To determine cloud cover during the day, meteorologists use visible images . Visible images are the most like photographs because they show the sunlight that is reflected from the Earth. On visible images, cloud cover reflects the most sunlight, appearing white against the darker areas of land and ocean.
Meteorologists use infrared images to determine the temperature in the atmosphere.
This information can then be used to infer cloud cover. Infrared energy radiates from objects on Earth below in relation to the temperature of the objects. The warmer an object, the more infrared radiation it emits. Clouds are usually cooler since they're high in the sky, so they emit less infrared energy than the ground. Satellite sensors measure the amount of infrared energy emitted by the clouds and ground below, and computers interpret the data, generating infrared images for meteorologists to use.