hey all let's talk about everybody's favorite Topic in Southeast Texas humidity this meme always reminds me of first moving to Southeast Texas because I always felt like it was humid the first year or two I moved here because the humidity along the Gulf Coast is so much higher compared to other places across the United States and that's something to talk about going forward first I want to tell you a little bit about absolute humidity versus relative humidity the first thing I'm going to talk about here the absolute humidity is the actual mass of water per unit volume of air okay so it's measuring how much water by weight is actually in a volume of air if you were to take like a sample box of the air itself so it's measuring the molecules within that volume this is important because it tells you if it is going to rain how much could actually fall the relative tells you the odds it's likely to come close to getting the air saturated and fall but this actually tells you how much rain could fall if you do reach that point of the air becoming totally saturated relative humidity however is what tells you how likely you are essentially to have the air be saturated because this is talking about the percent of water in the air given how much that air could hold at different temperatures and pressures so this is given by taking the actual Vapor content in the air divided by how much the air could hold at a given temperature and then we usually multiply that times 100 to give us the relative humidity as a percentage this is the one you see in your weather apps or when they talk about on the news and they show you a percent humidity that is talking about relative humidity in lab you may talk about using different terms but it's the same basic concept for actual Vapor content they talk about using the mixing ratio and then you're dividing it by the capacity okay so it's the same terms in lecture I'll probably use actual Vapor content divided by capacity and lab they may use the terminology mixing ratio divided by capacity and both of those again were expressing as a percentage and multiplying times 100 so the relative humidity is important because it tells you how saturated the air is compared to what it could hold so when we look at the relationships between temperature and how much it could hold it's like I've talked about already in class it takes a lot of energy to hold water as a vapor and so the higher the temperatures the higher the amount of moisture you can hold the lower the temperatures the lower the capacity of that air this is one reason why as you increase in elevation the water can no longer be held as a vapor because it gets colder as you go up in the troposphere and you don't have enough energy to hang on to that as a gas so let's look at some examples of how relative humidity works the first example I'm showing you here is what the potential or capacity is for that air to hold moisture at 80° so you can see it can hold up to 20 G on this day the actual Vapor content is 10 Gam so to figure out what the relative humidity is you just take 10 the actual divided by the potential or the capacity 20 that comes out to 0.5 and you would multiply that times 100 and that gives you a relative humidity of 50% now I want you to realize that the relative humidity changes with different conditions so if you heat that air up to 100° you now have increased its Vapor potential IAL content or its capacity to 30 G and so heating that air up has actually changed its relative humidity now you take 10 the actual divided by 30 the capacity times 100 and that gives you a relative humidity of 1 over3 * 100 or 33.3% now if we take that air and cool it down much more the capacity actually goes down remember the capacity de decreases the capacity of that air decreases as you cool it off so notice in all three of these examples the actual Vapor content is the same this is like what happens on a daily basis in most places when you don't have fronts and things that move through if you have relatively stable conditions in a given area it gets warmer during the day you peak in the afternoon and then it cools off at night so the relative humidity percentages change throughout the day even though the actual Vapor content may be pretty consistent so let's look at our final example here now we have our same actual Vapor content of 10 but our potential Vapor content has dropped to 15 or our capacity has dropped to 15 and so now we have a relative humidity of 66.66% I want you to note how relative humidity changed as air temperature changed okay so if you hold actual vapor content the same but increase the temperature like our example right here okay if you hold the vapor content the same but increase the temperature relative humidity goes down if you decrease the temperature and hold the vapor content the same relative humidity goes up so let's just summarize again what happened what happened to the potential Vapor content when the temperature decreased well we cooled the temperatures off and that meant that our capacity went down and therefore when we talk about the relative humidity when you decrease the temperature relative humidity goes up if the actual Vapor content Remains the Same so this is important for figure out what happens in different areas in areas like big band or areas out west you could have some humidity in the air you could have some decent absolute humid because the overall temperatures are pretty high however that air can also hold a lot more moisture the capacity is way greater and so your relative humidity winds up being much lower okay so hot air can hold way more cold air can hold way less in terms of water vapor so the question here and how this applies to weather and climate Beyond just calculating relative humidity and looking at absolute humidity and how much it could rain or if it's likely to rain or not what happens when actual Vapor content and potential Vapor content or the capacity are the same what happens when actual Vapor content and capacity equal each other well now we have our potential Vapor what we're saying is it's 10 G and we're filling that with 10 G of water well now we have a relative humidity of 100% once the air is saturated now we're likely to get condensation that occurs or deposition if we're below 32° F or 0° C so if we're above 32 degrees we get condensation that occurs the air is now totally saturated with moisture no more moisture can be held as a vapor or a gas and now it begins to condense out and it goes back from a gas to a liquid that is how fog forms that's how clouds form you're reaching points in the atmosphere or at the ground surface surf where the temperatures aren't high enough to hold any more water as a vapor and now it goes back to liquid form okay so condensation on your windows well in the morning when you wake up sometimes in Southeast Texas that means your windows are below the temperature that would allow all that moisture to be held as a gas clouds are telling you where you're reaching that 100% saturation level in the atmosphere itself so in summarizing relative humidity when we look at it it's actual divided by capacity okay now what I want to take from that is how we can reach a relative humidity of 100% because we want to know how we get clouds and how we get fog and those types of things so one way the most common way this usually occurs is by just changing the air temperature and to get 100% saturation the easiest way to do this is to drop the air temperature or decrease the air temperature because what happens as you decrease the air temperature is you're shrinking the size of that capacity box from the examples we were looking at you're decreasing the capacity of air and making it more likely that the actual water vapor in that air fills that air to capacity so how do we do that again we look at this example here and if we have this example of our 60° air and our 66.7% humidity that we had calculated before 10 over 15 what we can do is drop that air temperature to 40° and now we've changed our capacity to 10 gram and now that air is totally saturated okay and all we did to do that again remember is just decrease the air temperature so this is why sometimes you may not have fog you may not have precipitation during the day but when you wake up in the early morning hours you may find Dew there may be fog because that moisture is saturating that cooler air at that point the other way you can actually reach a relative humidity of 100% would be to increase the vapor content itself so for example in Southeast Texas maybe you get a bunch of moisture that gets blown in off of the Gulf of Mexico but you have to have the wind conditions and the atmospheric conditions just right for that okay so in that case you would be keeping the capacity the same but changing the absolute humidity or the actual water vapor to fill that box to capacity okay and so that case is not changing the temperature to do this this is changing the actual amount of moisture in the atmosphere itself if you were to weigh out that moisture and figure out how much is in there per unit volume of air so what happens with fog he we can tell this picture was taken earlier in the morning because the sun is pretty low on the horizon down here in Southeast Texas so what fog essentially is is where the relative humidity reaches 100% close to the surface or in other words water is condensing out near the surface because that air can no longer hold it there's not enough warmth or there's not enough energy in that air to keep it in gas form so it condenses out and we see that more moisture condensing out as liquid droplets keep in mind that there's moisture up there in the atmosphere all the time but when it's in gas form we don't see it we do not see that moisture until it is condensed into a liquid or deposited into a solid so I already answered this question for you this was taken early in the morning okay so what is important to understand is that most of the time actual Vapor content or the absolute humidity isn't usually changing much over the course of a day but we know the temperature changes over the course of the day right it's cooler in the morning it heats up during the afternoon we peak in temperature a little bit afternoon like we talked about with temperature lags and then once the sun goes down we start to get cooler through the night so it's temperature that is usually responsible for the major changes in our relative humidity now in lab you'll talk about using something called a sling pyrometer to actually measure humidity what it does is helps simulate water temperature wind and the amount of humidity in the air itself so what happens to evapo transpiration for example if you change the temperature and wind speed well you should have learned in previous lectures if you increase the temperature you're more likely to evaporate water if you increase the wind speed you're more likely to evaporate water right and so another way to counter that would be to increase the water vapor in the air you can slow things down by if the air is already saturated so we can simulate these different conditions using the sling cyclometer so the sling cyclometer has a dry bulb which measures the actual temperature of the air and we have a wet bulb which is saturated in water and simulates the effects of aapo transpiration if you lose more water from it when you sling it around that relates to more cooling because remember it takes energy to evaporate water so that heat energy goes with that water and therefore causes the wet bulb to cool down if there's a lot of vaporation so more water loss simulates conditions where there's much less humidity in the air and that wet bulb will cool down more as a result okay we sling the pyrometer around because that helps to simulate the effect of wind and allow those water molecules to leave more quickly so when we look at the difference between the wet bulb and the dry bulb after we sling the pyrometer around that can help us relate the amount of humidity that's in the air okay the bigger the difference between the two bulbs the dry bulb versus the wet bulb the lower the relative humidity so let's think about why that is well if there's a bigger difference between the wet bulb and the dry bulb that means there's a lot of cooling that occurred on the dry bulb which means a lot of water had to leave with its latent heat energy to move into the atmosphere okay so humid air and air that's cooler we can't evaporate as much water and so the temperature doesn't change as much for that wet bulb if you sling this around out in the middle of a desert or on a day where it's really dry and hot in Southeast Texas which can happen periodically more water evaporates therefore causing more cooling of the wet bulb so let's look at an example here A versus B the wet bulb depression or the amount of the temperature change of the bulb in a is 20° and in B it's 3° so the question is in which condition A or B is relative humidity the highest you should tell me the relative humidity is much higher in B meaning the air is already very saturated or close to Total saturation and that's why not as much water was able to escape the wet bulb and so it didn't cool down as much in letter A there must be a very low relative humidity because there's a lot of moisture escaping the sling pyrometer so then the question becomes under which condition would you be most likely to form fog applying this concept to something we see in real life well again you should answer B for this question because the air is already closer to saturation the relative humidity is higher that's why there's not as much moisture moving into the atmosphere and so B would be the condition where if you cool the temperature off a little bit more you're much more likely to form fog and saturate that air and you can look these things up in a lookup table and figure out how much wet bulb Depression Did you have what was the temperature of the dry bulb to start so for example if you had 60° as your starting temperature and you measured 6 degrees of wet bulb depression or 6 degrees of change compared to the dry bulb you wind up with a relative humidity of six 68° and we could look at another example where we had 18° of wet Bal depression so this is going to tell you right off the bat that you're likely to have a relative humidity that is going to be lower and so if we look at a 60° dry bulb starting temperature and 18 degrees of depression we have a 133% relative humidity that's something you might find on a winter day in the desert out west so for example Las Vegas you might get to 60° in January and you may have relative humidities of 10 to 20% okay so remember bigger wet ball depression lower the relative humidity because that era is already saturated sometimes and we'll have low wet bul depression and when it's not saturated you get lots of water evaporating okay now when we talk about humidity we also need to talk about a concept called dupoint dupoint is the temperature at which you reach that 100% humidity okay so if we think about our sling pyrometer example what's true of the wet bulb and the dry bulb temperatures at the dupoint temperature in other words what happens to wet bulb depression how does the wet bulb temperature change if you're at the D point you should tell me there's absolutely no change in the wet bulb if you're at the dupoint point because the air is saturated at that temperature meaning the air is already at 100% humidity it may actually be raining outside so there's no moisture escaping from that wet bulb at that point you're at total saturation when you have a dupoint temperature that has been reached okay so when we look at clouds and we think about the bottom of clouds the bottom of clouds when you look up on days where the atmosphere isn't super turbulent you'll notice the bottom of clouds is often relatively flat that's because that flat line that runs through here is essentially telling us where we've cooled down the air to the dupoint temperature and we've reached that saturation point in the atmosphere okay and so we have those flat bottoms as a result of that condition okay so that elevation in the atmosphere where you see the flat bottom of clouds tells you where the dupoint temperature is reached where the atmosphere is cooled off enough to saturate at those boxes of air to 100% humidity okay so what we need to remember is above that dupoint line in the atmosphere relative humid is 100% below that line that means relative humidity is less than 100% okay there's a video here that can help explain dupoint and why it's much more consistent and I'd like you to watch that in the PDF that you can find on blackboard so dupoint is much more consistent because it doesn't change substantially throughout the day relative humidity remember changes by temperature so relative humidity is high in the morning it's lower in the afternoon and then it gets higher again in the evening as temperatures cool down dupoint is much more consistent if you watch that video they'll talk about how do points above 65 to 70° tell you that the air is going to feel pretty oppressive to human beings that's because a 70° D point is telling us you only have to cool the air to a temperature of 70° to totally saturate it with moisture okay that means there's a lot of water around if you're only cooling it down to 70° to reach saturation okay and I want you to notice dupoint temperatures here have a high relationship or positive relationship with what we've looked at in terms of vapo transpiration so all these areas in the southeastern United States because they're located close to the Gulf of Mexico and a big source of warm water and evaporation all these areas have very high due points and so these areas especially during the summer when it gets hotter feel very uncomfortable to us because there's too much moisture in the air and we can't evaporate off our sweat into the atmosphere if it's highly saturated with water the other reason D point is often a better measure of humidity throughout the day is because it doesn't fluctuate so notice this blue line down here this one right here okay that line represents the dupoint temperature throughout the day it's very consistent okay meanwhile the temperature line or this red line this one right here is changing throughout the day as a results of temperature right an insulation it's coming in okay an insulation that's going out so where there's a surplus we're heating up where there's a deficit overnight we cool down like we've talked about notice how the green line is directly proportional to the temperature line the green line is relative humidity relative humidity is a relatively poor measure of humidity on a daily basis if you go out in the morning it's often going to be close to 100% because the air is cooled down during the night but what tells you how the air is going to feel better throughout the day is dupoint again do points that start to get over 65 start to feel pretty humid they feel very oppressive and very uncomfortable as you get above 70 and sometimes in Southeast Texas we'll get due points that are above 75 think about how much moisture you have to have to saturate air that is at 75° because remember that 75 degree air has a lot of energy compared to air that's say 20° okay so remember dupoint is more consistent it's one of the better ones we look at in weather and climate over time to see what's changing in different areas remember that relative humidity is important just for telling us if the air is close to saturation because that helps tell us if we're likely to get fog or precipitation or things like that but it doesn't really do a good job of telling us how we might feel on a given day so when we look at the heat index remember the higher the dupoint temperature the more moisture is around in the atmosphere remember I told you that dup points in Texas can reach 70 to 75 if you're in the Southeastern part of the state or closer to the Gulf Coast often times our air temperatures during our hottest months could reach close to 100 so during those times you're talking about heat index readings that could be in this Zone where we talk about them getting dangerous 100 to 100 or 108 to 114° is what that air will feel like because your sweat can't evaporate off you as quickly as it needs to to help cool you down so we wind up with stuff like this it gets so hot that we feel like we cannot cool down and if you've seen this scene in Anchor Man he's drinking milk and if you ever tried to drink something hydrating during the afternoon I wouldn't think milk is the best choice so to reach the dupoint temperature remember that the easiest way to do that is to cool the air down down okay water is pretty consistent on a daily basis and so how we get clouds to form is by cooling that air down as that moisture Rises okay so let's look at this example and think about how high above the surface we'll get the dup point if the air surface temperature is 80° and the dupoint on this day matches what we can get in Southeast Texas and is 73° remember that the average rate the atmosphere is going to cool off like I talked about just very briefly early on in our atmosphere lectures it's going to cool off if the atmosphere is pretty stable on a given day by 3 and a half degrees per th000 feet we call that the normal lapse rate and the environmental lapse rate and I'll show you more calculations in the lapse rate calculation video before we get to the assignment related to these things so what we're going to do is look at the change in elev ation okay and that's going to be equal to the change in temperature divided by the lapse rate so for our example here the difference between 80 degrees at the surface and a 73 degree due Point higher up in the atmosphere okay is going to be 7° so 7° of change divided by 3 and 1 12° per th000 ft is going to equal 2 times that th so 2,000 ft right times 3 and 1/ 12° of change per th000 ft is going to result in 7° of change so what that looks like visually is our surface temperature on a given day is 80 our dup point is 73 and that means that dupoint would cool or results in an elevation of 2,000 ft being where the boundary is because we would cool off at 3 and half degrees per th000 ft so that means the temperature of the air halfway up would be 7 6 1/2 it's cooled off 3 and 1/2 de on the way up and then it cools off another 3 and 1/ 12° and that results in that dup Point existing at 2,000 ft and that's where our clouds would exist you'd have a flat bottom Cloud right around 73 degree dupoint at 2,000 ft above the ground surface on this given day so in applying this concept which scene below that seen with low clouds on the left or the high clouds on the right where would relative humidity be highest near the surface hopefully you said this picture right here where you have low clouds that means you have a saturation existing or 100% humidity existing closer to the ground level that means in the desert areas where the dup point is very low and you may it do points around 20 or 30° that means you have to go way way way up for the air to cool down and to actually get clouds in desert environments that's why direct sunlight is more intense in desert areas versus diffuse because there are many days where there's not enough water vapor around and the D point is so low that during the summer and times like that you don't ever reach the D point and you have very clear skies in areas especially away from mountains where the air is getting forced up