Flashcards in Meteorology Deck (95):
What is troposphere
It is the first layer of the atmosphere. It extends up to 48,000 feet (14.5 km) over the equatorial regions. The vast majority of weather, clouds, storms, and temperature variances occur within this first layer of the atmosphere. Inside the troposphere, the average temperature decreases at a rate of about 2 °Celsius (C) every 1,000 feet of altitude gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain.
What is tropopause
It is a boundary at the top of troposphere. It traps moisture and the associated weather in the troposphere. The altitude of the tropopause varies with latitude and with the season of the year; therefore, it takes on an elliptical shape as opposed to round. Location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence.
What is coriolis force
The force created by the rotation of the Earth is known as the Coriolis force. In the northern hemisphere, the coriolis force deflects the airflow to the right (I.e. Westerly wind), and in the Southern Hemisphere, wind is deflected to the left.
What is jet stream
Jet stream is a strong, narrow current in the upper tropopause or stratosphere. The wind speed must be greater than 60 knots to be classified as a jet stream.
Why jet streams are westerlies?
On a global scale, surplus heating in the tropics creates north-south upper level temperature
gradients, which, in turn, creates upper-level high pressure at the equator. The resulting pressure gradient force causes upper air flow from the equator to the poles, which turns
eastward due to Coriolis Effect. So the winds in the upper troposphere are westerly at
The polar front jet forms in the region of strong temperature gradients between cold, polar air
and warmer air masses. The gradients increase with altitude because the warmer air columns
have a larger cumulative expansion than the colder air columns. The consequent pressure
difference in the upper troposphere will increase with altitude and cause strong winds.
Pressure lapse rate
with every 1,000 feet of increase in altitude, the atmospheric pressure decreases 1 "Hg.
-reduces quicker with height in cold air
ISA sea level pressure
ISA air temperature at sea level
Temperature lapse rate
1.98C per 1000ft
ISA temperature above 36090ft
Constant at -56.5C
How altitudes affect flight
-higher altitudes, thin air, more lift obtained by increasing speed, thus increasing takeoff and landing distance
-thin air, less efficient aircraft, thus decreasing climb rate
Isobars are lines drawn on the chart to depict lines of equal pressure.
Lowest pressure in the centre
NH: wind rotates counterclockwise around a low
Observed barometric pressure reduced to msl assuming ISA condition
-altimeter reads its height above sea level
Pressure at a given datum
Altimeter reads vertical distance above datum
With aerodrome QFE set in the subscale, it will read 0 after landing
Height indicated in the altimeter on landing at an aerodrome when the altimeter subscale is set to 1013.25mb
Used above FL150
Greatest pressure in the centre
NH: wind rotates clockwise
In the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure. This is known as anticyclonic circulation. The opposite is true of low-pressure areas
Factors that affect wind
1. Pressure gradient force
2. Coriolis force
Interrelation of coriolis force and pressure gradient
-when pgf=cf, wind blows on straight path along isobars
- cf>pgf, curved flow is around high pressure, wind curves clockwise around a high
-When the pressure gradient force is greater than the Coriolis force, the flow follows a curved path around low pressure. In the southern hemisphere this flow around a low pressure is in a clockwise direction.
-These relationships break down in the equatorial region when Coriolis force becomes so small that its influence is negligible. At the equator Coriolis force is zero, and is negligible from there to about 15 degrees latitude.
-In the atmosphere above the friction layer, only PGF and Coriolis force affect the horizontal motion of air
-Wind blows parallel to isobars when two forces in balance
At the surface of the Earth, all three forces come into play. As frictional force slows the wind speed, Coriolis force decreases. However, friction does not affect PGF. PGF and Coriolis force are no longer in balance. The stronger PGF turns the wind at an angle across the isobars toward lower pressure until the three forces balance.
Friction between the wind and the terrain surface slows the wind. The rougher the terrain, the greater the frictional effect. Also, the stronger the wind speed, the greater the friction. it is always acting opposite to wind direction.
The frictional drag of the ground normally decreases with height and becomes insignificant above the lowest few thousand feet.
Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. Wind shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the aircraft. While wind shear can occur at any altitude, low-level wind shear is especially hazardous due to the proximity of an aircraft to the ground. Low-level wind shear is commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong upper level winds (greater than 25 knots).
- the lighter the aircraft and its airspeed, the more vulnerable it is to wind shear
The most severe type of low-level wind shear, a microburst, is associated with convective precipitation into dry air at cloud base. Microburst activity may be indicated by an intense rain shaft at the surface but virga at cloud base and a ring of blowing dust is often the only visible clue. A typical microburst has a horizontal diameter of 1–2 miles and a nominal depth of 1,000 feet. The lifespan of a microburst is about 5–15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm)and headwind losses of 30–90 knots, seriously degrading performance. It can also produce strong turbulence and hazardous wind direction changes
Potential hazards of wind shear
-Violent air movement (up- or down-draughts or swirling or rotating air patterns)
-Sudden increase or reduction of airspeed
-Sudden increase or decrease of groundspeed and/or drift.
-Clear Air Turbulence (CAT), which may be very severe, is often associated with jet streams.
-Rotor action or down-draughts in the lee of mountain waves can create difficult flying conditions and may even lead to loss of control.
When meeting a shear zone on approach to land
Angle of descent steepens as indicated airspeed reduces
Most common situations where low level wind shear can be expected
- descending into a dense friction layer
-descending into a cold zone
-descending into the lee of sand dunes when a sea breeze blows
-descending where the reported wind is lighter and blowing from a different direction than above that level
-descending or ascending through a sea breeze boundary
During the day, land heats faster than water, so the air over the land becomes warmer and less dense. It rises and is replaced by cooler, denser air flowing in from over the water. This causes an onshore wind called a sea breeze.
at night land cools faster than water, as does the corresponding air. In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land, creating an offshore wind called a land breeze.
Sea breeze cloud
Mountain and valley breezes
-during the day the air above the mountain is heated and rises creating a valley breeze
-at night air above the mountain cools more rapidly and sinks creating a mountain breeze
Katabatic and anabatic winds
At night, When air over sloped terrain is cooled by conduction it becomes denser than near free air and drains to lower levels. The winds generated are known as katabatic winds.
The reverse effect occurs on slopes on sunny days. Air in contact with a slope warms by conduction and ascends (not necessarily following the slope). Such ascending winds are called anabatic winds. The upward flow will be strongest in the early afternoon and over sun facing slopes.
-anabatic wind is weaker because it has to work against gravity
Factors affecting Katabatic and anabatic winds
1.the degree of cooling along the slope (the colder the surface, the greater the potential for the generation of very dense air and hence greater wind speed);
2. the roughness of the slope (the smoother the slope the greater the potential for uninterrupted and thus stronger flow);
3. the steepness of the slope (gentle slopes are more favorable for katabatic development because steep slopes cause the wind to become turbulent, resulting in mixing with surrounding air and the consequential breakdown of continual downward movement of cold air).
While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar manner.
As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become.
Airflow over a ridge or mountain range may disturb flow downstream to a great height.
The flow upwind of the ridge is smooth and horizontal. At the ridge line the airflow is lifted and follows the shape of the ridge. On the other side a dramatic change in the flow takes place. The flow does not return to horizontal flow but continues as a wave that may be smooth or may contain dangerous turbulent zones.
If the air stream is sufficiently moist at any level, cloud may form in the ascending sections and produce almond or lens-shaped clouds called lenticular clouds. However if the air is dry the first indication of wave motion for a pilot may be a rapidly increasing or decreasing altimeter reading.
The conditions required for mountain waves
1. wind perpendicular or near perpendicular to the mountain;
2. wind strength of at least 25 to 30 knots near the mountain top;
3. wind speed increasing with height;
4. a stable layer.
Sea breeze time
Sets in around 10am
Peaks at 3pm
Cease before sunset
1. lower cloud base and precipitation on windward slopes;
2. higher cloud base on lee slopes;
3. higher temperatures and hence lower density at low levels in the lee of the mountain.
Atmospheric stability tells you how likely it is air will rise and form clouds and precipitation.
if, because of the vertical temperature distribution, a lifted parcel is cooler and therefore denser than the surrounding air, the parcel will tend to sink. Thus the environment is defined as being stable;
if, because of the vertical temperature distribution, a lifted parcel is warmer and less dense than the surrounding air, the lifted parcel will continue to rise. In this case the environment is defined as being unstable;
because of the vertical temperature distribution, a lifted parcel is the same temperature as the surrounding air, conditions are said to be neutral;
in some situations the atmosphere is stable for unsaturated parcels of air but unstable if saturated
Temperature lapse rate
The rate of decrease of temperature per unit increase of height I.e. C per 1000ft
The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location.
ISA: 1.98 °C/1,000 ft from sea level to 11 km (36,090 ft or 6.8 mi). From 11 km up to 20 km (65,620 ft or 12.4 mi), the constant temperature is −56.5 °C (−69.7 °F)
Air changes temperature due to change of pressure
No exchange of heat with surroundings
Dry Adiabatic Lapse Rate (DALR) is the rate of cooling of unsaturated air
saturated adiabatic lapse rate (SALR) is the rate at which the temperature of a parcel of air saturated with water vapour changes as the parcel ascends.
- cooling is partly offset by the latent heat of condensation
- 1.5°C/1000 feet
DALR,SALR < ELR
-negative lapse rate (temp increase with altitude)
-enhance stability(limit vertical movement)
-visibility below is fair and above is clear(weather and pollutants kept below the layer)
Below inversion layer
Risk of carburettor icing
Surface-based temperature inversions
occur on clear, cool nights when the air close to the ground is cooled by the lowering temperature of the ground. The air within a few hundred feet of the surface becomes cooler than the air above it.
occur with high-pressure systems that cause air in the upper levels to sink and thus warm. When the upper layer warms at a greater rate, an inversion is formed
-occur when warm air spreads over a layer of cooler air,
-or cooler air is forced under a layer of warmer air.
Fog is a cloud that is on the surface. It typically occurs when the temperature of air near the ground is cooled to the air’s dew point. At this point, water vapor in the air condenses and becomes visible in the form of fog.
-visibility less than 1000m
-air must be stable
Conditions required for fog
-adequate supply of moisture
-availability of condensation nuclei
On clear nights, with relatively little to no wind present, radiation fog may develop. Usually, it forms in low-lying areas like mountain valleys. This type of fog occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dew point. As the sun rises and the temperature increases, radiation fog lifts and eventually burns off. Any increase in wind also speeds the dissipation of radiation fog. If radiation fog is less than 20 feet thick, it is known as ground fog.
Conditions for radiation fog
1. Anticyclones and long clear night (clear sky for terrestrial radiation to escape)
2. Light wind for mixing of surface air
Dispersal of radiation fog
When solar radiation is introduced, relative humidity is reduced
When a layer of warm, moist air moves over a cold surface, advection fog is likely to occur. Unlike radiation fog, wind is required to form advection fog. Winds of up to 15 knots allow the fog to form and intensify; above a speed of 15 knots, the fog usually lifts and forms low stratus clouds. Advection fog is common in coastal areas where sea breezes can blow the air over cooler landmasses.
Difference between radiation fog and advection fog
advection fog :
-can form over sea
-requires a surface that is already cold rather than one that is cooling
-can persist throughput the day
Dispersal of advection fog
-decrease in moisture content of the moving air
-change in wind direction
Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range. This type of fog also requires wind for formation and continued existence. Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun but instead can persist for days. They can also extend to greater heights than radiation fog
Steam fog, or sea smoke, forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. This type of fog is common over bodies of water during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog.
Ice fog occurs in cold weather when the temperature is much below freezing and water vapor forms directly into ice crystals. Conditions favorable for its formation are the same as for radiation fog except for cold temperature, usually –25 °F or colder. It occurs mostly in the arctic regions but is not unknown in middle latitudes during the cold season.
What's the fog at Golden Gate Bridge
Advection fog forms when humid air from the Pacific Ocean swoops over the chilly California current flowing parallel to the coast. The fog hugs the ground and then the warm, moist air condenses as it moves across the bay or land. This is common near any coastline. Sometimes, high pressure squashes it close to the ground. By the way, the color of the Bridge is International Orange and was chosen in part because of its visibility in the fog.
-common in summer
How does cloud form
When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes of sublimation and condensation, moisture condenses or sublimates onto matter like dust, salt, and smoke known as condensation nuclei.
Types of cloud
-indicate unstable weather
Types of cloud formation
4. Frontal lifting
1. Low cloud
-below 6500 ft
-Stratus, cumulus, stratocumulus, cumulonimbus
2. Medium cloud
-6500 to 23000ft
-altocumulus , altostratus, nimbostratus
3. High cloud
-cirrus, cirrocumulus, cirrostratus
Orographic ascent is the forced vertical motion of air by mountains or raised terrain
-stratiform: air is moist and stable
-cumulus: air is moist and unstable
Air will rise from the surface by convection when surface air are heated
Suitable conditions commonly occur when:
strong surface heating occurs during the day; or
a relatively cold air stream moves over a warmer surface.
Cumuliform clouds form if the surface air is moist enough and the rising air is cooled to saturation at the condensation level. The depth of convection is determined by the moisture content of the rising air and the temperature distribution of the environmental air.
-general ascent of air over a wide area
-cold front: when cold air stream bumps into a warm one. Warm air forced to rise; creates cumulus
-warm front: warm air overtakes a cold one, warm air forced to rise; creating stratiform cloud
Turbulent mixing between atmospheric layers above and below the level of the dew-point temperature
-st and sc are usual
Dry ice crystal
Light-moderate turbulence in jet stream
Dry ice crystal
Dry ice crystal
Light-moderate turbulence in jet stream
Rain, snow, sleet
Can cause severe rime or glaze icing
Turbulence not common
Precipitation not common
Light to moderate icing
Light to moderate turbulence
Very light time ice
Turbulent if amy inversion is present
Light rain or drizzle
Light to moderate rime ice
Can occur at any level
Light to heavy showers
Hail in CB
severe glaze and rime ice in CB
Moderate to severe turbulence in CB
Persistent steady rain or drizzle
Moderate rime ice and possible glaze icing
Generally no turbulence
-warm air advances towards cold air
-predominantly stratiform clouds
-cb is possible if unstable
-speed 10-15 kts
-distance to 1000km
A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.
It is so dense, it stays close to the ground, sliding under the warmer air and forcing the less dense air aloft. The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds. The type of clouds that form depends on the stability of the warmer air mass. A cold front in the Northern Hemisphere is normally oriented in a northeast to southwest manner and can be several hundred miles long, encompassing a large area of land.
Effects of Cb
-Turbulence. Vertical movement within a Cb can be as much as 50kt. The interaction between strong updrafts and strong downdrafts causes wind shear and severe turbulence within the cloud. Strong surface winds, variable in direction and strength, are common at surface level in the vicinity of the Cb. These can be particularly hazardous to aircraft on take-off or landing.
-In-Flight Icing. Moderate to Severe icing can be expected, especially in the higher levels of the cloud.
-Electrical disturbance. Aircraft flying in the vicinity of Cb clouds may experience electrical disturbances effecting communications and navigation systems. The electrical phenomenon known as St Elmo's Fire, while not a threat to safe flight, is an indication of nearby Cb activity. Aircraft in the vicinity of a Cb are at risk of being hit by Lightning.
-Precipitation. Hail can cause significant structural damage to an aircraft. Other precipitation, such as snow, sleet, or rain, can contaminate airfield and runway surfaces creating a hazard to aircraft attempting to take-off or land.
-Extreme weather. Severe downdrafts, microbursts and funnel clouds such as Tornados are also features of cumulonimbus clouds.
What type of ice would you encounter when flying through CB?
How many miles above the CB?
beware of flying through or just under the anvil. Although the turbulence levels are low, often severe-icing conditions exist, including super cooled ‘liquid’ water. Generally the advice is to avoid cbs by a minimum of 5nm; however, to provide a margin for errors in detection (different radar systems and their responses in these conditions), rapid cloud formation and movement, anvil conditions, and misjudgment, then my experience suggests that aiming for a 15nm miss distance is safer.