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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
Measuring Temperature
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.
Measuring Pressure
A barometer
measures pressure.
Measuring Precipitation
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.
Measuring Winds
An anemometer
measures wind speed; A wind
vane measures wind direction.
Measuring Humidity
A hygrometer
measures humidity.
Measuring Temperature, Dewpoint, and Humidity (mobile instead of a
fixed station)
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.