1f. Dew Point and Nighttime Minimum TemperaturesKnowledge of the
current dew point temperature is a useful tool to the forecaster for the
prediction of the following morning’s low temperature. If no major wind
shifts or other weather changes are anticipated, the minimum
temperature will often approximate the dew point. Consider a
hypothetical evening with an air temperature of 15 °C (59 °F) and a dew
point of 5 °C (41 °F). The spread between the air temperature and the
dew point temperature is not very large, and a 10 °C (18 °F) lowering of
the air temperature is feasible. If the air temperature does indeed
drop to the dew point and there is little or no wind, a radiation fog
has a good chance of forming (Figure 5-1-1). The fog would then inhibit
further cooling, partly because latent heat is released and partly
because water droplets are extremely effective at absorbing longwave
radiation from the surface. Without the loss of radiation, the surface
temperature would remain almost constant, and the overnight low would
equal the dew point temperature.The relationship between dew point and
minimum temperature will not hold under certain conditions. The first
has to do with the changes in the big weather picture. Imagine, for
example, that a mass of warmer air is moving into the forecast region.
This large body of air can replace the one present at the time of the
forecast and bring with it higher nighttime temperatures. Similarly, the
passage of an advancing coid front (briefly described in Chapter 1 and
discussed in more detail in Chapter 9) can lead to significant drops in
temperature below the current dew point.Both heavy cloud cover and
strong winds inhibit a drop in air temperature, and their presence may
keep minimum air temperatures above the dew point. Cloud cover achieves
this effect because of its absorption and downward reradiation of
longwave energy. Strong winds prevent large temperature decreases at the
surface by vertical mixing. A shallow layer of cold air that would
otherwise develop is easily disrupted, leading to higher surface
temperatures and more uniform temperatures with height.Minimum
temperatures won’t go down to the dew point if the difference between
the air temperature and the dew point temperature is very large. One can
readily see how this might occur if a location has a high temperature
of 45 °C (113 °F) and a dew point of 0 °C (32 °F). Even with calm winds
and no cloud cover, a cooling of 45 °C is unlikely over the course of a
short summer night. Though the temperature won’t always drop down as low
as the dew point, it is always certain that unless a front passes
through or the wind direction changes significantly, the minimum
temperature will not fall below the evening dew point.Why are dew points
often good predictors of nighttime minimum temperatures?FIGURE 5-1-1
Dew Points and Minimum Temperatures. Dew points are often good
predictors of overnight minimum, temperatures. (a) Initially the absence
of cloud or fog droplets allows much longwave radiation to escape to
space. (b) When the temperature drops to the dew point, fog can form,
which releases latent heat into the air. (c) The presence of fog then
inhibits the escape of longwave radiation to space, inhibiting further
cooling... Get solution
2f. Dew Point and Nighttime Minimum TemperaturesKnowledge of the current dew point temperature is a useful tool to the forecaster for the prediction of the following morning’s low temperature. If no major wind shifts or other weather changes are anticipated, the minimum temperature will often approximate the dew point. Consider a hypothetical evening with an air temperature of 15 °C (59 °F) and a dew point of 5 °C (41 °F). The spread between the air temperature and the dew point temperature is not very large, and a 10 °C (18 °F) lowering of the air temperature is feasible. If the air temperature does indeed drop to the dew point and there is little or no wind, a radiation fog has a good chance of forming (Figure 5-1-1). The fog would then inhibit further cooling, partly because latent heat is released and partly because water droplets are extremely effective at absorbing longwave radiation from the surface. Without the loss of radiation, the surface temperature would remain almost constant, and the overnight low would equal the dew point temperature.The relationship between dew point and minimum temperature will not hold under certain conditions. The first has to do with the changes in the big weather picture. Imagine, for example, that a mass of warmer air is moving into the forecast region. This large body of air can replace the one present at the time of the forecast and bring with it higher nighttime temperatures. Similarly, the passage of an advancing coid front (briefly described in Chapter 1 and discussed in more detail in Chapter 9) can lead to significant drops in temperature below the current dew point.Both heavy cloud cover and strong winds inhibit a drop in air temperature, and their presence may keep minimum air temperatures above the dew point. Cloud cover achieves this effect because of its absorption and downward reradiation of longwave energy. Strong winds prevent large temperature decreases at the surface by vertical mixing. A shallow layer of cold air that would otherwise develop is easily disrupted, leading to higher surface temperatures and more uniform temperatures with height.Minimum temperatures won’t go down to the dew point if the difference between the air temperature and the dew point temperature is very large. One can readily see how this might occur if a location has a high temperature of 45 °C (113 °F) and a dew point of 0 °C (32 °F). Even with calm winds and no cloud cover, a cooling of 45 °C is unlikely over the course of a short summer night. Though the temperature won’t always drop down as low as the dew point, it is always certain that unless a front passes through or the wind direction changes significantly, the minimum temperature will not fall below the evening dew point.Under what conditions are dew points not useful predictors of nighttime low temperatures?FIGURE 5-1-1 Dew Points and Minimum Temperatures. Dew points are often good predictors of overnight minimum, temperatures. (a) Initially the absence of cloud or fog droplets allows much longwave radiation to escape to space. (b) When the temperature drops to the dew point, fog can form, which releases latent heat into the air. (c) The presence of fog then inhibits the escape of longwave radiation to space, inhibiting further cooling... Get solution
2f. Dew Point and Nighttime Minimum TemperaturesKnowledge of the current dew point temperature is a useful tool to the forecaster for the prediction of the following morning’s low temperature. If no major wind shifts or other weather changes are anticipated, the minimum temperature will often approximate the dew point. Consider a hypothetical evening with an air temperature of 15 °C (59 °F) and a dew point of 5 °C (41 °F). The spread between the air temperature and the dew point temperature is not very large, and a 10 °C (18 °F) lowering of the air temperature is feasible. If the air temperature does indeed drop to the dew point and there is little or no wind, a radiation fog has a good chance of forming (Figure 5-1-1). The fog would then inhibit further cooling, partly because latent heat is released and partly because water droplets are extremely effective at absorbing longwave radiation from the surface. Without the loss of radiation, the surface temperature would remain almost constant, and the overnight low would equal the dew point temperature.The relationship between dew point and minimum temperature will not hold under certain conditions. The first has to do with the changes in the big weather picture. Imagine, for example, that a mass of warmer air is moving into the forecast region. This large body of air can replace the one present at the time of the forecast and bring with it higher nighttime temperatures. Similarly, the passage of an advancing coid front (briefly described in Chapter 1 and discussed in more detail in Chapter 9) can lead to significant drops in temperature below the current dew point.Both heavy cloud cover and strong winds inhibit a drop in air temperature, and their presence may keep minimum air temperatures above the dew point. Cloud cover achieves this effect because of its absorption and downward reradiation of longwave energy. Strong winds prevent large temperature decreases at the surface by vertical mixing. A shallow layer of cold air that would otherwise develop is easily disrupted, leading to higher surface temperatures and more uniform temperatures with height.Minimum temperatures won’t go down to the dew point if the difference between the air temperature and the dew point temperature is very large. One can readily see how this might occur if a location has a high temperature of 45 °C (113 °F) and a dew point of 0 °C (32 °F). Even with calm winds and no cloud cover, a cooling of 45 °C is unlikely over the course of a short summer night. Though the temperature won’t always drop down as low as the dew point, it is always certain that unless a front passes through or the wind direction changes significantly, the minimum temperature will not fall below the evening dew point.Under what conditions are dew points not useful predictors of nighttime low temperatures?FIGURE 5-1-1 Dew Points and Minimum Temperatures. Dew points are often good predictors of overnight minimum, temperatures. (a) Initially the absence of cloud or fog droplets allows much longwave radiation to escape to space. (b) When the temperature drops to the dew point, fog can form, which releases latent heat into the air. (c) The presence of fog then inhibits the escape of longwave radiation to space, inhibiting further cooling... Get solution
