Random Temperature Variation
19
0
10
20
30
40
50
60
70
Time (s)
FIGURE 2.4. Air temperature 2 m above a desert surface at White Sands Missile
Range, NM. Measurements were made near midday using a 25 pm diameter
thermocouple.
differ substantially from the mean air temperature that one might measure
with a large thermometer. The relatively smooth baseline in Fig. 2.4, with
jagged interruptions, indicates a suspension of hot ascending parcels in a
matrix of cooler, descending air. Well mixed air is subsiding, being heated
at the soil surface, and breaking away from the surface as convective
bubbles when local heating is sufficient.
Warm air is less dense than cold air, and therefore has a lower index
of refraction. As light shines though the atmosphere, the hot and cold
parcels of air act as natural lenses, causing the light to constructively
and destructively interfere, giving rise to a diffraction pattern. Twinkling
of stars and the scintillation of terrestrial light sources at night are the
result of this phenomenon. The diffraction pattern is swept along with
the wind, so you can look at the lights of a city on a clear night from some
distance and estimate the wind speed and direction from the drift of the
scintillation pattern.
So-called "heat waves" often seen on clear days also result from refractive index fluctuations (Lawrence et al., 1970). The drift ofheat waves
can be seen, and wind direction and speed can sometimes be estimated
from the drift velocity. This phenomenon has been used to measure wind
speed (Lawrence et al., 1972). More extreme heating at the surface can
result in a mirage, where the heated, low-density air near the surface of
the earth refracts the light from the sky to the observers eye, making land
19
0
10
20
30
40
50
60
70
Time (s)
FIGURE 2.4. Air temperature 2 m above a desert surface at White Sands Missile
Range, NM. Measurements were made near midday using a 25 pm diameter
thermocouple.
differ substantially from the mean air temperature that one might measure
with a large thermometer. The relatively smooth baseline in Fig. 2.4, with
jagged interruptions, indicates a suspension of hot ascending parcels in a
matrix of cooler, descending air. Well mixed air is subsiding, being heated
at the soil surface, and breaking away from the surface as convective
bubbles when local heating is sufficient.
Warm air is less dense than cold air, and therefore has a lower index
of refraction. As light shines though the atmosphere, the hot and cold
parcels of air act as natural lenses, causing the light to constructively
and destructively interfere, giving rise to a diffraction pattern. Twinkling
of stars and the scintillation of terrestrial light sources at night are the
result of this phenomenon. The diffraction pattern is swept along with
the wind, so you can look at the lights of a city on a clear night from some
distance and estimate the wind speed and direction from the drift of the
scintillation pattern.
So-called "heat waves" often seen on clear days also result from refractive index fluctuations (Lawrence et al., 1970). The drift ofheat waves
can be seen, and wind direction and speed can sometimes be estimated
from the drift velocity. This phenomenon has been used to measure wind
speed (Lawrence et al., 1972). More extreme heating at the surface can
result in a mirage, where the heated, low-density air near the surface of
the earth refracts the light from the sky to the observers eye, making land