UNCOMFORTABLE DEW POINT FULL

UNCOMFORTABLE DEW POINT FULL

Is the glass of cold water half full or half empty? You can tell by the 'dew' on the outside of the glass. Find that hard to believe? Let me try to convince you. However, at this point all I want you to understand that evaporation and condensation events are taking place all the time, everywhere around you. (I should also mention here that solid to vapor and vapor to solid transitions are called sublimation and deposition, respectively.) Later in the course we’ll explore these “phase transitions” in more detail. Likewise, when transitioning from a liquid to a gas, the process is called evaporation. When transitioning from a gas to a liquid, water undergoes a process called condensation. Now we’ve established the properties of water vapor, we need to discuss the process by which water changes phase (namely to and from water vapor). We’ll discuss this idea in much more detail later in the course, but I wanted you to start thinking correctly about water vapor from the start. The implication of all of this is: Air does not “hold” water vapor.

This means that in our box of air, the oxygen molecules are acting independently of (and oblivious to) other molecules – including water molecules. However, each marble is acting independently from each other marble. These marbles (because they have a lot of energy) are zooming around, bouncing off the sides of the box and each other. This is very much like having a box of various colored marbles. Consider a situation where you had a box of “air” (containing all of the molecules normally found in the atmosphere). On a molecular level, water vapor behaves just like oxygen, nitrogen, carbon dioxide, etc. The next important thing to realize is that water vapor behaves just like any other gas. No one molecule can 'hold on to' another.

It is often helpful to think of air molecules as marbles. The “cloud” you can see isn’t technically steam either. In fact, this is only portion of the effluent that is “steam”, or super-heated water vapor. This is where the water exists in a pure vapor state. If you look closely, there appears to be a gap between the tea kettle’s spout and the visible cloud (here's an annotated image of the tea kettle). Within the effluent escaping from the spout, where is the water only in vapor form? (Hint: It’s not in the part you can see.) Although there is likely some water molecules remaining in the vapor state in this visible “cloud”, most of the water that you can see is actually in the form of tiny liquid drops. For example, check out this image of a steaming tea kettle. Often folks confuse what are actually condensed (liquid) water droplets as water vapor. Just like any other gas (oxygen, nitrogen, carbon dioxide, etc.) it is invisible. The first thing that you need to recognize is that water vapor is a gas. We are well familiar with water’s solid (ice) and liquid forms, but our understanding of its vapor state is a bit more elusive. Water is one of the rare substances that can exist in all three states naturally in our atmosphere. In the meantime, let me begin to paint a picture about the role of water in the atmosphere.Īn obvious starting point for this introduction is, “What is water vapor?” Now, I’m sure that everyone has learned at some point that matter exists in three states: solid, liquid, and gas. Rest assured, we will have this discussion later in the course. In the interest of full disclosure, moisture in the atmosphere is a complicated topic, the understanding of which will require an in-depth discussion that is not appropriate for this introductory lesson. It’s not that these variables don’t have their uses (indeed they do) it’s just that they don’t depend solely on the amount of water vapor in the air. This is a very important statement which sets dew point temperature apart from many of the other variables that describe moisture in the atmosphere. However, dew point temperature is much more than this seemingly straightforward definition, and to understand the details we have to delve a bit deeper.Īs it turns out, the dew point temperature is an absolute measure of the amount of water vapor in the air. By definition, the dew point is the approximate temperature to which the water vapor in the air must be cooled in order for it to condense into liquid water drops. This variable is called the dew point temperature. Thus, it’s no surprise that there is a variable relating to water plotted on the station model. Everyone will surely recognize that water is an important player in weather.

Uncomfortable dew point