Dr. Victor L. Martin
Before we can discuss our climate, there are several concepts and terms that will help it all make a little more sense. These are items that you likely know even if you don’t know you know them. The first one not only explains climate but most of what we observe and deal with every day.
· Energy, energy potential, and gradients – this is a concept that is the key to understanding not only weather and climate but why the world around us works as it does. We all know this, for example, water doesn’t flow uphill unless you add energy. Everything in our world wants to move from higher to lower energy or concentration levels and ultimately wants to reach equilibrium. The greater the difference of the gradient (the difference between the high and the low) the greater the movement along the gradient. The classic weather example is air moving from higher to lower pressure. When the difference between the high and low pressure areas is great we experience strong winds. We also use the concept of gradients in something like air conditioning. Before you read on, take a moment to answer this question: “How does an air conditioner work? Does it cool the hot air or warm the cold air?” If you understand potential and energy gradients you know the answer.
· Evaporation – the process by which a liquid is transformed from a liquid to a gas, in weather it is the changing of liquid water to vapor that enters the atmosphere and results in humidity. Water vapor enters the atmosphere from the soil, bodies of water, and even us as we sweat. Since water as vapor is a higher energy state than water as liquid it requires the addition of energy for it to occur. The energy comes in the form of solar radiation and heat added by the temperature of our surroundings. In areas with more rainfall and moisture available, say the traditional Corn Belt, the amount of energy consumed by evaporation helps keep temperatures lower, although the humidity may make it seem even worse. As a sidebar, the opposite is true. Water changing from a gas to a liquid or a liquid to a solid, gives up or releases energy (heat).
· Transpiration – the process by which water in plants is transferred as water vapor into the atmosphere. Typically, water moves into the plant from the roots through the xylem and throughout the plant and ultimately into the leaves and out the leaf pores (stoma) in response to a water vapor gradient between the atmosphere and the plant. As long as the plant can move adequate water the plant is okay, when it can’t keep up because of a lack of soil moisture or extreme heat and low humidity, the plant wilts. Wilting is a defense mechanism, a natural response to protect the plant. It is only when the plant can’t recover overnight or even worse, can’t curl up its leaves that real trouble begins.
· PET – Potential EvapoTranspiration is the amount of evaporation and transpiration that could take place given atmospheric conditions if water wasn’t limiting, i.e. all the water needed was available. As the actual ET and the PET diverge, we get into trouble.
So next week, let’s talk about our climate, the summer of 2011, and where do we go from here.