A weather thermometer measures ambient air temperature. In the United States thermometers are usually calibrated in degrees Fahrenheit, while in many other parts of the world they show degrees Celsius. Some show both. The most common types of thermometers measure temperature by measuring the rise or fall of a column of liquid, usually alcohol or mercury, within a glass column. The liquid rises as it's heated, and falls as it cools. Another type of thermometer uses a spring made of heat sensitive metal that moves a needle on a gauge as the metal expands or contracts.
Air has weight and presses down on the earth. How heavily it presses is called the atmospheric pressure, and is measured by an instrument called a barometer. One type of barometer consists of a glass vacuum tube placed over a bowl of mercury. The mercury rises or falls in the tube depending on how heavily the air presses down on it. The other commonly used type of barometer is an aneroid barometer, which measures changes in atmospheric pressure by the contraction or expansion of a small metal box inside it called an aneroid cell.
A wind vane, or weather vane, is a simple instrument for determining wind direction. It consists of a center post or shaft fitted with a spinning horizontal bar in the general shape of an arrow, with a pointer at one end, and a wider vane at the other. As the wind blows against the wider area, the bar moves so the arrow points toward the direction the wind is blowing from, which is also the direction the wind is named for. So a north wind is wind that blows from the north toward the south.
An anemometer measures wind speed. The simplest type has a freely spinning central pole with three or four horizontal bars radiating from it. At the end of each of these bars is a cup placed sideways to catch the wind. The faster the wind, the faster the cups and the pole spin. The number of revolutions per minute made by the center pole is recorded and used to calculate wind speed. Another type of anemometer resembles a windmill in appearance, and also uses RPMs to measure speed of the wind.
Rain gauges come in a variety of shapes and sizes, but are designed to calibrate the rain collected in volumes of cubic inches. The most common type of rain gauge, or standard rain gauge, consists of a funnel at the top feeding into a measuring tube contained within a larger cylinder set within a framework. The measuring tube can hold up to 2 inches of rain before overflowing into the larger tube.