind backs and lulls.
Winds at 1000 feet Veers and increases to become Geostrophic winds.
Winds above a height of 2000 feet has no effect.
From night to day the surface wind Veers, and increases in strength.
Winds at 1000 feet back and decreases in strength fill they are absorbed in the frictional layers,
Winds.
Winds at 2000 feet have no effect.
LULL- Decreasing in wind speed.
GUST- Increasing in wind speed lasting for a short period.
Squall- Increasing in wind speed above 25 knots, lasting for a long time. Squall brings in decrease in temperature and increase in pressure. It is accompanied by a phenomena like Thunderstorm, Dust Storm ($) etc.
Gale- Increase in wind speed above 33 knots. It is accompanied by a phenomena like Thunderstorm, Dust storm ($) or Cyclone. Gale also brings in decrease in temperature and increase in pressure.
Jet stream- Increase in wind speed above 60 kts, found at high altitude only.
They are very high speed of winds only found at altitude above 30,000 feet.
Hurricane- Increase in wind speed above 63 kts. They are associated with Cyclones.
Gustiness Factor = Change in Wind Speed ÷ Mean Wind Speed × 100
Example: If wind speed is gusting from 20 to 30 kts, find Gustiness Factor
10÷25 × 100 = 40 %
Therefore the Gustiness Factor is 40 %
Local Winds
Sea/Land breeze- Winds of coastal areas, tropical areas, summer regions. They blow due to temperature difference between land and sea and not because of pressure gradient.
Sea breeze– During day land gets heated, temperature over land increases and pressure decreases. (Wind tends to blow from high to low pressure) Therefore wind from sea blow towards land.
It reaches its maximum value by afternoon. It is cold and moist (due to formation from the sea) and brings down the surface temperature.
Wind speeds are of approximately 15-20 kts.
It may effect as must as 25 miles inside the land.
It withdraws by about 8 in the evening.
In evening when it withdraws it blows parallel to the coastline keeping land to the left in the Northern Hemisphere.
Land breeze– Land breeze blows at night due to difference in cooling of the land and sea surface. The land gets cooled quickly and sea water starts warming up by evening, when land cools temperature decreases and pressure increases (Wind blows from high to low pressure) Therefore wind blows from land to sea.
Katabatic winds- are mountain wind that blows down slope of a mountain at night. It occurs due to mountain cooling of the surface
It often reaches Gale (33kts and above) force.
Katabatic wind is stronger than Anabatic wind.
Anabatic wind- Anabatic wind blows up slope of a mountain at day time.
It occurs due to the heating of the surface.
Windscreen heating is an Anti icing device/Deicing agent.
It increases the impact strength of the windscreen, which is very beneficial in case of a bird hit on Take off roll.
Thermocouple is used to control temperature.
It has a cold and hot end.
Windscreen has a thermostat to control temperature.
It breaks the circuit and disconnects the heat at a particular temperature. It also defogs the windscreen.
Air Masses
Air lying over a large geographical area acquires uniform properties in terms of temperature and humidity horizontally.
Air Masses are associated with areas over which they generate.
Over land- Continental
Over Sea- Maritime
Polar Maritime Air Mass- At poles Cold and Moist.
Sub Tropical Maritime- At mid latitude Warm and Moist.
Equatorial Maritime- At Equator Very Warm and Very Moist.
Poles Continental- At poles- Cold and Dry.
Subtropical Continental- At Mid latitude Warm and Dry.
Front– When 2 different air masses lie next to each other, the line dividing them is called front.
In Northern Hemisphere the Cold air mass towards North and Warm air mass is towards South.
Warm air mass tends to glide over the Cold air mass and sometimes- Cold air mass tends to undercut the Warm air mass.
When Warm air mass goes into Cold air mass pressure decreases and a low pressure is created.
Air will blow from High Pressure to Low Pressure and wind will start blowing in an Anti clock wise direction in the Northern Hemisphere.
The low pressure will change into Depression.
(a) If wind speed around a low pressure is less than 22 knots. It is called a Low pressure.
(b) If wind speed is between 22-27 kts or less than 28 knots. It is called a Depression.
(c) If wind speed is less than 34 knots- it is called a Deep Depression.
(d) If wind speed is greater than 33 knot and less then 64 knots. (Wind speed between 34-63 knots). it is called a Cyclone.
(e) If wind speed is greater than 63 knots. It is called a Hurricane.
Wind speed below 22 knots – Low pressure
Wind speed between 22-27 knots – Depression.
Wind speed between 27-34 knots – Deep Depression.
Wind speed between 34- 63 knots – Cyclone.
Wind speed above 63 knots – Hurricane.
These Depressions are called Extra Tropical Depressions or Frontal Depressions, because they have well marked Cold and Warm front associated with them.
After forming these depressions start moving in an Easterly / Northern Easterly direction.
Gale moves in a straight direction (above 33 knots = Gale)
Cyclone takes place over sea (Not over land or lake) and air is in circulatory motion. Therefore Cyclone may cause Gale.
But Gale cannot cause Cyclone.
WARM FRONT– Front where Warm air replaces Cold air.
On a surface charts, it is shown by ½ moons in Red color.
COLD FRONT– Front where Cold air replaces Warm air.
On a surface charts, it is shown by hillocks in Blue color.
Hillocks and Moons are drawn on side towards which the Front is moving.
Cold front and Warm front have different characteristics and different weather conditions.
Warm Front- Slope of warm front is shallow (1 in 100 or 1 in 150)
When these depressions are approaching, high clouds (Cirrus) are seen on the Western horizon.
Because Front is coming from Westerly direction.
WARM FRONT– As the Depression approaches the cloud base starts lowering and also increases in thickness (all sheet type of clouds).
It starts raining 200 nm ahead of the Depression.
As warm Front approaches temperature increases and pressure decreases precipitation starts falling and as the Front moves ahead, the precipitation increases. Snow can also occur.
It has icing conditions, freezing rain, but smooth flying conditions, no turbulence.
When warm front passes over winds will Veer.
Veer- increasing in clockwise direction.
Cold front always follows the warm front.
COLD FRONT– When Cold Front approaches temperature decreases and pressure increases precipitation increases.
Clouds of vertical development with freezing rain, thunder, lightening, turbulence, heavy precipitation can be expected.
Slope of Cold Front is very steep (1 in 50) therefore Cold Front passes very quickly and improves quickly.
When Cold Front passes winds will Back.
Due to surface friction cold front slows down at the surface level.
This gives rise to circulatory motion of air and roll clouds, Which gives rise to increased Rain/Snow Fall.
Once the moisture is given out the Depression dies out, this is called OCCLUSION.
When OCCLUSION takes place, warm air is pushed up and its place is taken by Cold air. Then entire front moves up and dies out.
These Depressions form in the Atlantic ocean and also in the Mediterranean sea ocean.
This gives rise to phenomena in Europe and West Asia.