What is the definition of outdoor humidity? Simply put, outdoor humidity is the amount of water vapor present in the air outside. It is usually measured as a percentage of the total amount of water vapor in the air, and the amount of water vapor in the air is typically measured using a hygrometer. Outdoor humidity is important to keep in check because it affects the temperature of the environment. When the humidity is too high, it can cause temperatures to feel excessively hot, and when it is too low, it can create uncomfortable cold temperatures. It is for this reason that many people monitor outdoor humidity levels in their area, particularly before engaging in outdoor activities. Outdoor humidity levels also play a role in how well plants are able to absorb water, as it affects how much water vapor is available for them to draw from the air. Additionally, outdoor humidity can affect the growth of mold or mildew, as these organisms need certain levels of humidity to thrive.