Final answer:
An air mass is a large body of air with uniform characteristics of temperature and humidity, and it is a key factor in determining regional weather and climate. Relative humidity is a measure of the water vapor content in the air relative to the maximum it can hold at a certain temperature, affecting phenomena such as fog formation and the heat index.
Step-by-step explanation:
A large body of air that usually has uniform characteristics of temperature and humidity is known as an air mass. Air masses play a significant role in determining the weather and climate over a region. Characteristics such as temperature, wind, and rain are interestingly all part of climate, which is essentially a long-term statistical summary of these and other meteorological measurements like atmospheric pressure and humidity.
When discussing humidity, it's often in terms of relative humidity, which indicates how much water vapor is present in the air relative to the maximum amount it can hold at a certain temperature. When the relative humidity reaches 100%, the air is at saturation, and evaporation of water is inhibited. This can lead to the formation of fog if the temperature drops to the dew point and the conditions are right. Hence, meteorological concepts such as air masses, humidity, and the heat index are crucial for understanding biomes and climates as well as everyday weather phenomena.