EES215
Lecture 18
Land and sea breezes
Difference in specific heat between water (energy needed to raise temperature
of unit mass by 1 deg) water 4184 J/kg-deg; sand 840 J/kg-deg; air 1000
J/kg-deg
More energy needed to heat water, but water gives off more energy when cooled:
moderating influence on temperatures
Continentality: land heats and cools more quickly than water
Land masses are concentrated more in northern hemisphere: greater extremes in
northern summer and winter
Energy in wind; formula for wind extraction of energy:
E = ½ mv2
with
m = rAv è E = ½ rAv3
A - area swept out by wind rotor; v - wind speed; r - density of air
Efficiency of wind rotors - 40 %
Review of wind power: best locations in flat areas –
plain states in the
Global pressure distribution, associated with global wind
pattern: Fig. 1
Pressure changes with height; convergence - ascent - divergence: Fig. 2
Influence of land mass in northern hemisphere.
Distribution of actual wind patterns: Fig. 3
Intertropical convergence zone (ITCZ) follows roughly the equator, but
is influenced by distribution of land masses and change in seasons.
Upper geostrophic winds are westerly between subtropical high-pressure cells
(around 15oN/S) and polar low pressure center.
Betwen subtropical high-pressure and equator they are easterly.
Westerly circulation reaches maximum speeds of 45-67 m/s (100-150mph), twice
that in winter. Maximum speeds are concentrated at around 30o,
between 9000 and 15000 m - jetstream.
Variations in the jet stream: Rossby waves.
Alternation betwen cyclonic and anticyclonic curvature
(consequence of conservation of vorticity): Fig.
4
Air mass - large body of air whose physical characteristics
(temperature, moisture content, lapse rate) are roughly uniform (horizontally)
for hundreds of km
Source areas – cold: Arctic;
Polar; Warm: Tropic; Equatorial;
Each of them can be maritime or continental, describing the water content of
the air masses.
Example: cP - continental Polar air; mE - maritime Equatorial air.
Distribution and movement of air masses depends on seasons and on distribution
of land and oceans: Fig. 5
Collision of these air masses causes strong temperature gradients (à strong winds) and change in moisture
content (à precipitation).
Water content depends strongly on temperature: saturation mixing
ratio (Table 1).
Distribution of precipitation reflects global pressure distribution and wind
pattern: Fig. 6
Extreme lack of precipitation leads to formation of deserts: desert areas are
predominantly found in the subtropics (around 30o N or S). Other
causes are distance to water source (e.g.
Special effect of large water bodies in cold climate - Lake effect snow: Fig. 8
