128 B U L L E T I N A M E R I C A N METEOROLOGICAL SOCIETY

Estimating Dew Point from Minimum Temperature J . G E N T I L L I

University of Western Australia, Nedlands, W.A.

IT is generally accepted that, for the same air-mass, minimum temperature of the preceding night and the dew-point temperature are

closely related. In fact, many forecasters used to assume that the two temperatures were very nearly

equivalent. In the course of research work done at the Climatology Laboratory of the Johns Hop-kins University, the writer had reason to verify this supposed equivalence.

The relationship between minimum temperature

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FIG. 2. Correlation of dew-point and minimum tem-peratures by seasons, for locality above 2 0 0 0 feet. The departure from a straight-line relationship is very small in the cold months, but it may become very great in any of the hot months, when very dry air-masses may domi-nate regional weather.

and dew-point is studied here only from the point of view of the climatologist who may know the minimum temperature for many localities, but can very seldom know the dew-point, because only a few stations are equipped for the measuring of humidity data. The relationship studied here is that between the night minimum and the dew-point of the following morning. It is based upon monthly average values for U. S. stations, and independently upon actual daily readings at the Perth, W.A., Weather Bureau, which give the same line of regression.

In no case is this regression suitable for the forecasting of minimum temperatures during the following days, be it for whole regions or, worse still, for selected spots, such as valleys, basins, or individual orchards.

Even when the relative humidity is not known for some localities, it can be estimated within rea-sonable limits from data available at individual

FIG. 3. Correlation of minimum/dew-point ratio with relative humidity, monthly mean values for Yuma. The numbers show the months, the symbols the years. At low humidity the minimum temperature is much higher than the dew-point. The relationship may be expressed by a slightly curved line.

places in the vicinity. Use of the graphs given here will enable the climatologist to estimate the dew-point from this estimate of relative humidity and from the actual minimum temperature of the previous night.

As a preliminary step, minimum and dew-point temperatures for most stations in the United States, as published in the Weather Review, were plotted on graph paper (FIG. 1). The data were divided according to the main geographical divi-sions of the country, namely, "Plains and East," "Slopes and Plateau," and "Pacific Coast." The very anomalous values recorded on Mount Tamal-pais (2375 feet) on the West Coast were kept separate. A few records from Panama and the

FIG. 1. Correlation of dew-point and minimum temperatures by major regional units. Note the regularity of the relationship for maritime and eastern locations and its irregularity for continental and mountain locations. Very seldom is the dew-point higher than the minimum, and if so, by very little. In maritime locations a straight-line relationship is obviously acceptable.

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FIG. 4. Correlation of minimum minus dew-point differences with relative humidity, (left) mean monthly values for United States stations, and (right) actual daily (0900 hours) values for Perth, W.A. The curve on the left graph corresponds to the equation given in the text, and is repeated by the broken line on the right graph. The con-tinuous line on the right graph shows the line of best fit for the data shown. The difference is very small.

West Indies were added in order to extend the graph towards the higher temperatures.

FIGURE 1 shows that the readings for the Plains and East (shown by small diagonal crosses) fall along a straight line which may be described by Tdp — 0.97 Tmin for values expressed in degrees Fahrenheit, provided that the dew-point is not below 32°F. For colder conditions the equation changes and is represented by a steeper line, expressed approximately by Tap — 6 + 0.7 Tmin.

The values recorded in Panama (small vertical crosses) and the West Indies (squares) fall along the same line as the above-freezing readings for the Plain and East Coast, so that we may assume that the actual magnitude of the tempera-tures recorded does not affect the apparent rela-tionship. We may conclude that in the Plains and on the East Coast minimum temperatures may be taken as a reliable guide to the dew-point temperatures of the respective following day.

Records from the Pacific Coast (shown by

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small circles) mostly follow the relationship found for the Plains and East Coast, but in several in-stances depart considerably from it, and always in the same sense—with a minimum temperature higher than the corresponding dew-point tem-perature.

Records from the Slopes and Plateau (black dots) and for Mount Tamalpais on the West Coast (small triangles) depart far more con-siderably from the relationship observed. Thus a minimum of 6 7 ° F may be followed by dew-point at 26 °F, minima of 64 °F by dew-points at 4 1 ° to 4 9 ° F . The scatter of the points on the graph is such that no mathematical expression can represent it adequately.

This remarkable discrepancy between two parts of the country led to further analysis with the purpose of seeking an explanation and arriving at a general equation.

The fact that the anomaly was observed in the western regions, where many stations are at high altitude, led to the analysis of records (January-November 1 9 4 9 ) for all localities above 2 0 0 0 feet (FIG. 2). The records were also classified by seasons, so that it might be possible to detect a seasonal variation in the relationship, if any.

FIGURE 2 shows that January-March records are rather regular, April-May records less so, and June-August records most irregular. Phoenix and Yuma were the localities with the most striking seasonal variations in the relationship. All told, no generalization based on altitude or season was possible.

Data for Yuma were then graphed according to the Tmin/Tdp ratio and to the relative humidity, as usually given in percentage form (FIG. 3).

The data were selected over several years in order to cover extreme as well as normal conditions. The graph shows that undoubtedly there is a relationship, but a slightly irregular scatter is still noticeable.

A further correlation was then attempted based on the (Tmin — Tap) difference computed from monthly means, and on mean monthly relative humidity (FIG. 4a). The scatter is more regular, and a definite relationship may be deduced as a generalization, i.e.:

Tmin - Tap = 117.45 - 63.45 log RH.

It was thought at the time that this generaliza-tion, obtained from United States readings which are taken at 0730 and 1930 hours, would not ap-ply to Australian readings, which are taken at 0900, 1500 and 2100 hours.

In order to verify this under the most different conditions, actual 0900 readings for Perth, W.A., were taken from daily entries in the 1948 book of records (journal), by kind permission of the Perth Weather Bureau. Since extreme differ-ences are not very common, additional values were obtained from 0900 records of other years (FIG. 4b).

The relationship is almost identical with that obtained from United States monthly data, and confirms the view that only when the relative humidity is higher than 65 or 70 per cent may the minimum temperature be taken as an indica-tion of the dew-point for the following morning. In dry weather this assumption is not valid, and some estimate of the relative humidity is neces-sary in order to bridge the gap.

CORRESPONDENCE

Contract Weather-Station Operations * N O R M A N S . B E N E S

Route 5, Moultrie, Georgia

During World War II the U. S. Air Force contracted to civilian operators for the primary flying training of its pilots. With the outbreak of the Korean action it was again necessary to train many pilots quickly. Civilian contractors were once more asked to bid on contracts for training pilots and this time a portion of the contract stipulated that the successful bidders would also operate

* Read at Miami Beach Meeting, Nov. 1954.

a weather station in support of the training program. They had the responsibility of recruiting the trained personnel. The Air Force loaned the equipment and furnishings it considered necessary to operate the weather station.

At Spence Air Base, Georgia, the first contract weather station was established at a non-military base. The sta-tion had to be built from the bare walls. At present office and working space are sufficient. The pressing need is for adequate storage space and more library room. The station force consists of three meteorologists who are graduates of the University of Washington and Florida State University, which were cited by Beck [1] as preparing their graduates for careers in forecasting. Normal hours of station operation are from 0530E until 2030E while flying training is in progress. The bulk of

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