L12 Flight Over Mountainous Areas and Other Weather Hazards
Flight Over Mountainous Areas and Other Weather Hazards
Influence of Terrain on Atmospheric Processes
Meteorology typically deals with large-scale atmospheric conditions, but local factors like terrain can significantly modify weather.
Mountainous areas are particularly effective at altering weather patterns and creating localized conditions.
Rising ground forces air to ascend, leading to orographic lift.
Orographic lift can trigger precipitation, causing cloud bases to lower rapidly.
This phenomenon contributes to weather-related accidents where pilots encounter terrain unexpectedly due to reduced visibility and icing, leading to decreased aircraft performance.
Pilots flying into the ground due to terrain being hard to see results in controlled flight into terrain (CFIT).
Valley Winds and Inversions
Mountains and valleys can redirect airflow, either veering or backing the wind's direction.
Wind speed can increase considerably when funneled through a valley.
Examples of such winds include the Mistral in the Rhone valley and the Tramontane affecting the Spanish and French Mediterranean coast.
Katabatic winds flow down valley slopes, displacing warmer air and causing temperature inversions.
These temperature inversions can lead to radiation fog formation and persistence in the valley, even when conditions are clear elsewhere.
Temperature inversions decrease air density, reducing aircraft performance by affecting lift and engine thrust, requiring higher airspeeds.
Mountain Waves
Wind blowing over a mountain range can create large-scale atmospheric disturbances known as mountain waves, standing waves, or lee waves.
Mountains cause the airflow to undulate, creating waves in the atmosphere.
Glider pilots utilize the upward motion of mountain waves to gain altitude.
However, the downward motion of these waves can force aircraft to descend, even at full power and best climb airspeed, potentially reaching ground level.
Conditions favorable for mountain wave formation:
Wind speed of 15 kts or more at the surface, blowing at approximately right angles (or within 30°) to the ridge or mountain-range axis. Wind strength increasing with height, but consistent direction.
A stable air layer just above the ridge, bounded by less stable air. This could manifest as an inversion just above the ridge, more common in anticyclonic conditions.
Downdrafts on the lee side of high ground can exceed the best rate-of-climb performance of a light aircraft.
Clouds Associated with Mountain Waves
The presence of a mountain wave may not be immediately apparent until the aircraft's climb or descent rate deviates unexpectedly.
Lenticular clouds often form in mountain waves; these are smooth, elongated clouds that appear stationary at the crest of the wave.
Lenticular clouds form at the leading edge and dissipate at the trailing edge of the wave.
In favorable conditions, multiple lenticular clouds can stack on top of each other.
Venturi Effect
Daniel Bernoulli and Giovanni Venturi are key figures in understanding airflow over high ground.
Bernoulli's principle states that an increase in the speed of a fluid (air) is accompanied by a decrease in pressure.
A venturi is a constricted tube that forces air through a smaller opening, increasing its speed and reducing its pressure.
High ground acts as a large-scale venturi; the same quantity of air is forced through a narrower vertical gap above the mountain, increasing wind speed.
This relates to 'Topographical Altimetry Error,' where a reduction in pressure causes the altimeter to overread when flying over high ground.
Rotors
Flight within a mountain wave itself is usually smooth.
However, severe turbulence can occur in the rotor zone that forms downwind of high ground, beneath the crest of the waves.
A roll cloud, a ragged cumulus cloud, can indicate the presence of a rotor.
Roll clouds rotate around a horizontal axis, indicating violent turbulence.
The most intense turbulence is typically found in the first rotor downwind of the ridge.
The turbulence within a rotor can be strong enough to destroy a powered aircraft or glider.
Even in light wind conditions, the area downwind and below the level of a ridge is a dangerous area to avoid.
Safe Terrain Clearance Altitudes
Adequate altitude is the primary safety measure when flying over or near high ground in conditions conducive to mountain waves.
A rule of thumb is to double the elevation of the high ground to determine the minimum safe altitude.
Crossing high ground at an angle or parallel to a ridge (especially downwind) prolongs exposure to wave conditions.
Mountains and strong winds create a hazardous combination.
Thunderstorms over high ground are particularly dangerous due to the combination of thunderstorm hazards and mountainous terrain.
Orographic cloud formation increases the likelihood and severity of icing.
Foehn Effect
The Foehn (or Fohn or Föhn) effect describes conditions on the downwind side of a mountain range.
If airflow is moist and causes precipitation on the windward side, the downwind side typically experiences a higher cloud base, higher temperatures, and less precipitation.
Foehn conditions develop with stable air and widespread airflow against a mountain ridge.
The Foehn effect illustrates the difference between saturated adiabatic lapse rate (SALR) and dry adiabatic lapse rate (DALR).
When moist air rises to meet a mountain it cools at the DALR (approximately 3^{\circ}C per 1000 ft) until reaching its dew point and forming clouds.
If the air continues to rise it cools at the SALR (approximately 1.5^{\circ}C per 1000 ft).
Precipitation may occur on the windward side.
As air descends on the leeward side, it warms at the DALR (3^{\circ}C per 1000 ft) after passing its dew point.
The result is warmer and drier conditions on the leeward side compared to the windward side; this area is known as the 'rain shadow'.
Other Weather Hazards
Strong winds are associated with a steep pressure gradient.
The general advice to pilots is not to taxy, take-off, or land if the surface wind speed exceeds 50% of the aircraft's stalling speed.
For most light aircraft, surface winds above approximately 25 kts require extreme caution.
Most light aircraft have lower demonstrated crosswind limits.
Rapidly deepening lows (pressure decreasing by more than 1 mb/hPa per hour for more than twelve hours) are likely to produce especially strong winds.
Mountain waves can form with stable atmospheric conditions and a moderate wind flowing at approximately 90 degrees to a peak or ridge line.
Mountain side updrafts can occur due to solar radiation on the windward side of mountains.
Turbulence
Turbulence is any disruption of airflow that causes eddies and variations.
Turbulence becomes problematic when it affects aircraft handling or causes discomfort to occupants.
Low-level turbulence is generally caused by convective (thermal) activity or frictional (mechanical) forces.
Convective turbulence is most severe in light winds with strong surface heating, especially over urban areas or industrial sites.
Rising convection currents (thermals) may be capped by cumulus clouds.
Mechanical turbulence is most pronounced in strong wind conditions over irregular terrain, particularly downwind of hills or obstructions.
Increasing altitude typically leads to smoother air.
The aircraft's POH/FM will state a recommended turbulence or 'rough air' speed (V_a) for use during turbulence.
In turbulence, maintain the correct attitude and power setting for the desired performance, accepting minor airspeed fluctuations.
Turbulence can be problematic during final approach or just after take-off due to low-level airflow around buildings, trees, or local terrain.
Pilots should expect turbulence and windshear at their local airfield when the wind blows from certain directions.
A slight (5-10 kts) increase in approach speed is a worthwhile precaution when landing in turbulent conditions, provided runway length is sufficient.
Windshear
Windshear is the change of wind velocity over a distance.
Windshear becomes a significant hazard when there is a marked change in wind velocity within a small height band.
A strong windshear can dramatically affect an aircraft's airspeed.
Example: An aircraft transitioning from a 20 kts headwind to a 10 kts tailwind in a 100 ft height band.
As an aircraft descends at 90 kts airspeed with a 20 kts headwind, its groundspeed is 70 kts.
Upon encountering the windshear, the headwind becomes a tailwind; due to inertia, the groundspeed remains briefly at 70 kts, causing the airspeed to drop suddenly to 60 kts.
This loss of airspeed leads to a loss of lift and potential stall, which can be difficult or impossible to recover from at low altitude.
Heavier aircraft are more vulnerable to windshear.
Windshear and Thunderstorms
Windshear often occurs in association with thunderstorms and widespread strong winds.
Windshear can also develop with a marked temperature inversion near the surface.
Such inversions can occur on clear nights when surface air cools while air higher in the atmosphere remains warmer.
This separates the friction layer from the general airflow, potentially leading to strong winds at low altitude but calm conditions at the surface.
Upper wind forecasts may indicate the possibility of stronger winds above, even if surface winds are calm.
Turbulence and windshear may be marked when passing through the transition zone, which can be as low as a few hundred feet above the surface.
Windshear and Temperature Inversions
Low-level windshear is most often associated with thunderstorms, marked temperature inversions, and strong winds, especially in terrain that encourages its formation (valleys, ridges).
The primary strategy for dealing with windshear is avoidance.
If avoidance is not possible, approach at a slightly higher airspeed.
When experiencing a temperature inversion, the addition of wind shear adds another hazard as air density reduces and lift decreases.
Be prepared to execute an early go-around if windshear is expected or encountered.