MOUNTAIN FLYING Dr. Chip West National Weather Service

TERRAIN

WEATHER-RELATED ACCIDENTS NTSB Accident Data for 1995 Obscurations Thunderstorms Icing High Density Altitude Turbulence Low Ceilings Reduced Visibilities Adverse Winds Fatal AccidentsNon-Fatal Accidents

Valley Fog and Stratus Diurnal Cycle Outside vs Valley Observations Season – Fall to Spring

Fog and Stratus

Preflight.Observations – sources.ASOS/AWOS limitations.Valley fog/stratus.Winds aloft.Terrain effects on wind.Frost

Observations Types –ASOS –AWOS ASOS –“tunnel view –“Clr” < 12,000ft. –Cloud heights AGL –Point visibility

Weather Takeoff Factors Altitude Temperature Humidity Wind direction Worse case combination?

Enroute –To Where? Always leave yourself the option of turning to lower terrain or back Forecast/observed cloud heights vs wind direction and weather patterns Terrain/wind induced turbulence Mountain wave Venturi effect Valley winds Icing Approaching/crossing ridges How to fly valleys

Terrain effects on wind Up to a doubling of reported speeds when flow channeled between peaks Up to a doubling of reported speed when forced over or around a peak, ex. Grandfather mountain. Diurnal effects –Morning and evening winds

Terrain Effects on Wind Doubling or more of speed around peaks or between peaks

Turbulence over Ridges < 20 kts = Light to Moderate 20 – 30 kts = Moderate to occasional Severe > 30 kts = Moderate or frequent Severe

Mountain Waves

MECHANICAL TURBULENCE MOUNTAIN WAVE FAVORABLE CONDITIONS -WINDS 25 KTS OR GREATER ACROSS -WINDS INCREASING WITH HEIGHT -STABLE ATMOSPHERE STRATOSPHERE TROPOPAUSE TROPOSPHERE WAVE LENGTH 5 TO 30 MILES LENTICULAR CLOUD ROTOR TURBULENCE CAP CLOUD WIND ’s OF FEET

MOUNTAIN WAVE CLOUDS CCSL ACSL ROTOR

RESPONSE ITEM THE EFFECTS OF MOUNTAIN WAVES OFTEN EXTEND FROM THE SURFACE TO A.LESS THAN 5,000 FEET. B.THE ALTITUDE OF THE MOUNTAIN CREST. C.SLIGHTLY ABOVE THE TROPOPAUSE. A.LESS THAN 5,000 FEET. B.THE ALTITUDE OF THE MOUNTAIN CREST. C.SLIGHTLY ABOVE THE TROPOPAUSE.

Crossing Ridges

How to fly valleys

DOWNSLOPE FLOW WIND WINDWARD LEEWARD FAVORABLE CONDITIONS WINDS MODERATE OR GREATER ACROSS MOUNTAINS

RESPONSE ITEM IN ORDER TO AVOID AN ACCIDENT BY RAPIDLY DESCENDING INTO THE MOUNTAIN THE PILOT SHOULD: A. CLIMB 5,000 FEET OR MORE ABOVE CREST. B. DECREASE SPEED AT CREST HEIGHT. C. FLY AT CREST HEIGHT BUT STAY OUT OF DOWNDRAFT. A. CLIMB 5,000 FEET OR MORE ABOVE CREST. B. DECREASE SPEED AT CREST HEIGHT. C. FLY AT CREST HEIGHT BUT STAY OUT OF DOWNDRAFT.

Mountain Winds

Anabatic Wind or Updraft Wind Sun’s heating

Valley Wind Diurnal heating induces up valley wind

Mountain Breeze Evening cooling induces down valley or drainage winds

WARM COLD FRONTS WIND SHEAR TURBULENCE

FRONTS MECHANICAL TURBULENCE STRONG GUSTY SURFACE WINDS

Arrival/Landing “Localized” weather Wind, effects of local terrain on Time of day Surrounding terrain

Safety Summary Always remain in a position so you can turn toward lowering terrain – 45 & down Never fly beyond the point of no return – 180, idle