A sunny day happens when there are no clouds or only wisps of clouds in the sky.
Clear skies and sunny weather happen when the air in the atmosphere grows heavy and sinks toward the Earth. As heavy air slowly sinks downward, clouds and moisture in the air below are forced away. When a sunny day happens over one part of the Earth, a cloudy day is happening somewhere else.
Clouds form when there is enough moisture in the air and there is enough movement in the air to carry the moisture up into the sky. Clouds can not form when cold air high in the atmosphere compresses the warmer air below preventing moisture from rising. High-pressure is a term weather experts use to describe an area where the air cools, compresses, and sinks. Sunny days happen in high-pressure areas.
Weather–including sunny days–happens only in the troposphere. The troposphere is the layer of air in the atmosphere that is closest to the Earth. The atmosphere is made up of gases we call air. The troposphere is the layer of the atmosphere that contains the most moisture; that moisture or water vapor rises into the air when the Sun heats the Earth and its oceans. Condensation is the name for water that is heated and turned into the gas we call water vapor. Clouds and rainy weather depend on rising air and condensation. Air and water vapor can not rise under high pressure.
Air is always moving in the troposphere. Air is always moving because the Sun heats the Earth and the air in some places while cold air lingers in other places. The movement of rising warm air and descending cold air we call wind. Sometimes air moves fast and sometimes it barely moves at all. Some days are windy, and some days are calm. Sunny days usually happen on calm weather days.
Weather experts say a day is mostly cloudy when 5/8 to 7/8 of the sky is cloudy. Weather experts say a day is partly sunny when clouds cover 3/8 to 5/8 (more than half) of the sky. When less than 3/8 of the sky has clouds or is clear, that is a sunny day.
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