One of the most effective measures of the actual temperature is relative humidity. This indicates how much moisture is in the air and what the temperature actually "feels like". Most people know that humidity is the amount of water vapor that is presently in the air.
The measure of humidity itself can be obtained in a number of ways, including absolute, specific, and relative humidity levels. Absolute humidity is actually the volume of water contained in the air. Specific humidity is the water mass currently in the air. Relative humidity is probably the most accurate measure in most people's mind because it indicates how close to saturation the air is. When temperature increases, the amount of water in the air can actually remain the same while relative humidity decreases.
This is why relative humidity is such an important measure when discussing the ambient environment of any location, whether indoors or out.
Another related term is the dew point, which is the actual temperature when air can't hold any more water and it actually will condense back into liquid form which is actually 100% relative humidity. This is an interesting phenomenon since dew point and real temperatures are equal.
Heat and moisture are two of the common causes of structural decay in most building materials, which includes wood, metal and even some types of plastic which can breakdown due to these types of factors. This is why it is so important to be able to control the environment in any type of building.
Using a device called a hygrometer, it is easy to measure the level of humidity in the air, but often people can sense it on their skin when the levels are high. While heat and humidity are inter-dependent, the latter can be controlled through the use of a humidistat.
The amount of water vapor inside a building or more typically inside a room can be controlled through a humidifier or dehumidifier. The former puts more moisture into the air using steam or ultrasonically. The latter is essentially a collector that functions to remove excess water vapor from the air condensing it on coils for collection by a reservoir.
It is so important to control the humidity levels in almost any environment. A type of device that will control the water vapor is definitely a must have for any building or household that wants to establish a more constant environment.