Let the Rain Come Down
Crops aren't going to be very good this year in the midwestern United States and the Canadian prairie provinces. Here in Alaska, we'll probably see an increase in food prices; in other parts of the world dependent on American grain for food, there may well be starvation. Commentators have speculated that we may be seeing the first stages of the warming expected from the increase of carbon dioxide in the atmosphere. About that we can't be sure. What we do know is that the midwest drought is directly due to a large anticyclone --an area of high pressure and clockwise rotation -- stalled over central North America. But how does an anticyclone keep it from raining?
The problem is not lack of water vapor in the air. The absolute amount of water vapor in the air over the sun-baked midwest is probably greater than that over interior Alaska (where there's been more than enough rain this year). The problem is getting that water vapor to fall out as rain.
The key in making water vapor condense into clouds is cooling the air. The lower the initial relative humidity, the more cooling is necessary, but even the driest air will form clouds if it is cooled enough, and even the wettest air will not form clouds if it is not allowed to cool. Air can cool by being in contact with something cold or by radiating heat to space, but the first process is too small-scale to produce anything but local fog, and the second is very slow -- a degree or two a day at the most. Air also cools if it is lifted and expands, and almost all rain and snow results from this process.
Vertical movement of large masses of air has another effect. An air column moving upward is stretched as it moves, and becomes less stable. Air moving downward is compressed, and becomes more stable. This helps explain why downward-moving air is often associated with air pollution: smoke, particles and fumes tend to stay put.
In an anticyclone, air is moving downward. An anticyclone has higher pressure in its center than around its edges, so the air tries to flow away from the high-pressure core. But the rotating earth forces the air to turn to the right in the northern hemisphere, until it is blowing clockwise around the high pressure. Near the ground, friction interferes with this process, and some of the air near the ground succeeds in leaving the anticyclone. Air from higher up in the core of the anticyclone moves downward to replace this lost air, producing the general downward movement.
In a cyclone, with low pressure at its center, the flow is just the opposite. Inward motion at the base of the cyclone results in upward movement at the core, and a general destabilizing of the air. Clouds and precipitation are common, and the lack of stability encourages summer thunderstorms.
In the anticyclone over the midwest, the downward-moving air is warming and drying. At the same time, the air is becoming more stable, which keeps the heating of the ground by the sun from producing thunderstorms.
Now and then the anticyclone is disrupted enough to allow a few thunderstorms to form, but they aren't doing the crops much good. The longer a drought lasts, the more of a crust forms on the surface of the ground and the more weakly the dying crop roots can hold the soil. Rain from a thunderstorm arrives too fast for it to soak into this crust. Instead, much of the precious moisture runs off, all too often taking the topsoil with it. At this point, even a soaking rain would be too late to save many crops.
We really don't know why the anticyclone has planted itself where it has this summerr. Modeling of the effects of small increases in carbon dioxide is not yet complete enough to say whether this exact change is to be expected, though models with large increases in carbon dioxide do indeed predict midwest drought. We should at least consider the possibility, however, that this summer is a warning of what to expect in the future.
