Download Meteorology of Soaring Flight

An Introduction to the Meteorology of Soaring Flight
by Neil Parker - 1996
Edmonton, Canada
Introduction:
The ability of soaring birds to effortlessly employ air currents to their advantage remains one of the
wonders of nature. However, the physical properties of the atmosphere which soaring birds utilize are
quite well understood. The purpose of this paper is to provide the reader with an introduction to the
meteorology of soaring flight. This will add another dimension to bird watching and allow
knowledgeable decisions to be made regarding the best opportunities to observe soaring birds.
In order for soaring flight to be possible there must either be local upward movement of the air or the
air must have nonuniform horizontal velocity in space or time. The former includes the familiar
thermals and is the focus of this paper. The latter is more usually applied to sea birds and the
interested reader is referred to Cone (1962) for a discussion on the subject.
Definitions and Concepts:
Before proceeding it is necessary to present a few definitions and concepts. By necessity they will be
brief but will allow what follows to be more easily understood.
Moisture holding capability of air:
A unit volume of warm air can hold more moisture than the same volume of cold air. When a unit
volume of air is cooled to the point at which it cannot hold more moisture it is said to be saturated. If it
is then cooled further, some of the moisture will condense out of the air. That is why we see our breath
during cold weather. When we exhale, the air from our lungs is very moist. Once it enters the
atmosphere it is cooled. In cold weather the cooling is sufficient for it to become saturated. Further
cooling produces condensation which we then see. Clouds are the result of air being cooled below its
saturation temperature.
Lapse rate:
An understanding of the term lapse rate is crucial to understanding how birds soar during warm
summer days. As one proceeds in a vertical direction either by climbing a mountain or in a plane the
temperature of the air normally gets cooler. An exception often occurs in the winter when the lack of
solar heat allows very cold air to form near the surface of the earth. Under such cases the temperature
may increase with height. This situation is often encountered when skiing in the mountain parks. The
temperature on the ski hill may be 10 to 20 degrees celsius warmer than the air at the valley townsite.
This rate of change of temperature with height is called the lapse rate of the free atmosphere or more
generally, simply the lapse rate. It is constantly changing but the largest changes are associated with
the passage of major weather systems.
Dry Adiabatic Lapse Rate:
This is another crucial but not easily understood concept. It is a fact that as air expands it cools. This is
the principal of the air conditioner and why a propane bottle cools when in heavy use. Picture a very
thin balloon inflated with air which is gradually lifted up through the atmosphere. At the surface let us
suppose the balloon has a diameter of 1 foot. Its shape and size is defined by the pressure of the
atmosphere pushing inward on it. Because pressure decreases as one ascends in the atmosphere the
balloon will gradually increase in size as it rises higher and higher. This means the air in the balloon is
expanding and therefore cooling. If no heat from the surrounding air was allowed to enter or leave the
balloon, and we could measure the rate of cooling within the balloon we would find that it cooled at a
rate of 3 degrees celsius for every 1000 feet it ascended. Therefore if it was at 20C on the surface
and ascended 5000 feet it would cool 15 degrees (5 X 3 ) to 5C. This is the maximum rate of cooling
that is usually observed in the atmosphere and is called the Dry Adiabatic Lapse Rate. This is a
calculated value and may or may not equal the actual lapse of the free atmosphere defined above.
Stability and Instability:
Consider the previous described balloon and assume the material of the balloon has no weight but
exists only to contain the air. Accepting the fact warm air is less dense than cold air it is then logical
that if the air contained by the balloon is warmer than the surrounding air it will rise (this is the principal
of the hot air balloon). Furthermore, the parcel of air will continue to rise and cool by expansion at the
dry adiabatic lapse rate until it encounters a portion of the atmosphere warmer than itself. At that point
assent will end although the parcel will continue drift with the wind field and gradually become mixed
into the atmosphere.
A section of the atmosphere which allows the existence of such vertical currents of air is said to be
unstable. Conversely, an atmosphere in which the vertical temperature gradient is such that vertical
motions, either up or down, are suppressed is defined as being stable. The situation becomes more
complicated once condensation begins due to the large amount of heat released by the condensation
process.
Vertical Motion in the Atmosphere:
Vertical motion within the atmosphere occurs on a number of scales. Large scale assent and descent
of the atmosphere such as that associated with major weather systems is often used by birds during
migration and is briefly discussed in a later section. Significant changes to daily weather are generally
caused by large or synoptic scale weather systems such as the areas of high and low pressure shown
on the weather maps in the daily papers and on TV. Embedded within these large scale weather
systems, are mesoscale, or smaller scale weather events, some of which will be the subject of the
following discussion. Mesoscale systems include thermal updrafts resulting from heating of the earths
surface, the wind regime over hills and small mountain ranges, valley and mountain winds,
thunderstorms and chinooks to name but a few.
Flow over Obstacles:
When a horizontal flow of air encounters an obstacle whether it be a range of mountains or a stand of
trees it has two options: It can either attempt to go over the obstacle or go around it. Only the former
will be examined as our interest lies with ascending air over large or small objects.
Figure 2: Formation of the initial bubble.
Adapted from Cone (1962).
Figure 1: Flow of stable air over
a hill, a stand of trees, and a low
beach. Adapted from Cone (1962).
Figure 3: Flow within developing thermal.
Adapted from Wallington (1977).
Figure 1 is a modification of diagrams which appeared in Cone (1962) and indicates the type of flow
which might be expected to occur when a horizontal flow of stable, non saturated air encounters a
small hill, a stand of trees or a beach. The beach does not necessarily have to be associated with an
ocean. The lines indicate the direction the air might be expected to take while the distance between
the lines indicated the relative velocity of the wind. The closer the lines are together the stronger the
wind. In the case of the hill there is a noticeable acceleration near the top of the hill which decreases
with height above the hill. There is also an area on the lee side of the hill where the air is descending.
obviously not a favourable location for soaring. An increase in wind speed can be noticed on even
small hills making them ideal places for kite flying as well as local soaring.
Under certain atmospheric conditions lee or mountain waves develop downwind from a range of
mountains or high hills. These will not be discussed here but are well covered in Wallington (1977).
For large hills or mountains the keen observer will quickly be able to identify those cases where the air
is moist as cloud and rain may form when the air is forced upward. Additionally, if the free atmosphere
is unstable, the air forced upward by the mountain will continue to rise and large cumulus clouds,
showers and perhaps thunderstorms may form.
A stand of trees does not present an obstacle to the wind however, and a small area of lift will still
develop on the upwind side. Air near the surface enters the trees and the energy dissipated by
movement of the branches, leaves, etc.
The beach in Figure 1 presents an even smaller obstacle but will still produce a area of local upward
motion. However many ocean shorelines rise sharply and strong lift will extend for many miles.
The picture of the three frigates at the top of page 1 was taken along a low shore line but persistent
easterly trade winds allowed the birds to soar for hours without having to resort to flapping flight.
Dry Thermals:
In order for soaring land birds to scan large areas for food it is necessary for there to be a more
general source of lift than that provided by the static objects previously discussed. Fortunately nature
has provided such a source in thermal updrafts generated by the summer sun. The following
discussion will be limited to the case in which the air remains unsaturated throughout its assent and
subsequent cooling. That is, cloud is not observed to form.
Consider the case of a typical summer morning. Overnight the air near the surface has cooled and can
be considered to be stable. As the sun gradually warms the surface, heat is quickly transferred into a
shallow layer near the surface. As more heat is added by the sun this shallow layer will eventually
become warmer than the air above it and becomes buoyant. A portion of this warm layer will
eventually protrude into the cooler air above it and a thermal dome will form. More warm air will quickly
begin to flow into the area and the dome will grow as indicated in Figure 2. These domes form in a
random fashion much as bubbles form in a pot of boiling water.
As the warm air flows into the dome it begins to rise. Within a few minutes, the local source of very
warm air is exhausted, it pinches off at the base, and the thermal will rise as a bubble of warm air
detached from the surface. The bubbles or dry thermals will rise until they reach the top of the 'thermal
layer', or that portion of the lower atmosphere having a lapse rate equal to the dry adiabatic lapse rate.
This layer will initially be very thin. However, as heat is added by the sun and transported aloft by the
thermals, the base remains on the surface while the top of the layer will continually rise during the day
reaching 2000 to 3000 feet by early afternoon and frequently higher depending on how much heat can
be added to the atmosphere. This thermal layer defines the portion of the lower atmosphere in which
dry thermals will exist. As a thermal rises the air within it begins to rotate as shown in Figure 3. The
core of the thermal is filled with the warmest air and also contains the strongest updraft. As it rises the
thermal spreads laterally. Wallington (1977) states that the thermal will reach a diameter of 500 to
2000 feet when it between 1000 and 2000 feet above the surface. We are then talking of a area of
ascent in the order of 1/3 mile or 1/2 kilometre in width.
Figure 4 is an attempt to give more detail to the motions inside the
thermal and is based on a model presented by Cone (1962). Note
that because of the large size of the thermal a bird is able to soar
entirely within the centre updraft.
When examining Figure 4 one must keep in mind that the entire
bubble of air is rising and even the areas of downward motion
within the bubble may still be ascending relative to an observer on
the surface.
Figure 5 shows the lower portion
of the atmosphere during a warm
summer afternoon and may aid in
understanding a complex
situation. If, during the summer
months the weather pattern is not
changing (synoptic systems are
not moving across the area),
most of heat carried aloft one day
will remain overnight. In such a
scenario only a shallow layer of
air near the surface will have to
warmed the next morning before
strong thermals can redevelop.
Anyone who has flown in small
aircraft during a hot summer day
can attest to not only the
presence of, but the strength of
these dry thermals. They can also
make airport approach very
uncomfortable in a large
commercial aircraft.
The addition of moisture quickly complicates the situation as considerable amounts of heat are
released during the process of water vapour condensing into water droplets. The presence of cloud
will mark the location of the individual thermals. Keep in mind that these are not connected to the
ground so although one bird may be able to soar within an elevated thermal a similar bird at a lower
level may have to revert to flapping flight until it locates another thermal. These type of observations
assisted meteorologists in developing the theory of thermals.
Migration of Soaring Birds:
Heintzelman (1986) presents a diagram which shows strong correlation between cold frontal passages
and fall hawk migrations over Nevada. He also sites other studies which link fall migrations with the
passage of cold fronts over the eastern provinces and states. The reason being that an area of assent
develops along most cold fronts and moves southward with the front. Additionally, persistent northerly
winds normally follow the passage of cold fronts and the still warm earth heats the colder air producing
thermals. The literature examined did not contain examples from western Canada but a similar
situation may exist. A logical extension of the previously discussed isolated hill is a range of hills or
mountains. The Appalachian Mountains extend in a northeast to southwest line and with prevailing
westerly winds the various ridges and mountains that form the Appalachians serve as hawk migration
routes (Heintzelman, 1986).
The spring northward migration is not so easily correlated with weather conditions but warm southerly
winds have been shown to produce hawk migrations over eastern Canada and northeastern U.S.
The circulation around an area of high pressure is clockwise and counterclockwise around an area of
low pressure. Given the above, the large scale or synoptic weather pattern which would produce such
a flow east of the Rockies is an area of low pressure over southern British Columbia and higher
pressure over Saskatchewan and Manitoba. If an area of high pressure is pushing southward through
the northern prairies it is likely to generate northeast to easterly winds over central Alberta; an
unfavourable condition. As it moves southward into the east central prairies winds will swing into a
more south to southeasterly direction which should be more favourable. Major storms moving
eastward into Alberta are likely to produce cloud and possible precipitation which may suppress
migration. On the other hand relatively weak systems tend to deposit much of their moisture on
mountains while producing light to moderate southerly winds east of the mountains while patchy or
thinner cloud cover will still allow thermals to develop. The passage of such systems is often marked
by a rapid shift to a west or northwest wind.
The complete weather forecast issued by Environment Canada and available from some radio
stations, weather radio and off the Internet ( http://www.mb.ec.gc.ca ) generally provides enough
information to determine such days.
Summary:
Soaring birds may use rising air currents produced by objects such as hills, bluffs, stands of trees,
mountain ridges, etc for static soaring. Soaring birds will also use the lift produced by properly oriented
ranges of hills or mountain as a migration.
Thermals which develop during light or moderate wind conditions present a method for soaring birds to
move quite freely over large areas.
Studies show that at least over eastern North America the passage of a cold front will trigger a fall
southward migration.
In western Canada, spring migration most likely depends on a southern flow of air produced by large
scale weather systems. It would be enhanced by the presence of thermals. Such conditions most often
develop with the approach of an area of low pressure from the west and, or, the location of higher
pressure over the southern portions of Saskatchewan and Manitoba.
Finally, although the information presented in this paper should allow the complete local public
forecast to be interpreted so that the most favourable soaring days can be identified it will not replace
a few hours spent studying a good book on basic meteorology. Such books are available at most local
libraries and if not can be obtained on inter library loans. The Flight Strategies of Migrating Hawks by
Paul Kerlinger and Meteorology for Glider Pilots by C. Wallington are recommended. The books by
Wallington and Heintzelman and paper by Cone are available through the Edmonton Public Library.
Bibliography:
Cone, Clarence D., 1962: The Soaring Flight of Birds. Scientific American, April.
Heintzelman, Donald S., 1986: The Migration of Hawks. Indiana University Press.
Kerlinger, Paul, 1989: Flight Strategies of Migrating Hawks. The University of Chicago Press.
Sutton, O.G., 1960: Understanding Weather. Pelican Books.
Wallace, John M. and Peter V. Hobbs, 1977: Atmospheric Science, Academic Press.
Wallington, C. E., 1977: Meteorology for Glider Pilots. J. W. Arrowsmith Ltd.