Cumulus humilis is a low to middle cloud with small vertical extent (Family D1) that is commonly referred to as "fair weather cumulus". In hot countries and over mountainous terrain these clouds occur at up to 6000 meters altitude, though elsewhere they are typically found lower.

They are formed by rising warm air that has been heated by the ground, which in turn has been heated by the sun. They have a limited depth (technically known as showing no significant vertical development). This indicates that the temperature in the atmosphere above them either drops off very slowly or not at all with altitude (see Lapse rate). Cumulus humilis may be accompanied by other cloud types, and they are commonly seen beneath cirrostratus cloud, created as the heat from the sun needed for convection begins to cool, causing cumuliform clouds to flatten and become cumulus humilis. In this case, a warm front could be approaching and this means rain is likely in the next 12 to 24 hours. When they appear in a clear sky (see picture), they are an indicator of pleasant weather for the next several hours.

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.

Weather is driven by density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in Earth's orbit affect the amount and distribution of solar energy received by the Earth and influence long-term climate and global climate change.