Weatherwatch with Irv Lee

Topography

Lenticular clouds downwind of a mountain range. Beware severe turbulence in 'rotors' which can form at the bottom of the wave Photo: Irv Lee

A trick often missed by the powered pilot when navigation planning is the effect of hill and mountain ranges on the weather.

Depending on wind direction and stability of the atmosphere, the topography can have an marked effect on the comfort and even the safety of a flight.

Metform 214 with wind vectors and temperature by altitude, can be compared to the height and direction of hills or mountain ranges near, or upwind of your route. Look for the wind crossing a range approximately perpendicularly to the general direction of ridge line.

Atmospheric stability can also be assessed from Form 214. The more the temperature drops per 1000 feet, the more unstable the atmosphere. In unstable conditions, the vertical movement of humid air from even small hills can be the trigger for cumulus clouds downwind. At worst, towering cumulus and thunderstorms can result.

For example, take the North-South Pennines in a westerly airflow. With an unstable atmosphere, or weak 'breakable' inversion, the hills themselves and Yorkshire on the lee side would have large cumulus clouds, giving isolated heavy showers or even thunderstorms. Lancashire on the windward side might escape this completely, unless triggered by other factors.

In stable conditions, the airflow's vertical boost from the hills is quickly suppressed, and cloud is formed as the air ascends the hills, giving thick stratus on the windward side and over the top of the hills.

This gives widespread drizzle or light rain on the Lancashire side. It would not be unusual for the plain of York to escape rain, as the air is drier and slightly warmer by the time it descends.

In fairly constant strong airflow, with a reasonably stable atmosphere, pilots must be extremely wary of 'mountain waves' for many miles downwind of any high ranges (as found in Wales and Scotland).

In stable conditions, or if held down by a much higher inversion level, the wind undulates in a 'sine wave' flow after crossing the ranges. These waves can be visualised as bands of successive downflows and upflows parallel to the range on the leeward side.

Between the upflow and downflows are areas of extremely dangerous turbulence called 'rotors'. If humidity conditions are right, these rotors lying parallel to the mountain range, will be marked by caps of stationary lenticular cloud directly above, as airflow changes from upflow to downflow.

Rotors are not the only dangerous features on the lee side of mountains in stable strong wind conditions. The downflow areas themselves can be so strong, they are quite capable of overcoming the full power climb capabilities of light aircraft. If caught in such a downflow, the only way out is downwind. Upwind lies either the next rotor region or the mountain itself.

In stable conditions with a reasonably strong wind, where flight is envisaged parallel to ranges, the safer side is the windward side. If this is not possible, the 'lane' halfway between lines of lenticular clouds should have the least troublesome air. If crossing the ridge either way, keep well above to avoid the lee side downflow.

It's not just the higher ranges which cause pilots problems. The effects of long low ridges should be considered too. Take the South Downs in a southerly airflow from a continental blocking high pressure. Air will be fairly dry, having crossed land en route to the UK. Conditions are ideal for an inversion level higher than the low South Downs, with visibility below it poor due to haze.

These are exactly the warm stable days when pilots set off on longer journeys, heading east or west across southern England for a distant destination. Routes north of the Downs mean that controlled airspace keeps the pilot completely under the inversion to experience the poorer visibility but also the 'trapped' turbulence on the leeside of the hills. A pilot taking the windward southern side has a chance of climbing above the inversion for better visibility and, at any altitude, he will escape the lee side turbulence.

This article first appeared in FLYER magazine's August 2000 edition