Weather Data of September 18, 2007

I trust Dr. Holt will comment on these data relative to weather and the Broadwing flight of that September 18, 2007.

If you click on any of these graphs, they enlarge into the visible range.

Here are data from Buchanan for the month of September, 2007:



Here is the graph from Buchanan for 9/18/07:



Here is Lynchburg for the same day:


And Hollins:

Finally, Blacksburg:



Here is the count for 9/18/07 with weather mountain observations:


I continue my search for a representation of the weather that may explain flight. To that end, below are other graphs for that day which I trust Dr. Holt will comment on below (or possibly delete).

Roanoke airport:
This is "Frontal Positions":



Surface Pressure:



Frontal Pressure Tendency:


Surface wind:



Surface relative humidity:




Sorry about the graphics.

Dr. Holt responds:

I am overwhelmed as I am one who prefers to reduce the complex to its simplest terms and then build the theory from those simple terms incorporating the complexities into the complex whole the theory may entail.

Having said that, I am convinced that most people reading the above answer will be convinced that I am full of...

I am doing my own research of some data that I have selected and I will try to incorporate the data that Bill James has unexhaustively extracted from the web. But it will take awhile.

Meanwhile permit me to offer my theory of migration and how it coincides with the latest findings at the Harvey's Knob site.

I believe that because the migrating hawks especially the migrating buteos are driven to seek the means of soaring flight that the means of soaring flight are the prime determination of their behavior during migration.

There are two meteorological/topographical events that will provide the means for soaring flight. They are solar induced thermals and ridge lift. As the migrants move south they are constantly seeking either of these two means of soaring flight.

Since the solar induced thermal is the primary means of soaring flight the southbound hawk is usually moving southward via the solar induced thermal. If the southbound hawk does encounter mountainous terrain and ridge crossing winds then the hawk will elect to continue its southbound journey via the ridge lift means of soaring flight.

Mountain ridges are not leading lines for the migrating buteo unless the ridges are under the influence of ridge crossing winds. When the winds cross ridges and the ridges are aligned NE to SW then the ridge lift condition exists and the southbound buteos will be seen along the ridges. But, we must remember that the solar induced thermal condition is the rule while the ridge lift condition is the exception to the rule.

Taking that last fact into consideration requires that we consider that the migrating buteos are meandering southward on thermals most of the time. When strong winds occur in a ridge crossing direction on the few occasions they occur, the migrants will then gravitate to the ridges. They will remain in the vicinity of the ridges as long as those ridge crossing winds occur with any degree of consistency.

Therefore it is not only important to know what the weather conditions were on the day when the big flight took place it is important to know what the weather conditions were for several days before the big flight occurred.

We all know that westerly winds at the Harvey's Knob site will produce large flights but do we know what weather preceding the day of the big flight will produce an even larger flight?

Dave Holt

December 7, 2007 9:19:00 PM EST

BJ adds:

I may be wrong in trying to analyze the day of the big flight as opposed to equal scrutiny for preceding days. However, my thinking is that the Broadwings go by Harvey's Knob every year and the question is whether we can see them or not. When we do see them, it seems it is likely related to the conditions on that day. To some extent, we can research a lot of those days and, of course, we have even more data characterizing when they do not come into view. The difference between seeing them and not seeing them is certainly related to the weather and should be quantifiable. I am looking for variables that will help us quantify the 'ideal' day and I don't believe we have that model yet. To me, the biggest problem with studying hawk migration relative to weather is that you do not know how many hawks are out there. In that small window in mid-September, we know they are out there in big numbers and can use that time to effectively study hawk flight and atmospheric conditions.

I also wonder if there are not more possibilities for flight strategies. Couldn't hawks use thermals that are present on the mountain as well as in the valley? Can't they fly in a good wind through the valley without using the mountain?

If we could create the perfect day for hawk flight by Harvey's Knob, what would characterize that day?

Dr. Holt responds:


Bill and I are in total agreement about the migration in the vicinity of the Harvey’s Knob site. In my opinion the BWs are constantly moving through the latitude of the HK site during the last two weeks of September. Whether they are seen at HK site is indeed a matter of the weather conditions that occur while the watch is manned.

We must remember that we are only sampling a two mile patch of sky. A BW seen by the naked eye must be at most 3/4 of a mile away. Most of the BWs seen at the HK site are less than 3/4 mile away even those dots high overhead. Why are they in view in the first place is the question Bill and I are striving to answer.

As I mentioned above the birds are soaring on thermals during most of their migration period.
But, the heavily forested slopes of the Blue Ridge are not conducive to thermal soaring. The sun does not directly strike the western slope of the Blue Ridge during the third week of September and after the autumnal equinox it does not strike the western slope in the vicinity of the HK site at all. That leaves the crest of the ridge to be exposed to direct sunlight between the hours of 10:00 AM and 2:00 PM. The asphalt of the BRP, overlooks, and various openings and rock outcrops. A brief survey of the valley on each side of the HK site reveals a terrain rich in thermal activity. These thermal soaring possibilities on the mountain will produce large numbers of migrating Broad-winged hawks. Numbers that will continue excite the veteran hawkwatcher.

But, the average number of migrating Bws at the HK site is between 6 and 7 thousand. On 18 Sept, 2007 this number was exceeded in a single day. Also on a day during the third week of September in 1996, 5000 hawks were recorded. These are extraordinary numbers and there must be an explanation for their occasional appearance at the HK site. This is what I am interested in finding out and where Bill and I part ways a bit.

I am convinced that a continuation of ridge crossing winds for several days before the extraordinary day will influence the thermal soaring hawks out in the valley to gravitate to the ridge in numbers that when the proper conditions occur at the HK site an extraordinary single day count will occur. It doesn’t happen often and for that reason that is what I am after. I simply want to explain the extraordinary as I believe the normal may already be known.

Consider the following scenario: The bulk of the migrating BWs are migrating southward via the near constant thermal soaring condition. However the thermal density is such that fewer thermals occur within a purview of two miles at the HK site. Thermal density beyond that purview is greater therefore many thermal soaring migrants escape detection from the HK site.

What will bring the soaring migrants into view is a stiff ridge crossing wind. When such a wind occurs the thermals are shut down at ground level and ridge lift soaring becomes the dominate means of soaring. A wind speed gradient that always occurs may produce light winds at the valley floor and allow thermals to take place but they will not achieve much altitude before being blown away downwind. However, a thermal tilted by the wind toward the mountain will put even more migrants in view.

I am convinced that extraordinary days in the Broad-winged hawk migration season at the Harvey’s Knob site is due to ridge crossing winds either on the day in question or for a few days before the day in question. The data that Bill James has presented above offers absolutely no evidence to support my conclusion. I shall now invoke that overused phrase so often heard in the soft science circles; More work is needed.

Dave Holt
Dec. 8, 2007

BJ:

I am ready for more work. Perhaps my research into weather conditions on that day is premature. I am assuming there are a lot of BWs around during this period in September, likely thermaling through the valleys on both sides of us or riding the ridges to the North, and that they come into our view when conditions on a specific day are conducive to movement in our direction. If I understand your hypothesis, you suggest that a necessary ingredient for their appearance in numbers is a series of days where conditions bring them closer to the ridges and, more specifically, ridge crossing winds of substance. This appears to be testable.

But where do we look for that data for the test? We don't look at Harvey's since in the days before the big flight, they are not there. We would have to look to the North. Should I look for ridge crossing winds at Rockfish and Snickers in the week before our big flight? Or should I look for the passage of low pressure systems in the mid-Atlantic during that period? If I do, what would you suspect I will find?

12/9/07

Thanks for your suggestion to click on the graphs to enlarge them. I thought this method was only used on porn sites. Anyway, after review I find that there are no conclusions about the weather conditions for the month of September 2007 that can point to why the extraordinary flight of 18 Sept. Certainly the winds immediately preceding that day were not conducive to bringing Great Valley bound hawks to the Blue Ridge. They may have been conducive to bringing Piedmont bound hawks to the east to the Blue Ridge but that is a stretch.

Before we get into a discussion of wind blown hawks let me say emphatically that there is no such a thing as a wind blown hawk. Hawks are a flying object that has nearly zero drag. They are not wind blown. If they were those massive migrations in the Detroit area would be blown into the Atlantic Ocean when caught by the NW winds following a cold front passage. It is my belief that those hawks migrating in or near mountainous country sense the winds when they rise from their perch in the AM. If the winds are of a direction to cause ridge crossing winds the hawks will look to and gravitate toward the highest object in their purview.

The absence of ridge crossing winds of a consistency and strength during the days preceding the extraordinary flight of 18 Sept., lead me to believe that my theory above is completely wrong. And you and I have long since agreed that the hawks seen on 18 Sept., were strictly soaring on thermals. While I can account for the good viewing conditions on 18 Sept., I cannot account for the exceptional numbers of that day.

My theories have all been derived from my experience at the WRNC site during the Red-tailed hawk migration season. At that point close to the valley, when the hawks were soaring on thermals we would see them rise and catch a thermal and soar SW without moving toward the Blue Ridge. But if the winds were strong NW we would see them rise and immediately move toward the Blue Ridge to soar SW on ridge lift.

I recap that experience because I am coming to the conclusion that it is grounded in a very narrow view. Perhaps, an examination of the more northerly lookouts and their wind conditions are the next step. So if the boss (Bill James) is eager to work I would say go to the Rockfish Gap site and if possible to the Candlers Mt., site to look for something that put an extraordinary number of Broad-winged hawks in the vicinity of the Harvey’s Knob site on 18 Sept, 2007.

Dave Holt
12/10/07