1fs. Radiation Inversions and Human ActivitiesOn clear, still
nights, air near the ground can cool rapidly. Of course, air higher up
will not be subjected to the same cooling as air in contact with the
surface, and temperature differences of several degrees Celsius can be
observed over just a few meters in height. Thus, subfreezing
temperatures can exist near the ground, while just a short distance
above, temperatures may be safely above the freezing level. This has
some important effects on agriculture in the southern United States.When
temperatures drop to the freezing point, winter crops are vulnerable to
frost damage. If the ground is relatively flat, the cold air can
develop locally. In hilly regions, cooling of the air can occur at
higher levels and gravity may cause the chilled air to flow downward and
accumulate at the foot of the hills.To offset the frost problem,
growers often set large “wind machines” atop masts, as shown in Figure
6-3-1a. When temperatures near the ground fall dangerously low, the
machines are turned on to force the warm air in the upper part of the
inversion down toward the surface. Thus, the potentially damaging air
near the surface is replaced as warmer air circulates downward.At even
lower temperatures, growers may also activate smudge pots that burn
heating oil, as shown in Figure 6-3-1b. Although the emission of
longwave radiation by the smudge pots helps somewhat to protect the
crops, their greater contribution is to produce free convection. Like
the wind machines, the smudge pots produce a continual mixing of air
between low and higher levels, so that surface air stays above the
freezing point. Yet another tactic is to spray citrus crops with water.
As water freezes, latent heat is released, which keeps the fruit within a
few degrees of freezing, warm enough to prevent severe damage. Of
course, on very cold nights inversions are so deep and cold that none of
these remedies is effective. In such a case, frost damage can ruin an
entire crop.Problems associated with radiative cooling and the resultant
inversions are not restricted to agriculture. The strong stability of
an inversion can suppress the vertical motions that dilute the
concentration of pollutants near the surface. It is common for urban
dwellers to notice a low-lying layer of sooty haze near the surface on
cold, clear mornings when radiation inversions are most likely to
form.FIGURE 6-3-1 Wind Machines. (a) Agricultural wind machines blow air
downward during times of potential frost damage to force warm air in
the upper part of a radiation inversion toward the crops, (b) During
times of greater stress, growers also may employ smudge pots....By what
mechanism(s) do smudge pots and wind machines mitigate frost damage? Get solution
2fs. Radiation Inversions and Human ActivitiesOn clear, still nights, air near the ground can cool rapidly. Of course, air higher up will not be subjected to the same cooling as air in contact with the surface, and temperature differences of several degrees Celsius can be observed over just a few meters in height. Thus, subfreezing temperatures can exist near the ground, while just a short distance above, temperatures may be safely above the freezing level. This has some important effects on agriculture in the southern United States.When temperatures drop to the freezing point, winter crops are vulnerable to frost damage. If the ground is relatively flat, the cold air can develop locally. In hilly regions, cooling of the air can occur at higher levels and gravity may cause the chilled air to flow downward and accumulate at the foot of the hills.To offset the frost problem, growers often set large “wind machines” atop masts, as shown in Figure 6-3-1a. When temperatures near the ground fall dangerously low, the machines are turned on to force the warm air in the upper part of the inversion down toward the surface. Thus, the potentially damaging air near the surface is replaced as warmer air circulates downward.At even lower temperatures, growers may also activate smudge pots that burn heating oil, as shown in Figure 6-3-1b. Although the emission of longwave radiation by the smudge pots helps somewhat to protect the crops, their greater contribution is to produce free convection. Like the wind machines, the smudge pots produce a continual mixing of air between low and higher levels, so that surface air stays above the freezing point. Yet another tactic is to spray citrus crops with water. As water freezes, latent heat is released, which keeps the fruit within a few degrees of freezing, warm enough to prevent severe damage. Of course, on very cold nights inversions are so deep and cold that none of these remedies is effective. In such a case, frost damage can ruin an entire crop.Problems associated with radiative cooling and the resultant inversions are not restricted to agriculture. The strong stability of an inversion can suppress the vertical motions that dilute the concentration of pollutants near the surface. It is common for urban dwellers to notice a low-lying layer of sooty haze near the surface on cold, clear mornings when radiation inversions are most likely to form.FIGURE 6-3-1 Wind Machines. (a) Agricultural wind machines blow air downward during times of potential frost damage to force warm air in the upper part of a radiation inversion toward the crops, (b) During times of greater stress, growers also may employ smudge pots....Why does spraying crops with water decrease the likelihood of frost damage? Get solution
2fs. Radiation Inversions and Human ActivitiesOn clear, still nights, air near the ground can cool rapidly. Of course, air higher up will not be subjected to the same cooling as air in contact with the surface, and temperature differences of several degrees Celsius can be observed over just a few meters in height. Thus, subfreezing temperatures can exist near the ground, while just a short distance above, temperatures may be safely above the freezing level. This has some important effects on agriculture in the southern United States.When temperatures drop to the freezing point, winter crops are vulnerable to frost damage. If the ground is relatively flat, the cold air can develop locally. In hilly regions, cooling of the air can occur at higher levels and gravity may cause the chilled air to flow downward and accumulate at the foot of the hills.To offset the frost problem, growers often set large “wind machines” atop masts, as shown in Figure 6-3-1a. When temperatures near the ground fall dangerously low, the machines are turned on to force the warm air in the upper part of the inversion down toward the surface. Thus, the potentially damaging air near the surface is replaced as warmer air circulates downward.At even lower temperatures, growers may also activate smudge pots that burn heating oil, as shown in Figure 6-3-1b. Although the emission of longwave radiation by the smudge pots helps somewhat to protect the crops, their greater contribution is to produce free convection. Like the wind machines, the smudge pots produce a continual mixing of air between low and higher levels, so that surface air stays above the freezing point. Yet another tactic is to spray citrus crops with water. As water freezes, latent heat is released, which keeps the fruit within a few degrees of freezing, warm enough to prevent severe damage. Of course, on very cold nights inversions are so deep and cold that none of these remedies is effective. In such a case, frost damage can ruin an entire crop.Problems associated with radiative cooling and the resultant inversions are not restricted to agriculture. The strong stability of an inversion can suppress the vertical motions that dilute the concentration of pollutants near the surface. It is common for urban dwellers to notice a low-lying layer of sooty haze near the surface on cold, clear mornings when radiation inversions are most likely to form.FIGURE 6-3-1 Wind Machines. (a) Agricultural wind machines blow air downward during times of potential frost damage to force warm air in the upper part of a radiation inversion toward the crops, (b) During times of greater stress, growers also may employ smudge pots....Why does spraying crops with water decrease the likelihood of frost damage? Get solution
