Aviation Meteorology Part 1

1. Aviation Meteorology Part 1 AVN-1020 Fall 2010 Session 12

2. Overview • Winds • Atmospheric Stability • Clouds • Precipitation • Weather Hazards

3. The Atmosphere… • Composition: • 78% Nitrogen • 21% Oxygen • 1% Trace gases • Water vapor • Weather caused by: • Earth’s rotation • Differential heating from angle of sunlight (latitudes) • Differential heating due to surface absorbency • Water cycle (evaporation, condensation, etc.)

4. Atmospheric Layers • Troposphere - Surface to 5-8 miles; majority of weather • Tropopause - A thin, confining layer at the top of the troposphere - Keeps most water vapor & weather below • Stratosphere - Extends from tropopause to 19-22 miles - Certain types of clouds occasionally extend into it • Mesosphere & Thermosphere - Have little practical influence over weather

5. Atmosphere in Motion… • Convection - Circulating flow that results when warm air rises and is replaced by cooler air • Atmospheric Pressure - Unequal heating of the surface also causes changes in pressure - Points of equal pressure plotted /connected on weather maps are called: ISOBARS • Pressure Gradient: • Isobars far apart = “weak” or “shallow” gradient • Isobars close together = “strong” or “steep” gradient

6. Wind and Pressure Gradient • Weak pressure gradients = light winds • Strong pressure gradients = high winds

7. Coriolis Force • A deflective force that is created by the difference in rotational velocity over the earth • Equator rotates quickly • Mid latitudes rotate moderately • Poles rotate slowly • Without rotation, pressure gradient would propel wind directly from highs to lows • Coriolis force causes winds aloft to flow more parallel to isobars

8. Prevailing Wind Patterns

9. Pressure Systems • High: H - Circular area of high pressure surrounded by lower pressure - Good visibility, calm or light winds, & few clouds • Low: L - Circular area of low pressure surrounded by higher pressure - Low clouds, poor visibility, precipitation, gusty winds, and turbulence • Ridge - Elongated area of High pressure - Normally good weather • Trough - Elongated area of Low pressure - Weather may be very violent in areas of a trough

10. Surface Friction Effects

11. Combined Macro-Wind Effects • Pressure gradients – perpendicular to isobars • Coriolis force – shifts toward parallel • Surface friction – further shifts • In Summary (in Northern Hemisphere): • High Pressure System/Ridge Wind Flow: • Downward, outward and clockwise around • Low Pressure System/Trough Wind Flow: • Upward, inward and counterclockwise around • Friction causes winds to shift even more the closer we get to the surface

12. Micro-Wind Effects: Sea Breezes

13. Micro-Wind Effects: Land Breeze

14. Micro: Valley / Mountain Breezes

15. Water: Changes of States • Three states of water: • Solid: ice • Liquid: water • Gas: water vapor • Six change of states: • Evaporation: liquid to gas • Condensation: gas to liquid • Melting: solid to liquid • Freezing: liquid to solid • Sublimation: solid to gas /or/ gas to solid

16. Specific Heat • Specific heat: • Amount of heat required to change parcel 1o C • Specific Heat of water greater than N2/O2 mix • Result: moist air more difficult to cool or warm • Larger heat transfer in change of state This is why deserts have broad temperature swings from day to night

17. Relative Humidity • Percentagemeasure of amount of water vapor present compared to capacity of air to hold • Not a quantitative measure (ie, pounds per cubic foot) RH = Actual amount of moisture in air Total capacity • Amount of moisture air holds depends on temperature • Capacity roughly doubles every 11°C • Ex: water content RH 50% , 20°C = RH 100%, 9°C • If RH > 100%, air cannot hold excess water: • Condensation or sublimation 17

18. Dew-Point • Definition: Temperature at which an air parcel, if cooled, reaches 100% RH, or saturation point • Indicative of cloud / fog formation • Temp/dew-point spread within 4-5o F and decreasing • Easier with condensation nuclei present • Also indicative of Carburetor ICING conditions • Dew / frost / clouds: • Dew if collecting surface above freezing • Frost if collecting surface below freezing • Cloud if airborne particle 18

19. Atmospheric Stability • Stability: • The atmosphere’s resistance to vertical motion • Inhibits cloud formation, precipitation, and severe weather • Temperature & humidity directly influence stability • Heating from below causes decrease in density and tendency for air to rise • Adding moisture decreases density and causes air to rise • Greatest stability: • Dry air at lower temperatures • Greatest instability: • Saturated air at moderate & high temperatures

20. Pressure, Temp and Altitude • As air rises, pressure decreases due to gravity • As pressure decreases, temperature falls (Bernoulli) • As a result, when a parcel of air rises… • Pressure decreases • Volume Increases • Temperature Decreases* • Big variable is change of temperature, which is affected by overlying air masses and moisture content • Described by LAPSE RATE

21. Lapse Rates • Air’s lapse rate varies with moisture content: • Dry adiabatic: 5.4°F per 1,000ft • Moist varies: 2°F – 5°F per 1,000ft • Standard ICAO lapse rates: • Based on 58°F, 29.92” Hg at sea level • 3.5°F per 1,000 ft • 1” Hg per 1,000 ft • “K Index” • Comparative measure of stability • Compares actual lapse rate to standard lapse rate • Higher the number, the more unstable the air

22. Temperature Inversions • When normal lapse rate reverses • Gets warmer as altitude increases • Normally reverses at Tropopause (20-65K) • Higher during summer, lower during winter • Surface inversions • Clear, cool, calm nights - cooling via radiation • Cold air trapped in valleys by overlying warm air • Frontal Inversions • Warm air spreads over cold