Weather Theory

Understanding the weather is a key element of knowing what we, as pilots, are up against.  It can seem abstract at first, so let's take a look at the big picture...

Atmosphere
Its purpose is to absorb energy from the sun and provide a protected/moderate climate for life on Earth.

Composition: Nitrogen = 78%, Oxygen 21%, Argon and other gases make up remaining 1%.

There are 4 distinct layers within the atmosphere going from the surface to 350 miles away.

The Troposphere extends from SL to 20,000 ft. above the poles and up to 48,000 ft. over equatorial regions.  Most weather (clouds, storms, temperature changes) occur within this layer.  Standard lapse rate of 2ºC per 1000 ft exists and pressure decreases about 1" Hg per 1000 ft.

The Tropopause is a boundary layer which traps moisture and weather in the troposphere.

The upper layers have little influence on weather and are considered to be generally stable, though the atmosphere itself is in constant motion.

•Unequal heating of the surface creates most "weather."
•Warm air rises because air molecules spread apart when heated.  Expanding air is less dense and lighter.
•Unequal heating also changes air pressure (molecules have weight).  Pressure differs based on temperature, altitude, and air density.

Coriolis Effect - Generally areas of high pressure at poles want to go to areas of low pressure around the equator.  This is interrupted by rotation of the earth.  This force deflects air to the right in the Northern Hemisphere.

•The wind forces and actions are governed by 3 Cells
-Hadley Cell  (near equator)
-Polar Cell (near poles)
-Ferrel Cell (in between)

Atmospheric Pressure
In the U.S., our measurements of barometric pressure are in inches of Mercury (Hg).  Standards were developed for a reference point.
29.92" Hg=Standard Pressure or
1013.2 millibars (mb).
Standard Temperature = 15ºC

Temperature, altitude, and density can affect pressure.

•Remember that while there is still 78% oxygen at 10,000 ft, the pressure on that partial of air is significantly less - thus leading to risk factors like hypoxia.

Wind / Currents
Because air seeks out low pressure (and travels from areas of high pressure to low pressure), and because the earth's rotation deflects these currents to the right (in the northern hemisphere), air will flow Clockwise around a High Pressure System and Counter-Clockwise around a Low Pressure System.  Understanding this can help maintain favorable winds while enroute.

Convective Currents
Different surfaces radiate different amounts of heat.  Convective currents cause bumpy, turbulent air.  Land heats faster than water, so these currents are often found near land masses near water.  Circulation patterns are not the only cause of turbulence.  Obstructions ranging from airport hangars to mountains can reroute the flow of air, in a violent manner.
•Leeward sides of mountains are especially hazardous due to the strong downward pressure and eddies.

Low-Level Wind Shear
This is a hazardous condition that can occur with passing fronts, thunderstorms, or temperature inversions.

•Microbursts last about 15 mins, but can produce up to 6,000 fpm downdrafts, and take place often in areas of less than one mile.
•LLWAS systems detect wind changes of greater than 15 kts.


Stability
Stability deals with atmosphere's ability to resist vertical motion (convectivity).  When air rises, it enters an area of lower pressure and expands to a larger volume.  The expansion on molecules causes the temperature to lower.
•Water Vapor is lighter than air, thus moisture decreases air density, causing it to rise.  Moist air cools slower and is thus more unstable (requires more vertical movement to cool)
•Dry adiabatic lapse rate is 3ºC per 1000ft
•Moist adiabatic lapse rate varies from 1.1ºC to 2.8ºC per 1000 ft

Inversions
When temperature increases with altitude gain.  Top of inversion acts as lid, pressing down and keeping lower altitudes stable.

Temperature/Dewpoint/Humidity
•Relative Humidity refers to how much moisture a parcel of air is holding compared to how much it could hold
•Dewpoint is temperature of the air, where it could hold no more moisture (clouds, fog, or precipitation result)
•Dewpoint & Temperature converge at a rate of 4.4ºF per 1000 ft

Fog
Fog is one result of saturation of air.  There are numerous types resulting from varying causes.

Radiation Fog - Often occurs in low-lying areas like mountain valleys.  Forms at night when ground cools rapidly due to terrestrial radiation and surrounding air reaches dew point.

Advection Fog - occurs when winds push coastal air over cooler ground.

Upslope Fog - occurs when stable air is forced up sloping land feature.  Requires wind to form and can last for days.

Steam Fog - forms when cold, dry air moves over warm water.  As the water evaporates, fog forms.

Ice Fog - forms in cold weather when water vapor forms directly into ice crystals.

Clouds

Clouds are another result from a temperature/dewpoint convergence.  There are varied cloud types based on height, shape, and behavior.  They are separated into Low, Middle, or High clouds.

Cumulonimbus clouds are probably the greatest cloud hazard to pilots.  Because they contain extensive vertical development, they are extremely turbulent and can produce hailstones, lightning, tornadoes, and other hazards to flight.

Thunderstorms
There are 3 distinct stages of a thunderstorm= Cumulus, Mature, Dissipating.
Cumulus consists entirely of updrafts.  These strong updrafts prevent moisture from falling.
Mature - drops of moisture become too heavy and downward motion begins.   Precipitation falls.
Dissipating - Cloud is raining itself out, characterized entirely by downdrafts.

Air Masses

Air Masses are classified according to regions where they originate.  They take on characteristics of surrounding area.
•Air masses moving over warmer surface results in convective currents and instability.
•Air masses moving over cooler surfaces creates stable conditions with poor surface visibility.

Fronts
An air mass moving across water/land will eventually come into contact with another mass with different characteristics.  The boundary layer is a front.  There are 4 types: Warm, Cold, Stationary, Occluded.

Warm Fronts - warm mass of air replaces cooler air.  They move slowly (10-25mph).  Slides over cooler air and gradually pushes it out.  Often contains high humidity.  Precipitation is probable as is a reduction in visibility.

Cold Fronts - mass of cold, dense, and stable air advances and replaces warmer air.  Move rapidly (25-30mph).  They remain close to ground and act as plow, sliding under warmer air.  Generally more volatile than a warm front passage.  Fast moving cold fronts can produce squall line thunderstorms and turbulent winds.

Stationary Fronts - occur when two air masses are relatively equal and weather is a mixture of warm/cold fronts

Occluded Front - When a fast moving cold front catches up with a warm front.  Two types:
Cold Front Occlusion - When cold front is colder than air ahead of the warm front.  Cold air replaces the cool air and forces warm front aloft

Warm Front Occlusion - When air ahead of warm front is colder than air of the cold front.  Cold front rides up and over wam front.  Generally more unstable.

Squall Line
A narrow band of active thunderstorms that often develops ahead of a cold front.  Forms rapidly and often contains steady-state thunderstorms, which are extremely hazardous to aircraft.

Tornadoes
Thunderstorms that draw air into cloud bases when there is an initial rotation.  Tornadoes occur in both isolated and squall line thunderstorms.

Hail
Supercooled drops above freezing level begin to freeze.  When enough ice has latched on to the parcel, it becomes too heavy and falls.