Derived Weather Parameters - Wind Phenomena

Bise Index | Foehn Index | Moehlin Jet Index | Santa Ana Wind Index

Wind phenomena

Bise Index

This parameter computes an index to determine whether there is a bise and its strength. The bise is usually a cold wind from North-Easterly directions in the Lake Constance area which affects local weather conditions, including those of the Swiss Plateau. It is mainly triggered by a large scale air pressure difference between a low in the South-West of the Alps and a high on the northern side of the Alps. As a rule of thumb, if the pressure difference between Geneva and Constance is less that -2 hPa and the wind direction is North-East, then bise conditions are expected. If all conditions are met the value of the index is 1 and Bise is expected, otherwise the index is 0.

bise_index:idx

Example

Foehn Index

This parameter computes an index to determine the presence, strength, and direction of the Alpine Foehn. The Foehn is usually a strong, warm, dry wind crossing the Alps from the South-East to the North, but can also move in the the opposite direction. It mainly affects alpine valleys. Usually, it is triggered by large scale air pressure differences between a high in the South of the Alps and a low in the North-West of the Alps. As a rule of thumb, if the pressure difference between Lugano and Zurich is higher than 4 hPa, Foehn conditions are expected in the northern part of the Alps. If the pressure difference is less than -4 hPa, and the wind direction is South-East to the NorthFoehn, or vice versa, then foehn conditions are expected in the southern part of the Alps.

north_foehn_index:idx

If all conditions are met for North Foehn winds to be expected, the value of the index is 1, otherwise it is 0. Example:

south_foehn_index:idx

If all conditions are met for South Foehn winds to be expected, the value of the index is 1, otherwise it is 0. Example:

Moehlin Jet Index

The Moehlin Jet is a south-easterly local wind that originates from the Frick Valley and is responsible for dissolving fog in the region of Basel. This is the reason why sunshine duration there clearly exceeds the one of the Swiss midlands in fall. Favoring conditions are winterly high pressure situations over Switzerland, when the midlands act as a cold air pool. If the pool is full, the cold air masses spill over and follow gravity towards Moehlin, while the air is dried and heated on the way. The index takes values of 0 and 1, where 1 indicates the presence of a Moehlin Jet.

moehlin_jet:idx

Example

Santa Ana Wind Index

This parameter computes the presence and strength of the Santa Ana Winds in southern California. These are strong and extremely dry down-slope winds that originate from the desert and descend towards coastal areas. They develop if dry, cool high-pressure air masses are prevailing in the Great Basin and low pressure over the Pacific. Typically, these events occur in fall. Due to the extremely low relative humidities, such an event provides eligible conditions for wildfires. The index takes values in the range of 0 to 1. If the index exceeds 0.6, Santa Ana Winds can be expected.

santa_ana_wind:idx

Example