Meteorology

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The article below contains various topics concerning basic meteorology.

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The Atmosphere

Extent

  • It is impossible to give any definite value for the extent of the atmosphere
  • Half the total weight of the atmosphere is in the lower 18,000ft

Composition

  • Is a mixture of visible and invisible gases consisting of: 78% Nitrogen, 21% Oxygen and 1% Carbon Dioxide and Other Gases

Properties

  • The principle properties are mobility, capacity for expansion, and capacity for compression

 

Division and Characteristics

  • Divided into significant layers (with the suffix –sphere) on the basis of temperature and the nature of its changes 
  • The top of each layer is identified by the change of the nature of the temperature variation (with the suffix pause)

Troposphere

Characterized by its decrease in temperature

Varies in height ranging from 25,000’ to 35,000’ over the poles and 55,000’ to 65,000’ over the equator

Higher in summer than winter

Winds reach maximum speeds near the tropopause

Stratosphere

Temperature stays relatively constant at 56 degrees Celsius

Thickness varies but is thickest over the poles and narrow over the equator

Temperature begins to rise at the stratopause

Mesosphere

Temperature rises to 9 degrees then decreases to 108 degrees

Temperature rises again near the mesopause

The rise in temperature is due to the presence of ozone which absorbs the suns radiation

Thermosphere

Temperature rises again to over 3,000 degrees (not relative to the temperature in the troposphere)

Aurora lights are found in the top region of this layer

 

Heating of the Atmosphere

Radiation – the sun’s rays heat the ground which then heats the air above it

Convection – cold air being heated as it passes over a warm surface, then rises and distributes warmth to higher levels. A flow of cold descends to take its place.

Advection heating – cold air being heated as it moves over a warmer surface, then continuing to move horizontally

Turbulence – warm air mixed with colder air aloft

Compression – anticyclonic pressure systems or descending air is forced to compress due to pressure and warms up and this is also called subsidence

 

Cooling of the Atmosphere

Radiation cooling – occurs at night through the loss of heat which dissipates into the atmosphere

Advection cooling – warm air moving over a cold surface will be cooled by that surface and lose heat

Adiabatic cooling (expansion) – ait that is cause to rise will expand and cool, opposite to compression

 

The ICAO Standard Atmosphere

  • Is defined on the basis of average conditions in the lower 65,00ft at about 40 degrees latitude north
  • Mean Sea Level – 29.92 inches of mercury, 1013.2 hPa (mb) or 101.3 kPa
  • Mean Sea Level Temperature is 15 degrees Celsius
  • Lapse Rate of 1.98 degrees per 1000ft
  • Tropopause height 36,000ft 

 

Clouds

Composition and Formation

  • composed of tiny water droplets or ice crystals which form as a result of moist air being cooled

Most clouds form in rising air which has been cooled by expansion

 

Classification

  • Classified by their appearance or form
  • Clouds can be divided into 4 groups: Low, Middle High Clouds and Cloud of vertical development
  • Each group is subdivided on the basis of appearance:

Clouds with uniform bases and no markings are called stratus

Clouds with definite bases, patterns or structures are called cumulus

Nimbus type clouds are predominantly associated with precipitation

 

High Clouds: Cirrus (CI), Cirrostratus (CS), Cirrocumulus (CC)

Middle Clouds: Altostratus (AS), Altocumulus (AC), Altocumulus Castellanus*(ACC)

Low Clouds: Stratus (ST), Nimbostratus (NS), Stratocumulus (SC), Stratus Fractus (SF), Cumulus Fractus (CF)

Clouds of Vertical Development: Cumulus (CU), Towering Cumulus (TCU), Cumulonimbus (CB)

 

HC: FL200 – FL400 MC: 6,500 – FL200 LC: Base – 6,500ft CVD: Base – 6,500ft

 

Cirrus: fibrous wisps of delicate white cloud formed by ice crystals showing up against the blue of the sky

Cirrostratus: thin whitish veil, sometimes gives the sky a milky appearance other times tangles

Cirrocumulus: small rounded masses or white flakes, with slight or no shadows

Altostratus: grey, thick veil, sometimes steely or bluish, generally covering the entire sky

Altocumulus: a layer of series of patches of flattened rounded masses arranged in groups, lines or waves

Altocumulus Castellanus – rounded masses with turrets protruding outwards

Stratus – uniform layer of cloud resembling fog but not resting on the ground

Stratocumulus – a layer or series of patches composed of rounded masses which may show considerable shading

Nimbostratus – a low layer of dark grey cloud usually nearly uniform feebly illuminated

Cumulus – dense clouds of vertical development appearing clear cut, puffy

Cumulonimbus – heavy masses of CVD extending to well above the freezing level. ASOC TD and PRECIP

  

Pressure

Station Pressure

‐ Is the actual pressure at the elevations of the observing station

 

Mean Sea Level Pressure

‐ Is station pressure reduced to a common level and is universally expressed in millibars

‐ MSL pressure is calculated by using an average surface temperature from the last 1 hours for the imaginary column of air extending from the elevation of the station to sea level

 

Altimeter Setting

‐ Is also pressure reduced to sea level but is based on the temperature at sea level always being 15 degrees Celsius

‐ It is expressed in inches of mercury (N. America)

 

Horizontal Pressure Difference

‐ Weather analysts and forecasters draw line (isobars), connecting point of equal pressure

‐ These lines form definite patterns and are an important factor of being able to forecast and follow weather

‐ Isobars are drawn at 4 millibar intervals above and below the value of 1,000mb

 

Depression of a Low or Cyclones

‐ Region of relatively low pressure with the lowest pressure in the center

‐ The size of a low may vary from less than one mile to many hundreds of miles

‐ Airflow always rotates counter‐clockwise and winwards around a low pressure area (N. Hemisphere)

 

Trough of a Low

‐ Indicated by isobars extending outwards from a low in such a way that pressure is lower along the trough lines than either side of it

 

Secondary Low

‐ Is often found at the end of a trough, being smaller than the primary low which it satellites

 

Buys Ballot’s Law

‐ If you stand with you back to the wind the low pressure area will be to your left

 

High Pressure Area or an Anticyclonic

‐ Is a region of relatively high pressure at the center, with the isobars usually being farther apart

‐ The airflow always rotates clockwise and outwards around a high (N. Hemisphere)

 

Ridge or Wedge of a High

‐ Like a trough to a low, a ridge is to a high

‐ In general, the isobars are more rounded in a ridge than a trough

 

Col

‐ Is a neutral area between 2 high and 2 lows

‐ Usually consists of unstable weather

 

Pressure Gradient

‐ Is the difference in pressure in the horizontal, measured at right angles to the isobars

‐ A steep of strong pressure gradient indicates isobars which are close together (high wind)

‐ A shallow or weak gradient indicates isobars which are more spaced out

 

Pressure Levels and their Variation in Height

‐ When you are flying at a constant altitude ASL, without adjusting your altimeter, you are actually flying at a constant pressure level and your distance AGL will vary

‐ From HIGH to LOW, lookout below! (actual altitude will be lower than indicated.) Pressure Levels and Their variation in Height due to Hot and Cold Air

‐ From HOT to COLD lookout below (actual altitude will be lower than indicated)

  

Wind

Hemispheric Prevailing Winds

‐ The most direct rays of the sun strike the earth at the equator

‐ This causes greater heating of the earth which creates vertical currents which in turn produce prevailing westerly winds over the greater part of the Canadian land mass

 

Upper Level Winds

‐ Two main forces which affect the movement of air in the upper levels:

o Pressure Gradient: causes air to move horizontally (high to low)

o Earth’s rotation deflects the wind to the right in the N. Hemisphere and causes the air to flow parallel to the isobars (Coriolis Force)

 

Low Level Winds

‐ Another force affects the movement of the air close to the earth’s surface and that is friction

‐ The result is lower wind speeds and the direction changes so the wind blows across isobars

 

Veering and Backing

‐ When the direction from where the wind is blowing is changing in a clockwise direction, the wind it said to be veering and conversely, when it changes in a counter‐clockwise direction, it is said to be backing

 

Wind Gust

‐ Is a comparatively rapid and brief increase in the winds speed. 2 main causes are:

o Mechanical Turbulence: eddying motion caused by manmade objects

o Unequal heating of Earth: if one of the earth’s surface becomes heated, the surrounding cool air will rush in

 

Squalls

‐ Occur when the speed increases rapidly and maintains the speed for a duration of minutes

 

Diurnal Effect

‐ During the day, the wind veers and increases due to daytime heating and mixing with the 3000ft winds

‐ At night, the wind backs and decreased through the loss of daytime heating

 

Land and Sea Breezes

‐ Effect coastal area where at night the land cools rapidly, cooling the air it is in contact with, the colder air moves from land to water creating a land breeze

‐ By day, the earth heats up quicker and more so then the sea which creates a cool sea breeze (sea to land air flow)

 

Katabatic Effects

‐ Blow down ice‐free slopes at night and ice‐covered clopped by night and day

‐ At night the sides of the valley will cool by radiation and the more dense air will slide down the slope

 

Jet Stream

‐ Are narrow bands of exceedingly high wind speeds

‐ They are found at altitudes of approx. 20 to 40,000ft

‐ There is usually 2 – 3 jet streams over N. America at a given time

‐ Tend to form along areas of low pressure

 

Barrier Effect

 ‐ The wind may be deflected to blow parallel to high terrain as a mountain range

 

Moisture and Temperature

Moisture in the Air

‐ There is a limit to the amount of water vapour that can be present in the air

‐ This is governed by the temperature of the air

‐ If there is a maximum amount of water vapour at a certain temperature, it is said to be saturated

‐ If there is a maximum amount of water vapour at a certain temperature, it is said to be saturated

‐ If the air is already saturated and then cooled, it will have more water vapour than it can hold

‐ Therefore the excess water vapour transforms into water droplets and cloud or fog form, called condensation

‐ This process can actually not take place unless there are tiny sub‐microscopic particles on which the water vapour can condense

 

Dew point

‐ Is the temperature at which the air would become saturated if cooled at a constant pressure

 

Relative Humidity

‐ the relationship between the amount of water vapour that is actually in the air and the amount of water vapour which could be in the air at that temperature and pressure (in a %)

 

Saturated Adiabatic Lapse Rate

‐ when the air is saturated, it tends to retain its heat and only cools 1.5 degrees Celsius per 1000ft

 

Dry Adiabatic Lapse Rate 

‐ when the air is dry, it tends to cool quicker at 3 degrees per 1000ft

 

Stability and Instability

Definition

‐ an object is unstable if it does not attempt to return to its original position after it has been displaced

 

Characteristics

Stable Air: Shallow Lapse rate, layer type cloud or fog, uniform precipitation including drizzle, poor vis

Unstable Air: Steep Lapse rate, heap type cloud, showery precipitation, good vis except in showed, BLSN

 

Lapse Rate and Instability

‐ the lapse rate in the part of the air which is not rising is one of the most important factors which determine the stability of the air

‐ if the lapse rate is steep, there is cold air aloft which will tend to sink to the surface if the air is disturbed

‐ because the lapse rate in non‐rising air is seldom uniform with height, there may be a considerable variation in the degree of stability or instability

 

Inversion

‐ is a condition when the temperature increases with altitude

‐ there is no tendency for vertical motion to develop

‐ usually results in reduced visibility as smoke and fog get trapped in lower levels

‐ can form on clear, still nights when the cold ground cools the air directly above it

‐ results in stable air

 

Isothermal Layer

‐ is a condition when the temperature remains constant with an increase in altitude

‐ results in stable air

 

Lifting Agents

Convection – is the result of unequal heating of the earth’s surface, adjacent land and water areas give the most pronounced differences

Orographic Lift – refers to lift by some prominent topographical feature like a mountain range

Friction (Air and Ground) – disrupts the lower airflow into a series of eddies (mechanical turbulence)

Frontal Lift – associated by an advancing cold air mass pushing warm air aloft

Convergence – near a low pressure area, the wind blows across the isobars and inwards towards the low, the air then accumulates and has nowhere to go but upwards

Note‐Divergence (opposite to convergence) – is the downward and outward flow of air, associated with a high

Air Masses

Definition

‐ the troposphere may be divided into section of uniform, properties of temperature and moisture (air masses are separated by a relatively narrow transition zone called Fronts)

‐ air masses are often several thousands of miles across (in the winter, arctic air may cover more than half of the N. American continent)

‐ the degree of horizontal uniformly within an air mass varies considerably in the lower levels and is due to lakes, terrain and cloud cover

‐ the main weather determinant are moisture content, stability, and the cooling of the air

 

The Formation and Classification of Air Masses

‐ uniform conditions of an air mass develop when an extensive section of air comes under influence of a large part of the earth’s surface that has uniform characteristics

‐ an air mass which has formed over a large body of water is referred to as a maritime air

‐ an air mass which has formed over a large land area is referred to as continental air

Fronts

Definition

‐ the transition region between air masses (it may be many miles in width)

 

The Polar Front

‐ is the edge of the cold air from the north and marks the transition zone between this air and

warm tropical air from the south

‐ the polar front is usually associated with a jet stream

 

The Structure of Fronts

Cold Front

‐ is a cold air mass advancing on warm air

‐ has a relatively narrow frontal zone

‐ because cold air is heavier, it lifts warm air above it

‐ moves relatively quickly compared to a warm front

‐ meteorological symbol: blue triangles on a line

Warm Front

‐ is the area where warm air is rising over the retreating cold air

‐ has a relatively wide frontal zone

‐ because warm air is less dense it will rise over cold air

‐ moves relatively slow compared to cold front

‐ meteorological symbol: red semi‐circles on a line

Upper Cold Front

‐ is the result of a cold front which advances over an area of shallow colder air trapped below it

‐ meteorological symbol: blue triangles not coloured in on a line

Upper Warm Front

‐ is the result of a warm front which advances over an area of shallow colder air trapped below it

‐ meteorological symbol: red semi‐circles not coloured in on a line

Stationary Front

‐ is an area where a cold air mass is neither advancing nor retreating

‐ meteorological symbol: red moons and blue triangles on either side of a line with the blue triangles pointing to the side with cold air

Occluded Cold Front

‐ is a cold front which is over taking a warm front

‐ cold air is colder than the air ahead of it

‐ cold air lifts above the warm air aloft to form a wedge

‐ meteorological symbol: alternating red/blue moons and triangles on the same side of a line

Occluded Warm Front

‐ is a cold front which is over taking a warm front

‐ the cold air is not colder than the air ahead of it

‐ the cold air lifts the warm air aloft to form a wedge

‐ meteorological symbol: same as occluded cold front

Trowal

‐ is the wedge of warm air which is pushed aloft at an occluded front

Frontogenesis – formation of a front

Frontolysis – diffusion of a front

 

Line Squall or Squall Line

‐ is a long line of squalls and or thunderstorms which sometimes proceeds a fast moving cold front

‐ may be anywhere up to 300 miles ahead of the front

‐ it is a line of low, black, roll type clouds from which heave rain or hail may fall

‐ thunder and lightning are frequent

‐ it is also accompanied by sudden veering of the wind, drop in temperature and rise in pressure

‐ usually only lasts several minutes

‐ can be very devastating

 

Wind Shift as a Front Passes

‐ With the passage of a front, the surface wind changes from that in the cold air to warm air for a warm front and warm to cold air in a cold front. This change will always be a veer.

‐ The wind shift is such that alteration of heading to the right is required to stay on course, no matter which way you fly through it.

 

Weather at a Cold Front

‐ Surface winds will back in advance of the front and veer abruptly upon passage

‐ Temperature will drop

‐ Clouds in advance and at the front will be in the form of cumulus, altocumulus and towering cumulus

‐ Precipitation will be in the form of moderate to heavy showers

‐ Visibility will be generally good after a cold front passage, but poor in the rain showers

‐ Pressure will drop in advance of the front and then rise after frontal passage

Factors that determine cold front weather: moisture content in air mass, stability of warm air mass, speed of cold front, steepness of the frontal surface.

 

Weather at a Warm Front

‐ Surface winds will veer more gradually upon the passage of the front

‐ Temperature will gradually increase

‐ Clouds will be in the form of cirrus, altostratus or altocumulus, and nimbostratus

‐ Visibility is generally poor due to fog or low stratus type cloud and steady precipitation

‐ Pressure change is a relatively rapid decrease ahead of the front and then, often a slower decrease after passage

‐ Icing is often frequent at the warm front when upper level temperatures are near freezing level

Factor which determine the weather at a warm front: moisture content of warm air mass, stability of warm air mass, degree of overrunning (determined by pressure gradient.)

 

Weather at a Trowal

‐ The cloud (from the warm side) pattern is similar to that of warm front, however the weather is more complex

‐ If you approach from the cold air side, the cloud structure may resemble a cold front

‐ Depending at what altitude you fly through the trowel, the wind may change direction twice

Factors which determine the weather near a trowel: the mature of air masses involve (moisture and stability), the height of the trowel above the ground.

 

Weather at the Upper Front

‐ Same as regular fronts, except that the winds, temperature and pressure may change well after the front passes

  

Fog and Visibility

Definition

‐ fog is essentially a cloud formation in the shallow layer of air next to (touching) the earth’s surface

 

Condensation and its Requirements

‐ condensation is the process by which water vapour changes state to become a liquid

‐ in the atmosphere this results in the formation of cloud droplets and drizzle but not rain

Requirements for condensation are: high relative humidity, nuclei (tiny particle which water vapour can cling to), cooling of the air

 

Precipitation

‐ the main way heavy rain droplets is by collecting on tiny ice crystals in the cloud, then growing and increasing in weight and eventually falling to the ground

‐ the size of the water droplet depends on the vertical activity (instability) within the cloud

‐ clouds of vertical development will produce larger water droplets then stratus type cloud

 

Sublimation

‐ occasionally cooling results in water vapour changing directly into a solid form, without becoming a liquid first

 

Types of Fog and Characteristics

Radiation Fog

‐ follows a cloudless night, where light winds and moist air is present

‐ as night falls, the earth’s heat dissipates into the atmosphere, the cold ground then cools the air directly above it condensing the water vapour into fog

‐ this type of fog tends to drain into areas of low terrain such as valleys and ravines

‐ the presents of smoke assists in the formation of fog

‐ radiation fog never forms over the sea

Advection Fog

‐ forms when warm moist air moves horizontally over a cold surface which cools and condenses the water vapour

‐ it may be found with winds of 15 miles or more

‐ this type of fog is often wide spread and will remain as long as the wind direction remains constant and these is insufficient heating to dissipate it

‐ common on coastal areas where warm, moist air is blown inland

Upslope Fog

‐ formed by the cooling of the air due to expansion, as warm moist air is forced to rise over rising terrain

Steam Fog or Arctic Sea Smoke

‐ forms when a body of water is warmer than the air moving over it

‐ the water which is escaping from it saturates the cold air above which condenses to form fog

‐ this is an example of raising the dew point to air temperature

Frontal Fog

‐ forms when cold air becomes saturated by evaporation from rain falling from the warm air aloft

‐ can also form due to advection cooling after the passage of a warm front

Ice Fog

‐ may form at very low temperatures when the air may become full of ice crystals which have formed by sublimation 

Thunderstorms/Ice Accretion

Requirements for the formation of a Thunderstorm

‐ unstable air to high levels

‐ high relative humidity

‐ a lifting agent

 

Structure and Development of a Thunderstorm (3 Stages)

Cumulus Stage

‐ Initially every thunderstorm is just a cumulus cloud

‐ If the 3 requirements of formation are present, the cumulus cloud will develop an updraft throughout the cell

‐ The maximum vertical speed may reach 3000ft/min near the upper portion of the cell late in the cumulus stage

‐ At every level within the cell, the temperatures are higher than the temperatures at the same level outside the cell

Mature Stage

‐ Updraft reaches greater heights and the presence of ice crystals and water droplets lead to precipitation

‐ The appearance of rain at the surface marks the transition from cumulus stage to mature stage

‐ The onset of rain forms a downdraft starting in the middle of the cell

‐ The cell may grow to 40,000ft to 60,000ft in height

‐ Downdraft may reach speeds of 2,000ft/min

‐ The friction also causes the formation of roll cloud

‐ Hail may be present in the cloud or well outside the clear air around the cell

‐ Positive electricity builds near the top of the cell and negative electricity builds in the base of the cell

Dissipating Stage

‐ Downdraft spread through the entire cell except the very top portion which frays to an anvil shape

‐ Rainfall decreases in intensity and finally ceases

 

Effects of Icing on Aircraft Performance

‐ Frost or ice on the surfaces of the aircraft can reduce lift by up to 30% and increase drag up to 40%.

Wings and Control Surface – alter and reduce aerodynamic lift and may impede movement

Propellers – reduce thrust, and pieces of ice are dislodged and may damage other components

Windscreen and Canopies – reduce and/or distort visibility

Radio Antennas – ice may collect quickly, increase weight, vibrate and brake off

Pitot Tubes and Static Ports – may become partially or fully blocked rendering some instruments unreliable

Carburetors and Air Intake – may become partially or fully blocked leading to engine failure

 

Types of Icing

Rime Ice

‐ Is an opaque, whitish, granular type of ice often resembling crusted snow

‐ Often forms into a knife edge facing into the airstream

‐ The temperature of the aircraft’s skin must remain below zero degrees Celsius

‐ Forms quickly on impact when small super‐cooled water droplets strike the aircraft without spreading back

Clear Ice

‐ Is a clear, glassy, hard type of ice that often spread unevenly over the aircraft’s surface

‐ If flight in clear icing is continues, it will build to a blunt formation along the leading edge of the wing

‐ When the skin temperature is at 0 degrees Celsius, large, super‐cooled water droplets only partially freeze on impact and have time to spread aftwards before freezing entirely

Hoar Frost

‐ Is a white, feathery type of frost which can cover the entire aircraft following a clear night

‐ Forms by sublimation, when the surface of the aircraft is considerably below freezing and the air is warm and moist

‐ Commonly develops during clear, cold nights (may be encountered in flight)

 

Rate of Catch

‐ Is the amount of water intercepted by an aircraft in a given time

‐ This rate varies with: liquid water content of the cloud at a particular FL, size of water droplet, airspeed, shape of wing

‐ Small water droplets will tend to follow the airflow around the camber of the wing

‐ As airspeed increases, so does the rate of catch

‐ Thin wings catch more droplets per square inch than thicker wings

 

Cloud Type and Icing

Cumulus Cloud

‐ Highly variable, although sever icing is common especially in the upper levels of cumulonimbus clouds

‐ Icing may be present at temperatures up to ‐25 degrees Celsius

Stratus Cloud

‐ Usually less sever (on avg.), although serious icing may be encountered if the cloud is high in water content

‐ Usually icing is worse near the top of the cloud 

Mountain Waves, Turbulence and Wind Shear

Mountain Waves

‐ Airflow which crosses a mountain range forms a series of waves which are spaced between 2 and 20 miles apart

‐ The strongest wave is the one closest to the mountain

‐ In Canada, mountain waves are most common just east of the Rockies

‐ Downdrafts of 2,000ft / min are common and are usually the strongest on the downwind slope of the range

‐ If the air is sufficiently moist, clouds form in the vertical currents associated with the turbulence

‐ 3 type of clouds are associated with the mountain wave: cap, rotor and lenticular

‐ The greatest turbulence will most often be between the ground and the tops of the rotor clouds

‐ Altimeter errors are cause by the increased flow of air and cause altimeters to read too high

‐ Icing can be serious and unpredictable as strong vertical currents create large super‐cooled water droplets and the freezing level varies

 

Turbulence 

Mechanical Turbulence

‐ Is caused by friction between the air and the ground

‐ The strength depends on the strength of surface wind, nature of the surface and stability of the air

‐ Can be caused by buildings, trees, ravines, escarpments, hills etc,.

Thermal Turbulence

‐ Is caused by the rising warm air which is created by the sun’s unequal heating of the earth’s surface

‐ The strength of this turbulence depends on time of day, amount of sunshine and the nature of the earth’s surface

Frontal Systems

 

‐ Turbulence is created by the rising warm air and by the friction between the cold and warm air

Wind Shear

‐ Is a form of turbulence created by the wind shifting direction or intensity very suddenly

‐ It is associated with thunderstorms, mountain waves, fronts, valley winds, convective thermals, low‐level inversions and microburst’s

‐ It is most hazardous within 1000ft AGL during take‐off and landing

‐ Generally can be noted when the following deviations are noted below 1000ft AGL: 15kt in airspeed, 500fpm vertical speed, 5 degree pitch attitude, one dot displacement from the glide slope, GPWS signals wind shear

Microburst

‐ Is a strong downdraft which may last 2‐4 minutes

‐ Vertical winds may be as high as 6,000ft / min

‐ Create low‐level wind shear of horizontal winds reaching 45 knots

‐ Downbursts describe a severe down rush of air and its outburst of damaging winds on or near the ground. These events are classified as either microburst or macroburst. These two types of event differ in terms of their size, with radial outflow at earth’s surface lasting between 3 to 20 minutes. Downbursts can occur wherever convective weather conditions exist. Approx 5% of thunderstorms produce a microburst.

‐ Frontal wind shear is present in both cold and warm fronts, but exists in a different relative location in each type of front. Because the cold front boundary slopes back behind the frontal surface, the windshear line also slopes back. The frontal boundary of the warm front slopes upward ahead of the surface front, so the same is true of the wind shear. Significant wind shears can be expected if there is a large difference in surface temperature across the front and if the front is moving rapidly at more than 30 kts.

‐ Friction Windshear describes large wind speed changes near the ground in various meteorological situations, including frontal conditions. Terrain irregularities or buildings which interrupt the wind flow can produce significant windshears.

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