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Water allows flowers to bloom and plants to grow and is essential to life on Earth. The water cycle is the cyclical movement of water between Earth's surface and the atmosphere through evaporation, condensation, precipitation, and surface runoff. Clouds store water in the atmosphere and periodically release the water back to Earth's surface. The water cycle regulates the amount of water present on the surface of Earth, thereby helping flowers to grow while mitigating droughts and floods.
Condensation is when water vapor suspended in the air turns into liquid water. In minute amounts, these water droplets are light enough to remain suspended in the air and cannot be seen from the ground. Droplets of water can also adhere to particulates suspended in the air, such as dust, salt, smoke, or chemicals. The particulates combine with water to form cloud droplets, which amass into visible structures in the sky.
Clouds in the atmosphere provide thermal insulation for Earth. They reflect heat from the ground back to the surface as well as reflecting radiation from the sun back into space. The atmosphere consists of air that decreases in density at higher altitudes. Dense air stores and radiates thermal energy more efficiently than less-dense air, so air that's higher up is colder.
Condensation can happen throughout the atmosphere at different altitudes. When the humidity of air is high, this means that water vapor is present in larger quantities than during periods of low humidity. Humid air that touches colder surfaces, such as cool or frozen ground or cold-weather plants and flowers, can produce droplet clusters at low altitudes, known as fog or ground fog.
Condensation can form on a variety of surfaces. For example, turning on the heat in a car on a cold day can cause water vapor to obscure the windshield and windows. This condensation forms when warm, humid air touches the cold glass. Similarly, eyeglasses can fog up on cold days when warm air travels upward as you exhale.
Evaporation involves the upward motion of water vapor from Earth's surface. When rising water vapor reaches an altitude where the air is at or below the dew point, the evaporation rate falls below the condensation rate. The water vapor adheres to nearby particulates in the atmosphere, forming large numbers of suspended cloud droplets. Amassed cloud droplets form visible structures in the sky called clouds. A cloud reaches saturation when the water molecules in the cloud are packed together too densely. Once the cloud is oversaturated, the water molecules combine to form droplets that fall from the cloud as rain, snow, or other forms of precipitation. Rain returns water to Earth's surface, supporting the growth of plants and flowers.
Cloud-like structures called contrails can form around airplane engines. When a jet airplane flies at a high altitude, the heated exhaust from the engines can mix with the cold air in the atmosphere, causing water vapor to condense around the particles of exhaust and producing cloud-like structures in the sky.
Although a cloud can weigh more than a million pounds, the cloud is dispersed over a large area. This means that the molecular density of the cloud is similar to the molecular density of air, enabling the cloud to float in the atmosphere.
The three major types of clouds are:
A cumulus cloud appears as a bushy, white formation at low altitudes in the sky. Cirrus clouds are thin, wispy structures that are located high in the atmosphere, while stratus clouds form sheet-like structures that blanket the sky at low altitudes. A common cloud combination is the cumulonimbus cloud, which has large vertical extensions and often produces lighting and precipitation.
The water cycle is the cyclical movement of water between Earth's surface and the atmosphere. Parts of the water cycle include evaporation, condensation, precipitation, and surface runoff. Evaporation is the upward movement of water vapor, largely from water sources such as rivers and oceans. Condensation is the formation of liquid water molecules from water vapor and particulates in the atmosphere. Precipitation is the release of water droplets from clouds, while surface runoff is the movement of water along Earth's surface toward bodies of water, where it is then taken back up into the atmosphere through evaporation.
A rainbow is the refraction of sunlight by water molecules and particulates in the atmosphere. As sunlight passes through water in the atmosphere, the light wavelengths bend, separating into an array of colors. Rainbows can form anywhere in the sky, but they are most easily visible after precipitation and around clouds, where water droplets are more densely packed in the sky.