Weather affects almost every known activity of man either directly or indirectly; and the probability of success of many enterprises, civil and military, can be noticeably increased if the weather factor can be counted as a "known parameter." The various weather services are constantly striving to provide this knowledge, either in the form of actual data concerning current weather or in the form of a forecast of future conditions. In either case, the information provided is only as good as the weather data available to the meteorologist.
It is well known that there is in operation a worldwide weather data collection network supported by most of the civilized nations of the world. Furthermore, these data are freely disseminated to all nations participating in the collection program. While there appears to be a great wealth of information available, it is 1unfortunately true that there are large areas of the world (e. g., the oceans and the polar regions) from which very little day-to-day weather information is forthcoming. The only means presently available for filling these gaps in weather data is reconnaissance by aircraft or the positioning of weather observing ships. At best, even with such a great and expansive effort, only spotty information can be provided which by its very nature lacks one quality of observation most necessary to synoptic meteorology, that of complete continuity in time and space.
A third possibility, however, has been suggested 1-4 that might indeed provide the necessary information about remote regions of the earth. This is weather reconnaissance by means of satellites. The questions that one might ask about such a system are:
1 Greenfield, S. M., Synoptic Weather Observations From Extreme Altitudes, The RAND Corp., paper P-761, February 15, 1956.
2 Widger, W. K., and C. N. Touart, Utilization of Satellite Observations in Weather Analysis, Bulletin of the American Meteorological Society, vol. 38, No. 9, pp. 531-533, 1957.
3 Wexler, H., Observing the Weather From a Satellite Vehicle, Journal of the British Interplanetary Society, vol. 13, pp. 269-276, 1954.
4 National Aeronautics and Space Act, hearings before the Special Committee on Space and Astronautics, U. S. Senate, 85th Cong., 2d sess., pt. 2, H. Wexler, p. 369.
William Stroud and his associates at the Signal Corps Engineering Laboratories have developed a scanning system for the Vanguard satellite. 5
In this type of operation almost all of the regular quantitative measurements usually associated with synoptic meteorology tend to fall by the wayside. It will be impossible to do more than make an intelligent guess at the values of temperature, pressure, humidity, and other conventional meteorological parameters. However, some added information can be gathered if an ability to look at the Earth and the atmosphere in the infrared part of the spectrum is also provided.
The basic limitation to early weather satellites, then, is the degree to which meteorologists can utilize data that w ill be largely qualitative. Since clouds are the objects most readily discernible from extreme altitudes, they become the predominant item of observation and must be utilized to the utmost in forming a weather picture.
While cloud data alone cannot tell everything about the current weather situation, it does appear that with theoretical knowledge and meteorological experience with satellite data, an accurate cloud analysis can produce surprisingly good results in areas where no other information is available. Further, in areas where good data are currently obtained from the surface, satellite cloud observations can provide continuity and completeness that are not given by the present weather-observing network.
The visibility of clouds from satellites
On the basis of a rather extensive body of evidence, it appears that except for the limited cases of snow background or water illuminated by the sun when low in the sky, clouds will have a brightness at least twice as great as that of the general Earth background. 6-9 A contrast of this extent between Earth and clouds means that clouds will be observable with television or photographic equipment in a satellite.
Cloud photography of useful quality can be obtained with an observation system exhibiting even rather poor resolution. For example, gross cloud cover can be obtained with a system that can resolve ground dimensions of the order of 1 mile or more from an altitude of several hundred miles. Study of cloud pictures taken from rockets, such as figure 1, indicates that it is necessary to resolve ground dimensions of the order of 500 to 1,000 feet in order to identify individual cloud types. Such resolution is attainable with current television and optical systems, although communication links of rather large capacity would be required to relay data to the ground if large areas are to be viewed. Therefore, recognition and identification of clouds is readily feasible by satellite weather reconnaissance, but
5 Annals of the International Geophysical Year, vol. VI, pts. I-V, 1958, Pergamon Press (London-New York-Paris) pp. 340-345.
6 See footnote 1, p. 192.
7 Glaser, A. H., and J. H. Conover, Meteorological Utilization of Images of the Earth's Surface Transmitted From a Satellite Vehicle, Harvard University Blue Hill Meteorological Observatory, October 1957.
8 Hewson, E. W., Quarterly Journal of the Royal Meteorological Society, vol. 69, p. 47, 1943.
9 Hewson, E. W., and R. W. Longley, Meteorology, Theoretical and Applied, John Wiley & Sons, Inc., New York, pp. 73-75, 1944.
communication of the information rapidly to ground stations will be a limitation in first vehicles.
The use of cloud observations
Cloud observations from a satellite could provide the meteorologist with a view of the entire world weather pattern that he can hardly achieve by present indirect methods. In addition, some detailed information can be extracted from cloud photographs. 10 Wind direction
10 See footnote 1, p. 192.
may be estimated in several ways: (1) From present meteorological models it is established that certain definite weather situations will produce certain sequences of clouds preceding or following them. Using these models for preliminary orientation, the wind direction may then be approximated through a knowledge of the theoretical circulation associated with a given weather situation. (2) Cumulonimbus clouds may extend from as low as 1,600 feet up to 40,000 feet, and their slope becomes an indication of wind variation with altitude. (3) Use can be made of the fact that cumulus clouds form on the lee side of mountains. (4) The direction of movement of atmospheric pollutants such as industrial gases, etc., will indicate the direction of winds at low altitudes.
Temperatures may be estimated by starting with the statistical normal for the time of year. This preliminary estimation may then be modified by the various affecting conditions. Cloud systems, wind direction, and even forms of general ground cover (snow, etc.) will aid the analyst in deciding whether the area under observation is being affected by relatively cold or warm air. Upper air temperatures may be estimated in the same manner, clouds indicating the boundary between air masses (fronts). The slopes of vertically developed cloud forms will also aid in determining the temperature gradient of the surrounding area.
No quantitative measures of barometric pressure can be obtained from observations of the types under consideration. Furthermore, it now appears to be virtually impossible to make even a qualitative estimate, other than to determine whether the area is under the influence of a high- or low-pressure system.
Infrared and other electromagnetic measurements provide a very powerful tool for investigating and observing our atmosphere. By proper choice of the regions of the spectrum to use for observation, it is possible to obtain temperatures at various altitudes in the atmosphere. The altitudes would actually be fixed as the tops of various layers of atmospheric gas that absorb radiation in different parts of the spectrum. Examples are the ozone layer at approximately 100,000 feet, and the region at about 40,000 feet where atmospheric water vapor becomes very tenuous. By very careful observation it may be possible to determine not only the temperatures at the tops of these layers, but also the quantities of various gases that exist in the atmosphere.
Measurement of atmospheric gas content would be of considerable value. The atmosphere can be likened to a large heat engine with the sun providing the power and the air acting as the working fluid. The reactions of the atmosphere, and the resulting visible manifestation that we call weather, are a function of the amount of energy absorbed by the various gases that make up the atmosphere. Thus, the variations in the amounts of the various gases available are necessary parameters in any research leading to an improved ability to forecast weather. More immediately, the local moisture content or water-vapor content of the atmosphere is needed in forecasting the advent of clouds and precipitation.
It is also of interest to note that a comparison of the amounts of solar radiation entering and leaving the atmosphere will yield a better
measure of the "heat balance" of the atmosphere. Better determination of the heat balance should shed new tight on the way in which the sun-atmosphere heat engine drives the massive circulation cells that are known to exist in the atmosphere and which serve to distribute energy around the Earth.
Improved forecasting through satellite observations
It is not completely clear that the first meteorological satellites will markedly improve forecasting ability. However, there are several areas where one might expect at least a modest improvement. These are:
An improvement in forecasting depends to a large degree upon the opportunity to observe more or less continuously over a very wide area since weather is a dynamic phenomenon and the ability to forecast its development depends upon the ability to observe its behavior repeatedly. For most weather phenomena, the maximum cycling time is of the order of 24 hours. Wide coverage within such a period of time is possible with observation satellites.
Dissemination of satellite weather data
To be useful, weather information must be quickly available to the various meteorological services. Its value decreases very rapidly with time after it is first obtained.
For maximum usability, the maximum time from the instant of acquisition to actual dissemination should be held to within something like an hour. This rate of information flow suggests a rather elaborate pickup and relay system. One might visualize a "Satellite Weather Data Center" where incoming data would be processed to extract the maximum amount of usable meteorological information for immediate dissemination.
The problems of data analysis and dissemination warrant at least as much thought and effort as the design and operation of the data-gathering vehicle.
The Advanced Research Projects Agency (ARPA) has initiated a meteorological satellite development program with the following main divisions: Rocket vehicle, to place the satellite in orbit at an altitude of about 300 miles; satellite packaging, containing the sensors, storage, power, and radio communication equipments; ground tracking and data readout network; data handling and processing system; data analysis procedures. Parts of the complete system are classified for
the present; however, a description of the data to be obtained has been released. 11
The primary observations will be of the patterns of cloud cover, obtained by miniature vidicon television cameras designed by the Radio Corporation of America. One camera will sweep out a path some 1,200 miles wide and about 6,000 miles long. Ten pictures 1,200 miles on a side will be taken on each orbit revolution, and these will overlap to make the strip. Since there will be 500 television lines in each picture, the ground resolution will be about 2.5 miles. The cameras will be most sensitive in the red part of the spectrum in order to reduce the blue light which is scattered from the atmosphere. In addition, in order to obtain some cloud pictures with higher resolution for comparison, two other cameras with longer focal lengths will be provided. These will cover a smaller area on the ground, though the number of higher resolution pictures will be greater - 34 pictures per orbit revolution with each camera.
The pictures are to be taken at a point in each orbit which is most suitable with respect to sunlight and satellite orientation. Pictures are then to be stored on magnetic tape and subsequently read out while the satellite passes over a ground station - a period of at least 4 minutes is required for the readout.
The transmitted pictures will be handled in two ways. On the ground the video signal will be stored on magnetic tape, and a direct television display will be photographed as the pictures are received. Thus, pictures can be available within minutes from the time of taking. The principal use of these pictures will be made after they have been rectified and located geographically by a central data processing center.
A variety of infrared sensors will also scan the Earth and give a new and unique set of observations. These sensors have been designed by the Army Signal Research and Development Laboratories (ASRDL), and are a direct outgrowth of the IGY "weather satellite" built by ASRDL. From these observations, the heat budget of each area observed can be estimated.
The value of these infrared measurements, though still untested, is expected to be substantial.
The two agencies responsible for analysis and use of the data are the Geophysics Research Directorate of the Air Force's Cambridge Research Center and the United States Weather Bureau.
Preliminary experiments in meteorological observation from a satellite are planned in the Vanguard program with photocells as sensors in a total payload weight of about 20 pounds. 12
The desirable objectives of future meteorological satellite developments, as indicated above, include capabilities to measure cloud motion, atmospheric temperatures, total moisture content, total ozone content, and total radiation flow into and out of the atmosphere.
11 Minutes of the first meeting of the Committee on Meteorological Aspects of Satellites, Space Science Board, National Academy of Sciences, September 26, 1958.
12 Satellites Will Advance Knowledge of Weather, Department of Defense, Office of Public Information, news release 871-58, September 26, 1958.
It must be clearly understood that many of the ideas discussed here have not been completely proved by experiment. Enough information is at hand, however, to clearly indicate the desirability of employing satellites for weather reconnaissance.
The first weather satellites will be exploratory, and as more experience is gained in the interpretation of these radically new kinds of observations their usefulness should grow. It is conceivable that in the not so distant future satellites may actually supplant a part of our present weather network.
In addition to direct experiments with meteorological satellites, certain IGY experiments should cast light on this area. Specifically, useful results should flow from the cloud-cover experiment of the Army Signal Engineering Laboratory and the radiation-balance experiment of the University of Wisconsin.
Much has been written and said in the past few years on the subject of weather control. The reason for such speculation, as much of it has been, is obvious when one considers the effect that weather and its vagaries leave on our day-to-day living (both economically and physically). The ability to control the weather will not be easily acquired. Rather it will probably grow out of a better understanding of the basic processes in the atmosphere. From this understanding will come an indication of the chain of physical events that lead to a particular weather phenomenon. Not until we completely know and understand this chain and the part that each link plays will we have any hope of influencing the particular phenomenon. The part that space experiments can play at present in weather control is that of providing a research tool, such as the meteorological satellite, which will aid in gaining the necessary better understanding of the atmosphere.