Grand Duke Ferdinand II of Tuscany was the first to notice that barometric pressure dropped during storms. And in 1660 the barometer was used to predict the weather for the first time by Otto von Guericke, mayor of Magdeburg, Germany.
In 1844, the first barometer that didn’t use a liquid was patented by Lucien Vidie, a French scientist who based his invention on a small vacuum chamber attached to a pointer that would rise and fall as the atmospheric pressure made the chamber expand and contract. He called his barometer an aneroid, from a Greek word meaning “without water.” Vidie’s invention is still in use today, as are mercury-filled varieties, but the electronics revolution has helped create a much more advanced version of the barometer.
An electronic barometer uses an electrical sensor to measure minute changes in atmospheric pressure, and sends the readings to a digital display. Originally very expensive, these barometers are now available to home users either as a stand-alone instrument or as part of a home weather station to help you predict coming weather conditions. Barometers have changed quite a bit since Pascal’s brother-in-law climbed Puy de Dôme with one in 1648!
Of course, the more instruments you have, the more accurate your forecasts will be. Determining how much moisture the air contains is one of the most important measurements you can make. The instrument used to measure humidity is called a hygrometer, invented in 1780 by Horace de Saussure, a Swiss meteorologist and geologist. De Saussure based his invention on the fact that hair becomes longer when the air is humid and shorter when it’s dry. He attached small levers to human hairs that measured the change in length, correlating to how much water vapor the air held.
Relative humidity can actually exceed 100 percent. When the temperature drops below the dew point, air can become supersaturated with moisture, raising the humidity to a higher percentage. But the condition is just temporary: the moisture will condense into fog or dew fairly quickly, and the relative humidity will once again be 100 percent.
The hair hygrometer was rather inaccurate, leading to the invention of the sling psychrometer: two thermometers mounted side by side. One is covered with a wick that is moistened with water, while the other is kept dry, and the unit is spun around for a few minutes. Water evaporating from the wet side will cool the thermometer, lowering the reading below the actual air temperature. The drier the air, the greater the evaporation, and, therefore, the greater the difference between the two temperatures. The difference tells the reader how much water the air can hold versus how much it’s currently holding—the relative humidity. The sling psychrometer has largely been replaced by the electronic hygrometer.
This is probably a good time to address the dew point. You know it’s important because weather forecasters mention it often, but it’s rarely well explained. Just remember that the warmer the air is, the more water vapor it can hold. When the air can no longer hold any more water, condensation occurs. When this change happens up in the clouds, it forms rain; on the ground it makes dew. So the dew point is the temperature at which the vapor in the air will begin to condense. For instance, if the current temperature is 80°F and a forecaster says the dew point is 70, you know it has to get ten degrees cooler before dew can form.
The instrument that tells you how fast the wind is blowing is an anemometer. Leonardo da Vinci usually gets credit for inventing the first one, which he designed around 1500. But the first working model was put together by Robert Hooke in 1667. Hooke’s version used a hanging plate of metal hinged at the top; when the wind blew, the plate would swing out at an angle and the wind speed could be read off a scale attached to the side of the instrument. Today’s anemometers use three or more cups mounted on a vertical shaft. The speed of the cups’ rotation translates directly to wind speed; the data can then be transmitted to an electronic base station that displays the information.
The oldest weather instrument is no doubt the wind vane, which reveals which way the wind is blowing. The earliest known wind vane stood atop the Tower of the Winds in Athens, Greece, which was built around 48 B.C. to honor the god Triton. The vane featured the head and torso of a man attached to a fish’s tail, and is thought to have been from 4 to 8 feet long.
The Vikings used bronze weather vanes on their ships in the ninth century, and about that time the pope reportedly decreed that every church in Europe should have a rooster on its steeple as a reminder of Luke 22:34, which states that a cock would crow after the disciple Peter had denounced Jesus three times. You’ll still see roosters on weather vanes today.
Before 1850, weather vanes were produced by hand. But growing interest in the devices brought about factory production to meet the demand. Most of the major weather vane producers went out of business during the Great Depression, but their products are in demand today as highly sought-after collectibles.
In the digital age, the weather vane has been combined with the anemometer in an instrument called the aerovane, which looks rather like an airplane with no wings. The propeller part measures the wind speed, while the tail keeps the unit pointing into the wind, indicating which way it’s blowing. Data on wind velocity and direction are constantly fed to a display unit and recording station.
The rain gauge is another important tool in the meteorologist’s array of instruments. A modern gauge consists of an outer cylinder, a measuring tube that can record as little as a hundredth of an inch of rain, and a funnel. The gauge can directly measure up to 2 inches of rain; when more than that falls, the extra water flows into the outer cylinder. The observer then pours the excess from the outer cylinder back into the measuring tube to determine the total rainfall amount.
If you find a radiosonde, send it back to the National Weather Service for reconditioning (instructions for returning it are printed on its side) to save the Weather Service the cost of a new unit. Currently only about 20 percent of the approximately 75,000 radiosondes released each year are found and returned to the NWS.
Another kind of rain gauge is the tipping bucket type, which tips and empties itself when it has accumulated 0.01 inches of rain. Immediately another bucket moves into place to continue recording rainfall, and an electronic signal is sent to a recording unit. Rainfall is measured by adding up the number of tips in a given period and multiplying by 0.01. The advantages are obvious: no one has to go outside and empty the gauge each day, and there is no water lost to evaporation. In a downpour, however, the tipping bucket gauge may not be able to empty fast enough, so its readings can be inaccurate.
Weather scientists need to know what is happening up in the atmosphere. To discover this, they attach an instrument package to a balloon and send it up. The instrument package, called a radiosonde, contains sensors that measure temperature, pressure, and relative humidity. Radiosondes and balloons have been used for decades, and were vital tools in the study and prediction of weather during World War II. The National Weather Service (NWS) is undertaking a program to replace its older radiosondes with new units that employ Global Positioning System (GPS) receivers, which will be cheaper and easier to maintain and will deliver much more accurate and reliable data.
Although the balloons used to loft radiosondes haven’t changed much since World War II, the units themselves have undergone several improvements. Wind speed and direction can now be determined by tracking the balloon as it rises, and additional data streams are sent down from the radiosonde to a ground station. This data is processed through a computer before being released for distribution. The balloons can ascend to a height of more than 19 miles before they pop; a parachute then gently brings the equipment to the ground so that it can be recovered and flown again.