• Silver Iodide Seeding and Radar Echoes from Convective Clouds

      Battan, Louis J.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1967-10-15)
      Summer convective clouds over a fairly isolated mountain range over southeastern Arizona were seeded by means of airborne silver iodide generators. The selection of days to be seeded was made according to a randomization scheme involving pairs of days. A 3-cm vertically scanning radar set was used to observe the maximum echo height over the "target" area at 30 minute intervals. The data so obtained were used to examine the effects of the seeding on the vertical extent of the cloud echoes. Although there is a suggestion that the silver iodide nuclei may have initiated precipitation in some clouds and caused small vertical echo growths, the statistical analyses, for the most part, showed that the observed differences could easily have been caused by chance.
    • Detection of Hail By Means of Doppler Radar

      Battan, Louis J.; Theiss, John B.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1968-05-01)
      The doppler spectrum obtained by means of a vertically-pointing Xband pulsed doppler radar may yield information on the size distribution of hail in the free atmosphere. It is concluded that the quantity A in the size distribution curve for hail, N = N0-AD e is sometimes more than twice the values 2.27 or 2.93 cm -1 proposed by earlier investigators. It is also shown that when both rain and hail are present the variance of the 2 doppler spectrum may exceed 20 m2 sec-2.
    • Recent Studies on Hail and Hail Modification in the Soviet Union

      Battan, Louis J.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1965-10-05)
    • Observations of Convective Clouds by Means of Pulsed-Doppler Radar

      Battan, Louis J.; Kassander, A. Richard, Jr.; Theiss, John B.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1963-01-15)
      A new pulsed-doppler radar set designed for measuring the vertical distribution of the particle size and vertical-velocity spectra is briefly described. The X-band radar set employs a vertically-pointing antenna producing a 1.3 degree beam. Measurements of the properties of the hydrometeors are made at 500-ft. intervals from 500 ft. to a maximum altitude of 63,000 ft. The intensities of the radar return from particles in each of ten velocity channels are recorded digitally. The analysis of echo returns from showers is discussed in some detail. The data allow inferences of the change of particle size as a function of altitude and time, as well as of the vertical air motions in the cloud. New observations of so-called "angel echoes" are presented. Most often the vertical velocities of the angel echoes were positive. These observations can be used to infer information about clear air convection.
    • Measurement of Atmospheric Water Vapor by a Spectrophotometric Technique

      McMurry, Earl W.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1962-09-21)
      A spectroscopic technique was used to measure the total amount of water vapor between the point of observation and the sun. Measurements were taken continuously throughout the day, yielding a detailed picture of water vapor variations as a function of time. Results from 28 days of observation over a period of a year show a wide variety of situations ranging from days of relatively stable and constant precipitable water vapor W to days in which large changes occur, sometimes quite abruptly. The median value of the range of W within successive one-hour periods was found to be 0.05 in, with ten per cent of all ranges exceeding 0.12 in, the latter amounting to about fifteen per cent of the annual mean of W at the locality in question. It is concluded that significant W-fluctuations are commonly missed in routine radiosonde practice. Natural variations of Ware, on the average, somewhat greater than the instrumental errors of radiosondes. Crude estimates of moist 11 bubble11 diameters of 0.5 to 12 kilometers were obtained.
    • Evaluation of Effects of Airborne Silver-Iodide Seeding of Convective Clouds

      Battan, Louis J.; Kassander, A. Richard, Jr.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1962-03-01)
      Observations of the convective clouds over the mountain ranges of southeastern Arizona show that a large fraction of them reach temperatures far below freezing but fail to produce precipitation. During the summer periods of the years 1957 through 1960, a randomized cloud seeding experiment was conducted to test if these clouds could be modified by airborne silveriodide seeding. The essential features of the experimental design were the following: 1) An objective technique was evolved for predicting days with suitable clouds; 2) one of a pair of two adjacent days was seeded on a random basis; 3) measurements on all days were made with rain gages, radar and a pair of ground-located aerial cameras mounted at the ends of a 3-mile base leg, and visual observations were made of cloud-to-ground lightning; 4) the signrank test was used to make statistical evaluation of the effects of seeding. The results after four years are the following: 1, The rainfall statistics do not allow a conclusion that seeding had any effect. Statistically, the rainfall on both seeded and not-seeded days was the same. However, the sensitivity of the tests to changes in the quantity of precipitation was quite low. 2. The frequency of occasions of large thunderstorm echoes was statistically the same on seeded and not-seeded days. 3. The cloud-census and radar studies suggest that in some cases, seeding may have led to the initiation of precipitation echoes. 4. Although there were more cloud-to-ground lightning strokes on seeded days, the differences between their occurrence on seeded and not-seeded days were not statistically significant. Evidence is presented indicating that precipitation in convective clouds is not initiated by the ice-crystal process. Samples of days with heavy rain and light rain were compared in order to study the factors governing the quantity of rain. It was not possible to show that there was any relation between the region of echo initiation and the quantity of precipitation. On the other hand, it is clear that on days with heavy rain there were many more large clouds. The observations are best explained if it is assumed that the quantity of rain is governed by those properties of the atmosphere which determine the number and size of the convective clouds. The microphysical processes which determine the region of precipitation initiation do not appear to be as important as was once suspected. A new program of seeding tests is described which should be more sensitive to changes in quantity of rainfall. It differs in several fundamental aspects from the program conducted during the period 1957 to 1960.
    • The Low-Level Wind Field at Phoenix and Tucson

      Frenzel, Carroll W.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1961-10-02)
      Resultant surface winds presented as monthly averages for each hour of the day for two years have been computed for Tucson and Phoenix and are displayed as hodographs showing the diurnal variation of the surface wind. Resultant winds for these two years were computed from upper air observations for all times and all levels of observation up to and including 4000 meters above mean sea level for both Phoenix and Tucson, and are presented in the form of tabulated listings as monthly averages. In addition, mean speeds of the upper air winds are presented and a tabulation of the occurrence of calm winds at the surface vs. month and hour is given. Results of this study show that a distinct diurnal variation in the surface winds is present at both Tucson and Phoenix. The most persistent feature of the wind field at both of these stations is the drainage wind which occurs during the early morning hours. During the warmer portions of the day air motion tends to be toward higher elevations. Thus, in general, the air motion conforms to what is expected on the basis of theory. Mean air motion for each month is presented. Considerable difference between months and between the same months of the two years is noted. High frequencies of reported calms at Phoenix for certain months lead to rather small net air movement. The diurnal variation in the number of reported calms is seen to be related to the diurnal reversal in wind direction associated with the mountain-valley circulation. The relationship of these resultant winds to the air pollution problem at these two cities is discussed.
    • Back-Scattering of 3.21-cm Radiation by Water Bubbles

      Battan, Louis J.; Herman, Benjamin M.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1960-08-15)
    • Calculations of Mie Back-Scattering from Melting Ice Spheres

      Herman, Benjamin M.; Battan, Louis J.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1960-09-01)
    • Evaluation of Potential Vorticity Changes Near the Tropopause, and the Related Vertical Motions, Vertical Advection of Vorticity and Flow of Radioactive Debris from Stratosphere to Troposphere

      Staley, D. O.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1960-03-26)
      Individual potential vorticity change, vertical motion, vertical advection of vorticity and flow from stratosphere to troposphere are evaluated at different levels and for different times in an extratropical disturbancec Vertical motions are obtained from trajectories on three isentropic surfaces for two different times. The isentropes pass through or near a front in the upper troposphere, the lower stratosphere, and the high troposphere on the cold side of the front. Vertical motion is also evaluated at 500 mb from an adiabatic method designed to give instantaneous values to the extent that the system moves without change of shapeo Negative extremes of 8 to 10 cm sec-1 occur in the upper air front, with positive extremes of the same magnitude a short distance to the northeast, coinciding with the eastern exit of the frontal zone. From theory it is shown that potential vorticity is not conserved if there is either a gradient of diabatic heating or a component of curl, normal to the isentropic surface, of frictional force. The sum of these two effects is evaluated over the United States for two 12-hr periods on two isentropic surfaces which were common to parts of the lower stratosphere, upper troposphere and a front in middle and upper troposphere. Generally large, positive potential vorticity changes occur in the lower stratosphere and in the upper troposphere on the cold side of the front. Large negative values occur in the frontal zone and around the entire periphery of the positive area, or around the periphery of the trough in the upper air. The potential vorticity changes are related to simultaneous stability and vorticity changes of like sign. The potential vorticity changes are positive in the region of the so-called 1 tropopause funnel 1 ; changes everywhere appear attributable to vertical gradient of diabatic heating rather than curl of frictional force. ' ( I The terms in the vorticity equation which contain vertical velocity (vertical advection of vorticity and tilting terms) are shown by means of the thermal wind equation to depend only on the vertical motion and temperature fields in an isobaric surface. For the frequent case where negative motion is centered in the baroclinic or frontal zone and rising motion centered at the exit of the zone, each of the vertical motion terms has the same characteristic distribution. Positive values occur on the cold side of the zone and to the right of the downwind exit of the zone, and negative values occur on the warm side of the zone and to the left of the downwind exit of the zone. The vertical advection and the sum of vertical advection and tilting terms are evaluated at 500 mb. Magnitudes obtained compare with those of the divergence term, although magnitude depends considerably on the distances over which finite differences are evaluated. Isentropic trajectories trace air initially in the lower stratosphere downward to within 5000 ft of the surface within 24 hr. Diabatic incorporation into the troposphere is also noted. The total adiabatic mass flow into the troposphere associated with the number of typical upper air disturbances in existence at any time is estimated and found to be sufficient to give the observed short residence times of a few months for radioactive debris injected into the stratosphere by nuclear detonations.
    • Distribution of Relative Humidity and Dew Point in the Southwestern United States

      Sellers, William D.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1960-02-01)
      This report consists of three basic parts. In the first, average monthly values of relative humidity are presented for between one and four hours of the day at 126 weather observing stations in the southwestern United States, including all of Arizona, New Mexico, Colorado, Utah, and Nevada, and parts of California, Texas, Idaho, and Wyoming. The nature, accuracy, and import of these data are discussed briefly. It is shown that the month with lowest average humidities varies systematically from March in central Texas to August in northern Nevada, antedating the westward movement of the Atlantic high pressure system, with its moist unstable air. In the second section, the distribution of dew point over the Southwest is discussed. High values are found on the windward sides of most of the larger mountain ranges, near bodies of water, and in heavily irrigated farmland; low values are concentrated at higher elevations and in the dry desert regions bounded by the Sierra Nevada Mountains to the west and the Rocky Mountains to the east. Hourly data for Arizona indicate that the diurnal variation of dew point is small, with a tendency for the lowest values to occur in the midafternoon in dry regions and in the early morning in moist regions, i.e., those with a surface snow cover or with considerable crop irrigation. The final section outlines a method of estimating the mean relative humidity from the mean temperature. Ordinary linear regression techniques are used, with a correction added to account for the systematic geographical distribution of regression errors.
    • Calculations of Mie Back-Scattering of Microwaves from Ice Spheres

      Herman, Benjamin M.; Battan, Louis J.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1959-12-31)
    • Characteristics of Summer Radar Echoes in Arizona, 1956

      Ackerman, Bernice; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1959-07-08)
      This report describes an investigation of the radar data obtained during the summer of 1956. Descriptions of the evaluation and analysis techniques, of the basic data and of the findings of this study are given in considerable detail. For the convenience of the more casual reader, all discussion and a summary of the findings are contained in sections VI and VII.
    • Seeding of Summer Cumulus Clouds

      Battan, Louis J.; Kassander, A. Richard, Jr.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1959-07-01)
      A description is given of a program of randomized seeding of orographic cumuli over southeastern Arizona. This investigation, started in 1957, is still in progress. The aims have been to learn more about natural cloud processes and to ascertain if airborne silver iodide seedings can modify them. It will be shown that analyses of cloud photographs, radar observations, lightning observations and rainfall data suggest that cloud seeding modified the natural cloud processes. However, the changes observed so far have not been large enough to conclude that effects have positively been established.
    • Randomized Seeding of Orographic Cumuli, 1957: Part II

      Battan, Louis J.; Kassander, A. Richard, Jr.; Sims, Lee L.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1958-10-01)
      During July and August 1957, orographic cumuli over the Santa Catalina Mountains of southeastern Arizona were seeded from an airplane with silver iodide. The experimental design of the program involved the randomized seeding by pairs of days; one of two days with suitable clouds was seeded on a random basis. On seven pairs of days, observations were made of the cumuli with a pair of K-17 cameras on a 1.3 mi baseleg. At the same time, radar observations were made with a 3-cm vertically-scanning radar set. On the basis of the analysis of the camera and radar observations an investigation has been made of the occurrence of precipitation as a function of cloud size and temperature. It was found that there is a large variability in cloud behavior from year to year. Natural clouds in the arid southwest do not produce precipitation until their vertical thicknesses are of the order of at least 8 to 10,000 feet. From a comparison of seeded and non-seeded clouds it appears that the silver iodide particles may have produced changes in the precipitation formation mechanisms in orographic cumuli.
    • The Annual and Diurnal Variations of Cloud Amounts and Cloud Types at Six Arizona Cities

      Sellers, William D.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1958-09-01)
      The annual and diurnal variations of cloud types and cloud amounts are described for six first-order weather stations in .Arizona. These stations are Flagstaff, Winslow, Prescott, Phoenix, Tucson, and Yuma. Most of the data analysed were obtained on punched-cards from the National Weather Records Center at Asheville, North Carolina. The period of record was from January 1, 1948 through December 31, 1955. This information was supplemented by cloud amount data for the period from July 1, 1956 through March 31, 1958 taken from the supplement to the Local Climatological Data for the six stations. After a brief outline of the terrain features in the vicinity of the stations (section II), the type, quantity, and quality of the data are discussed. Emphasis is put on the homogeneity of the records, both temporally at a single station and spatially from one station to another. In section rv the annual variations of cloud amounts, cloud types, and thunderstorms are described, without any reference to the more detailed diurnal variations, which are discussed later, month by month, in section V. Part of the summary (section VI) is devoted to an investigation of the relation between the average cloud cover and the frequencies of occurrence of both clear and overcast skies at a "typical" .Arizona city.
    • Cloudiness Over the Southwestern United States and its Relation to Astronomical Observing

      McDonald, James E.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1958-02-24)
      A number of types of cloudiness statistics for Weather Bureau stations in the southwestern United States are analyzed in terms of their implications for astronomical observatory site-selection. In all but one of the analyses, Yuma proves to be distinctly superior to other stations with respect to clearness of skies. Lack of nearby mountain peaks extending above haze and dust layers plus poor seeing due to inevitably high thermal instability throughout much of the year render the immediate vicinity of Yuma astronomically unattractive, however. Hence the difficult task of comparing the relative cloudiness of the region roughly concentric with Yuma is the practical problem confronting the astronomer seeking new observing sites. Inherent limitations in available types of meteorological observations are disctissed; but these limitations are not precisely defined, since they are not quantitatively known to the meteorologist at present. In view of these uncertain limitations in each individual type of data, the safest procedure becomes that of assembling all possible types of independently observed data and assessing the site problem in terms of the overall implications of all of these data. ~e present report consists chiefly in such an assembly and assessment of meteorological data. In addition, a review of past studies of the site-selection problem is given. It is concluded that during the winter half-year (more significant to astronomical observing than the sunnner half-year for night-duration reasons), the area extending out about 200 miles northeast and east of Yuma 1s the best portion of Arizona for observing sites. Sites in this area will have clearer winter skies than those over coastal southern California, and will: be somewhat superior to those north of the Mogollon Rim where more frequent migratory cyclonic storms increase the mean winter cloudiness to values higher than those found to the south. The southeastern corner of Arizona is unattractive in the summer rainy season (July-August) due to high-frequency of thunderstorms, and equally undesirable in summer is the whole Mogollon Rim whose thunderstorm frequencies are nearly as high, on the average, as those of extreme southeastern Arizona. A rapid westward diminution of summer thunderstorm activity across southern Arizona (due to upper-level flow conditions governing moisture distribution over the Southwest) makes summer conditions increasingly more favorable from Tucson westward to Yuma; and in the winter the entire border area west of Tucson to and beyond Yuma is quite favorable. Haze and dust tops average about 5000 to 6000 ft. msl. in winter and probably 8000 to 10,000 ft. msl. in summer in southern Arizona. Areas of agricultural cultivation, as the Salt River Valley area around Phoenix, have a locally severe transparency problem. The general altitude of the haze and dust layers plus other seeing difficulties leave only a few peaks in southwestern .Arizona as feasible sites. A peak at 5672 ft. in the Harquahalas and Kitt Peak (6875 ft.) in the Quinlans seems to offer the the meteorologically best possibilities in .Arizona. Several California peaks considered by Irwin may be almost as favorable as those in southwestern .Arizona, but no first-order Weather Bureau station data are available for those areas. However, the only peaks above 6000 ft. msl. in the area. west of Yuma are those just south of San Jacinto Peak and these ranges undoubtedly have about as high a winter cloudiness as San Diego, which proves to be distinctly higher than either Tucson or Yuma.
    • The Distribution of Clouds at Tucson, Arizona, with Respect to Type, Amount, and Time of Observation

      DesJardins, Robert B.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1958-01-01)
      More than 25,000 Weather Bureau cloud observations for Tucson, Arizona, for the nine-year period from 1 July 1945 to 1 July 1954, were statistically analyzed. It was immediately obvious from this analysis that the cloud data for this period were not homogeneous. A study of the heterogeneities in the data indicated that they were probably caused by two changes which had been made in the Weather Bureau cloud observing procedures. These changes were: (1) In 1949 the descriptive material for cirrostratus clouds was changed when the revised version of the Manual of Cloud Forms and Codes for States of the Sky was published. !!his change is believed to have caused a large increase in the annual number of observations of cirrostratus clouds and a correspondingly large decrease in the annual number of observations of cirrus clouds. (2) On July 1, 1948 a new procedure for recording cloud observations on WBAN Form lOB was adopted. This new recording procedure resulted in cloud observations which were not as detailed as were the observations made before July 1, 1948. The change from the old "cloud family" recording procedure to the new "cloud layer" recording procedure resulted in a decreased number of recorded observations of cumulonimbus and altostratus clouds. The frequencies of occurrence were computed for eight types of clouds (stratocumulus, cumulus humilis, cumulus congestus, cumulonimbus, altostratus, altocumulus, cirrus and cirrostratus) and the annual and diurnal variations of these frequencies were determined. The average monthly tenths of cloud cover were also computed and the annual and diurnal variations of the amounts of cloud cover were determined. A comparison of the annual variations of these parameters showed that they were essentially the same for six of the cloud types but were notably different for the cumulus humilis and cumulus congestus cloud types. The annual variations of the frequencies of occurrence of the most common cloud types were also compared to the annual variations of three synoptic parameters (the number of frontal passages, the amount of precipitation and the amount of precipitable water). These comparisons seemed to indicate that there was a relationship between the general synoptic situation and the amount of cloudiness during some months. However, the number of synoptic parameters included in this part of the study need to be increased before any definite conclusions can be reached. For each month, the year-to-year variations in the amounts of high, middle and low cloudiness at Tucson were compared with the year-to-year variations of the average amount of precipitation for the Tucson area. In general, changes in the amount of cloudiness were of the same sign as changes in the amount of precipitation, but there were many noteworthy exceptions to this general pattern. In addition, the rank correlations -2- between the monthly amounts of precipitation and the monthly amounts of cloudiness were computed. In general, the precipitation was best correlated with the amount of low cloudiness and the highest of these correlations occurred during the winter months. However, there were many interesting deviations from this general pattern.
    • A Study of the Trajectories and Diffusion Patterns of Ground-Generated Airborne Particulates under Orographic Wind Flow Conditions

      Kassander, A. Richard, Jr.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1957-05-15)
      In late June and early July, 1956, a series of experiments were performed to determine the wind structure over an orographic barrier and to ascertain whether ground-generated aerosols reach cloud heights in such a situation. Zinc sulphide particles were dispersed in an oil fog from a ground-located fog generator and collections were made using airborne impactors. Particles were found at elevations of at least 14,000 feet at distances out to at least 20 miles. However, "plumes" were found to be considerably broader than had been anticipated and the structure of the wind over a mountain ridge is judged to be substantially more complex than has been assumed in cloud-seeding operations.
    • A Critical Evaluation of Correlation Methods in Climatology and Hydrology

      McDonald, James E.; Institute of Atmospheric Physics, The University of Arizona (Institute of Atmospheric Physics, University of Arizona (Tucson, AZ), 1957-01-25)
      The sampling properties of three types of correlation coefficients are examined critically with reference to the practical problem of selecting a statistic optimally suited to correlation analyses in climatology and hydrology. Reasons are presented for concluding (1) that undue concern has been given the problem of non-normality of underlying frequency distributions, and (ii) that application of significance tests and determinations of confidence limits for the product-moment coefficient, r, are quite adequately achieved through simple use of the standard error of r. It is suggested that the growing tendency in climatology and hydrology to employ unnecessarily elaborate methods stems jointly from characteristic emphasis upon theoretical refinements found 1n most statistics references and from neglect on the part of users of statistics to examine the numerical magnitudes of these refinements in relation to basic data-precision and to desired precision of inference. The properties of a tetrachoric and a rank correlation coefficient are discussed, and it is concluded that in many geophysical applications, especially in cases of moderate sample sizes, the Spearman rank-difference coefficient should be regarded as the preferred correlation statistic. Em- . pirical comparison of these three coefficients are presented for a sample of precipitation data taken from a region (Arizona) where non-normality of precipitation frequency .distributions .. is known to be extreme. The rank difference coefficient is found to lie within one standard error of r for eleven of fourteen cases in this sample. The tetrachoric coefficient is found to be a much poorer estimate of r, yet in thirteen of fourteen cases it yields (on an approximation basis) the same inferences as does r with regard to existence of correlation.