Local Wind Conditions

Regarding survey planning, the position of the site within the landscape may be important for the choice of platform or for navigation planning because it may influence local winds or cause windward and leeward situations. For kites, the wind direction always must be taken into account when choosing the launching site, but for most other SFAP platforms, too, wind susceptibility is more or less crucial. Both wind direction and speed may vary at different heights above the ground, especially in alpine terrain, as discussed in the next section.

Usually the wind direction varies during the course of a day. Early in the morning until a short time after sunrise, land or mountain winds, respectively, are dominating. During the day the wind system goes into reverse so that from noon onward sea or valley wind, respectively, may be expected. Due to thermal updrafts, wind velocities are usually higher, so the preconditions are best for the use of the kite as camera platform. Toward evening the wind is likely to calm down in most situations.

Seasonally varying wind systems mainly have to be considered in the subtropics and marginal tropics. In North and West Africa the Harmattan provides excellent flight conditions in the dry period in winter, the same applies to the Cierzo in northern Spain and the Mistral in the western Mediterranean area. In summer these regions are dominated by thermally induced local winds or by calm. Similar seasonal winds are encountered in many parts of the world, for example the Santa Ana, a hot desert wind from the east or northeast in southern California. This should be taken into account if a monitoring project involves conditions in different seasons. In such cases, a combination of camera platforms for different wind velocities including a system for calm conditions has proven of value for the authors. For long-term monitoring purposes it is easier to take the pictures always in the same season and to adapt the camera platform to the wind velocity prevailing at that time.

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