How Weather Gadgets Work

Weather Forecasting Devices

An anemometer spins in the breeze, measuring wind speed.
An anemometer spins in the breeze, measuring wind speed.

In solving a murder, a detective compiles a number of elements to determine the murderer and the method. It's much the same with the weather, except meteorologists attempt to uncover a future event, not a past crime. ­

­To forecast what tomorrow will bring, weather detectives gather evidence: air temperature, air pressure, humidity, wind speed and rainfall. In the last section, we covered thermometers, but how do the rest of these weather forecasting devices work?

Barometer: This device measures air pressure with a glass tube and a small quantity of mercury, much like a thermometer. This time, however, an airless tube is inserted into a plate of mercury. The top of the tube is sealed, while the bottom is open. Air pressure pushes down on the mercury, which in turn pushes it up the tube against the vacuum in the tube. In this regard, a basic barometer works like a scale. Air pushes down on one side, mercury rises on the other. By measuring the position of the mercury in the tube, we can determine current atmospheric pressure.

Hygrometer: If you want to monitor the humidity outside or inside your home, then this is the gadget for you. A psychrometer uses a wet-bulb and dry-bulb thermometer. The user simply directs air over the two bulbs and checks the temperatures. The difference between the two reveals humidity. Dew point hygrometers use a cooled metal plate. Whatever the metal's temperature is at the point of condensation is the dew point. Electrical hygrometers measure moisture-related changes in electrical resistance through a semiconductor. Finally, mechanical hygrometers actually use human hair (or similar tissue) to determine humidity. Since hair expands or contracts depending on humidity, these machines grip the hair tightly and register minute changes in tension.

Anemometer: Often mounted beneath weather vanes, which determine wind direction, anemometers typically measure wind speed with either a small propeller or a kind of pinwheel with cups on the end. Sonic anemometers transmit ultrasonic waves between two pairs of transmitters. These stick up in the air like antennae, several inches apart from each other. Wind passing through this space will either speed up or slow down the ultrasonic waves. The anemometer records these changes to determine wind speed, direction and pressure.

Rain collector: These gadgets can be as simple as a measuring cup placed outside during precipitation. More advanced models record precipitation mechanically, automatically emptying at preset levels.

If these gadgets won't satisfy your weather obsession, keep reading to find out what the pros use.