Climate indicator - Geostrophic wind

Direct wind measurements have been made at some stations since the 19th century. During the 1950s, anemometers (wind gauges) became common at airports and coastal stations. In 1996 there was a network of no less than 130 automatic stations with identical wind equipment. The anemometers have thus changed technically over time and surrounding changes in vegetation and construction have a very large impact on the wind speed. This means that there are no long homogeneous time series of measured wind speed. Therefore, instead of direct wind measurements, the climate indicator is based on geostrophic wind, an idealized average wind speed calculated from air pressure observations.

Comment on the result

In general, it is difficult to see any clear trend over time of the geostrophic wind speed. It applies to the whole year as well as to different triangles. However, for some of the triangles, the number of cases of geostrophic wind above 25 m/s have decreased since the beginning of the 20th century.

The potential geostrophic wind energy has also been calculated. For at least the two southernmost triangles, it has had a decreasing tendency since the beginning of the 20th century.

How is the indicator geostrophic wind defined?

The wind we experience and which is measured with anemometers is often influenced by local factors such as friction, turbulence and various obstacles. Comparing wind measurements between different locations is therefore not simple.

Geostrophic wind is a somewhat idealized average wind speed, determined by a balance between the pressure gradient force and the Coriolis force. The geostrophic wind speed is dependent on the difference in air pressure between different points. The geostrophic wind speed can therefore be calculated using barometric pressure observations.

Why is this indicator important?

Strong winds associated with storms and hurricanes can cause severe damage and disruption to society. Historically, storms and hurricanes are among the worst storms in terms of loss of life and material damage. This applies both in Sweden and globally. Therefor, predicting storms has been perhaps the main driving force behind developing weather forecasts.

Very strong winds can form partly in connection with large-scale low pressure, partly in connection with small-scale phenomena such as tornadoes and thunderstorms. It is the more large-scale winds that are captured in the geostrophic wind climate indicator.

Prolonged periods of light wind can also cause problems. For example, in connection with wind power production and sailing ships.

How has the indicator been calculated?

Direct wind measurements are often influenced by local factors. Over the years, varying and not always completely comparable methods for wind measurement have been used. Barometric pressure observations, on the other hand, have been very stable for a long time. As the geostrophic wind can be calculated using air pressure observations, the geostrophic wind indirectly also becomes a stable parameter over time.

Air pressure measurements are available for at least three times a day since 1879, therefore we also have a data series of geostrophic winds for the period 1879 and onwards.

Data for the years 1879-1938 have not undergone the same quality control as data after 1938 and may therefore contain inaccuracies. Data before the year 1900 is chosen not to be displayed as the quality is questionable.

The geostrophic wind has been calculated for nine triangles over Sweden . For the two southernmost triangles, data since 1900 is included, for other triangles since 1939 or 1940.