Clouds and Weather
Think back to the hydrologic cycle - there are a lot of clouds involved! Cloud formation plays a key role in driving the hydrologic cycle, and therefore, in shaping Earth's weather.
Water Vapor
Clouds are made up of water vapor, the gaseous state of water. We also know from the Atmosphere Module that water vapor is an atmospheric gas.
Variable Gas
First, water vapor is a variable gas because the amount of water vapor is highly variable throughout the atmosphere across space and time. It can range from very little (less than 1% by volume in some arid desert environments) to 4% by volume in tropical areas. Water vapor also varies seasonally, with many areas on Earth experiencing drier seasons with less water vapor/precipitation, and wetter seasons with more.
Greenhouse Gas
Water vapor is also a greenhouse gas because it absorbs heat energy! Remember, greenhouse gases absorb longwave/terrestrial radiation, a.k.a. heat energy, emitted by Earth. In this way, greenhouse gases help regulate surface temperatures, producing the greenhouse gas effect that warms the Troposphere and supports life on Earth.
Humidity
The amount of water vapor in the air is referred to as humidity, and is mainly determined by temperature. Relative Humidity is the ratio (usually expressed as a percentage) of the amount of water vapor that is in the air, compared to the maximum amount that is possible at a given temperature.
So, relative humidity varies depending on both how much water vapor is in the air and how the temperature is changing. For instance, higher temperatures increase evaporation rates from water surfaces, increasing humidity. On the other hand, cooler temperatures increase condensation rates onto water surfaces, lowering humidity.
In other words, humidity is the result of a phase change of water between (typically) its liquid and gaseous states. At hotter temperatures, liquid water absorbs heat and evaporates into the air as water vapor, increasing humidity. At cooler temperatures, water vapor in the air releases heat, resulting in condensation, changing to liquid and lowering humidity. (This is how dew forms, and why water droplets appear on the outside of a cold glass on a warm day!)
In addition to these functions, water vapor is the source material for cloud formation and precipitation, and thus for weather.
Cloud Formation
Clouds are aggregations of tiny moisture droplets and ice crystals suspended in the air in a large enough volume and concentration to be visible.
Cloud formation is driven by condensation (see the water cycle visual above!). However, condensation cannot occur without something for the liquid water to attach to! Clouds form around cloud-condensation nuclei – microscopic particles present in the atmosphere such as dust, soot, ash, aerosols/particles from pollution, and sea salts over ocean areas.
Cloud Types
Cloud classification is determined by altitude (how far above Earth’s surface clouds are in the troposphere) and shape (what form the cloud takes). There are four altitude classes of clouds: low, middle, high, and vertically developed (meaning extending beyond low into middle/high altitudes). There are also three main shapes: flat (stratiform), puffy (cumuliform), and wispy (cirroform).
Together, there are ten basic cloud types. For this class, we will focus on one cloud type: Cumulonimbus.
Cumulonimbus Clouds
Cumulonimbus clouds are towering, vertical-structure storm clouds. Colloquially, they are often referred to as ‘thunderheads’. Cumulonimbus clouds start as a cumulus cloud – think the smaller, fluffy cloud that are the idealized cloud shape we often see in illustrations and art.
Cumulus clouds are typically located in the low altitude. However, updrafts and downdrafts circulating within a cumulus cloud can create a towering vertical structure stretching from low to high altitude - see the difference between the cumulus and cumulonimbus clouds in the cloud type illustration above!
The important thing to note about altitude is that temperatures are lower at higher altitudes. If you have ever hiked or driven up a mountain, you may have noticed how much colder it is at the top. (This is known as the environmental lapse effect.)
Remember the relationship between heat energy, water, and phase changes? At higher altitudes within the vertical structure, water vapor cools down, releasing heat energy and returning to a liquid state (condensation). As that liquid water precipitation falls back down through the vertical structure, it warms up at lower altitudes, causing it to absorb heat energy and changing back to water vapor (evaporation).
These updrafts and downdrafts violently circulate to create the towering vertical structure of a cumulonimbus. Cumulonimbus clouds also have a characteristic anvil shape, caused by high altitude winds at the top level of the troposphere. These windshear off the top of the vertical cloud column, creating the anvil shape.