Weather Theory

Font Size: Larger /Smaller


The Atmosphere

The Troposphere
Starts at the surface and goes up to an average of 37,000 feet
20,000 ft at the poles to 65,000 at the equator
It is higher in the summer than winter
Lapse rate is 2 C/1000 or 3.5 f/1000

Boundary layer between troposphere and stratosphere
Signaled by an abrupt change in the temp lapse rate
Isothermal air temp remains constant -57º C
The jet stream exists in occasional breaks in the tropopause.

Starts at about 39,000 ft
Temp doesn’t change much creating an inversion
This inversion keeps the Troposphere’s upward currents down
High amounts of ozone absorb the sun’s ultraviolet radiation and gives off heat
At 66,000 temps starts to rise because of the ozone absorbing energy

Narrow river of wind
50-150 knots
100-400 miles wide
1 mile thick
Subtropical jet 33,000 to 43,000
Polar Front jet 25,000 to 35,000Stronger in winter than summer
Farther south in winter than summer
Occurs at breaks in the tropopause
Has to be 50 kts or greater to be classified as a Jet Stream

Primary Cause of all Wx
Variation of solar energy received and reradiating is the primary cause of all weather
Insolation is when the suns rays warm the earth
Different surfaces radiate and receive at different rates
Terrestrial radiation is when the earth radiates the energy back into the atmosphere
This is referred to as Longwave radiation
The sun’s energy is referred to as shortwave radiation
Diurnal Variation is difference between day and night (coldest temp just after sunrise)

Increase in temp with an increase in altitude
Can occur at any altitude in troposphere
Most common occurrence is on calm clear cool night in which the ground cools air close to it.
This is caused by terrestrial radiation and is referred to as a Radiation Inversion
The inversion is associated with a stable layer of air
Little or no mixing of the layers
Generally poor visibility due to fog, haze or low stratus clouds

Rotates counterclockwise
Area of rising air
Usually clouds present
Bad weather

Rotates clockwise
Area of descending air
Usually no clouds
Good weather

Apparent Coriolis Force
Earth’s rotation transforms straight line motion into curved motion for an outside viewer.
The Coriolis force explains this apparent curvature of winds to the right due to the earth’s rotation under the winds.
The earth rotates at about 15° longitude an hour.
Thus if say a missile were airborne for an hour flying from the north pole toward the equator, it would appear to deflect toward the southwest.

Flow would normally be 90º to isobars except for Coriolis Force
Causes a deflection of winds to the right in the Northern Hemisphere
To the left in the Southern Hemisphere
The deflection turns the winds parallel to the isobars at altitude
Near the ground, the deflection depends on surface friction
The more surface friction the lesser Coriolis force becomes
Over land 45 degrees to the isobars
Over water 10 degrees to the isobars
So if winds are southwesterly at altitude, surface friction will cause surface winds to be more southerly
The magnitude varies with the speed of the wind and the latitude
As speed increases Coriolis increases
As latitude nears the poles, Coriolis increases

Pressure gradient = difference in pressure / distance
Sets up a flow from high to low
The closer the isobars, the stronger the pressure gradient force and the stronger the wind
Pressure gradient force is at a 90 degree angle to the isobars
It is Coriolis’ arch enemy because it acts in the opposite direction to Coriolis
It is the interaction of these 2 forces that causes flow parallel to the isobars at altitude

Moisture and Precipitation
When water vapor condenses you get clouds, fog or dew
Dew point is the temperature at which the air must be cooled to become saturated
However condensation does not necessarily happen when the temperature reaches the dew point
Unstable air and the presence of updrafts can enhance the possibility of cloud formation
The warmer the air, the more moisture can be present
This is the result of net evaporation outpacing net condensation

Stable and Unstable Air
Stability is the resistance of vertical motion
Stable air
Stratiform clouds
Poor vis
Steady precipitation
Smooth ride
Remember if air is lifted and there is no Latent heat it cools at the same rate or faster and will be stable
Unstable air
Cumuliform clouds
Good vis
Showery precipitation
If air is lifted and there is Latent heat released through condensation, the lapse rate will be less and therefor warmer than surrounding air

Stable and Unstable Air
The lapse rate is the measurement of stability
The average lapse rate is 2°C/1000
The dry lapse rate is 3°C/1000
Cumuliform clouds form in unstable conditions
Moist air moving over warmer ground
Stratiform clouds form in stable conditions

High clouds 16,500 – 45,000
Middle clouds 6,500 – 23,000
Low clouds surface – 6,500
Extensive vertical development

High clouds – cirrus, cirrocumulus, cirrostratus
Middle clouds – altocumulus, altocumulus castellanus, altostratus nimbostratus
Low clouds – stratus, stratocumulus, cumulus
Extensive Vertical Dev. – cumulus, cumulonimbus, cumulo congestus

nimbo or nimbus = rain
2 classifications overall stratus and cumulus
The high cloud family is least likely to produce structural icing on aircraft
Mostly made up of ice crystals
Cumulonimbus clouds have the greatest turbulence
Strong updrafts and downdrafts up to 6000 fpm
Standing lenticular clouds also have a great potential for turbulence
They form due to strong winds being lifted orographically
Fair weather cumulus indicate turbulence below the cloud level

1. Cool air to the dew point
2. Add moisture near the ground
In addition, there must be sufficient condensation nuclei for the fog to form
Industrial areas provide an abundance of nuclei

Types of Fog
1. Radiation fog
Forms on clear, cool, calm nights
2. Advection fog
Needs a wind to exist
Common along coastal areas where moist air is driven inland
3. Upslope fog
Needs a wind to exist
Orographic lifting cools the air to the dew point producing fog
4. Precipitation-induced fog
Warm rain or drizzle falling through colder air
5. Ice fog
Needs a very cold environment to form
6. Steam fog
May form over a warm lake under calm wind conditions

Fog, Dew and Frost
Dew point is the term given to that temperature at which the air becomes saturated
It will always be the same as or lower than temperature
Usually when the temperature is cooled to the dew point condensation occurs
If this happens near the ground it’s fog
You have to have condensation for fog to form
For dew to form the collecting surface must be at or below the dew point
For frost to form the collecting surface must be at or below the dew point and below freezing
It is not necessarily true that clouds, fog or dew will form with 100% relative humidity

Air Masses and Fronts
Air Mass – Body of air that has fairly uniform temperature and moisture
Cold air mass is generally defined as being colder than the ground it is moving over
Likewise a warm air mass is warmer than the ground it moves over
Source Region – The region which an air mass acquires its particular properties of temp and moisture
mP, mT, cT, cP, A

Discontinuities are changes in properties from 1 front to another
Temperature – usually a temp change
Dew Point – Temp dew point spread will usually change
Wind always changes across a front usually both in speed and direction
Beware of wind shear in this area
Cold front passage pressure rises
Warm front passage pressure falls
Notice the Frontal Waves at right

The Frontal Wave
These form on slow moving cold fronts or stationary fronts

1. Convective currents
2. Obstructions to wind flow
3. Wind shear
Whatever the cause
Slow to Va
Accept variations in speed and altitude
Report turbulence to ATC
Get a block altitude if necessary

Stable vs Unstable
To determine the stability or instability of the atmosphere we look at the lapse rate
Unstable air
If a parcel of air is forced upward and cools at a slower rate than the surrounding air
Warming from below, high condensation rates
Stable air
If a parcel of air is forced upward and cools at a faster rate than the surrounding air
Cooling from below, low or no condensation

Stratiform clouds
Smooth air
Poor vis
Continuous Precipitation

Cumuliform clouds
Good vis
Showery precipitation

1. Water Vapor
2. Unstable Lapse Rate
3. An initial upward boost or lifting action

1. Cumulus
Mostly updrafts
2. Mature
Greatest intensity
Precipitation starts
3. Dissipating
Mostly downdrafts

You’ve got to have lightning to have a thunderstorm
A Squall line is a narrow non-frontal band of thunderstorms
These usually form in the Midwest out ahead of a cold front
These generally produce the worst conditions like hail and destructive winds
Keep in mind the outflow from a thunderstorm is going to be from the center out, in a radial pattern
Expect higher winds downwind from the direction the thunderstorm is moving

Thunderstorm Hazards
Squall Lines
Embedded Thunderstorms
High winds
Wind Shear/Microburst

Characteristics of microbursts include:
The microburst downdraft is typically less than 1 mile in diameter.
It descends from the cloud base to about 1,000 – 3,000 feet above the ground.
In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.
The downdrafts can be as strong as 6,000 feet per minute.
Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst.
These strong horizontal winds occur within a few hundred feet of the ground.
Some extreme microbursts have clocked in at 150 kts.
Most microbursts seldom last longer than 15 minutes

Flying into a Microburst
A pilot flying into a microburst must anticipate sudden and strong changes in wind direction and speed.
Initially a headwind is encountered that lifts the plane, followed by a strong downdraft, and when leaving the storm a tailwind causes a loss of altitude.
If encountered on takeoff a normal jet aircraft may have only 5 to 15 seconds for recognition and recovery.

1. Increases weight
2. Reduces lift – changes the shape of the airfoil
3. Decreases thrust – effects prop
4. Increases drag – sticks up into the wind

1. Must have visible water
2. Aircraft must be below freezing

1. Clear – cumuliform clouds
Large droplet size, spreads then freezes
Accumulates at the highest rate
2. Rime – stratiform
Small droplet size freezes before spreading out
3. Mixed – cumuliform or stratiform

Freezing Rain
Can happen any time you are flying in rain at or below 0°C
Results in the highest accumulations of icing
Ice Pellets – frozen droplets of rain
Indicates a layer of freezing rain above the altitude ice pellets are encountered

Doing an approach with ice
Use a faster approach speed
Do not use flaps
If you get an uncommanded roll, increase speed and enter a controlled descent
Turn off the autopilot
If you know your getting ice, periodically turn off the auto pilot
Watch for tail ice
It accumulates at about 3 times the rate on the main wing
This will likely be the first aerodynamic surface to accrete ice
Signified by a change in elevator force or an uncommanded nose down pitch
Use of flaps will antagonize the situation
If you lower flaps and you get buffeting on the tail, retract the flaps and use a higher speed

The first signs of ice should show up on probes and antennas
If you have deicing equipment use it at the first signs of ice
To get out of icing conditions initiate a climb to colder temps
Look for temps in the range of -10 to -15
Apply your lapse rate formula to determine how much your going to need to climb
If your MEA is below freezing this may be your only option so consider this before going
If your MEA is above freezing you always have the option to descend

May be at any altitude
May be at any direction
Basically anytime there is a wind shift or high gradient
You may find wind shear:
Low level temperature inversions
The bigger the temp difference the higher the chance of shear
Watch for a sudden change of airspeed when transitioning the inversion
Frontal zones
With warm fronts it happens before the front passes
With cold fronts it happens just after the front passes
Clear air turbulence
Associated with the jet stream
Worst is with an upper level trough on the polar side of the jet
Low level wind shear usually associated with thunderstorms
Found all around and under the thunderstorm

Print Friendly, PDF & Email