1f. Why Clouds Have Clearly Defined BoundariesThe next time you see a
cumulus cloud, notice its base and edges. You will see that the
boundaries of the cloud are marked by flat, sharply defined bottoms and
edges that are clearly distinct from the surrounding air. This clear
definition of the cloud base is partly due to the rapid growth of
droplets as they form just above the lifting condensation level.Recall
that liquid water at the base of a cloud initially forms onto
condensation nuclei, of which there are a finite (but very large)
number. Within a few tens of meters of the lifting condensation level,
all the available condensation nuclei have attracted moisture, and
further condensation occurs only onto existing droplets. When droplets
first form, they are very small; but they quickly attain diameters of
about a micrometer, which makes them effective at scattering visible
light—hence the clearly visible base. (If instead the droplets grew
slowly, there would be only a gradual increase in the number capable of
scattering visible light from the base of the cloud. The cloud would
therefore have a faintly visible base that would gradually become more
discernible with height.)Another factor is that cloud droplets evaporate
within a very short distance of the cloud base. To see why, consider
that if a droplet falls into unsaturated air below the cloud, it shrinks
by evaporation. Because they are so small, cloud droplets fall slowly
to begin with and fall even more slowly as they evaporate. As a result,
the distance a cloud droplet can fall without evaporating is minuscule,
on the order of a centimeter. Viewed from a great distance, this will
look like a flat surface. (By way of contrast, raindrops have survival
distances measured in kilometers because they are so much larger.)The
sharp boundaries along the sides of cumulus clouds are the result of
entrapment. When unsaturated air just outside the margins of the cloud
is drawn into the cloud by turbulence, water droplets rapidly evaporate,
leaving behind a distinct boundary between the unsaturated, ambient air
and the large droplets remaining within the cloud.How does the size of
cloud droplets forming at the base of a cloud create the sharp boundary
between the cloud and dry air? Get solution
2f. Why Clouds Have Clearly Defined BoundariesThe next time you see a cumulus cloud, notice its base and edges. You will see that the boundaries of the cloud are marked by flat, sharply defined bottoms and edges that are clearly distinct from the surrounding air. This clear definition of the cloud base is partly due to the rapid growth of droplets as they form just above the lifting condensation level.Recall that liquid water at the base of a cloud initially forms onto condensation nuclei, of which there are a finite (but very large) number. Within a few tens of meters of the lifting condensation level, all the available condensation nuclei have attracted moisture, and further condensation occurs only onto existing droplets. When droplets first form, they are very small; but they quickly attain diameters of about a micrometer, which makes them effective at scattering visible light—hence the clearly visible base. (If instead the droplets grew slowly, there would be only a gradual increase in the number capable of scattering visible light from the base of the cloud. The cloud would therefore have a faintly visible base that would gradually become more discernible with height.)Another factor is that cloud droplets evaporate within a very short distance of the cloud base. To see why, consider that if a droplet falls into unsaturated air below the cloud, it shrinks by evaporation. Because they are so small, cloud droplets fall slowly to begin with and fall even more slowly as they evaporate. As a result, the distance a cloud droplet can fall without evaporating is minuscule, on the order of a centimeter. Viewed from a great distance, this will look like a flat surface. (By way of contrast, raindrops have survival distances measured in kilometers because they are so much larger.)The sharp boundaries along the sides of cumulus clouds are the result of entrapment. When unsaturated air just outside the margins of the cloud is drawn into the cloud by turbulence, water droplets rapidly evaporate, leaving behind a distinct boundary between the unsaturated, ambient air and the large droplets remaining within the cloud.What process works to give the sides of clouds well-defined boundaries? Get solution
2f. Why Clouds Have Clearly Defined BoundariesThe next time you see a cumulus cloud, notice its base and edges. You will see that the boundaries of the cloud are marked by flat, sharply defined bottoms and edges that are clearly distinct from the surrounding air. This clear definition of the cloud base is partly due to the rapid growth of droplets as they form just above the lifting condensation level.Recall that liquid water at the base of a cloud initially forms onto condensation nuclei, of which there are a finite (but very large) number. Within a few tens of meters of the lifting condensation level, all the available condensation nuclei have attracted moisture, and further condensation occurs only onto existing droplets. When droplets first form, they are very small; but they quickly attain diameters of about a micrometer, which makes them effective at scattering visible light—hence the clearly visible base. (If instead the droplets grew slowly, there would be only a gradual increase in the number capable of scattering visible light from the base of the cloud. The cloud would therefore have a faintly visible base that would gradually become more discernible with height.)Another factor is that cloud droplets evaporate within a very short distance of the cloud base. To see why, consider that if a droplet falls into unsaturated air below the cloud, it shrinks by evaporation. Because they are so small, cloud droplets fall slowly to begin with and fall even more slowly as they evaporate. As a result, the distance a cloud droplet can fall without evaporating is minuscule, on the order of a centimeter. Viewed from a great distance, this will look like a flat surface. (By way of contrast, raindrops have survival distances measured in kilometers because they are so much larger.)The sharp boundaries along the sides of cumulus clouds are the result of entrapment. When unsaturated air just outside the margins of the cloud is drawn into the cloud by turbulence, water droplets rapidly evaporate, leaving behind a distinct boundary between the unsaturated, ambient air and the large droplets remaining within the cloud.What process works to give the sides of clouds well-defined boundaries? Get solution
