Meteorology II

METEOROLOGY –LOCAL WIND FORCES


*Currents generated by varying surface conditions.


Effect of Obstructions on Wind
Another atmospheric hazard exists that can create problems for pilots. Obstructions on the ground affect the flow of wind and can be an unseen danger. Ground topography and largebuildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. These obstructions range from manmade structures like hangars to large natural obstructions, such as mountains, bluffs, or canyons. It is especially important to be vigilant when flying in or out of airports that have large buildings or natural obstructions located near the runway.

*Turbulence caused by manmade obstructions.

Mountains:  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.

Due to the effect terrain has on the wind in valleys or canyons,downdrafts can be severe. Before conducting a flight in or near mountainous terrain, it is helpful for a pilotunfamiliar with a mountainous area to get a checkout with a mountain qualified flight instructor.

*Turbulence in mountainous regions.

COLD AND WARM DOWNSLOPE WINDS (KATABATIC) CAN BE UP TO 50 KNOTS.

DO NOT FLY IN OR NEAR MOUNTIANS WITHOUT SPECIAL FLIGHT INSTRUCTION. SERIOUS CONSEQUENCES IF YOU ARE NOT FULLY PREPARED FOR THIS AND OTHER HAZARDS (I.E. LENTICULAR CLOUDS)

METEOROLOGY –WEATHER PATTERNS STABILITY 

Stability is the resistance to vertical motion.  Florida is not a stable environment except during winter. CONVECTION (heat rising and producing high instability) Stable air cools at a rate less than standard lapse rate, unstable air cools at rate greater than standard lapse rate.

INVERSIONS work backwards.   Increase temperature with altitude to the “roof”of the inversion layer.

Atmospheric Stability

The stability of the atmosphere depends on its ability to resistvertical motion. It is determined by the “Adiabatic”lapse rate 

Adiabatic heating and adiabatic cooling 

Rising air expands and cools due to the decrease in air pressureas altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed.

The rate at which temperature decreases with an increase in altitude is referred to as its lapse rate. As air ascends through the atmosphere, the average adiabatic lapse rate of temperature change is 2 °C (3.5 °F) per 1,000 feet

Since water vapor is lighter than air, moisture decreases air density, causing it to rise. Conversely, as moisture decreases, air becomes denser and tends to sink. Since moist aircools at a slower rate, it is generally less stable than dry air since the moist air must rise higher before its temperature cools to that of the surrounding air. The dry adiabatic lapse rate (unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 feet.

Inversions 

Inversion layers are commonly shallow layers of smooth, stable air close to the ground. The temperature of the air increases with altitude to a certain point, which is the top of the inversion. The air at the top of the layer acts as a lid, keeping weather and pollutants trapped below. If the relative humidity of the air is high, it can contribute to the formation of clouds, fog, haze, or smoke, resulting in diminished visibility in the inversion layer.

Characteristicsof stable and unstable air

METEOROLOGY –WEATHER PATTERNS MOISTURE

EVAPORATION	Liquid to Vapor
CONDENSATION	Vapor to Liquid
SUBLIMATION	Ice to Vapor
DEPOSITION	Vapor to Ice
FREEZING	Water to Ice
MELTING	Ice to Water

HUMIDITY–Amount of moisture in air.  “Relative Humidity”is the actual moisture compared to the total that could be present in the air.

DEWPOINT–Temperature where air can no longer hold water (just above +4F,expect fog) Calculation for expected cloud base AGL= ((Temp(F) –Dew point(F)) / 4.4) * 1000

We divide by 4 as an average because unsaturated air cools at 3/1000ft, two point decreases at 1/1000 ft.  Thus in convective current the dew point converges at about 4.4 degrees (2.5C)/1000 ft Ex: temp 82F, dewpoint 62 [diff=20). Divide by 4=5 * 1000 give expected level clouds at 5000 AGL.

FOG OCCURS WHEN TEMPERATURE IS WITHIN 2 DEGREES OF DEWPOINT !! Fog is a cloud that begins within 50 feet of the surface.  More later on Fog.

METEOROLOGY –WEATHER PATTERNS CLOUDS

Stratus clouds (Stratoform) -no vertical development –flat but can be puffy 

Cumulous clouds (Cumuloform) -vertical development –puffy 

Cirrus clouds –very, very high and composed of ice –wisp appearance.

Low Level clouds –SL to 6,500 feet 

Middle Level clouds –6,500 to 20,000 feet  (clouds usually prefixed “alto” 

High Level clouds –above 20,000 feet  (clouds always cirrus [cirroform])

•Cumulus—heaped or piled clouds 

•Stratus—formed in layers 

•Cirrus—ringlets, fibrous clouds, also high level clouds above 20,000 feet 

•Castellanus—common base with separate vertical development, castle-like 

•Lenticularus—lens shaped, formed over mountains in strong winds 

•Nimbus—rain-bearing clouds 

•Fracto—ragged or broken 

•Alto—meaning high, also middle level clouds existing at 5,000 to 20,000 feet

BASIC CLOUD TYPES

METEOROLOGY –WEATHER PATTERNS LOW LEVEL CLOUDS

Ice FogTemperature is much below freezing (-25F) and water vapor forms directly into ice crystals.

“Aspertus”-1stdiscovered 2003 –Stratoform, now more prevalent in Midwest and other parts of the world.

You can learn by yourself with the material below:
- Pilot's Handbook of Aeronautical Knowledge [PHAK]
- Online Meteorology Guide

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