Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 63

called cold air drainage . Often this cooler air passes
under warmer air, pushing the warmer air above it and
forming an inversion , in which a warmer layer of air
becomes “sandwiched” between two layers of colder air.
In some locations, the cold pocket of air can lead to frost
formation and plant damage, whereas the warm air inver-
sion just above it stays significantly warmer. This pattern
of local temperature variation is illustrated in Figure 5.5.
The planting of frost-sensitive citrus between 500 and
1000 ft elevation on the lower slopes of the foothills of
the Sierra Nevada Mountains of the Central Valley of
California is a good example of how farmers have
learned to take advantage of a wintertime inversion layer
of warmer air that is forced up by the drainage of colder
air into a valley floor below.

RESPONSES OF PLANTS TO TEMPERATURE

All physiological processes in plants — including germi-
nation, flowering, growth, photosynthesis, and respiration
— have limits of tolerance for temperature extremes, and
a relatively narrow temperature range at which functioning
is optimized. Thus the temperature regime to which a plant
is exposed is ultimately connected to its yield potential.
For example, temperature conditions may allow a plant to
establish and grow, but then a sudden change in the
weather (e.g., a cold spell) might prevent it from flowering
and setting fruit and producing seed.
Farmers must carefully adapt their practices to the
local temperature regime, taking into account diurnal
variations, seasonal variations, moderating influences,

100 100

990
880
770
660
550
440
330
220
110
0

FF°

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Stockton, CA (37° 54' N) San Francisco, CA (37° 37' N)

FIGURE 5.4 Monthly average daily high temperatures at San Francisco, and Stockton, CA. Both cities are at nearly the same
latitude and elevation, but coastal San Francisco has a maritime climate, and Stockton, 100 km to the east, is under more of a
continental influence. (Data from Conway and Liston, 1990. The Weather Handbook . Conway Data: Atlanta.)

Rapid nighttime
cooling

Cold air
flows
downslope

Warmer air

Cold air

FIGURE 5.5 Cold air drainage and inversion layer. Cold air can drain into valley bottoms at night and pool beneath a layer of
warmer air.