Temperature gradient

“ In an ascending mass of air, from the beginning of the condensation onwards, the potential temperature steadily increases by reason of the loss of moisture, but in a descending mass of air it remains constant at the maximum value attained by it at the highest point of its previous path. In general the potential temperatures of the upper strata of the atmosphere are higher than those of the lower. In general the so-called vertical temperature gradient is smaller than would correspond to the adiabatic rate for the dry stage. ”

“ Study carefully, as in the previous problems, the relation of the different types of temperature distribution in the atmosphere to the weather conditions as shown on the daily weather maps.
Observations made in different parts of the world, on mountains and in balloons, have shown that on the average the temperature decreases from the earth’s surface upwards at the rate of about 1° in 300 feet of ascent. The rate of vertical decrease of temperature is known in meteorology as the vertical temperature gradient. ”

“ Differences in the conductivity of rock are an obvious source of varying underground temperature gradients. If an outer formation conducts heat more freely than those below, it tends to lower the gradient within itself and to cause a relative rise in the gradient just below. If a lower formation is more conductive than that above, it tends to lower the gradient within itself, and to raise it in the one above, because it carries heat to the outer one faster than the latter carries it away. ”

“ The circulations which take place are accompanied by certain changes of temperature in a vertical direction, called temperature gradients, which are characteristic of them. If a cubic centimeter of air at the sea level is lifted up to higher levels, so that it cools simply by the expansion of its own mass, and there is no mixture of warmer or colder air with it from the outside, then the temperature will fall 9.87° Centigrade for every 1,000 meters. ”

“ Quantitative measurements in temperature, based upon the calorie, have a definite place in physics; and those based on the British thermal unit have a broad application in various economies. Human sensitiveness to heat does not pertain to quantity but to intensity. A large block of ice at 30° may contain more heat, quantitatively considered, than a red-hot horseshoe. If carried into a room whose air was far below freezing, the ice might warm the room to a greater degree than would the horse-shoe. ”

“ So you at once see that the difference between the readings of the wet and dry bulb thermometers indicates the amount of moisture in the air. This amount the observers express in percentages of 100; and thus we read of "humidity, 60 per cent." Under ordinary circumstances, when the humidity gets close to 100, the point at which the air is soaked with moisture, it is going to rain. The temperature, however, and also the wind, have a good deal to do with this. The form in which the weather observers use these two thermometers is called the whirling psychrometer. ”

“ In winter, when the temperature is below freezing, the muslin of the wet-bulb thermometer should be moistened with water a little while before a reading is to be made. The amount of water vapor which air can contain depends on the temperature of the air. The higher the temperature, the greater is the capacity of the air for water vapor. Hence it follows that, if the temperature is lowered when air is saturated, the capacity of the air is diminished. ”

“ It is equally true that, in both cases, the facts may be consistent with such an irregular distribution of the atmosphere in successive layers, or strata, of very unequal temperature as was apparent in most of Mr. Glaisher’s balloon ascents. What is completely proved is that it is only under exceptional conditions that the hypothesis of an uniform rate of decrement of temperature, directly proportional to height above the sea-level, is approximately correct for observations in the temperate zone, where there is a considerable diurnal and annual range of the thermometer. ”

“ At depths of from 1,500 to 2,000 feet, the thermometer placed in a freshly drilled hole will show 130 degrees. Very large bodies of water have run for years at 155 degrees, and smaller bodies at 170 degrees. The temperature of the air is kept down to 110 degrees by forcing in fresh air cooled over ice. ”

“ Molecular structure affects temperature change Heat measurement units, as we have seen, are defined in terms of a temperature change brought about by heating. But the same amount of heat applied to different materials does not necessarily bring about the same temperature change—it is well known that some materials can be heated or cooled more quickly than others. ”

“ The mean heat of the earth, above the stratum of constant temperature, is determined from that of springs; and, if the spring be on elevated ground, the temperature is reduced by computation to what it would be at the level of the sea, assuming that the heat of the soil varies according to the same law as the heat of the atmosphere, which is about 1° of Fahrenheit’s thermometer for every 333·7 feet. ”

“ Nevertheless, they are held together by a powerful force. Thus, the heat used in converting one pound of water at 212° F to a vapor at 212° F would raise 967 pounds of water one degree in temperature. Measured thus, in terms of heat units, great power is required to overcome molecular attraction. ”

“ One circumstance, however, deserves to be mentioned, which may in some measure account for the innocuous effort of so low a temperature. I have occasionally, during the summer, been up the greater part of the night at my observatory, with the internal as well as the external thermometers as low as freezing point, without being particularly warmly clad, and yet not feeling the least cold; and in the winter, the thermometer, on similar occasions, has been at 24° and 26° without my suffering the slightest inconvenience. ”

“ In order to procure a fixed starting-point, the thermometer tube is placed in ice, with a scale attached; the temperature of ice never varies, it is always at 32 degrees. When, therefore, the mercury has sunk to the lowest point it can do by exposure to this degree of cold, the place is marked off in the scale, and represents that position in the graduated scale where the freezing point of water is indicated. ”

“ Also one can determine how much colder one day is than another, noting how many degrees the water ascends and descends. One can compare the greatest heat and cold of one year with those of another year. One can ascertain how much hotter one room is than another; one can maintain a room at an equal temperature by making the water of the thermometer stand always at a certain degree. One can test also the intensity of fevers; in short, one can know pretty nearly to what extent air is rarefied in the greatest heat, and so forth. ”

“ The heat no longer affects the thermometer, because it is wholly employed in converting the ice into water. As the ice melts, the caloric becomes latent in the new-formed liquid, and therefore cannot raise its temperature; and the thermometer will consequently remain stationary, till the whole of the ice be melted. ”

“ Materials with high heat capacities can absorb and lose large quantities of heat without much temperature change. Conversely, relatively little heat is needed to change the temperature of materials with low heat capacity. At ordinary temperatures, the heat capacity of air is about 0.017 Btu/ ft3/°F—little heat is needed to change its temperature. ”

“ On dry, winter days, the flagstones of a sidewalk frequently absorb enough heat to melt ice and snow, even though the temperature of the air is as low as 20°; and occasionally side walks and hard-paved streets are slushy with the thermometer scarcely above 25°. ↑ A moist surface, and very moist air as well, does not have a temperture materially higher than the wet-bulb thermometer; a dry surface, or very dry air, acquires a temperature approximating that registered by the black-bulb thermometer. ”

“ A reflecting body, heated or cooled in the interior of such an enclosure, will acquire the surrounding temperature more slowly than would a non-reflector, and will less affect another body placed at a little distance, but will not affect the final equality of temperature. Apparent radiation of cold, as from a block of ice to a thermometer placed near it, is due to the fact that the thermometer being at a higher temperature sends more heat to the ice than it received back from it. ”

“ Importance of Self-Registering Thermometers.—Heat being apparently the most effective agent in producing meteorological phenomena, the determination of the highest temperature of the day, and the lowest during the night, is a prime essential to enable an estimate of the climate of any place to be formed. ”

“ We need not consider the formula that we have just given as applicable to very great changes in the volume of gases. We have regarded the elevation of temperature as being in inverse ratio to the specific heat; which tacitly supposes the specific heat to be constant at all temperatures. Great changes of volume lead to great changes of temperature in the gas, and nothing proves the constancy of specific heat at different temperatures, especially at temperatures widely separated. ”

“ When the common rack and manger are preferred, the rack staves should be straight, and brought nearly down to the manger, and this may easily be done without the necessity of a hay loft, and the manger may be made deep and wide as I have described it.
The heat of a stable should be graduated by a thermometer, always kept there for the purpose. Our feelings are but a feeble guide to our judgment in measuring temperature. It would be well that the stable heat should seldom reach, but never exceed, 50 degrees of Fahrenheit in winter, or 65 in summer. ”

“ Note C.—We see no reason for admitting, à priori the constancy of the specific heat of bodies at different temperatures, that is, to admit that equal quantities of heat will produce equal increments of temperature, when this body changes neither its state nor its density; when, for example, it is an elastic fluid enclosed in a fixed space. Direct experiments on solid and liquid bodies have proved that between zero and 100°, equal increments in the quantities of heat would produce nearly equal increments of temperature. ”

“ Thermometric Substances.—Thermometers for meteorological and domestic purposes are constructed with liquids, and generally either mercury or alcohol, because their alterations of volume for the same change of temperature are greater than those of solids; while being more manageable, they are preferred to gases. ”

“ No feats of discovery, not even the search for the North Pole or Stanley's expeditions in the heart of Africa, present more points of fascinating interest than the attempts now being made by scientists to explore the extreme limits of temperature. We live in a very narrow zone in what may be called the great world of heat. The cut on the opposite page represents an imaginary thermometer showing a few of the important temperature points between the depths of the coldest cold and the heights of the hottest heat—a stretch of some 10,461 degrees. ”

“ When the air temperature ceases to rise or fall, and begins to change at the same rate in the opposite direction, the thermometer will fail to record the true maximum or minimum temperature by an appreciable error depending upon the rapidity of the change, and will follow the new temperature changes with the same lag. ”

“ It will thus be seen that all over the world there is an increase of temperature at a rate which, on the average, is about 1° for every 100 feet. There are 5280 feet in a mile; therefore, if this rate of increasing temperature were maintained, at a depth of 100 miles the temperature -84- would be perhaps 5000° F.; a temperature at which steel would melt and boil away into vapour. ”

“ This is of great convenience in calculating refrigeration required to cool and de-humidify air. For example, suppose it is required to find the refrigeration necessary to cool 1 lb. of air containing 98 grains of moisture and having a dry bulb temperature of 95 deg., to a final temperature of 40 deg. saturated. We find from the chart that the wet-bulb temperature is 75 deg. The total heat corresponding to a saturation temperature of 75 deg. is 37.8 B.t.u., while the total heat at 40 deg. is 15.3 B.t.u. The difference, 22.5 B.t.u. is the refrigeration required per pound of air. ”

“ Provided both hot and cold junctions are located so as to attain the same atmospheric temperature in the absence of radiations, a given quantity of energy impinging on the hot junction will always produce in it the same excess temperature, and will therefore give rise to the same deflection at all ordinary atmospheric temperatures. ”

“ If any other physical change be chosen, such as the expansion of a solid, or the increase in resistance of a metal, and a temperature scale be based on the supposition that the change in question varies directly as the temperature, the results obtained would differ considerably from the absolute standard. For this reason the practical standard of temperature now universally adopted is an instrument based on the properties of a suitable gas. ”

“ With an average temperature of seventy-two degrees it has no extremes of heat and cold, and the thermometer seldom varies more than twenty to twenty-five degrees during the entire year. In the so-called winter season the mercury rarely goes below sixty-five degrees and the summer heat does not usually exceed eighty-five degrees. ”