Anywhere you go, the locals will tell you: “wait five minutes and the weather will change”. But what makes the weather so changeable? Most everyone learns about the phase changes of water early in their education, but few know the intricacies of these phase changes and how they create weather. This post uncovers the details.
What You’ll Learn In This Post
- How energy varies between the three primary phases of water
- Why we usually refer to net phase changes rather than the phase changes of individual water molecules
- How water phase changes depend upon interactions with the environment
- Why your freezer has to work overtime after you place an ice cube tray filled with liquid water in your freezer
- How the phase changes of water relate to clouds
Ultimately, the weather is mostly driven by one molecule: water. Water can readily change phase in the atmosphere between solid, liquid, and gas, and these phase changes contribute to changes in the weather that we observe in the environment. These phase changes also produce precipitation. In doing so, they generate severe weather, such as large hail, strong winds, and tornadoes.
Wait! Where exactly can I find water in the atmosphere? (Click for answer)
Everywhere! Water exists in the form of water vapor throughout the atmosphere, and it’s ever-present indoors as well. Wave your hand through the air, and it’s colliding with trillions of water vapor molecules! Even a cubic meter of hot, dry desert air contains over one quadrillion(!) water vapor molecules. While water vapor is everywhere in the atmosphere, liquid droplets and ice crystals are found in clouds.
The Three Phases of Water
Water can take three forms: water vapor (gas), liquid water, and ice (solid). When water is in the vapor state, the individual water molecules carry the most kinetic energy: they are independently zinging through the air. In liquid droplets, water molecules in the liquid phase are joined together by hydrogen bonds (attractions between opposite charges on neighboring molecules), but these molecules linked by hydrogen bonds have the freedom to jiggle around next to their neighbors. In the solid state (ice), the individual molecules are constrained in a rigid six-sided lattice, and they have a lower internal energy than both water vapor and liquid droplets.
Answer the question below to test your knowledge of water’s energy states.
Phase Changes are not the movement of individual molecules
What does it take for a water molecule in a liquid droplet to become a water vapor molecule, zinging around the atmosphere? (Click for answer)
The molecule in the droplet must break the bonds with its neighbors by gaining energy from the surrounding environment.
The video above shows the formation of small, liquid-like clusters from the vapor state. In the atmosphere, individual molecules are changing phase all the time, moving back and forth between phases. On the one hand, zinging water vapor molecules settle down and join a cluster of molecules in a liquid droplet, and on the other, individual molecules from those droplet clusters are leaving the tiny droplet and striking off on their own water vapor adventure.
The question is not the behavior of the individual molecules; it is how the net phase changes compare to one another: for example, how the sum of all molecules evaporating compares with the sum of all molecules condensing. In a cloud made of liquid droplets and water vapor, two phase changes are possible: evaporation and condensation. At all times, some individual molecules are evaporating while others are condensing. However, one process usually dominates over the other: the dominant phase change is the net phase change. In general, this is called a net process.
The most well-known example of conditions supporting a net phase change is saturation (discussed in more detail here). When we say that condensation is occurring in the cloud, we mean that condensation is the much more dominant process than evaporation or any other phase change that may be occurring. For example, if both ice crystals and liquid droplets were present, there could be as many as six phase changes taking place among the three phases of water.
Is it possible for evaporation to occur inside a cloud? (Click for answer)
It’s not only possible, it’s common! Evaporation is an important process in forming the downdraft in thunderstorms. Evaporation cools the air, making it denser. Dense air sinks, forming the downdraft. We’ll study this in the Thunderstorm chapter.
On the next page, we’ll explore how the phase changes of water involve an energy exchange between the water molecules and the environment. Please advance to the next page using the links below .
