when it's really hot and humid outside it can be really uncomfortable and make even the simplest of tasks a lot of hard work humidity is a huge factor in driving how clouds are formed and weather systems in general so it's going to be very important to understand how it all works [Music] hi i'm grant and welcome to the fifth class in the meteorology series in this class we're going to be taking a look at the last of those fundamental elements of the atmosphere which is humidity humidity is a huge player in the formation of clouds and precipitation and we'll be looking a bit more at how that actually happens in the next couple of classes i'm sure the water cycle is something you're pretty familiar with so every day countless tons of water evaporate into the atmosphere the water rises up condenses down and forms into water again in the form of rain and if it freezes then it forms into ice or snow particles when the condensing gas becomes liquid it gives off heat and in the process of freezing it also gives off heat this is a release of heat in order to get to these states and in the other process if we melt and we evaporate we're taking in heat from the environment in order to break down bonds and create these less structured forms of water you can also have the direct process of changing from gas straight to ice or ice straight to gas or water vapor to solid and solid to gas that sort of thing and this is known as sublimation and so you think of sublimation from gas to ice again we're going to be giving off heat and from ice straight to gas we're going to be taking in a lot of heat so the water vapor in the air has a certain pressure to it which it exerts onto the air molecules and the air around it pressure can be kind of a confusing term because it's more like the amount of water vapor that is in the air but because it has a weight which is a force and it's exerting that over certain areas you can think of it as a pressure because pressure is force over area the air has a fixed amount of water which you can hold which i have represented here by this grid if you think about each molecule taking up one square of this grid there's only a certain amount of squares before there is no more room which is what happens when we've got saturated air you can see all these squares are filled in we've got regular air molecules nitrogen oxygen trace gases and then you've got the water molecules in the air and it gets full and then any more water molecules are then condensed out into liquid form so you've got water vapor pressure which is the amount of water in the air saturated vapor pressure is the amount that it can hold and the saturated air would be when the water vapor pressure and the saturation vapor pressure are equal and then any more water that is added in condenses out and forms liquid and clouds in essence so the saturation vapor pressure this grid varies according to the temperature basically what happens is energy in the form of heat is used to break the chains of liquid water molecules apart into the individual molecules which is what water vapor is if the air is warmer then there is more energy and the air can break these chains of molecules into individual separate molecules more easily what this means in practice is that warmer air can hold more water vapor it has a higher saturation vapor pressure than cold air this in essence is the theory behind cloud formation the water vapor rises into colder air and the saturation vapor pressure comes down to meet the level of actual water vapor pressure in the air and it condenses out and starts to form clouds another way to think of saturation vapor pressure is if you had like a towel and you're wiping up some water you can only wipe up so much water before the towel becomes saturated and any more water just leaks out of the towel there's a few different flavors or types of humidity the most common type that we have is relative humidity this is the amount of water vapor in the air compared to the max that the air can hold as a percentage so it's the actual water vapor pressure the water vapor in the air compared to the saturation water vapor pressure we divide one by the other and then we multiply by 100 to get percentage so as the temperature changes the saturation vapor pressure will change as well um which will have an impact on the relative humidity that was that graph with the line we saw earlier as it gets hotter the amount of saturation vapor pressure will go up we can hold more water in the air which means that if we have the same actual vapor pressure but we get warmer the relative humidity will change think about this as being the 100 we would have the actual water vapor pressure and the saturation water vapor pressure is the same would be the same amount but then as we get warmer the saturation water vapor pressure will um increase but the actual water vapor pressure remains the same so our relative humidity will go down there's a phenomenon called a dew point a temperature called a dew point which is the temperature of air where the saturation vapor pressure and the water vapor pressure are exactly the same what happens is that line that we saw reduces down until the saturation vapor pressure matches the actual water vapor pressure and clouds start to form the water starts to condense out you can think of it as basically where the relative humidity becomes 100 the humidity mixing ratio describes how many grams of water vapor are held within a specific kilogram of air as air increases in temperature it will be able to hold more grams of water per kilogram the saturation water vapor pressure will go up which means more water vapor molecules can come in and therefore the weight increases we can say that this massive here whilst having a relative humidity of 100 has a humidity mixing ratio of 16 grams per kilogram for example note that 100 of relative humidity doesn't mean that all of the air is water it means that it's a hundred percent saturated that's what i like to think of as it's a percentage of how much air would take to saturate so that's 50 of the weight being saturated that's 100 of the way to being saturated so in this case with a relative humidity of 100 we've got 16 grams per kilogram we could imply that a relative humidity of 50 we'd have 8 grams per kilogram that would be our humidity mixing ratio absolute humidity is we take the mass of the water vapor but then we compare it to the volume of air it's kind of like the water density in the air and it will vary a lot with geography of where you are somewhere close to sea is bound to have more water in the air per cubic meter than in the desert for example so whereas relative humidity is the level of saturation in the air the absolute humidity is a very quantifiable amount it's the mass of water per volume and so is the humidity mixing ratio mass per kilogram diurnal variation is a fancy way of saying the day night cycle so the influence of the sun and the heat it gives off in terms of temperature um influence the relative humidity basically it gets cold at night and that influences what happens to the relative humidity so if we take the night time example this means that the saturation vapor pressure also changes remember warmer air can hold more water before coming saturated so at night the value for saturation vapor pressure reduces so if we have the same amount of water vapor pressure that means that we're dividing by a smaller number and that sends the relative humidity up so at night if we have the same amount of water vapor pressure the saturation for pressure comes down and makes it more relatively humid and then the converse happens during the day so the saturation vapor pressure goes up we heat up the air we can hold more water so if we have the same amount of water vapor we're divided by a bigger number which makes our relative humidity smaller so it forms a line that looks something like this as the temperature reduces to the dew point the saturation vapor pressure reduces and we would get a relative humidity of 100 and this is where the water vapor would condense out into forming clouds so at night as the temperature is reducing the relative humidity can shoot up shoot up and if it reaches a hundred percent we start to condense out and form clouds even if we're at the surface and this is what fog and mist is and this happens at the coldest point of the night which is just before sunrise and that is why you sometimes get early morning fog forming if it gets particularly cold at night to summarize them we've got the water cycle we've got the gas liquid and solid if we move from gas to liquid we are condensing and then freezing if we're going from solid to liquid melting then evaporating and if we skip the liquid in the middle we sublimate sublimation between the two the concept of humidity relies a lot on water vapor pressure and saturation vapor pressure water vapor pressure is the amount of water in the air saturation vapor pressure is the amount it can hold that's the grid that i was talking about or you could think of it as a towel a towel can only hold so much water once this grid is full or once that towel is completely wet it is saturated it can't hold anything else and any more water will start to condense out and form liquid the saturation vapor pressure varies with temperature if it's hotter we can hold more water in the air in the form of water vapor and if it's colder we come down and we can't hold as much when the temperature reduces to the point where our actual water vapor pressure and our saturated vapor pressure are equal we get to the point which is known as the dew point and this is where the water vapor pressure equals the saturated vapor pressure this is the basis behind cloud formation you get water vapor rising it starts to cool down and the saturation vapor pressure comes down to the point where the amount of water in the air the water vapor that is rising goes into area where it can't be held anymore and it starts to condense out and form clouds there's a few types of humidity the most common one you see is relative humidity this is the amount of water in the air compared to the amount that it can hold water vapor pressure divided by the saturation vapor pressure and then you multiply that by 100 to get it as a percentage the humidity mixing ratio is the mass of water vapor per kilogram of air quantifiable amount and the absolute humidity i think of it as the water vapor density because it's the massive water vapor per volume of air per meter cubed of air usually and then you get the diurnal variation so this is basically the interaction between the saturation water vapor pressure and temperature so at night when it gets nice and cold the saturation vapor pressure reduces which means we're dividing by a smaller number and the relative humidity goes up if we have the same amount of water vapor pressure throughout the day the same thing happens well sorry the opposite thing happens um during the day so when it's as hot as temperature just after 12 the saturation vapor pressure goes up a lot and that would send the relative humidity down so you end up with a graph which is almost the inverse to the temperature