Dear Gramps,
Why is it that you can have three sunny days in a row with no clouds, and the first day the temperature will be in the 80s, the next day in the 70s, and the final day in the 100s? If there are no clouds to block the sun, then why are some days hotter than others?
Robert, from Grants Pass, Oregon

Dear Robert,
Air temperature is a rather complex function of insolation, density, humidity and direction of flow. Perhaps a principle influence on air temperature is density. When a parcel of air ascends, it expands, and therefore becomes lighter. As it becomes lighter it cools off. If there is no heat added to or extracted from a parcel of air as it rises–a phenomenon called the adiabatic lapse rate–the expanding air cools at a rate of 5,4 deg F per 1000 feet increase in altitude. That’s why it’s colder on the mountain tops than in the valleys, and why the last snow to leave in the spring is at the higher elevations.
A complicating factor to this cooling off process is that as air cools its moisture carrying capacity decreases. When the relative humidity in the air reaches 100% of its capacity, that is called the dew point, i.e., the temperature and pressure where condensation occurs, i.e. the altitude point of the base of clouds.
Let’s follow a parcel air over a plowed field on a warm afternoon. Because the plowed field is much darker than the surrounding green fields more of the solar radiation is absorbed and less reflected than in the surrounding green fields. Therefore, the ground heats up and warms up the air over it. As the air over the dark field expands with heating it becomes lighter than the surrounding air and begins to rise. As it rises the relative humidity in the parcel increases until it finally reaches 100% and it becomes a cloud. However, when the included water vapor changes to water droplets, the water vapor releases a LOT of heat. Because the air can’t hold any more vapor, it condenses into water droplets, and this change from a gas to liquid releases 540 calories of heat per gram of water. The volume of a gram of water is one cubic centimeter. 540 calories is enough heat to heat up 5.4 grams of water from freezing to boiling!
So when the air reached 100% relative humidity and the water vapor condenses it immediately becomes a lot hotter, and thus expands even more than the expansion rate it had been experiencing. As it goes up more rapidly it quickly cools down again, and the process feeds on itself. That’s what causes thunderheads. As the air continues to rise rapidly and continues to cool down, the water droplets turn to ice. The ice particle continues to grow in size by impacting the high level of humidity (100%) until the particle becomes so large that it cannot be supported by the upward moving air, and falls out of the cloud as hail. Often, on the way down, the piece of hail encounters more strong updrafts and is blown upward again, accumulating more ice. If you pick up a hail stone and break it open, you can count the number of layers of ice of which it is composed. The number of layers represents the number of times that it has fallen and been blown back up by stronger vertical winds.
Now with all that air being sucked up into the thunderhead it has to be replaced by something. So the air around the clouds is pulled toward the ground to replace the rising air in the cloud. So there is a strong wind shear near the edges of these towering thunderheads. Many airplanes that either inadvertently or foolishly have flown into these areas have had their wings torn off by the wind shear. Vertical winds speeds in thunderheads often reach speeds of more than 200 MPH.
That is just one small beginning to the answer to your question. We won’t go into the rotating air systems–low pressure areas called cyclones and high pressure anticyclonic areas. Because of Coriolis effect of a free flowing object traveling in northerly or southerly directions over the rotating spherical earth, low pressure areas in the northern hemisphere tend to rotate in a counter clockwise direction and high pressure areas in a clockwise direction, and in opposite directions in the southern hemisphere. That’s also why water swirls down the sink in a clockwise direction in the southern hemisphere and in a counter clockwise direction in the northern hemisphere. This force is so pronounced that even at moderate latitudes, if you put you hand in the water and swirl it in the opposite direction to which it is moving and then remove your hand, you will often see the rotation stop and reverse itself before the water is all gone.
I must now confess that when I mentioned that we would not go into rotating air systems and then felt impelled to explain what is was that we were not going to talk about, I committed an apophasis. Don’t bother to look that word up unless you have that great historical dictionary of the English language, the Oxford dictionary, because you won’t find it. That most elegant and useful word was omitted from Webster’s Third New International Dictionary, that was edited in the 1950s, on the flimsy excuse that they had found only two citations of the word from other references–one published in 1657 and the other 1753. Hopefully they will soon come to their senses and stop this irrational impoverishment of the English language.
Apophasis–Function: noun
Etymology: Late Latin, repudiation, from Greek, denial, negation, from apophanai to deny, from apo- + phanai to say.
– the raising of an issue by claiming not to mention it (as in “we won’t discuss his past crimes”)
Gramps