Clouds of the Day - April 7, 2020
/Stratocumulus - A Story to Tell
This morning dawned with fog but after a couple of hours visibility improved leaving these clouds behind. The clouds are all that remained after the fog dissipated.
As temperature increases, the relative humidity decreases - the fog evaporates. We see the fog thinning until all that remains are the clouds you see in the photos. Fog is a stratus (flat/layered) cloud at ground level. Cumulus puffy or lumpy clouds caused by rising air currents. The clouds in the photos have both flat and lumpy shapes; hence the name stratocumulus.
For the scientific explanation of what happened scroll down below the photos. View the pictures below and then scroll down to see the temperature, relative humidity, and dew point traces from our weather station this morning. The traces illustrate what happened. When reading the explanation keep in mind that dew point is a measure of the amount of moisture in the air. If it increases (decreases) then moisture is increasing (decreasing) in the air. Relative humidity is the relative amount of moisture in the air compared to the capacity of the air to retain moisture. As temperature or dew point rise the air gets closer to saturation - the point at which the relative humidity becomes 100%. Clouds form when relative humidity approaches 100%.
Stratocumulus from dissipating fog
Stratocumulus from dissipating fog
Stratocumulus from dissipating fog
Stratocumulus from dissipating fog
Looking South at 12:15 p.m. after mixing evaporated most of the clouds.
Looking North at a cumulus cloud formed by rising warm air currents.
End of the Day - Cumulonimbus (thunderstorm) southeast
Cumulonimbus (thunderstorm) - Hiding behind the trees - Evening
What happened last night and this morning demonstrates how temperature, dew point, and relative humidity are related. Temperature is a measure of heat energy in the air. Dew point is a measure of the moisture in the air. Relative humidity is a measure of how much moisture is in the air compared to its capacity of contain moisture.
For example, if moisture remains steady and temperature increases (decreases), the relativity humidity decreases (increases). If dew point increases (decreases) the relative humidity increases (decreases). If temperature increases (decreases) and dew point increases (decreases) combinations of relative humidity are endless. Relative humidity depends on the values of both temperature and dew point which may exist together in any combination. Relative humidity depends on both temperature and relative humidity but temperature and dew point do not depend on each other.
Follow the traces on the chart. Temperature is red. Dew point is the green dotted line. Relative humidity is the blue dashed line. Temperature and dew point were almost identical all night which meant the relative humidity was near 100%. The air was nearly saturated and fog (a cloud near the ground) formed. After sunrise the temperature increased and relative humidity decreased. Notice how the temperature and dew point lines diverged. As the difference between temperature and relative humidity increases relative humidity decreases. As relative humidity decreased the fog began to thin - it evaporated.
What ultimately happened was sunlight began to heat the earth increasing the temperature. That caused the air to rise which began to mix the air. Warmer air near the ground began to rise and drier air aloft descended. This mixing warmed and dried the air enough to cause evaporation of the fog. The layer of moist air near the ground was very shallow. In the end the moist air became mixed and the relative humidity in the lower levels of the atmosphere dried enough for the fog and most clouds to dissipate.
The entire process takes place in front of our eyes. The bottom left photo, taken at 12:15 p.m. shows only a few remnants left over from the mixing process that evaporated the fog and finally the left over stratocumulus. The rising air currents and sinking air currents stirred the air, mixing in drier air. The rising motion is not being sustained because the air is stable after mixing so the sky is clear. On the other hand, looking north at the bottom right photo, there is a band of cumulus clouds partly explained by a storm system to the north that is sustaining rising motion in a slightly unstable air mass. This is maintaining upward motion and cumulus clouds. Clouds do have a story to tell.