Why can warm air carry more moisture? Warm air can carry more moisture because more water evaporates when the air is warm and there is more space between the water molecules to hold that moisture. Saying that warm air “holds” more water is a little bit inaccurate because the air doesn’t actually hold the water. It is more that they exist in the same place at the same time.
Recently, the global temperature is increasing and the amount of rainfall in certain areas is drastically increasing. There have been devastating flash floods and hurricanes are not only becoming stronger, they are carrying far more rain. As the global temperature continues to increase, the amount of water that the atmosphere can hold will increase and the amount of rain will also increase. Why is that?
The reason why there is more water in the air when the air is warm is all down to energy. When you have heat, you have to have thermal energy. Thermal energy is the amount of energy contained within something and heat is what is produced when that thermal energy is transferred from one body to another. For example, the frying pan has a lot of thermal energy after you have used it. If you accidentally touch the pan, that thermal energy will be passed to your hand, which is heat. Thermal energy makes the molecules in a material move more and adding thermal energy can change a substance from one state of matter to another. With water, for example, the state with the least thermal energy is ice. This is the solid state. The molecules don’t have much energy and don’t move much, staying rigid. If you add more thermal energy, the molecules start to move more and you get water. This is the liquid state. If you keep adding thermal energy, the molecules start to move even more, and at some point, they have so much energy that they break away from the substance. With water, this state is steam, which is the gas state.
When the air over water, mostly the sea, is warm, the seawater gets enough thermal energy to move out of the sea and become water vapor. This process of evaporation removes the molecules with the most energy and it lowers the average temperature of the sea. However, when the global temperature rises, the air gets warmer and the air puts more energy into the sea than it takes out. This means that the water keeps on becoming water vapor.
Another reason that warm air can hold more water is because there is more space for that water vapor to be in. When air is cold, the molecules have less energy and they are closer together. When air is warm, the molecules move more and there is more space for the water vapor.
Warm air can continue holding water vapor until it is completely saturated, but that takes a lot of energy. Generally, with every 1-degree Celsius rise in the temperature of the air, the air can hold 7% more water vapor. This means that if we can hold the rise in global temperature to 1 degree, we can limit the amount of water the air can hold. That isn’t looking likely though.
What problems are there is the air can hold more water. Well, the first and most obvious problem is that there will be more rain and more floods. We have seen the number of powerful storms increase over the last few years and each time there is record rainfall. This is happening because the air is warmer and holding more water. So much rainfall leads to floods that often do far more damage than the storm.
The second problem is that the rainfall doesn’t fall evenly over the whole world. Water can be sucked up from the sea, but it is also sucked up from the land. The winds blow and the rain falls in other places. When there are droughts, the earth gets compacted and is no longer able to absorb water. If it does rain there again, the water just rolls off the land into the rivers. Nothing can grow. Droughts will become more extreme, storms will become more extreme, and rainfall and floods will become more extreme. In the United States, annual rainfall has increased by 5 cm a decade since 1901. This trend doesn’t show any sign of stopping and is even speeding up. And this is what I learned today.
Photo by GEORGE DESIPRIS: https://www.pexels.com/photo/big-waves-under-cloudy-sky-753619/
Sources
https://scied.ucar.edu/learning-zone/how-weather-works/humidity
https://www.lsop.colostate.edu/wp-content/uploads/sites/6/2014/10/WhyDoesWarmAirHoldMoreWater.pdf
https://physics.stackexchange.com/questions/773239/is-heat-energy-actually-kinetic-energy
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