This month I'll be talking about a subject near and dear to my heart. Meteorology---the study of the weather---is my chosen profession. 'Weather' describes the state of the atmosphere at any given moment. I'm sure you're familiar with the 'weather report': for example, temperature is 50 degrees with cloudy skies and rain, relative humidity 98 percent, wind north at 10 miles an hour, pressure 30.12 inches of mercury and falling. Everything in that report describes some aspect of the atmosphere at the time of observation.

Because weather has always had such an important impact on humanity, from rain or the lack thereof affecting the growth of crops to the destructive winds of tornadoes and hurricanes, references to it can be found in fragments of the earliest writings. Ancient man worshiped numerous deities associated with various weather phenomena, from the all-powerful king of the gods, Zeus, who hurled deadly thunderbolts, to the wind gods (Aeolus, Boreas, Zephyr, Notus, and Eurus), Iris (goddess of the rainbow), and Athena (protector of agriculture), among many others. The earliest known scientific writings dealing with weather are Aristotle's Meteorologica and the writings of his pupil Theophrasus on winds and weather signs. For the next 2000 years, very little progress was made in the study of weather. The development of the science of meteorology had to wait until the 17th Century and the invention of instruments which could be used to measure the primary physical elements, and the development of the elementary physical laws of gases, liquids, and solids in the 17th and 18th centuries.

1607 -- Galileo invents thermoscope (a primitive gas thermometer) to measure air temperature

1643 -- Torricelli invents the barometer to measure air pressure

1659 -- Boyle's Law discovered: if the temperature is kept constant, the volume (V) of a gas decreases when the pressure (P) exerted on it is increased (PV=constant)

1714 -- Gabriel Fahrenheit invents first mercury thermometer; on his scale, water freezes at 32^oF and boils at 212^oF

1735 -- George Hadley makes first attempt to explain the trade winds as the effect of the earth's rotation on the atmospheric winds

1742 -- Anders Celsius invents the Celsius temperature scale, where water freezes at 0^oC and boils at 100^oC

1778 -- Lavoisier discovers that the air is composed of respirable oxygen (25% by volume) and non-respirable nitrogen (75%)

1783 -- Lavoisier's years of experiments with combustion result in the discovery that water is composed of the two gases, hydrogen and oxygen

1787 -- Charles' Law discovered: at a constant pressure, the volume of a given mass of air is directly proportional to its absolute temperature (T)

Boyle's and Charles' laws can be combined into the ideal gas law: PV=RT, where R is the constant of proportionality.

1800 -- John Dalton explains the variations of water vapor in the atmosphere and the relation between the expansion of air and atmospheric condensation

1801 -- Dalton formulates his law of partial pressures for gases

1848 -- Lord Kelvin proposes temperature scale with absolute zero being the coldest temperature possible

The 19th Century saw the creation of networks of surface weather observing stations, the development of synoptic meteorology (where the movement of air masses, fronts, and low pressure systems over large areas is tracked on daily synoptic charts), and the beginning of modern weather forecasting. By the middle of the century, the characteristic global patterns of surface pressure, wind, and weather had been mapped and empirical rules established for their development, movement, and the accompanying sequence of weather changes. Between 1850 and 1875, many nations established meteorological services based on synoptic observations from networks of weather stations.

1871 -- national weather station network established within the U.S. Army's Signal Corps

1891 -- Signal Corps weather observing network moved to Department of Agriculture under new U.S. Weather Bureau

Meteorological advances during the 20th Century resulted from study of the upper atmosphere via the use of instrumented balloons that rise through the free atmosphere (these became the radiosonde in the 1930's when they were equipped with transmitters to radio their data back to the surface), with aircraft, and after World War II by rockets and weather satellites. The importance of the Jet Stream, as a steering current for surface lows and fronts, was realized. The two world wars accelerated both the need for weather data (especially upper air data) and the development of technology that could be applied to meteorology.

Last month I started a discussion of meteorology---the study of the weather. This month, we'll pick up where we left off, with a list of important dates and events in the history of meteorology, then we'll cover some of the basics of the subject.

1902 -- Léon Teisserenc de Bort discovered that the Earth's atmosphere has at least two different layers: the troposphere and stratosphere

1904 -- Vilhelm Bjerknes publishes Weather forecasting as a problem in mechanics and physics, one of the first scientific studies of weather forecasting

1921 -- V. Bjerknes's paper, On the dynamics of the circular vortex with applications to the atmosphere and to the atmospheric vortex and wave motion, shows that the atmosphere is made from sharply differentiated air masses

1950 -- von Neumann leads team using ENIAC (one of the first computers) to make first computerized 24-hour weather predictions

1960 -- U.S. launches TIROS 1, the first weather satellite

1968 -- USAF scientists show that radar can be used to detect wind shifts and precipitation, although weather radars were in use 10-20 years earlier

Weather Basics

Energy from the sun is the ultimate force that drives the earth's weather. Solar radiation (or insolation, i.e., sunlight) heats the ground (radiative heat transfer), and the ground heats the air immediately in contact with it (through conduction). Thus, temperature decreases with height in the troposphere. If the ground gets warm enough, it will heat the surface air enough that it expands, becomes lighter than the air around and above it, and rises, much like the water at the bottom of a pan being heated on a stove boils to the top of the pan. This is the basis behind convection which gives rise to cumulonimbus clouds and thunderstorms.

The noon sun is directly overhead in the tropics, which results in maximum concentration of sunlight on the surface of the earth, and hence, maximum heating. The noon sun is lower in the sky at higher latitudes, and this lower sun angle results in the same amount of sunlight being spread over a larger area, a lower concentration of sunlight, and hence, less heating. Thus, on average, the tropics are warmer than the poles. The atmosphere strives to balance out this heat (if it didn't, the poles would completely ice over and the tropics would eventually boil away), so we have cold air masses coming out of Canada into the U.S. and warm air from the Gulf of Mexico marching north across the Great Plains (advection).

The earth's axis of rotation is tilted 23 ½ degrees, so the noon sun is directly overhead the Tropic of Cancer at the northern summer solstice and over the Tropic of Capricorn at the winter solstice. The amount of heating varies throughout the year, resulting in the seasons.

The atmosphere is three dimensional. The warm tropics generally have rising air which cools, water vapor condenses, and there is heavy rain. Once the air gets high up into the atmosphere it has to go somewhere. It goes poleward, eventually descending around 30 degrees N and S. Descending air warms and dries, there is little precipitation, and you get deserts like the Sahara.

Poleward of the subtropical deserts, the weather systems are controlled by the Jet Stream, which is a "river" of fast-moving air at roughly 30,000 feet altitude. The mid-latitude Jet Stream changes shape and location and strength throughout the year and exerts a powerful influence on weather systems at the surface. If the Jet loops northward, it creates a ridge over the U.S. which is associated with warm advection, descending air, and dry conditions. If it loops southward, it forms a trough which is associated with cold advection, rising air, and wet conditions. Thus, the Jet Stream controls the movement of air masses and warm and cold fronts, and the creation, strength, and movement of Low Pressure systems or cyclones which can cause wintertime blizzards. This makes monitoring and prediction of the Jet Stream extremely important in weather forecasting.

References:

Encyclopedia Britannica, Volume 15, "Meteorology", pp. 277-300. 1972 edition.

Hamilton, E., Mythology, Penguin Books, New York, 1940 ( © renewed 1969).

Hellemans, A. & B. Bunch, The Timetables of Science, Simon & Schuster, New York, 1988.

Lutgens, F.K. & E.J. Tarbuck, The Atmosphere: An Introduction to Meteorology, Prentice-Hall, Englewood Cliffs, 1979.

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