The amount of water vapor in the air.
The ratio of the amount of water vapor in the air to the maximum amount of water vapor the air can hold at a set temperature.
The condition of the atmosphere at a certain time and place.
The change of state from a gas to a liquid.
Cool temperatures, and large amounts of rain and/or snow
Thunderstorms, heavy rain, or snow.
Many days of cloudy, wet weather.
Here's an interesting quiz for you.