I am opening three maps today with two purposes in mind. Two of the maps provide a countless number of illustrations about the close relationship between (Total) Precipitable Water (TPW) and air temperature anomalies on this one day. I have added the third map because it contains a striking illustration related to two other points of interest that can both use some clarification. Cloud cover is a key part of both points. For one, any combination of cloud cover and sunshine is known to have a cooling effect at the surface, simply because a portion of the incoming solar energy will be reflected back to space before it ever reaches the surface. This effect has peaks and valleys, depending on numerous factors including the angle of sunlight and its hours of duration. The illustration I have today suggests that at this time of year, even in the mid-latitudes, the realized amount of cooling from this effect may tend to be of little significance. To that end, focus your attention on the shape of the long and large band of relatively clear sky in the top half of this map:
Next, compare that shape and its position on the map with the shape and position of the long and large cold anomaly that stands out so clearly on this next map. Then ask yourself, if clouds were to move into this region and start blocking off sunlight the temperature should just become colder yet, shouldn’t it? Maybe so, if those clouds were only condensed from existing water vapor, but what if they moved in from the outside? Then everything changes. Or what if, to begin with, there was only a little water vapor on hand to make condensing feasible? Moreover, what about the many nearby regions on all sides that are well clouded over and even precipitating, yet showing up with rather significant warm anomalies—despite no help from the sun? Take a look:
The best answer I can give is that maybe there is something else, in this kind of situation, that has a decisively more significant effect of temperature anomalies than any amount of cloud cover. Perhaps I can even present vivid imagery that helps to make the point. Just look for the long and large area of relatively low level of TPW that coincides with the shape and position of the two clear-air and cold-anomaly images we’ve just viewed, best delineated on the western end but still complete via darker shading in the gray part:
The relative absence of PW certainly provides a good reason for the cold anomaly and also helps to explain why the skies are clear, assuming only that clouds have had no reason to form in this area. A reminder may be useful here that apparently low levels of TPW can become associated with warm air anomalies whenever the situation moves northward, as we see in abundance in these images. Less and less additional PW is needed to warm things up by a given amount when the normal amount of TPW over the surface keeps declining for entirely natural geographical reasons.
Clouds may or may not be present as part of a TPW product mix. Another point of interest I have been wondering about is whether this factor alone makes any difference in the greenhouse energy effect of a given weight of TPW above a given surface, all else being equal. It is impossible to get an exact answer without having an abundance of now-missing data, but it never hurts to look for a better understanding. The above two maps of TPW and anomalies provide many opportunities to prove how powerful the relative effect of TPW on temperature can be. Whenever you are checking out an example it is always a good idea to hop over to the third map to see what the cloud situation is for that area, and whether or not it may be making a difference in the result.
Carl