Sandstorms, Mountain Waves and Significant Temperature Inversion

Sandstorms

Avoid flying in active sandstorms whenever possible. When on ground, the aeroplane should ideally be kept under cover if dust storms are forecast or in progress. Alternatively, all engine blanks and cockpit covers should be fitted, as well as the blanks for the various system and instrument intakes and probes. They should be carefully removed before flight to ensure that accumulation of dust is not deposited in the orifices which the covers are designed to protect.

Mountain Waves

Mountain waves and downslope windshear are caused by a significant airflow crossing a mountain range together with special atmospheric conditions. The strong vertical and horizontal wind shears, rotor turbulences, represent a danger at low heights as well as the strong downslope wind at the lee side of the mountains.

Frequently, a second rotor will form up to 100 NM from the lee side of the mountain, producing original wave action. Flight crews should be aware of the potential hazard at airports within the flow regime of the wave. Depending on the moisture content of the air, lenticular (lens-shaped) clouds may be present.

When approaching a mountain range from upwind side, there will usually be a smooth updraft. Therefore, it is not quite as dangerous an area as the lee of the range. Expected downdrafts from the leeward side can exceed the climb capability of the aircraft. Flight crews should always be prepared to cope with a downdraft and turbulence. When overflying mountainous terrain in strong winds applies additional terrain clearance margin.

On some airports, relief or obstacles may cause special wind conditions with severe turbulence and windshear on approach or during take-off. Special procedures or recommendations are indicated on aerodrome charts when appropriate. They must be taken into account by the flight crews for the choice of the landing or take-off runway.

Significant Temperature Inversion

General

In meteorology, air temperature at the earth’s surface is normally measured at a height of about 4 ft above the ground. From that temperature which is reported by ATC, take-off performance will be defined. All along the take-off flight path, aeroplane performance is computed considering the altitude gained, the speed increase, but also implicitly considering a standard evolution of temperature, i.e. temperature is considered to decrease by 2°C for each 1.000 ft. Although most of the time temperature will decrease with altitude in quite a standard manner, specific meteorological conditions may lead the temperature evolution to deviate from this standard rule. With altitude increasing, marked variations of the air temperature from the standard figure may be encountered. In that way, air temperature may decrease in a lower way than the standard rule or may be constant or may even increase with altitude. In this last case, the phenomenon is called a temperature inversion.

As described below, this may particularly affect the very lower layer of the atmosphere near the earth’s surface. There are many parameters, which influence air temperature and may lead to a temperature inversion. Close to the ground, air temperature variations mainly result from the effects of:
• Seasonal variations;
• Diurnal/nocturnal temperature variations;
• Weather conditions (effect of clouds and wind);
• Humidity of the air,
• Geographical environment such as:
   - Mountainous environment;
   - Water surface (sea);
   - Nature of the ground (arid, humid);
   - Latitude;
   - Local specificity.

As a general rule, valid for everywhere, low wind conditions and clear skies at night will lead to rapid cooling of the earth and a morning temperature inversion at ground level.

Morning Temperature Inversion

In the absence of wind or if the wind is very low, the air which is in contact with a cold earth surface will cool down by heating transfer from the warm air to the cold ground surface. This transfer of heat occurs by conduction only and consequently leads to a temperature inversion which is limited in altitude. This process needs stable weather conditions to develop.

Schematically, during the day, the air is very little heated by solar radiation and the earth is much more. But the lower layer of the atmosphere is also heated by contact with the ground, which is more reactive to solar radiation than the air, and by conduction between earth and atmosphere. At night, in the absence of disturbing influences, ground surface cools down due to the absence of solar radiation and will cool the air near the ground surface. In quiet conditions, air cooling is confined to the lowest levels. Typically, this effect is the biggest at the early hours of the day and sunshine subsequently destroys the inversion during the morning.

Similarly, wind will mix the air and destroy the inversion.

Magnitude: This kind of inversion usually affects the very lowest levels of the atmosphere. The surface inversion may exceed 500 ft but should not exceed 1.000 to 2.000 ft. The magnitude of the temperature inversion cannot be precisely quantified. However, a temperature inversion of about +10°C is considered as quite an important one. Usually, within a temperature inversion, temperature regularly increases with altitude until it reaches a point where the conduction has no longer any effect.

Exposed Areas: This kind of inversion may be encountered world-wide. However, some areas are more exposed to this phenomenon such as arid and desert regions. It may be also encountered in temperate climate particularly during winter season (presence of fog). Tropical regions are less sensitive due to less stable weather conditions. In some northern and continental areas during winter in anticyclonic conditions, the low duration of sunshine during the day could prevent the inversion from destruction. Thus, the temperature of the ground may considerably reduce and amplify the inversion phenomenon. In a lower extent, this may also occur in temperate climate during winter, if associated with cold anticyclonic conditions.

Wind change: The air mass in the inversion layer is so stable that winds below and above tend to diverge rapidly. Therefore, the wind change, in force and direction, at the upper inversion surface may be quite high. This may add to the difficulty of flying through the inversion surface. In some conditions, the wind change may be so high as to generate a small layer of very marked turbulence.

Other Types of Temperature Inversion

The morning temperature inversion process is considered as the most frequent and the most sensitive. However, as also mentioned above, other meteorological conditions of a less frequent occurrence and magnitude may lead to temperature inversions. For instance, the displacement of a cold air mass over a cold ground surface may lead to turbulence resulting in a transfer of heat to the lower levels of this mass, thus, also creating a temperature inversion in the lower levels of the atmosphere below this air mass. Usually, this kind of inversion has lower magnitude than the previous case described above. In any case, pilot experience, weather reports or pilot reports will be the best way in identifying such weather conditions.

Effect on Aeroplane Performance

A temperature inversion will result in a reduction of the thrust only when performing a maximum take-off thrust during hot days, i.e. the actual ambient temperature is above TREF (Flat Rating Temperature).

In the event of temperature inversion, the climb performance will be affected in the cases where the thrust is affected. However, to affect the aeroplane performance, a temperature inversion must be combined with other factors. During a normal take-off with all engines operative, the inversion will have no effect since the actual aeroplane performance is already far beyond the minimum required performance. Then, the actual aeroplane performance could be affected only in the event of an engine failure at take-off. However, conservatism in the aeroplane certified performance is introduced by the FAR/JAR Part 25 rules, to take account for inaccuracy of the data that are used for performance calculations. Although not specifically mentioned, temperature inversions can be considered as part of this inaccuracy.

Therefore, a temperature inversion could become a concern during the take-off only in the following worst case with all of these conditions met together:
• The engine failure occurs at V1;
• Take-off is performed at maximum take-off thrust;
• OAT is close to or above TREF;
• The take-off weight is limited by obstacles;
• The temperature inversion is such that it results in the regulatory net flight path margin cancellation and leads to fly below the regulatory net flight path.

In all other cases, even if the performance is affected (inversion above TREF), the only detrimental effect will be the climb performance to be lower than the nominal one.