on the
very highs

contents...

M0BLF Homepage
QSL Card
Articles & Archives
DXCC List
IOTA List
Foundation Licence

sections:

1. Introduction
   
2. Temperature Inversions
   
3. Continental Air-Mass Inversions
   
4. Localized Convection Inversions
   
5. Smoke and Dust Layers
   
6. Cloud Stalking
   
7. Diagrams of a 'normal' wave and one undergoing 'bending' (Supplied by myself, not the article)

'On The Very Highs'
Conducted by E.P. Tilton, W1HDQ
Source: QST, July 1944, Pages 42-45
Copyright: ARRL 1944, Expired 1994.

[Notes for the modern amateur beginning in this field are provided as hyperlink footnotes, with hyperlinks to return the reader to the paragraph he was reading.]

Shortly after the outbreak of war, we proposed, in these pages, a study of aerology as a substitute for amateur operating activity -1-. The response we received indicated a considerable interest in the subject, but this was at the time when the submarine menace along our coasts was at its height and, as a result, we were scarcely even permitted even to print the word "weather," much less to go into a discussion of the means by which weather may be foretold by observation of v.h.f. and u.h.f. -2- signal variation. Our first complete column devoted to the subject, presenting material similar to that which follows, was withheld from publication at that time, in compliance with censorship.

Now, with wartime developments accentuating the rôle that v.h.f. and u.h.f. portions of the spectrum are destined to play in the future of all forms of radio activity, it behooves -3- us to learn as much as we can about the effects of weather on propagation -4- at these frequencies. So if the experienced weathermen in the audience will find something else to do for a few minutes, we shall attempt a sketchy review of some of the points where observing amateurs in both aerology and radio meet on common ground.

Like all sciences, aerology can become very involved in mathematics if one goes into it deeply enough; but, unlike most scientific pursuits, it also affords many opportunities for interesting work for anyone who is equipped with nothing more pretentious than a barometer, a thermometer and a good pair of eyes. If the person so equipped happens to be a ham who is casting about for something to do to while away a spare hour now and then, he will do well to get hold of a receiver which is capable of tuning the frequencies above 30Mc. -5- With nothing to be heard on Five, -6- and with probable reception on 112-Mc limited to the WERS test periods, -7- the f.m. broadcast band now offers undoubtedly the best opportunity for observation of variations in v.h.f. conditions.

By the end of 1941, most of us had already become used to regarding strong v.h.f. signals from beyond visual distances -8- as an advance warning of a storm on the way, but many were content to leave the matter there. Perhaps there would have been more interest in daily operation on 56 and 112Mc in the dear, dead days prior to December 7th -9- if more of us had taken the correlation between radio and weather a little further.

Temperature Inversions

As most extensions of the normal operating range of the v.h.f. stations result from a temperature inversion of one sort or another, we should first understand what this term means in order to recognise the visible and audible effects associated with it. We all know that air temperature normally drops with an increase in altitude. The year-round average rate of decrease for the world's atmosphere is about 3F -10- for each 1000ft of altitude. Whenever the rate of decrease (commonly called the lapse rate) is less than this figure, an inversion may be said to exist, even though the air aloft may not actually be warmer than that at ground level. Under certain conditions, to be outlined later, the temperature up to several thousand feet may be several degrees higher than the ground reading.

Bending of v.h.f. waves arises from the fact that the top of the radiated wave hits warmer hits the warmer and rarer -11- medium first, and thus is accelerated. As the warmer air seldom is at any great height the bending usually serves merely to keep the wave travelling more or less parallel to the earth's surface, although in some cases something approximating a skip-zone -12- is noticed. A pronounced inversion at lower altitude, a common occurrence in warm weather, may increase the strength of a signal from the other side of town; while the maximum distance at which atmospheric bending, uncomplicated by other factors, has resulted in two-way work -13- on 56 Mc. is somewhere between 350 and 400 miles. It is at distances between 75 and 200 miles that the effect of temperature inversions seems to be most pronounced, and the degree of variation in signal strength from night to night is greater with increasing frequency.

Temperature inversions may be said to fall into two general classifications: those resulting from air-mass movement which may be continental in nature, and the more limited type resulting from localised atmospheric convection. This is not the place to go into an involved discussion of air-mass analysis -14- other than to say that a temperature inversion results when a mass of warm, moist air (such as that originating in the Gulf of Mexico or the Pacific) over-runs a mass of cool, dry air of polar origin -15-. This type of inversion can develop anywhere in the United States, at almost any time of the year. As warm, moist air is of low density, it over-runs the heavier cold air whenever the two types come into contact. The sloping "discontinuity" -16- thus produced means interesting times for the v.h.f. enthusiast.

Continental Air-Mass Inversions.

A typical weather cycle may consist of a period of cool, clear weather, with high barometer and good visibility for two or three days, followed by gradually increasing cloudiness and warmer weather until the advent of a storm. Toward the end of the fair stage, the strength of v.h.f. signal from points beyond the visual horizon will show a gradual improvement, increasing until such a time as the storm breaks, wiping out the inversion condition -17-. Almost everyone knows what direction storms come in his own locality. By watching the clouds -18- and listening for the variations in strength of stations in that general direction, the approach of a change in the weather may be foretold as much as several days in advance.

If one is fortunately situated as to altitude of the receiving location, this correlation is particularly striking. An example is the prewar location of W1HDQ, which was several hundred feet above the surrounding countryside with a clear path of fifty miles or more toward the southwest. Here the signal-weather relationship showed up beautifully on 56Mc. and 112Mc. and, to a certain extent, on 28Mc. also.

During the evening following a clear day in spring, for instance, 56-Mc. signals might be heard from points as far southwest at Washington, D.C. - more than 300 miles away. Stations all along the line to Philadelphia, about 200 miles in the same direction, would also be very strong, but signals from the New York area, 125 miles away, would be only slightly above normal. This would place the maximum inversion area below New York.

By the following evening, we would have lost the stations below Philadelphia as the rain moved up from the southwest, wiping out the inversion as it came towards us. By this time, the amateurs in Springfield and Hartford would be working into the New York metropolitan area, with ordinary low-elevation locations -19- at both ends of the 100-150 mile path.

Around this time, we would be noticing an increase in the strength of signals from the Boston area and up into New Hampshire, some 90 miles to the northeast. If our period of observation was continuous, we would note a sudden drop in the strength of the New York signals, coinciding almost exactly with the arrival of rain in that area. Just preceding this some of the New York stations would have been working up into the Boston area. Cloudiness would have become general over Western New England by this time, and Boston-area signals would be very strong, even though 1200-foot elevations intervene along the path. Until the storm broke signals from the east and northeast would remain strong, though it would have become impossible to hear any but the strongest stations to the southwest.-20-

Thus far we have been dealing only with the continental air-mass type of inversion, the idealised case described above occurring most frequently during the late spring or early fall -21-. In winter we find the air-mass inversion in a practically pure state, although the extremes of bending are not so great as in warmer weather. In midsummer the picture can be complicated by so many factors that it took several months of operation from our Wilbraham Mountain location before we could resolve any very reliable system for predicting weather by radio conditions, or vice versa.

With certain variations, which each observer will have to learn for his own locality, the appearance of the clouds and the movements of the barometer furnish quite reliable warnings of the approach of good v.h.f. weather. With the exception of midsummer, a rising barometer means increasing signal strength, while a falling or below barometer indicates lower signal levels. -22- The appearance of cirrus clouds, those beautiful wispy "mare's tails" high in an otherwise blue sky, often gives advance warning of the approach of a storm by as much as 36 hours. The old saying about "mackerel scales and mare's tails" is useful for v.h.f. enthusiasts as well as for mariners. A hazy ring around the moon, evidence of warm, moist air aloft, has been recognised as a good sign by five-meter men since the earliest days of work on that band; and the periods around both full moon and new moon are invariably times of improved signal strength. "New moon, full moon, and over thirty -23- on the barometer" is a good rule-of-thumb basis for predicting good times on the very-highs.

Localized Convection Inversions

We come now to the more localised type of inversion, a product of hot weather, which gives the boys fortunate enough to be located along our sea coasts, or near the Great Lakes or other large bodies of water or even adjacent to heavily forested areas, some of their most exciting moments.

We all know that heated air rises; when it does, other air must come from somewhere to take its place. Cool air is heavier (at higher pressure) and therefore will tend to flow in to replace air which has risen as a result of heating by direct radiation from the sun or reradiation from the earth. Thus, when we have a clear, hot day along the sea-coast, by noon there is a fresh cooling breeze coming in from over the ocean, the air inland having been heated sufficiently to cause it to rise. The heated air, in turn, flows out over the ocean at high levels creating an inversion condition which holds well into the evening of practically every fair summer day -24-.

V.h.f. men who operate from points remote from the ocean know that mid-afternoon is the time during which the lowest signal strength of the whole day occurs, but the enthusiast located on Cape Cod, Long Island or the California Coast will find things interesting practically all day long in summer time.

For the man located further inland, the two hours around sunset will show strong signal peaks any sunny day in warm weather. As the earth cools more rapidly than the air above it, an inversion develops close to the ground shortly after sunset in almost any locality. The chilly spots frequently encountered when riding through open country at dusk after a hot day constitute a familiar example. The effects of seacoast inversion described above may show up at points as much as 100 miles or more inland as the evening wears on; and when this sort of thing is combined with an inversion resulting from the approach of a tropical air-mass, v.h.f. enthusiasts experience an evening which goes down in the history of "big nights." -25-

The coastal inversion is a phenomenon well known to the occupants of the 112-Mc. band along the Californian coast. The tremendous updraft of warm air from the superheated desert country far inland moving out over the relatively cold Pacific produces a degree of bending of 112-Mc. signals seldom equalled elsewhere in the United States. So pronounced is the bending that flea-powered -26- pack sets operating in the hills near Los Angeles frequently worked into San Diago, more than 100 miles away, and two-way work between home stations in San Diago and Los Angeles, using nothing more than simple receiving-tube rigs -27- and half-wave antennas, -28- was a common occurrence during the peak of the inversion season. -29-

The coast of Northern New England is blessed with very cold water, as Maine vacationists will testify -30- . Although the country inland is not heated to the extent that prevails in California, the "inversion by subsidence" is a daily affair in midsummer. A rare combination of subsidence and air-mass inversions prevailed on that now-famous date of August 21, 1941, when W2MPY made the long climb to the summit of Mt. Katahdin in the heart of Maine, arriving on the scene at the peak of the bending. The result is now history: a dozen or more contacts beyond the former 255-mile record for 112-Mc. work, and a 335-mile QSO -31- with W1JFF at Newport R.I., -32- - a record which will take some beating when we get back to such things again.

In describing his Katahdin experiences, W2MPY reported that on his trip, and on many others when he had worked long distances from high elevations, he was actually able to "see the inversion", a phenomenon also observed by your conductor in his mountain-climbing days -33-. It took an aerology textbook -34- to give us the answer to this one. In practise, its appearance has turned out to be one of the most reliable visual warnings of the existence of both subsidence and air-mass inversions.

Smoke and Dust Layers

During hot weather, particularly after a dry spell, the rising heated air carries aloft a considerable amount of dust -35-. When this dust strikes an overrunning layer of moist, tropical air it can rise no further, with the result that it spreads out at the bottom of the warm layer, clearly marking the height at which v.h.f. signals are bent. In industrial areas, smoke in the air serves the same purpose, with the added advantage that the smoke generally rises whether there is any appreciable warming of the earth's surface of not. We have observed the occurrence of this brownish-grey smoke/dust layer on many occasions, both in winter and in summer, and it has never yet failed as a warning of a storm on the way. Its appearance may give 36 hours or more warning of a change in the weather (from fair to stormy) in the summer, when air-mass movement is slower, and 12 to 24 hours in winter. It should be an equally good harbinger -36- of periods of strong v.h.f. signals.

The foregoing is just a meagre glimpse of the vistas which will be open to the v.h.f.-minded amateur after the war, but it will serve to show that operation on the very-highs can be a whole lot more than a nightly round of friendly chats with the gang in our own locality, important though these contacts loom in our picture of amateur radio as a source of relaxation and enjoyment.

The events of December 7, 1941 interrupted ten years of continuous operation on the v.h.f. bands on our part, but we've found this weather business a splendid antidote for that empty feeling we used to get when we looked across the Connecticut River Valley to that tower -37- of ours high up on Wilbraham Mountain.

Cloud Stalking

Most of the time during our six-month sojorn [sic] -38- in Key West, now concluded, we kept an eye peeled for interesting cloud formations, trying all the while to image what operation on 56 and 112 Mc. would have been like down there. There, as elsewhere in open flat country, the general trends were much easier to discern than in rugged country like our native New England; and therein lies the beauty of cloud-chasing as a hobby. If one takes the trouble to learn a little about aerology (and he need not get to the calculus-and-slide-rule stage) he will still have to figure out the conditions for his own locality himself when it comes to tying weather and radio "signs" together.

Since Pearl Habor we've been finding an ever-increasing interest in books dealing with the weather, and we've been stalking clouds with a camera whenever we could get the necessary film. We find this a most enlightening pursuit, especially when carried on in conjunction with a daily log of radio and weather observations, even though the latter may include nothing more than an occasional glance at the barometer and a check on the gyrations of the weather cock on the steeple of the town-hall. Operation on the very-highs is going to be a source of even greater pleasure and satisfaction, as a result of this enforced interlude, when the current unpleasantness is over -39- .

Before this material appears in print we shall have returned to Boston, where we will remain for a short period, following which we expect to be off on another assignment afield, this time outside the continental limits of the United States.

We shall continue to submit copy whenever time and conditions permit, and we will especially appreciate hearing from any of the amateur fraternity who cares to write. For the time being, the mailing address had better be: c/o ARRL, 38 LaSalle Road, West Hartford 7, Coan.

Cloud types:

Early morning radiation fog, as it appears here filling the valleys in Vermont's Green Mountains, indicates a mild temperature inversion of the local type, with fair weather and moderate v.h.f. bending in prospect.

The familiar fair-weather cumulus, appearing at middays or in the early afternoon as a result of warm air rising at a moderate rate, will dissipate in the late afternoon. Prediction: fair weather and moderate v.h.f. bending.

The rapidly billowing wind-blown cumulus pictured hereindicates turbulent conditions aloft. Thunderstorms may follow in warm weather, accompanied by high noise levels and generally poor radio conditions. V.h.f. hams should watch for openings in early summer, however.

Cirrus "mares-tails," tell-tale vanguard of an advancing cold-front, presage a change from fair to stormy weather and may possibly portend strong v.h.f. bending.

Nature's best but least-known visual warning of the presence of a temperature inversion resulting from an overruning tropical air-mass is the clearly defined smoke or dust layer marking the base of the warm moist mass. Look for it before sunrise or at dusk in clear calm weather, at any season. When you see it, rain is 36 to 48 hours away in summer; rain or snow 12 to 24 hours away in winter. Excellent radio conditions with strong v.h.f. signals may be expected at any time of year.

A thickening alto-stratus haze veiling the sun, as shown above, is the result of moist, warm air aloft. This is the stage following that picture at the bottom of this page, (the smoke/dust layer) occuring immediately preceeding preceipitation.

(1) At this time, due to World War II, amateur licences had been mainly suspended.
(2) ie at frequencies of 30-300MHz and 300-900MHz
(3) 'It is right for us to learn...'
(4) The way a radio wave travels
(5) 30Mc = 30MHz. At this time, such receivers were still quite new. A modern enthusiast will find an FM broadcast radio (circa. 100MHz) very suitable.
(6) The American amateur radio five metre band (56MHz) was not available to amateurs while America was at war.
(7) The 112MHz Amateur Radio Band was being used for tests to experiment with these relatively frequencies.
(8) We now know that this should read 'visual distances + one third'.
(9) Dec. 7 1941 was the date of the attack on Pearl Harbor and, therefore, the entry of the U.S.A. into World War II.
Footnotes 1-9: Return to paragraph

(10) 3F = approx. 1.5C
(11) ie less dense
(12) an area where no signals are heard, between the source and an area where it is received - an effect normal on the lower frequencies.
(13) contacts
(14) NB. FOOTNOTE 1 IN ORIGINAL TEXT. "Weather And the Ocean of Air" by Major W. H. Wenstrom (Houghton-Mifflin Co.) provides the most readable discussion of air-mass theory we have yet found. More strictly technical, but still understandable, is the paper "An Introduction to the Study of Air Mass Analysis," by Jerome Namias and others, contained in the Bulletin of the American Metreological Society, Vol. 17, Nos. 6. and 7.
(15) In the U.K., the Gulf of Mexico mass may be approximated to one from the direction of the Azores. A polar air-mass may be one over Scandinavia and the two might meet over Britain.
(16) The border between the types of air (see diagram at end of document)
Footnotes 10-16: Return to paragraph

(17) In the U.K., we rarely get such storms. The point is that the increase in signals occurs as the high pressure decreases and clouds start to appear, before an area of lower pressure or front bring rain, arrives.
(18) NB. FOOTNOTE 2 IN ORIGINAL TEXT. The accompanying photographs show the appearance of typical formations... (Due to the very high quality required to show these black and white images, I have decided not to reproduce them on the web. The captions are, however, provided at the end of the text and it is advised that anybody interested look in a meteorology book for an idea of the formations)
(19) ie not necessarily on hills
(20) In order to visualise this, think of a front on a weather map. As the front crosses certain places, signals from those areas disappear, whilst those from just places which the front is just about to reach, become stronger.
(21) American English for 'early autumn'.
(22) Personally, I would prefer to phrase this '...a rising barometer means the possibility of increased signal strength immediately as it starts to fall, while a barometer from not such a high figure or low barometer indicates lower signal levels.', since I would consider this more accurate.
(23) Approx. 1000mB at sea level. Personally, I would go for 1015mB, at least - 30-and-a-half to the older generation!
Footnotes 17-23: Return to paragraph

(24) True in the U.S., in the U.K. the variable and more localised weather conditions make everything much more unstable. Therefore, do not expect this to happen on even half of hot, sunny, summers days!
(25) Perhaps the greatest "big night" in Western Europe, and especially southwest England, of recent years was towards the end of October 1996, when stations in eastern Europe were being heard, perhaps a thousand miles away.
(26) ie with hardly any power from batteries etc.
(27) ie transmitters whose operation is based around a valve for a receiver. These were, by their very nature, low-powered.
(28) ie a piece of wire cut to half the wavelength (in this case about 1m40, say)
(29) ie late spring, early autumn
(30) ie people who take their holidays in the State of Maine
(31) QSO = radio 'jargon' for contact
(32) Newport in the State of Rhode Island, just south of Boston
(33) It should be noted that long distance operation does not work above the inversion layer, since the signal no longer bends. In the U.K., due to the geography, inversion layers tend to occur at low levels, so you are probably better off staying in your own home at the bottom of a valley than on top of the nearest mountain!
(34) NB. FOOTNOTE 3 IN ORIGINAL TEXT. "Weather and the Ocean of Air," Chapter VII.
Footnotes 24-34: Return to paragraph

(35) Much less in the U.K. as in the heart of America but it is occasionally possible to see the inversion due to pollution - the principle is the same as that described here.
(36) ie predictor
Footnotes 35-36: Return to paragraph

(37) ie large antenna mast
(38) ie stay. This should be spelt as in the French word 'sejour' with an 'e' not an 'o'.
(39) ie World War II, which was not to finish in Europe for another thirteen months, in Asia, longer.
Footnotes 37-39: Retrun to paragraph