Horizon superelevation

Example of a fisheye shot in New York showing the elevation of the horizon of a half spherical hemisphere. The buildings narrow the view of the sky.

As horizon overhead (engl. Sky view factor), a in the microclimatology referred important (u. A. Urban and Bioklimatologie), cartography, meteorology and geography reference size of the visible region from one point of the sky. It can be given mathematically as a dimensionless parameter between 0 and 1. One level gives the value 1, the entire upper half of the sky (hemisphere) is visible. In narrow valleys only part of the sky is visible, the factor is therefore less than 1.

calculation

The sky-view factor is the proportion of the visible sky (Ω, gray area) above a certain observation point. Here in a two-dimensional representation (cold lake in the South Dinarides)

The example of a closed canopy photographed with a fisheye lens shows a sky view factor of almost 0.

The classic method of measuring the narrowing of the horizon is done with the help of a horizontoscope . Numerical calculations of the elevation of the horizon use specialized programs in terrain topography today that evaluate a circular fisheye image with a 180 ° viewing angle. To calculate the elevation of the horizon from remote sensing data, digital elevation models are required, which can be generated from radar data, for example.

meaning

The elevation of the horizon is important in various scientific contexts. In microclimatology, the elevation of the horizon determines, among other things, the effectiveness of inverse cold air lakes in ground depressions, as this determines the nocturnal radiation of long-wave radiation. In cartography, the diffuse incidence of light from spatial points, due to their position in relation to the elevation of the horizon, can be used to obtain an expanded display space for the classic application of cartographic hillside shading to visualize the relief.

Bioclimatology

The deepest frosts in Germany are measured in the closed depression of the Funtensee. A relatively high sky view factor is indispensable for this, as it significantly determines the intensity of the long-wave nocturnal radiation.

In addition to sealing, the elevation of the horizon plays a central role in the causes of the formation of urban heat islands. By narrowing the horizon in urban canyons, the effective long-wave radiation is reduced. In addition, the factor of the elevation of the horizon plays a dominant role for the intensity of frosts in inversion cold-air lakes of closed mountain hollows (including sinkholes). A high sky view factor , i.e. a low elevation of the horizon, can contribute to the lowest regional and subcontinental minimum temperatures in cold-air lakes during winterly Siberian cold air incursions and starry nights. Such extreme microclimatological phenomena occur in Austria in Grünloch and in Germany in Funtensee .

Web link

• Relief factors of cold air lakes

credentials

1. ↑ Fredrik Lindberg, Björn Holmer 2010: Sky View Factor Calculator. Göteborg Urban Climate Group University of Gothenburg, Department of Earth Sciences. (PDF)
2. ↑ Zaksek, K., Ostir, K., Kokalj, Z. 2011: Sky-View-Factor as a Relief Visualization Technique. - Remote Sensing 3: 398-415
3. ^ Dobrowski, SZ 2011: A climatic basis for microrefugia: the influence of terrain on climate. - Global Change Biology 17: 1022-1035.
4. ↑ Whiteman CD, T. Haiden, B. Pospichal, S. Eisenbach, and R. Steinacker, 2004b: Minimum temperatures, diurnal temperature ranges, and temperature inversions in limestone sinkholes of different sizes and shapes. - J. Appl. Meteor. 43: 1224-1236.
5. ↑ Steinacker, R. Whiteman, CD, Dorninger, M., Pospichal, B., Eisenbach, Holzer, AM, Weihs, P., Mursch-Radlgruber, E., Baumann, K. 2007: A Sinkhole Field Experiment in the Eastern Alps. - Bull. Americ. Meteo. Soc. May 2007 88 (5): 701-716.
6. ↑ Zaksek, K., Ostir, K., Kokalj, Z. 2011: Sky-View-Factor as a Relief Visualization Technique. Remote Sensing 3: 398-415.
7. ↑ Andreas Matzarakis 2001: The thermal component of the urban climate. Reports of the Meteorological Institute Freiburg, 6. (PDF) ( Memento of the original from July 17, 2013 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.