Weather Theory

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Commercial Ground School AVF 221


One common way to classify the atmosphere is by temperature
Since every physical process of weather is accompanied by or the result of a temperature exchange it makes sense

Starts at the surface and goes up to an average of 36,000 ft
It ranges from about 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
The air gets colder because of less terrestrial radiation

Signaled by an abrupt change in the temp lapse rate
Boundary layer between troposphere and stratosphere
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
This creates an inversion that keeps the Troposphere’s upward currents down.

This is the boundary layer between the Stratosphere and the Mesosphere
It exists at about 164,000 ft
This one gets colder the further out
It exists from about 164,00 to 280,000 ft (31 to 53 miles high)
99% of the atmosphere exists below this layer
There is little ozone up here therefore temps decrease with an increase in altitude.
Lowest average temp found at about 53 miles at -90º C or -130º F

This one gets warmer with an increase in altitude.
It exists from about 280,000 to 1,637,000 ft (53 to 310 miles)
Radiation excites the oxygen molecule causing a temp increase.
There are so few molecules, even a small amount of radiation can cause high temperatures
Temps higher than 1000°C are found up here.
Note however that the density is so minute that it would not feel warm to your hand.

1,637,000 ft (310 miles) and further
Consists of atoms and molecules in loose orbit sometimes shooting off into space.

Atmospheric Pressure
Similar to earth’s atmosphere, the pressure at the base of this column of air results from the weight of the gasses above.
The earth’s atmosphere, however, has a greater density of gases at its base due to gravity.

Temperature and Elevation
When two columns of air are equal in elevation and density, they are at equilibrium.
When flying across these two columns without resetting the altimeter, the flight will be of level true altitude.

Temperature and Elevation
Adjusting temperatures by cooling (or heating) increases (or decreases) air density.
As you move up in altitude, in the colder column less vertical distance is required to transit the same amount of density.
When flying across air masses of different temps the airplane will be higher in the higher temp column.
The half way point in each column moves relative to the height of the column.
The true altitude in the warmer column will be higher.

Temperature and Elevation
At the surface, equilibrium is maintained, but the taller column has greater upper-level pressure, and winds are generated.
Wind always flows from high pressure areas to low pressure areas.
Remember too that in heat transfer warm moves to cold.
Warm air aloft is associated with High pressure systems while cold air aloft is associated with Low pressure systems.
The fact that there are two different pressures in adjacent air masses sets up a pressure gradient.

Pressure Fluctuations
Solar heating of ozone gasses in the upper atmosphere, and of water vapor in the lower atmosphere, can trigger oscillating thermal tides of sea-level pressure change.
For these reasons, all other variables being equal pressure will normally fall on hot summer days.
We normally see this occurrence right here in the Columbia Basin on hot summer days when our weather is influenced by a strong dominate high pressure system.

Surface and 500mb Maps
Surface maps chart pressure contours, highs and lows, and wind direction.
Winds blow clockwise around highs, called anticyclones.
Winds blow counterclockwise around lows, called cyclones
500 mb maps reveal patterns that on average are 5600 m above the surface, where westerly winds rise and fall across ridges and troughs.

Forces and Motion
Pressure forces are only one influence on the movement of atmospheric air.
Air responds similarly as water to this force, moving from higher pressure to lower pressure.
Centripetal, friction, and apparent Coriolis are other forces, determining winds.

Pressure Gradient Force
Change in pressure per change in distance determines the magnitude of the pressure gradient force (PGF).
Pressure gradient force is what causes the winds to blow.
High pressure over a short distance means higher wind speeds.
Greater pressure changes across shorter distances creates a larger PGF to initiate movement of winds.
Remember the flow from a High is down and out.

PGF vs Cyclonic Winds
Pressure gradient force (PGF) winds acting alone would head directly into low pressure.
Surface observations of winds, such as the cyclonic flow around this low, reveal that PGF winds are deflected by other forces.

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.

Coriolis Effect
While this force causes a right turn north of the equator, it causes a left turn for the south.
As you can see by the formula, it is influenced by the mass of the object, the earth’s angular rotation rate, wind speed, and latitude.
O is 7.29×10-5 radians per second (Earth’s angular Velocity)
The higher the latitude, the higher the Coriolis Force.
The faster the speed, the more distance it covers in a shorter time thus, the higher the Coriolis Force.

Actual and Observed Paths
Latitude effects Coriolis by increasing the angular deviation the higher the latitude.
Since the earth is a sphere, a straight line will only be congruent with the equator.
Anywhere north of the equator will have an ever greater degree of divergence from the starting line of latitude.
Since the 60° circle is smaller than the 30° circle, a tangent line drawn to it will represent a greater angle.
Therefore greater Coriolis angle due to latitude would be present the higher the latitude.

Geostrophic Wind
Geostrophic winds are winds where the pressure gradient force and Coriolis effect have found equilibrium and there is no net acceleration.
Winds have direction and magnitude, and can be depicted by vectors.
Observed wind vectors are explained by balancing the pressure gradient force and apparent Coriolis force.
These upper level geostrophic winds are parallel to pressure contours.
Surface friction causes a reduction in Coriolis force turning the winds 10° over water and as much as 45° over land to the isobars.
it’s a battle between pressure gradient force and Coriolis effect.
Pressure gradient’s ally is surface friction.
When the wind blows parallel to curved isobars it is called a gradient wind

Wind Speed and Pressure Contours
Just as a river speeds and slows when its banks narrow and expand, geostrophic winds blowing within pressure contours speed as contour intervals narrow, and slow as contour intervals widen.

Isobars and Wind Prediction
Upper level pressure maps, or isobars, enable prediction of upper level wind direction and speed.
In this snipit of an upper level contour map which way would you expect the wind to blow?
Where would it be the fastest

Centripetal Acceleration and Cyclones
Acceleration is defined by a change in wind direction or speed, and this occurs as winds circle around lows (cyclones) and highs (anticyclones).
Centripetal force is the term for the net force directing wind toward the center of a low, and results from an imbalance between the pressure gradient and Coriolis forces.
Centripetal force is greatest in tornadoes.
In this case PGF is way way stronger than Coriolis (because PGF acts inward in a low)
Remember, flow is reversed in a High so pressure gradient force moves outward.
In this case Coriolis must be stronger, however the only way Coriolis can be stronger is with faster winds so you might conclude that winds on a high are stronger than a low
Not the case! Usually isobars are more tightly packed on a low which gives a strong PGF

Friction and Surface Winds
Surface objects create frictional resistance to wind flow and slows the wind, diminishing the Coriolis force and enhancing the effect of pressure gradient forces.
The result is surface winds that cross isobars, blowing out from highs, and in toward lows.
The atmospheric layer influenced by friction is referred to as the friction layer or the planetary boundary layer.

Sensing Highs and Lows
The location of high and low pressure centers are estimated by detecting surface wind direction and noting pressure, Coriolis, and friction forces.
If you’re on the ground turn your back to the winds aloft, the low is on your left the high is on your right.
Do the hokey pokey cause that’s what its all about.

Vertical Air Motion
Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules.
These updrafts may cause cloud formation.
Likewise, diverging air molecules from a high pressure area are replenished by downward winds.

Single Cell Circulation Model
The basis for average air flow around the earth can be examined using a non-rotating, non-tilted, ocean covered earth.
Heating is more intense at the equator, which triggers Hadley cells to redistribute rising heat from the tropical low to the polar highs.

Three Cell Circulation Model
A rotating earth breaks the single cell into three cells.
The Hadley cell extends to the subtropics, the reverse flow Ferrel cell extends over the mid latitudes, and the Polar cell extends over the poles.
The Coriolis force generates westerlies and NE trade winds, and the polar front redistributes cold air.
As you can see, the equator has an area of low horizontal pressure gradients.
This combined with year round warmth and moisture can spawn huge cumulus clouds in the afternoon.
These thunderstorms are called convective hot towers and serve to drive the Hadely cell circulation.
Since the air doesn’t move much here this area is referred to as the Doldrums.

Subtropical Highs
As the air moves pole ward, it cools and piles up which causes a semi-permanent high to develop at 30° latitude.
As the air descends, adiabatic compressional heating takes effect and promotes generally clear skies.
Over land this means desert, over the ocean it means trouble for sailing ships.
In the old days the horses on board had to be thrown overboard or eaten, thus the term Horse Latitudes was born.
Yummy, horse meat.

Intertropical Convergence Zone
As the air at the horse latitudes descends, some air moves back toward the equator.
This air is deflected by Coriolis force causing the flow to be from the northeast above the equator and the southeast below the equator.
These winds are known as the trade winds.
Where the northeast trade winds collide with the southeast trades, the area is referred to as the ITCZ.
The ITCZ is an area of semi-permanent low.

The Prevailing Westerlies
Some of the air moves northward away from the equator toward the poles as it descends.
Again Coriolis takes over and turns the wind toward the west.
The U.S. lies in this area which provides us with the prevailing west wind that dominates our weather structure.
Of course the caveat to all of this discussion is that at times, surface highs and lows may change or divert this flow causing local deviations in the planetary wind model

Polar Flow
As the air travels northward, it encounters southward moving air from the poles at about 60° latitude.
Since the air moving north is mild and the air moving south is cold, they don’t mix very well.
This is called the Polar Front.
A zone of low pressure sets up as both air masses move upward, creating the subpolar low.
At this point, the air splits, some moving south and some north.
The south moving air meets up with the air at the horse latitudes and completes the Ferrell Cell circulation.
The north moving air moves toward the pole creating a Polar high.
As the air descends, Coriolis once again creates a deflection and the winds move to the southwest.
This circulation is the Polar Cell.

Observed Circulations in January
Observed average global pressure and winds have increased complexity due to continents and the tilted earth.
There are 4 semi-permanent pressure systems in the Northern Hemisphere.
The Bermuda-Azores High
The Pacific High
The Icelandic Low
The Aleutian Low
Differential ocean-land heating creates areas of semi-permanent high and low pressure that guide winds and redistribute heat.

Observed Winds in June
Global pressure and wind dynamics shift as the Northern Hemisphere tilts toward the sun, bringing the inter-tropical convergence zone, the Pacific high, and blocking highs in the southern oceans northward.
There are other systems that occur on a regular basis but are not classified as be semi-permanent.
The Siberian High
The Canadian High eh

Jet Stream
High velocity Polar and subtropical jet stream winds are located in the lower tropopause, and they oscillate along planetary ridges and troughs.
That puts them at 30,000 to 45,000 feet.
Jets also form in the stratosphere, upper mesosphere and thermosphere.
In addition, over the central plains of the U.S. a low level jet may form creating winds of 60 kts.

Jet Stream
The Subtropical Jet is situated where the Hadley and Ferrell Cells meet at roughly 30° latitude and exists at about 43,000 feet.
The Polar Front Jet hangs out at about 60° latitude and exists where the polar cell and the Ferrell cell meet.
Since they are both found at the tropopause, they are referred to as tropopause jets.
Sometimes the polar jet will split into two parts.

300mb Winds and Jets
300 mb pressure surface maps illustrate lines of equal wind speed (isotachs) as the jets meander.
Jet streaks are the maximum winds, exceeding 100 knots.
The worst turbulence induced by the jet occur on the polar side in sharp bends caused by surface lows and highs
Temperature differential is greatest on the polar side setting up a steep gradient

Simulation of Clear Air Turbulence
Clear turbulence is created by steep gradients of changing wind speed near the jet, called shearing winds, which generate fast flowing particles in thi
s simulation.
The blue is the jet while the yellow is high velocity turbulence.
The Rockies are shown in white.
Wouldn’t this be handy on your HUD?

Polar Jet Formation
Steep gradients of temperature change at the Polar front trigger steep pressure gradients, which then forces higher velocity geostrophic winds.
This is the trigger for jet stream flow.
However, this is just part of the story.

Wind and Angular Momentum
The conservation of angular momentum is the second player in the jet stream.
At the equator, the earth rotates close to 1000 kts.
Since the air above also rotates at roughly the same speed, it moves in a circular pattern and has angular momentum.

Wind and Angular Momentum
Angular momentum is the product of mass, velocity, and the radius of curvature and it must be conserved.
As air is heated along the equator it rises up to the top of the troposphere, where it is capped by the inversion of the stratosphere and spreads out laterally flowing northward.
Northward-flowing air experiences a smaller radius and since mass remains unchanged, velocity must increase to conserve the momentum.
During the winter months this process shifts south towards the equator
Temperatures drop in the north which intensifies the whole process
Thus the jet moves south in the winter and gets stronger due to higher pressure gradient force caused by the colder temperatures

If the parcel is warmer than the ambient air it will continue to rise – thus unstable
if the parcel is the same temp as the ambient air it is neutrally stable
if the parcel is cooler than the ambient air it will sink – thus stable
Warming from below will decrease stability
Cooling from below will increase stability
The lapse rate is how we measure stability

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

Temp and dew point converge at a rate of about 4.4º F or 2. 2º C per 1000 feet
Given: Temp 14º C Dew Point 3º C
11/2.2= 5000

1. Cumulus
Characterized by continuous updrafts
2. Mature
Characterized by the start of rain from the cloud base
Downdrafts have started
Cold rain within the cloud creates downdrafts
3. Dissipating
Characterized by downdrafts
Anvil head forms
Rain ends
Cloud dissipates

Types of Fog
1. Radiation fog
Clear, cool, calm nights
Terrestrial radiation
2. Advection fog
Needs a wind to exist
Common in coastal areas
3. Upslope fog
Needs a wind to exist
Air cooled adiabatically
4. Precipitation-induced fog
Found along frontal boundaries
Warm rain falling through cooler air
5. Ice fog
Found in the arctic
Vapor sublimates directly into ice crystals
6. Steam fog
Moisture evaporates into cooler air
Usually found over lakes
Calm winds

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