fronts are the border between one weather system and another and they come in various shapes and sizes but what happens when these fronts approach and pass over our location let's find out [Music] hi i'm grant and welcome to class 15 in the meteorology series today we're going to be taking a look at weather fronts weather fronts bring with them a change in the weather conditions it could be for the better could be for the worse but in any case it is a change and we can often see these fronts approaching from quite far away so it can help us to make predictions about what's going to happen to the weather in our local area for example over the next couple of days or even couple of weeks when air masses move over the earth they sometimes meet other air masses which are hotter or colder and because of the temperature difference that means that there's a density difference and the air masses tend not to mix and instead there is a quick transition from one air mass to the other which we call a front this is the border between two different air masses and fronts come in a few different shapes and sizes the first one is a warm front a warm front is when warm air moves into the area of colder air unsurprisingly the line on the earth will look like this and have sort of semi circles think of as half sun shapes is what i do to remind me that it's a hot front and it's usually red in color and that'll show the line at the earth's surface and the front itself doesn't stay vertical it instead slopes up towards um the colder air because the hot air is rising above the cold air the ratio of what it does this is about one to one hundred fifty so we go up one meter and a long 150 meters so it's quite shallow and it spreads out quite far it could be in the region of about 600 700 nautical miles in total length that this warm front um covers and the warm front travels at a certain speed and it's usually a third of the speed of the 2000 foot wind and why the 2000 feet wind well at 2000 feet we're sort of outside the friction layer so it's the free stream flow of air and the warm front moves a third of that speed the opposite to a warm front is a cold front and this is where cold air mass moves in and forces its way under warmer air due to it being higher density and it's given this symbol which is sort of like icicles i think of it as and it'll be blue in color the slope of a cold front is much steepers normally has a ratio of 150 so we go forward one and up 50 which is a lot steeper than the warm front and this means that it's not as spread out typically uh only nears a distance of around 100 nautical miles in total length and a cold front can often be held at the surface by surface friction and it can lead to an overhang forming which is known as a cold nose and this cold nose repeatedly forms and collapses and forms and collapses which leads to sort of gusty wind conditions as it's doing that the speed of travel of a cold front is faster than a warm front it's typically two-thirds of the two thousand foot wind or two-thirds of the wind outside of the friction layer occasionally a cold front will catch up to a warm front this is because the cold front is obviously moving at two-thirds the wind speed and warm front is only one third the wind speed so occasionally they catch up and we get something known as an occluded front an occluded front basically forms a v shape of warm air in the middle where the two fronts become connected and ahead of it you get cold air behind it you get cold air just with this little sandwich of warm air in the middle an occluded front front is normally just associated with wet weather conditions so weather is formed at the fronts because some air mass is being forced to rise above another air mass because of those different densities with a warm front we see stratiform weather so the ascending air is forced up quite slowly because of this gradual slope and we'll see fairly stable stratiform conditions and it will lead the actual line of the front on any charts because the line shows it on the surface and obviously we're sloping towards that so you'd start off with maybe cirrus clouds very high up getting towards alto stratus clouds again stable in that mid layer nimbostratus around sort of this region and eventually stratus behind it and you'll basically see the cloud base the lowest level of cloud lowering as the front approaches your position if we were standing here and this is passing over us our cloud base starts all the way up here and starts to lower as this front approaches in contrast a cold front forces its way underneath warm air at quite a quick rate at 1 in 50. and so it causes the warm air located here to rise quite rapidly and makes that unstable conditions which is cumulaform weather producing as a result so you get cumulus nimbus clouds those storm clouds you get cumulus clouds and eventually you get altocumulus up in this sort of region up here and with an occluded front we see a wedge of warm air produced in between the two colder air masses and as a result we get a bit of a stratiform area leading in and a cumuliform area after the front and again you get nimble stratus and cumulative just associated with wet weather in general if we think of a few columns of air where a front passes we can easily figure out what happens to the pressure as these fronts approach so first we'll have a little look at a warm front so each column of air will have a different proportion of warm air and cold air due to the slope of this front you can see this has a lot of cold air whereas this has a lot of warm air warm air is less dense which means there are fewer particles per unit area so this column on the right has a small proportion of air with not many particles in it and the particles are being pulled down by gravity and imparting a pressure onto the surface of the earth that surface pressure that we're going to measure as we move to the left or you can think of it as this warm front passing over this same column and this just being three time shots of that same column and basically more and more of the column is made up of this warmer less dense air and that means that there's fewer particles in the air and that means that there's fewer particles being pulled by down by gravity and imparting that pressure onto the surface so overall you can see that as a warm front approaches the pressure drops as a result you get one zero one zero one double eight one two six and so on and so on as the pressure drops as the front approaches a cold front is a bit different because the cold air is forcing its way under the warm air and if we have a look at the columns of air then as the front approaches nothing's actually happening it's only once the front hits that we start to get parts of these columns made up of that colder more dense air because it's more dense there's more particles more particles get cooled down by gravity therefore more pressure as a result and you will see that it will increase the pressure rapidly just after the front's passed and it'll be rapid because you've got that steep um steeper angle so quickly you get more and more of these columns made up of colder basically higher pressure air so both fronts will do something interesting to isobars let's just take a look at the cold front for example so we know that the pressure increases rapidly after the front has passed so we've got 1006 here and here and then it goes to 1008 1 0 1 0 and so on so if we were to draw these individual pressures joined up in isobars then we'll see something quite interesting so once the lines are all joined up on all these spot pressures joined up we can see that there's a very definite change as we pass over the cold front in terms of the direction of these isobars there's a very sharp angle at the front and that basically means the wind direction changes as well so at the surface you would get one that's not exactly aligned with the isobars because in the northern hemisphere the wind backs as it goes down but then if you still take a fairly standard backing of maybe like 40 degrees and apply over here as well you can see that the wind has changed in angle by the same proportion as the isobar has changed so as a cold front passes the wind veers round and the same thing happens when a warm front passes as well so across the world there are different air masses that dominate certain regions such as the tropical maritime region and the polar maritime region over the north atlantic ocean and where these air masses meet a front is formed and depending on the time of year this polar front this boundary will be higher in latitude or lower in latitude as the warming areas warm and cold areas sorry fluctuate with the temperature and the seasons they will also vary with pressure patterns and other factors such as competing ear masses and this front changes and it's very rarely a straight line like this and it can cause bumps and indentations into each other's air spaces and something that forms sometimes is called a polar front depression which we're going to have a look at just after i've explained something else another common frontal zone exists around the equator in something known as the intertropical convergence zone so what happens is at the equator it's very hot so that surface heating causes air to rise out which ascends up it spreads out starts to cool down and then descends back to earth in the hadley cells i've mentioned them before in previous classes this creates high pressure areas around the tropics air likes to flow from high to low pressure so it flows back in towards the low pressure area caused by this surface heating at the equator as it starts to flow it gets pulled round to the right in the northern hemisphere and to the left in the southern hemisphere this creates some predictable winds known as the trade winds which are north east in direction in the northern hemisphere and south east in direction in the southern hemisphere where these winds meet a front is produced because the air masses will have slightly different temperature and moisture levels and it will vary throughout the year in the same way as this polar front does through the seasons so i talked earlier about how the front between the tropical and polar maritime air masses kind of fluctuate and bulge and sometimes a frontal depression will form a polar front depression we call it this is when a warm air mass or the tropical maritime air mass bulges up into the cold air and the wind starts to curve around the air the bulge as a result and a low pressure area is formed at the tip of the warm air and forms a shape that looks like this the air masses will have distinct boundaries with the warm air moving northeast and the colder air mass moving southeast and a front forming at the cold and the warm areas respectively this low pressure area is a depression and it has fronts in it hence the name frontal depression and the fronts will then be moved with the prevailing wind around the depression and the depression itself will move in the direction of the wind in the warm sector so you get these fronts starting to move round the low pressure zone like this but the overall bulge will move horizontally like this and this is why in europe we experience a lot of these frontal depressions these waves of weather coming in and you might associate places like ireland uk with constantly having rain and storms and stuff like that this is one of the reasons why because these fronts move in and hit ireland in the uk all the time the weather associated with a frontal depression is the same as if we had a warm front then followed by a cold front so at the warm front we will see slowly rising air over a long distance which means we get cirrus clouds up top and then altostratus and nimbostratus and stratus clouds the cloud base will be lowering as the front approaches our position and as the front passes over our position we'll see a temperature increase as we go into the warm sector we will see an increase in the humidity of the air and therefore a higher dew point as a result and we'll also see the pressure falling off as we saw earlier and again with that pressure and that frontal drop off we'll see the isobars changing so we'll get a change in the wind direction in the warm sector we'll see low stratus clouds as the warrior is maritime air it's full of moisture but the low the high temperature air is slightly more stable so we don't associate with very cumuloformed clouds and as the cold front approaches and passes we will start to see the rapid ascent of this warmer air leading to unstable conditions thunderstorms heavy rain showers etc and as the front passes we'll see the pressure increasing the temperature will fall as we go back into the colder air which means the dew point falls and the humidity falls as well eventually we will get occluded fronts forming where the cold front catches up to the warm front this happens first where they're closest together so it'll be at that tip of the um the sort of triangular pattern and then they'll start to spread down and eventually you end up with a fully occluded front it's almost like if you zip up the polar front depression that's how the occluded fronts form so just as there are depressions with fronts in them frontal depressions there's also some without fronts in them which are non-frontal depressions surprisingly there is one sort of non-frontal depression that there is worth adding a bit of extra information about and which is called a thermal depression so these form in certain areas year round like the equity equatorial lows and hadley cells that we looked at previously but they can also form in specific areas depending on the season so thermal lows form because the surface temperatures heat up and cause the air to rise creating an area of convergence at the surface and rising unstable air in the center of the low pressure area something that also happens is as a cloud is formed by the rising air and the um condensing air into clouds some latent heat is released from the air mass itself and that means that the rising air is now warmer than the surrounding air because of this latent heat release which can cause even more unstable conditions a common place that this happens is somewhere like india throughout the summer gets very hot and as a result there's a large low pressure area which is formed over the land and it can move over the ocean lead to a lot of air rising over the ocean moisture filled air huge clouds and storms form as a result towards the end of the summer which is the monsoon season tropical revolving storms or hurricanes cyclones or typhoons are another version of these thermal depressions but in its most extreme form so a very deep depression will form in a warm equatorial region over the ocean these areas of low pressure start to move away from the equator and if they move over dry land then there's not enough evaporation to form any storms so it's no issue but if they move over the ocean then there's more moisture in the air it's a more humid environment and it rises and it condenses to form clouds this releases a large amount of latent heat and causes even more instability in the atmosphere once this area of low pressure and cloud moves sufficiently far away from the equator about 500 kilometers then it starts to feel the effect of the coriolis force and a spin starts to form this spinning motion starts to spread the clouds away from the center and an eye form the eye of the storm and at the edge of the eye we have the most violent weather conditions experienced so violent in fact that of course it causes massive disruption to places like the caribbean or you know japan gets hit by these quite hard and it's only certain places that these form because we need very specific conditions we need a warm ocean with a low pressure area that is very large so they tend to happen normally at the end of summer as well because the oceans had a large amount of time to heat up and form this low pressure area so over the atlantic when they hit places like the united states and the caribbean we call them hurricanes what happened towards the end of summer in july october time in the pacific in general in japan and places like the philippines they would call them typhoons usually and again it would be the northern hemisphere end of summer so july to october and in the indian ocean over the south of the indian ocean they tend to call them cyclones and they'll happen in the at the end of the summer for the southern hemisphere which would be sort of january to march time but if they hit the indian ocean in the northern hemisphere it would happen again at the end of summer which would be july to october but still called cyclones in summary then you've got a warm front which moves at one third of the 2000 foot wind speed it's got a slope of about 1 to 150 it's given this little red half sun symbol it slopes up very slowly and gradually leading to stratiform conditions which lead ahead of where this cold front sorry the warm front actually is on the surface you get cirrus altostratus nimbostratus and stratus clouds typical stable sort of conditions and drizzle that sort of thing a cold front is the opposite it moves at two-thirds of the 2000 foot wind speed it's got a slope of one to 150 and it forces its way under warmer air causing that warm air to rise rapidly it can lead to very cumulaform clouds as a result and cumulonimbus big storm clouds and it's given the symbol of this sort of pointy blue line and most of the weather will be in and around the cold front itself and because it's not got this very low gradual slope leading ahead of it it actually slopes behind it because of the cold front moving faster than the warm front eventually the cold front will catch the warm front ahead of it and you get an occluded front as a result which basically means you get warm front weather followed by cold front weather and the warm sectors kind of squeezed up into this v shape it's given the symbol which is just a combination of the two for cold front and warm front so as the front approach interesting things happen to the pressure think of the columns of air and the warmer air being less dense colder air being more dense uh air that is less dense fewer particles to push down on the earth and create pressure so if we look at the warm front first which is these figures up the top we'll see that as the warm front approaches more and more of these columns are made up of hot air and or warm air and the pressure drops as a result quite gradually a cold front would be the opposite we see that the pressure increases rapidly after the cold front has passed because there's more cold air in each of these columns more dense more particles more amount of particles pushing down on the earth's surface and it's faster change than the slope because of the it's a faster change than the warm front because of the slope difference between the two so you get various global frontal activities you get the polar front which moves north and south throughout the year it's the board line between the tropical maritime and the polar maritime you get the equatorial or sorry the inter-tropical conversion zone which is formed by the equatorial lows and the hadley cells the warm air the equator rises creates low pressure zone that air starts to spread out falls back down to earth creating high pressure the tropical zones air flows from high to low gets pooled around with the coriolis force we get the trade winds as a result which are north easterly in the northern hemisphere southeasterly in the southern hemisphere where those winds meet into tropical convergence zone a front is formed there we also get more local versions of this sort of thermal effect and where we get a lot of heating over land causes a lot of air rising or even over the sea and the most extreme version would be a tropical revolving storm we got a large area of low pressure lots of rising air because it's so full of moisture those clouds start to condense out as they condense it creates heat that heat leads to these parcels of air being even more unstable more rising and as it moves away from the equator it starts to spin causing a lot of damage as it does so hurricanes typhoons cyclones whatever you want to call them tropical revolving storms so as you get non-frontal depressions we get frontal depressions frontal depressions basically form from a bulge in the polar front or indeed it could be the intertropical conversion zone and essentially you see weather from a warm front then a warm sector where there's low stratus clouds and then you see weather from a cold front so as one of these passes you'll see the pressure start to drop it'll start to stabilize for a bit and then it will increase rapidly behind you get all the weather that's associated with it you get the wind direction changing um and this sort of thing happens a lot to northern europe these polar front depressions blow over the atlantic and we've got lots of frontal activity in northern europe as a result