Description

A valley exit jet is a strong, down-valley, elevated air current that emerges above the intersection of the valley and its adjacent plain. These winds frequently reach a maximum of 40 kts at a height of 130-660 ft above the ground. Surface winds below the jet are significantly weaker so the presence of a valley exit jet above an aerodrome may not be indicated by the published surface wind vector.

There are 2 types of valley exit jet:

• Dynamically driven exit jets: produced by the channeling of moderate to strong background winds from aloft down a valley axis.
• Thermally driven exit jets: produced by the cascade of cold air out of the valley exit fed by nocturnal down-valley flows.

Thermally driven exit jets

The presence of strong nighttime down-valley air flows has been documented at the mouth of many Alpine valleys that merge with basins, such as the Inn Valley in Austria (potentially affecting traffic into LOIK and LOXS), where the jet is strong enough to be heard from the ground. In the USA, exit jets have been observed at the North Fork Gunnison River at Paonia, Colorado; the exit of South Boulder Creek south of Boulder, Colorado; Albuquerque, New Mexico at the mouth of Tijeras Canyon (affecting traffic into KABQ); and the mouth of the Spanish Fork Canyon in Utah (potentially affecting traffic into KPVU).

Factors which act to increase exit wind speeds are the acceleration of winds originating inside the valley as they travel to lower elevations downvalley, and the process of cold valley air sinking and ejecting into the plain.

Flight safety considerations

The impact of an encounter with a valley exit jet will depend on the direction of flight relative to the valley exit and the strength of the jet. 

A sudden strong headwind encounter on final approach will lead to an increase in indicated airspeed (IAS) and a pilot may be tempted to reduce power; as the aircraft comes out of the influence of the jet on short finals, the IAS will reduce and there may be a significant loss of height. 

Moderate turbulence can be expected in all circumstances when encountering a valley exit jet.