A greenhouse thermometer is used to find the temperature inside the enclosure. There are digital thermometers that are highly accurate, but they are expensive and not really necessary, although they often incorporate humidity levels and barometric pressure, which is good for forecasting weather. A regular mercury thermometer is cheap and accurate enough for most greenhouses.
It is best to have a few greenhouse thermometers in different areas inside the greenhouse. This way temperature differences inside the single greenhouse can be detected. Small temperature differences are called micro-climates and they can greatly affect a plants growth, for better or for worse. Knowing where these micro-climates are can allow for many different varieties to be grown.
Many greenhouse thermometers also have humidity sensors that tell how much the air is saturated with water. Too humid of an environment and molds and mildews start to grow and propagate, while a dry environment is not desirable for most plants and allows heat to affect them more, causing scorching and dry leaves.
