In-Class Notes for Meteorology

In-class Notes for Meteorology

LIFTING AGENTS

Orographic Lift:

Air blowing against a range of hills or mountains is forced upward into a region of lower pressure, expands and cools. Condensation will occur when the dew point is reached.

Frontal Lift:

A mass of warm air is forced aloft and rises over the cold air. The rising warm air cools by expansion and clouds are formed.

Convection:

As the sun heats the earth unevenly, rising currents will occur. This is convection. The rising air expands and cools. This produces condensation and cumuliform cloud forms at the top of the column of air. Further ascent and cooling causes rain.

Convergence:

Air is forced into a region, as at the center of a low, convergence is occurring. The excess air is forced to rise, as it rises it expands and cools. When the condensation level is reached, clouds form.

Turbulence:

As wind blows over an uneven or rough surface or when uneven heating occurs, vertical currents are created. Air in the upward motion cools by expansion and if enough moisture is there condensation occurs. This results in cumulus cloud.

Precipitation and Cloud Type:

Ice Prisms

Heavy cumulus

TYPES OF FOG:

Radiation fog:

Usually forms on clear nights. Conditions that favor the formation of radiation fog are:

• clear skies
• moist air
• light winds

The ground cools losing heat through radiation. The air in direct contact with the earth's surface is cooled. If this air is moist and the temperature is lowered to the dew point, fog will form.

Advection fog:

Forms when warm moist air moves over a cold surface.

Upslope fog:

Caused by the cooling of air due to expansion as it moves up a slope. A light upslope wind is necessary.

Steam fog:

Forms when cold air passes over a warm water surface. Evaporation of the water into the cold air occurs until the air becomes saturated. The excess water vapour condenses as fog.

Frontal fog:

Often found near fronts. Warm front fogs are the most extensive and are caused by the cold air becoming saturated by evaporation from rain falling from the warm air. It is also known as precipitation-induced fog.

Precipitation-induced fog:

Caused by the addition of moisture to the air through evaporation of precipitation. (Example – pre frontal fog)

Ice fog:

At very low temperatures, the air may become full of ice crystals. The ice crystals, which form by sublimation, may suddenly appear when an aircraft engine has been started (the exhaust contains water vapour, condensation nuclei and the engine creates mixing).

THUNDERSTORMS HAZARDS

TURBULENCE

• could overstress aircraft or cause loss of control
• downdrafts as strong as 2000 feet per minute and updrafts as strong as 6000 feet per minute that can severely cripple aircraft
• strongest between 12,000 – 20,000 feet in mature stage
• can be experienced, in clear air up, to 20 miles away from severe cells
• can be severe in micro/macrobursts and at gust front.

WINDS

• danger of gusts up to 80kts with rapid changes in direction
• gust front generates strong, gusty winds near the surface which can change direction by 180° and gust up to 50 kts in seconds.

HAIL

• could cause serious structural damage
• can be encountered outside of cloud as it is thrown upward and outward by active cells.

ICING

• abundance of supercooled water droplets, will cause severe icing
• most severe during the mature stage.

LIGHTNING

• hampers vision for 30 – 50 seconds at a time
• greatest likelihood of strike at temperatures between -5C and + 5C)
• solid state circuitry is particularly vulnerable to strikes
• electrical circuitry may be disrupted
• possibility of igniting fuel vapour in fuel cells.

PRESSURE

• rapid changes in pressure cause unreliable altimeter readings.

TYPES OF ICING:

Frozen Dew

• Sometimes dew will form on an aircraft parked outside at night. If the aircraft skin temperature falls below freezing this dew will freeze
• Must be removed before take-off

Hoar Frost

• A white, feathery, crystalline formation that covers the entire surface of the aircraft
• Forms by sublimation on cold clear nights
• Must be removed before take-off since it can increase stall speed and reduce lift
• Can form in clear air when a cold aircraft enters warmer, damper air during a steep descent
• May obscure vision by coating the windshield

Rime Ice

• An opaque or milky white ice that forms on an aircraft

• Forms by the almost instantaneous freezing of small supercooled water droplets
• Has no great weight
• Alters the aerodynamics of the airfoils
• Chokes off the orifices of the carburetor and the instruments

 Is very brittle and easily dislodged by de-icing equipment

Clear Ice

• A coating of glassy like ice
• Forms as large supercooled water droplets freeze slowly and spread
• Can form a strong, solid sheet of ice which is difficult to dislodge
• Can increase drag by as much as 300% to 500%
• Increases the weight of the aircraft
• Disrupts the smooth air flow over the wings and tail surfaces decreasing lift
• Unequal loading may cause vibrations
• As large blocks break off, the structure of the aircraft may be impaired.

TYPES/CAUSES OF TURBULENCE

Mechanical Turbulence (EDDIES):

Friction between the air and the ground especially from irregular terrain and man made obstacles, causes “EDDIES”

Thermal Turbulence:

Happens on hot sunny days when the sun heats the earth’s surface unevenly. Isolated convection currents are set in motion and this results in turbulence.

Frontal Turbulence:

The lifting of the warm air by the sloping frontal surface and friction between the two opposing air masses produce turbulence in the frontal zone.

Wind Shear:

Marked changes in wind direction and speed either vertical or horizontal. If the change in wind is great you can expect severe turbulence.

Weather Signs

Look For Unsettled Weather When:barometer is falling

• temperature at night is higher is than usual
• clouds move in different directions at different levels
• cirrus clouds increase. A large ring appears around the sun or moon and stays there until the overcast clouds thicken and obscure the sun or moon

• summer afternoon clouds darken.

Look For Steady Precipitation When:

• there have been signs of unsettled weather
• the wind is southeast to northeast and the pressure is falling. Rain or snow within a day if the pressure is falling slowly. If falling rapidly, rain or snow soon, with strengthening winds.

Look For Showers When:

• cumulus clouds develop in a westerly wind
• cumulus clouds develop rapidly in spring or summer early afternoon

Look For Clearing Weather When:

• the barometer rises
• wind shifts into west or northwest
• the temperature falls

Look For Continued Bright Weather When:

• sun sets like a ball of fire
• barometer is steady or slowly rising
• afternoon cloudiness decreases after 3PM to 4PM
• morning fog breaks within two hours after sunrise
• light breeze from west or northwest
• there is a red sunset

Sea Level Pressure (definition)

Sea level pressure in the atmospheric pressure at sea level or at an aerodrome. SLP will vary from aerodrome to aerodrome depending on the elevation.