Much of the surface weather observations (95% of them, really) are taken by automated machines called AWOS or ASOS. The larger airports still have the sites manned to backup the machines. For more info on how surface observations are taken (temperature, dewpoints, sky conditions, visibility, winds, etc...) click on this official FAA site regarding ASOS and AWOS systems
Pictures (from University of Illinois) of many instruments and descriptions
A thermometer measures temperature (max/min). The National Weather Service keeps the thermometer six feet above ground in a white box thus keeping the sensor in the shade. Here's an example of what that looks like.
A barometer measures pressure.
A rain gauge measures precipitation. There are two major types of rain gauges used in weather observations. One type is a tipping bucket and the other is a weighing rain gauge. Generally, 10" of snow for 1" of rain can work as a fairly good ration but not always. The NWS chart can be found here.
An anemometer measures wind speed; A wind vane measures wind direction.
A hygrometer measures humidity.
Measuring Temperature, Dewpoint, and Humidity (mobile instead of a
A sling psychrometer measures wet bulb and dry bulb from which you can get temperature, dewpoint and relative humidity.
Measuring Upper Air Weather Conditions
A radiosonde (or rawinsonde) is a package of weather instruments attached to a weather balloon that radios back to earth upper air observations.
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