Weather Theory (Instrument)

om 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
Pressure
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

3 MAIN CAUSES
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

STABLE AIR
Stratiform clouds
Smooth air
Poor vis
Continuous Precipitation

UNSTABLE AIR
Cumuliform clouds
Turbulence
Good vis
Showery precipitation

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

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

Thunderstorms
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
Lightning
Squall Lines
Embedded Thunderstorms
Hail
High winds
Wind Shear/Microburst

Microburst
Characteristics of microbursts include:
Size:
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.
Intensity:
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.
Time:
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.

Icing
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

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

THREE TYPES OF ICING
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

ICING
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

Icing
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

Icing
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

WIND SHEAR
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

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