Sources of rising air

Some Sources of Rising Air Conducive to Hawk Migration

Migrating hawks are extremely dependent upon a feature of meteorology that I will call rising air. That is that certain areas of the earth’s surface are more capable of causing parcels of air to rise than others for a variety of causes including man-made. The dependency of the migrating hawks upon the rising air is due entirely to the fact that of all the bird of prey species the hawk species are the most capable of soaring. That is, they can achieve and maintain altitude with a minimum of wing flapping and in most cases no wing flapping at all. So, it would help the hawkwatcher to know the sources of rising air and how they may be found. The following discussion intends to do just that.

The sources of rising air are first listed in the order of their strength. It must be remembered that the strength of the rising air is dependent upon the concentration of rising air with reference to the area of the earth’s surface that causes the air to rise. As with all energy sources the density of the source is the prime factor. Here are the sources I have observed and if others exist this blog is open to your input.

1. Solar induced thermals
2. Ridge lift
3. Cold front passages
4. Sea breezes
5. Atmospheric low pressure cells

Let us examine each one of these sources of rising air to further round out our understanding of rising air. It begins like this:

Solar Induced Thermals: When the sun heats the earth’s surface unevenly it causes the latent heat of the surface to be uneven. The air in contact with the surface is therefore heated unevenly. Thus, parcels of air are warmer than the surrounding air and those parcels of air will rise. While those parcels rise they are replaced with the surrounding air rushing in to fill the void. That air is then super-heated and also begins to rise. Thus a fairly narrow column of rising air is created as long as the sun strikes that particular patch of ground. The width of the column is dependent upon the amount of terrain that became super-heated. But, for our purposes, it will be a somewhat narrow high concentration of super-heated air. The solar induced thermal will provide the most energetic and constant supply of lift energy of all the sources of rising air.

A: Man made thermal: The cooling towers of a nuclear power plant produce super-heated air, rising from a small area which is just the exact description of a solar induced thermal and yet it is independent of whether the sun shines or not. Even the ever present, cumulus cloud will cap the air rising from a cooling tower. By all means these towers do produce man made thermals that have all the characteristics of a solar induced thermal.

Ridge Lift: Ridge lift describes the condition when a high pressure cell and low pressure
Cell straddles a large area of terrain thus causing horizontal winds to blow. These winds are particularly interesting to the hawkwatcher station in the mountain ranges. As the wind blows across the mountain ridge it is deflected upward on the windward slope. This deflection creates rising air on the windward slope. The energy of this source of rising air, while not as concentrated as a solar induced thermal, can be spread all along the windward slope of the ridge thus making this source of rising air as important to the hawkwatcher as the solar induced thermal. It should be noted that solar induced thermals and ridge lift are not compatible. Horizontal winds will spread the sun’s heat more evenly over the earth’s surface thus defeating the condition necessary for the generation of a solar induced thermal. But, it should be noted that solar induced thermals are the condition of rule and ridge lift winds are the exception to the rule.

A: Mountain Waves (Lee Waves): Mountain waves are a phenomenon of ridge lift. They occur at a time when the ridge lift winds are strong and steadily blowing across the ridges. Other weather factors combine to create the mountain waves to the extent that sailplanes reach altitudes of 30,000 feet over the western mountains and 16,000 feet over the eastern mountains. The conditions for mountain waves are rare but they do occur and anytime a lenticular cloud is seen over the mountains there is a good chance that mountain waves are occuring. If the hawks utilize this condition they will not be seen by the hawkwatcher as the maximum lift air will be displaced to the lee side of the mountain and the hawks will be very high. The mountain wave is probably the best condition for migrating hawks to escape the hawkwatcher's view when the hawk is within the two mile purview of the hawkwatcher.

Cold Front Passages: A front is the boundary between a cold air mass and a warm air mass. Cold air masses usually are in motion descending from the north. As they advance the denser cold air is hugging the ground and it displaces the less dense air of the warm air mass upward thus creating vertical instability in the atmosphere near the earth’s surface. It is such instability in the atmosphere that makes for rising air and should always be of interest to the hawkwatcher. The cold air acts as a wedge and pushes the warm air upward all along the front thus there is a great deal of energetic rising air along the front of a cold air passage. The problems for the hawkwatcher are the conditions ahead of the advancing cold front. They are usually rain or snow or any other condition limiting visibility. The advance of a cold front may provide good hawkwatching but the ability to extract that data is severely limited by the intensity of the frontal passage. Nay, It is what follows the cold front passage that enhances the hawk counts. Especially in the mountains as ridge lift conditions are usually created with the passage of a cold front.

Sea Breezes: At the coast the sun is heating the ground inland and the water out to sea simultaneously. The ground absorbs more heat and therefore becomes super-heated, i.e. it becomes warmer than the air over the sea. Super-heated air is less dense and therefore constitutes a low-pressure area. The air over the sea is cooler and therefore constitutes a high-pressure area. Therefore air moves from the sea to the land. This movement of the air will contain a windward move from sea to land but more importantly it constitutes a diminished frontal movement, where the cooler air is moving underneath the warmer air thus producing rising air. The folks at the coastal watches swear by this effect. It occurs often in that environment.

Atmospheric low-pressure cells: A low-pressure cell defines an area where the air pressure is lower than the surrounding areas. If the air pressure is lower in this area then that must mean that air is leaving that area. The air cannot leave such an area horizontally because the surrounding air is much denser. Therefore the air in a low- pressure cell is rising. But a low-pressure cell can occupy several tens of miles, perhaps, several hundred miles, therefore the rising air from a low-pressure cell is relatively weak.

But, it is rising and it will rise so long as the low-pressure cell exists. In short it is a definite source of rising air for a long enough period of time to be operational for soaring hawks. It is my belief that such a condition existed when several large Broad-winged hawk flights occurred when visibility was low due to rainfall and fog.

As far as the hawkwatcher is concerned these sources of rising air are pertinent to the soaring species. All hawks are capable of soaring but those species occupying the buteo genus are absolutely dependent upon rising air as they spend their entire time in migration utilizing rising air wherein they can soar. In the southern Appalachians where I have gathered much data, the Broad-winged hawk and the Red-tailed hawk, are the species that produce the greatest numbers in migration. I have seen very few of these species flap their wings during their migration flight. They prefer to soar and they actively seek soaring conditions. Soaring conditions include rising air, ergo the importance of understanding the sources of rising air.

David Holt

Dec. 9, 2006