Environmental Engineering Reference
In-Depth Information
5 541 0·79 −17·5 1220 1·9 2·0
Notes: P
, pressure; ρ, density;
T
, temperature; SWR, short-wave radiation (calculated assuming an
overhead sun); AH, absolute humidity; ωρ, vapour pressure.
adjacent
free atmosphere
. Latent heat exchange triggered by orographic effects provides
an additional heat source which may exceed advection (sensible) heat flux, except in
windward coast ranges. Latent heat release on condensation reduces adiabatic cooling by
6°-8° C at 5 km in mid-and high-latitude mountains and by 3°-5° C in low-latitude
mountains, where clouds are less extensive vertically. Atmospheric density and thickness
decrease with altitude, whilst transparency increases as aerosol density and absolute
humidity fall as they become involved in cloud formation. Both short- and long-wave
radiation fluxes thus increase with altitude, the former by 7-10 per cent km
−1
in the Alps.
Gains exceed losses, since the long-wave flux remains more constant with lower ambient
temperature. This may be countered above snow and ice surfaces. With albedos between
0·4 and 0·9, they reflect more short-wave radiation and consume sensible heat during
summer melt. On balance, mountains may form high-altitude heat sources. The Tibetan
plateau, for example, experiences temperatures 4°-6° C above the zonal average free
atmosphere at 5 km. Increasing latitude sees a general decline in radiation balance and
temperature. Stronger
seasonal
contrasts towards the poles compare with stronger
diurnal
contrasts at the equator.
MOISTURE BALANCE AND PRECIPITATION
The principal mountain impact on moisture balance is
orographic enhancement
of
precipitation by triggering conditional instability during forced ascent. This was formerly
thought to be exaggerated, since precipitation from onshore winds occurs anyway at the
coast, but it is now appreciated that mountains also retard mid-latitude cyclones and
thereby accentuate convergence and uplift. Windward coast mountains undoubtedly
experience some of Earth's highest precipitation levels and intensities. Precipitation may
increase up to 3-4 km in middle latitudes at frontal disturbances but only up to 2-3 km in
the tropics, marking the more limited vertical extent of warm tropical clouds. Mean
precipitation maxima are found between 0·5 km and 1·0 km, 0·7 km and 1·5 km, and at 3
km in equatorial, tropical maritime and high-latitude mountains respectively. Above this
the atmosphere becomes dryer, as available water vapour is consumed in cloud and
precipitation formation, and may be arid. Mountains may also enhance convection,
especially inland in summer, and provide high-level moisture sources. Effective wind
speed increases over exposed surfaces and either has a desiccating effect or provides a
source of moisture by advection and sublimation (Plate 25.3).
