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Temperature and water-vapor content soundings for some famous dates.
Most v.h.f. operators develop weather consciousness before they have been in t he game very long. They know that the daily weather maps can give indications of possible favorable propagation. Here W2QBB shows the actual upper-air conditions needed for 2-meter DX, and presents some soundings taken in areas where long-distance contacts were made.
The v.h.f. man is well aware that a "temperature inversion," "steep water vapor gradient" or some such thing is necessary for tropospheric propagation of his signals over extraordinary distances.
W2BAV in his article, "Painless prediction of two-meter band openings" (QST, October, 1949), pointed out the correlation between surface weather conditions and some good 2-meter openings. Similarly, this article will show the vertical distribution of temperature and moisture for times of good 2-meter DX.
Consider first the variation of temperature and water vapor with height as shown in Fig. 1. There are no temperature inversions (that is, an increase of temperature with height(1)), and no sharp changes in the water vapor curve - just a steady decrease with altitude from a maximum at the earth's surface.
Fig. 1. U.S. standard atmosphere temperature curve. The water-vapor curve is one that would result if the relative humidity were 70 per cent from the ground elevation to 12,000 feet. Figures in parentheses in this and following drawings are values of mixing ratio.
If a radio wave is to remain near the earth's surface and not be lost to outer space, some downward refraction is necessary. There would be no v.h.f. DX with the average sounding of Fig. 1, but the refraction under such conditions is sufficient to extend the v.h.f. range somewhat beyond the line of sight. A condition known as superrefraction (that is, more than ordinary refraction) is needed for real tropospheric DX.
Now that we have seen what won't produce 2-meter DX, let's look at a sounding that has a superrefracting layer in it. Fig. 2 is a plot of an actual upper-air sounding obtained at Joliet, Illinois, on September 6, 1950. The data were obtained from a radiosonde instrument released at 2200 EST. The moisture content of the air is shown by plotting the ratio: grams of water vapor per kilogram of dry air. This is called the mixing ratio. In this sounding and those to follow, the altitude scale shows the height above sea level of the significant points. Therefore, the first point on a curve is at the elevation above sea level of the station and not necessarily at zero altitude. No water vapor scale is shown as it would be different at each altitude. At certain minimum values of relative humidity, the radiosonde instrument transmits only a very low audio frequency. Where this occurs, MB (motorboating) is shown on the water vapor curve.
Fig. 2. Upper-air sounding made at Joliet, Ill., September 6, 1950, at 2200 EST. Superrefraction resulted from the sharp decrease in water-vapor content (CD). Points marked (MB) indicate motorboating in the radiosonde unit at low relative humidity levels.
Superrefraction occurs with either (1) a temperature inversion exceeding 2.8° centigrade per 100 feet or (2) a rate of decrease of water vapor exceeding 0.5 gram per kg. per 100 feet.(2)
In Fig. 2, the temperature at the inversion AB is seen to increase from 13°C to 18°C through an altitude difference of 800 feet. The gradient is therefore (18-13)/8 = 0.6°C per 100 feet and is insufficient for superrefraction. At CD on the water-vapor curve, there is a decrease from 6.2 grams per kilogram to 1.9 grams per kilogram through an altitude difference of 400 feet. The gradient is (6.2-1.9)/4 = 1.1 grams per kilogram per 100 feet, giving us a level of superrefraction.
Now let's see how some soundings check out with various 2-meter band openings.
On the evening of September 6, 1950, W2NLY, Oak Tree, New Jersey, worked W9EQC, Aurora, Illinois, a distance of nearly 750 miles. The nearest available sounding for the eastern end of this path is one made at Albany, New York. It is shown in Fig. 3. The sounding of Fig. 2, already discussed, is representative of conditions at the western end of the path.
Fig. 3. Upper-air conditions at the eastern end of the W9EQC-W2NLY path are shown by this sounding made at Albany, N. Y., on September 6, 1950, at 2200 EST. Superrefraction at 4000 feet altitude is indicated by the water-vapor curve.
On October 30, 1950, W4HHK, Collierville, Tennessee, contacted W3NKM, Pittsburgh, Pennsylvania, about 650 miles. The Pittsburgh sounding is plotted in Fig. 5, while the Nashville, Tennessee, sounding in Fig. 4 shows the probable conditions at Collierville.
Fig. 4. Some idea of the upper-air conditions at Collierville, Tenn., can be gained from this upper-air sounding made at Nashville, Tenn., on October 30, 1950, at 2200 EST. The water-vapor gradient at 5000 feet was more than four times that necessary for superrefraction when W4H11K worked W3NKM, Pittsburgh, Penna.
Fig. 5. Conditions at the eastern end of the W4ITHK-W3NKM QSO are shown by this sounding made at Pittsburgh, Penna., on October 30, 1950, at 2200 EST. The rate of decrease of water vapor with height beginning around 2000 feet altitude is about 75 per cent greater than needed for superrefraction.
In none of these soundings do the temperature inversions meet the criterion for superrefraction, but in every case there is a superrefracting water-vapor gradient.
With only surface weather data available to him, W2BAV expressed the view that a South Carolina-Michigan contact was apparently missed for lack of activity on July 23, 1949. While Michigan conditions were favorable as shown by the Toledo, Ohio, sounding in Fig. 6, conditions in South Carolina were something different. The Charleston sounding (Fig. 7) does not show any water-vapor gradient steep enough nor any temperature inversion strong enough to produce superrefraction.
Fig. 6. Upper-air sounding made at Toledo, Ohio, on July 23, 1949, at 2200 EST. The water-vapor gradient between 3000 and 4000 feet is more than three times that needed for superrefraction.
Fig. 7. This sounding made at Charleston, S. C., on July 23, 1949, at 2200 EST shows why a Michigan-South Carolina contact was not made in spite of good conditions in the northern states. Only weak temperature and water-vapor gradients are evident.
From this it may be seen that upper-air sound ings are the only reliable source of information as to where and when tropospheric DX may be worked. The isobars (lines of common atmospheric pressure) shown on daily weather maps provide good clues, but they are not infallible, as any experienced and observant v.h.f. man will testify.
Notes
- An inversion is sometimes considered to exist if the temperature lapse rate (decline in temperature with altitude) is less than 3 degrees Fahrenheit for 1000 feet of altitude. - Ed.
- H. G. Booker, Compendium of meteorology, pages 1290 to 1295, published by the American Meteorological Society, Boston. Mass.
James S. Collier, W2QBB.