Final answer:
Humidity is the amount of water vapor in the air relative to what it could 'hold' at that temperature and pressure. Relative humidity reaches 100% when the air is saturated and can't hold more water vapor, leading to condensation and potentially fog. The vapor pressure of water is what determines how much water vapor air can contain at a given temperature.
Step-by-step explanation:
When we talk about humidity, we're generally referring to relative humidity. This represents the amount of water vapor present in the air compared to the maximum amount at the same temperature. When relative humidity reaches 100%, it indicates saturation, and the air cannot contain any more water vapor in the gas phase. This saturation point is dependent on temperature, causing relative humidity to rise in the cooler evening air potentially reaching the dew point, where condensation can result in fog formation.
Understanding the relationship between temperature and humidity is important. For instance, blowing hot air to dry your hair is more effective because warm air can hold more water vapor, facilitating evaporation. Conversely, as temperature declines, the air's capacity for water vapor decreases, and condensation is more likely, indicating the intimate link between vapor pressure and relative humidity.
The notion that air can 'hold' water vapor is a common misconception. In reality, the amount of water the air can contain is solely controlled by the vapor pressure of water at a given temperature, not by any inherent property of the air itself.