Weather Theory
Objective
Motivation
Timings
Elements
- Composition of the atmopshere
- Layers of the atmopshere
- Constant motion of the atmosphere
- Hot air = lower pressure = rises
- Ambient pressure: ~14.7 psi at sea level
- Barometer: Measures pressure
- International standard atmosphere is a common reference
- Barometric pressure decreases with altitude
- On average, 1" Hg per 1000'
- Wings, engine, propellers are all less efficient with less pressure
- In the northern hemisphere:
- High to low pressure, clockwise: anticyclonic circulation
- Low to high pressure, counterclockwise: cyclonic
- Convective heating of the air
- Paved areas, plowed fields, dirt absorb and give off more heat quickly
- Trees, water, vegetation give off heat more slowly
- This uneven heating makes more warm pockets of air that cause turbulence
- Sea breeze vs land breeze
- Sea breeze: Land heats quickly during the day, the air heats and rises, creating low pressure which draws cool air from the sea near the surface (on-shore breeze)
- Land breeze: Land cools faster at night, so moist warm air over the water rises creating low pressure, which draws air off-shore (off-shore breeze)
- Turbulent flows created by obstructions
- Near the ground: hangars, buildings
- Mountains, ridges, bluffs
- Low-level wind shear
- Microbursts: convective
- Advisory Circular (AC) 00-54, FAA Pilot Wind Shear Guide
- Surface pressure maps
- Closer the contours: Greater the pressure differential
- Surface wind speeds are less
- Atmospheric stability
- The ability of the atmosphere to resist or encourage vertical motion
- Adibatic cooling: Air loses temperature as it rises, since the ambient pressure decreases
- The average lapse rate is 2° per 1000'
- Moisture decrease air density. Moist air cools at a slower rate
- The dry adiabatic lapse rate is 3 °C (5.4 °F) per 1,000 feet
- In summary:
- Cool, dry air is very stable and resists vertical movement
- Warm, moist air produces the most instability
- Temperature inversions: Layers where temperature increases with altitude
- Often occurs on clear, cool nights, when the ground cools the air above it
- Can trap pollutants
- Moisture
- Every 20°F increase in temperature increases the capacity of water the air can hold
- The relative humidity is the percentage of water present vs the total amount the air could hold
- Dewpoint is the point at which the air would be completely saturated by the current level of humidity
- Clouds often form when unstable air rises and cools to the dewpoint
- Saturated air bring clouds, fog, and precipitation
- Dew and frost: form when surfaces cool beyond the dewpoint and water condenses on the side
- Fog: Ground clouds that form when the ground temperature is low
- Radiation fog: Clear, windless nights, the ground cools and cools the air above it
- Advection fog: Warm, moist air moves over a colder surface at night. This requires wind (usually up to 15 knots) to move the air
- Upslope fog: Forms when air is forced up a slope and cools
- Steam fog: Cold, dry air moves over water
- Clouds
- Three ingredients for clouds to form: Moisture, cooling, condensation nuclei
- Moisture condenses onto minicsule particles of matter
- Low clouds, middle clouds, high clouds
- Towering cumulus clouds contain very turbulent air and potential for thunderstorms
- Air masses
- Form from large "source regions", where conditions may develop for days (deserts, oceans, large lakes, polar caps)
- Cold over warm: unstable
- Warm over cold: stable
- Fronts
- Fronts are boundaries between two different air masses
- As a front pass, the pressure will rise and climb, the temperature will change, and the wind direction will change
- Warm front
- Warm, often moist air that slides slowly over a colder air mass (shallow frontal slope)
- Ahead of the front, cirriform or stratiform clouds and light precipitation
- Poor visibility, haze as the front passes
- Cold front
- Cold, dense, stable air advances and quickly slide under and replaces a warmer air mass (steep frontal slope)
- Prior the passage of a warm front, cumulonimbus clouds are common
- A fast-moving cold front may produce a concentrated band of precipitin and thunder storms
- Squall lines may form ahead of a fast-moving cold front
- Stationary front
- Air masses with relatively equal forces can remain stationary for several days
- Weather is a mixture of cold front/warm front conditions
- Occluded front
- Occurs when a cold front overtakes a slow-moving warm front
- Cold-front occulusion: Fast-moving cold front air is colder than the cooler air ahead of the warm front
- Mixture of cold/warm front weather, relatively stable
- Warm-front occulusion: Fast-moving cold front air is warmer than the cold air ahead of the warm front
- The cold front "rides up" over the warm front, and instability occurs. This can sauce severe thunderstorms, rain, and fog
- Thunderstorms
- Three things for a thunderstorm for form:
- Instability
- Lifting action
- Moisture
- Three distinct stages:
- Cumulus stage: Air rises, strong updrafts occur
- Mature stage:
- Moisture is too heavy for cloud to support, precipitation starts falling, this causes a downdraft
- Vertical motion is stalled, and top of the cloud forms the anvil shape
- Dissipating stage:
- Downdrafts spread and replace updrafts
- It is impossible to fly over most thunderstorms, especially in the light aircraft
- Circumnavigate a thunderstorm radar echo by at least 20nm
- Hazards
- Heavy rain
- Engine water ingestion
- Hail which may be thrown miles from the storm
- Violent turbulence in the storm and in the vicinity
- Wind shear turbulence and microbursts near the surface
- Supercooled water droplets that can freeze on impact with an airplane
- Lightning can be temporarily blinding and induce radio interference and magnetic compass errors
References
Pilot's Handbook of Aeronautical Knowledge pg. 12-1