Geographical Controls of Mountain Meteorological Elements

1. Geographical Controls of Mountain Meteorological Elements Latitude Continentality Altitude Topography

2. Latitude Determines length of day and angle of incoming sunlight and, thus, amount of solar radiation received • In equatorial regions, day length & solar angle change little with season. Little seasonal variability, mostly diurnal changes. • In polar regions, the sun does not rise at all in winter. In the summer it never sets, although remaining low in sky. Big seasonal changes, small diurnal changes. • In mid-latitudes, seasonal and diurnal changes. Also determines site’s exposure to latitudinal belts of high and low pressure • High pressure - subsidence • Low pressure - convection

3. Day length vs latitude (Whiteman, 2000)

4. Impacts of Latitude Net radiation (incoming – outgoing) and temperature decrease as latitude increases Elevation of treeline/snowline decreases poleward Belt of alpine vegetation and permanent snow and ice are lower on mountains at high latitude versus the tropics

5. Snow lines and timberlines

6. Continentality • The degree to which a point on the earth’s surface is in all respects • subject to the influence of a landmass. • Continental locations experience larger diurnal and seasonal • temperature changes than locations on or near large bodies of water • because land surfaces heat and cool more quickly than oceans. • Interior locations experience more sunshine, less cloudiness, less • moisture and less precipitation than coastal areas. • Precipitation is especially heavy on the windward side of coastal • mountain ranges oriented perpendicular to prevailing winds from • the ocean. Marine air lifted up a mountain range releases much of • its moisture as precipitation. As a result, far less precipitation is • received on the leeward side.

7. Continentality Arises from differences in heat capacity and heat conduction of soils vs. water • Water able to store more heat • Soils less Degree of continentality expressed by annual range of mean monthly temperature

8. Continentality High mountains also protrude into the middle of the troposphere where the atmospheric circulations may differ considerably from that at sea level. Mountain ranges located in semi-arid macroclimatic zones, may have distinctly different climatic and vegetation characteristics from the adjacent lowlands. Extensive mountain massifs and high plateaus set up their own large-scale and local-scale circulations. Such large-scale effects on diurnal and seasonal circulations (‘plateau monsoons’) have been demonstrated for the Tibetan Plateau and the mountain ranges of the south-western US (Tang and Reiter 1984).

9. Continentality The summer-time plateau wind circulation in the Great Basin area of the US, reverses diurnally over a depth of 2 km. Gao and Li (1981) show that the Tibetan Plateau creates a lateral boundary layer that enlarges its effective dimensions.

10. Altitude Incoming solar radiation increases with altitude • Changes in air temperature at high altitudes are small, however, because of smaller amount of land area at higher altitudes • Air temperature usually decreases with altitude (-6.5°C/km) • Moisture in air usually decreases with altitude • Wind speed usually increases with altitude • Air density and atmospheric pressure decrease exponentially with altitude

11. Altitude • Distribution of state variables (p,ρ,T,u) depends strongly on height in free atmosphere and as function of terrain height • Vapor pressure of water and radiation also vary strongly with height (Whiteman, 2000)

12. Solar Radiation at Altitude Mountain observatories were of special importance in early studies of solar radiation and the solar constant. Sonnblick Observatory, Austria

13. Solar Radiation at Altitude In 1875, J. Violle made the earliest mountain-top measurements on Mont Blanc. Langley (1884) made actinometer observations on a special expedition to Mt. Whitney, California in 1881, but their estimates of the solar constant were higher than the currently assumed value of 1368 W m-2. Early estimates of the solar constant were obtained by extrapolating actinometer (later pyrheliometer) measurements of integrated solar radiation, made at different path lengths. The alternative method of spectrobolometry, pioneered by S. P.Langley on Whitney (4420 m) was gradually perfected by C.G. Abbott. This involved observations of relative intensity in narrow spectral bands at different solar angles. Transmission coefficients are thereby determined for each ray.

14. Optical air mass The path length of the solar beam through the atmosphere expressed in terms of optical air mass, m m = 1/sin θ where θis the solar altitude. At sea level the relationship between optical mass and solar altitude is: for m = 1, θ = 90°, m = 2, θ = 30°,m = 4, θ = 14°. For comparative radiation calculations at different altitudes, the absolute optical air mass M= m (p/po) where p is the station pressure and po = 1000 mb, is used for the effects of air density on transmission. Thus at 500 mb a value of 2 for M corresponds to m = 4 and θ = 14°. For an ideal (pure, dry) atmosphere, the direct solar radiation received at the 500 mb level (5.5 km) is 5-12% greater than at sea level. This corresponds to an average increase of 1-2 % km-1.

15. Optical air mass (Barry 1992)

16. Solar radiation vs. Altitude (Barry 1992)

17. Seasonal variation of short and longwave radiation (Barry 1992)

18. (Barry 1992)

19. Solar Radiation at Altitude Following new spectrobolometer measurements with UV filters on Mt. Whitney in 1909-10, Abbott and Fowle (1911) estimated a mean solar constant of 1343 W m-2 . Pyrheliometer data obtained 30 years later by US Weather Bureau on Mt. Evans, Colorado gave 1349 W m-2. These estimates are within 2% of modern value derived from satellite measurements.

20. Solar Radiation at Altitude The depletion of the direct solar beam irradiance by atmospheric absorption and backscatter is referred to as the relative opacity of the atmosphere or its turbidity. Valko(1980) shows that the turbidity at Swiss mountain stations is typically four to five times less than that in the lowlands. Dirmhirn (1951) made in-depth studies of the effect of altitude on diffuse (sky) radiation. Under cloudless skies, the sky radiation decreases with altitude owing to the reduction in air density and therefore scattering, but multiple reflections from adjacent peaks may obscure this to some extent, especially when there is snow cover.

21. (Barry 1992)

22. Role of Topography

23. Topography- Wind Speeds on Mountain Summits Wind observations on mountain summits and in the free air was carried out by Wahl (1966). From data for European stations, in general, speeds on summits average approximately half of the corresponding free-air values.

24. Topography- Temperature on Mountain Summits

25. Topography- Temperature on Mountain Summits During the eighteenth century there was still considerable controversy as to the cause of the general temperature decrease with height. De Sussure, was the first physical scientist to approach a realistic explanation of the cause of cold in mountains. (Barry, 1978). Read: Barry 1978