we know that the sahara desert is a hot dry and sunny place but why is this the case let's find out [Music] hi i'm grant and welcome to the 16th class in the meteorology series today we're going to be having a quick look at climatology climatology is a study of why the weather is the way it is in a certain geographic location and hopefully by the end of this quick class you'll have a good understanding of some typical climates that we see across the world climatology is the study of global general weather phenomena and patterns the main driving force behind the predictable weather patterns over the seasons is the movement of air within the troposphere caused by surface heating we've already seen this phenomenon in previous classes when talking about air masses and pressure patterns and they're called hadley cells basically what happens is we see surface heating at the equator causing air to rise the air rising causes low pressure it rises up to the tropopause spreads out cools down and starts to descend in the subtropical regions leading to high pressure the inverse of this is true at the poles the surface cooling the very cold temperatures of the poles create areas of high pressure that pool down the air and it sucks in air from the mid-latitudes say we end up with three hadley cells in north and three happy cells in the southern hemisphere this is obviously a cut through view in reality they would be like donut shape these hardly cells drive the various weather conditions year-round or climates such as sunny hot deserts and stormy equatorial regions which we're going to have to talk about just after we've covered the winds here air flows from high pressure to low pressure and then it feels the effects of the geostrophic or coriolis force this means that we have some winds that are fairly consistent year round if we look at the equatorial region we see air flowing um towards the equator from the subtropical highs the coriolis force then pulls it round to the right in the northern hemisphere and in the southern hemisphere it goes around to this left this means that in the northern hemisphere around the equator we get north easterly winds and in the southern hemisphere we get south easterly winds which are known as the trade winds where these winds meet we call it the inter-tropical convergence zone the itc-z and the position of this inter-tropical convergence zone will not always be directly along the equator this is because the equatorial lows that are driving this whole system won't always be directly along the equator either they will move north in may to august due to the northern hemisphere summer and move south in november to february during the southern hemisphere summer this means that the line of the intertropical convergence zone and the resulting trade wind convergence will also move north and south with the seasons we'll also see fluctuations due to the difference levels of heating on the land versus water land heats up and cools down better so we see large rises in temperature over the land in the summer and basically in the southern hemisphere there's a lot less land than in the northern hemisphere so we see the inter-tropical convergence zone fluctuate a lot more when it actually hits the land compared to when it's over the sea and that means that the trade winds move as a result of this so other than the trade lines in the mid latitudes we see air masses moving from the subtropical high towards the mid-latitude low pressure areas and again the coriolis force is acting on these air masses and it pulls it round to the right in the northern hemisphere and then it's high to low in the southern hemisphere and it pulls around to the left so we get generally westerly direction winds in the mid latitudes you get wind blowing over the atlantic and blowing over the indian ocean in the southern hemisphere especially these can get up to very fast speeds again because of that lack of land in the southern hemisphere it means that there's nothing to create too much surface friction and these can quickly carried away which is why sailing in the southern ocean around antarctica is renowned for being very difficult because of these high winds defining climates is quite a difficult thing to do because there's a lot of nuance detail and local factors that can be involved in a specific climate for a specific area but there are quite a lot of different climates and one of the easiest ways to break them down is based off of latitude so the first one we're going to start with is the equatorial zone or the tropical zone it can also be split down into rainforest and savannah so the rainforest is located closest to the equator and we see very little in the way of any seasonal variation we see the low pressure areas caused by these hadley cells which means we get high levels of convective weather and high levels of precipitation and storms year round if we move slightly further away from equator it's a little bit cooler than the rainforest just because it's a little bit further away from the equator and it results in a little bit more of a seasonal variation and we get distinct dry and wet seasons and again with quite convective weather so you see storms coming to the savannah when it's the wet season and in the dry season and not that many storms and yeah animals struggle to find water and things like that next furthest away from the equator you would get the arid or the dry zone this can be in the form of either a desert or a step region basically what happens is these hadley cells are pushing down causing a very low level of rising air and rising moisture because of that low level of rising air rising moisture the sun is able to penetrate and hit the surface of the earth make it very hot to make it very dry but in the step regions you are a bit further away from the equator so you do get a slight bit more um precipitation and rain as a result next out you get the temperate zone which is where the majority of the world's population live and it can be either cool or warm which is sometimes referred to as mediterranean in the cool temperate zone we see quite high seasonal variation with lots of precipitation year round and we get the four distinct seasons of spring summer autumn and winter in the mediterranean or the warm temperate zone there is still high seasonal variation but it's not to the same extreme levels as in the cool temperate zone and we often get hot dry summers and warm wet winters next furthest away we get the snow or the sub-arctic region and it's where we have a large seasonal variation predominantly down to the large surface heating effects caused by the large areas of land predominantly russia and canada there's a lot of land there a lot of land to get hot a lot of land to get cold so we see very hot summers and also very cold winters and they last a very long time after that you get the polar regions the polar zones and this is where we get very extreme seasonal variations featuring incredibly cold winters we also get constant daylight in the summer and constant night during the peak winter because we are in the arctic circle so within the large climate latitude zones there are factors that will affect the local weather which can vary from day to day such as pressure patterns fronts moisture levels etc there's also some more long-term month-to-month factors which can have an effect on the local climate as well one of the main ones is called the monsoon which is experienced in india for example basically what happens is there's different levels of surface heating in the summer between the land and the ocean this causes a large low pressure area on land when compared to the ocean and a large flow of air inland it's essentially a large scale sea breeze it brings with it a lot of moisture filled air and heavy precipitation storms as it does this in the winter the process is reversed and the land cools down more than the ocean it has a higher pressure and we see air flowing out into the indian ocean with very little water because it's been overland it's nice and dry monsoon climate is primarily associated with india but it's not exclusive to india they're also experienced by north australia west africa and east asia so mountain chains or other topographical features may affect the weather and produce seasonal wind patterns that can affect the local climate for a period of the year there will be some specific ones for wherever you live but some common ones that are experienced in europe where i live are the shiroko wind which is a hot dry and dusty wind which blows north from the sahara you get the mistral wind which is a northerly breeze which blows down the rhone valley you get the bora wind which is a katabatic wind flowing down over the mountains in the balkans towards the mediterranean and you get a lot of phone winds which is a warm wind that is formed by the adiabatic warming of the air after crossing mountain peak so the climate is very often driven by latitude season and local factors such as mountain ranges or large masses but the weather won't always follow the predictions that come with the climate you're in they should generally follow that pattern for sustained periods of time but you will see variations from day to day so in summary then you get surface heating causing low pressure areas around the equator leads to rising air which spreads out and falls back down to the earth in what is known as hadley cells these hadley cells produce high pressure in the subtropical regions low pressure in the equatorial regions air mass flows from high to low gets pulled around to the right in the northern hemisphere or to the left in the southern hemisphere creating the trade winds the same thing is happening at the poles but it's the reverse process the poles are very cold so they pull air down creating high pressure and this pools in air from the mid-latitude creating low pressure area again the high pressure area the subtropical regions and the low pressure area at the mid-latitudes causes a flow of air which is then pulled down to the right nor in the hemisphere or to the left in the southern hemisphere so we get general westerly winds in the mid latitudes the climate is based on a number of factors but one of the main ones would be the latitude that we're at you get equatorial dry or arid temperate subarctic or snow and polar in the equatorial regions we can break it down into rainforest which is right by the equator it's very hot very convective a lot of rain in the form of storms and uh yeah storms really in the savannah it's a little bit further out you still get convective weather but you get a distinct dry and wet season in the dry regions we can break it down into desert or step conditions deserts are hot high pressure very high daily variation because there's not much cloud cover and it means that there's very low precipitation as a result the step region is slightly further out we still get high daily variation in temperature between the night and the day and there's a little bit more moisture so we get slightly more weather and rain sorry than the desert regions in the temperate region where most of the world's population live we get mediterranean climates and cool climates mediterranean climates have four seasons we get hot dry summers warm wet winters and a spring and an autumn in between and in the temperate cool zones we get high seasonal variation four distinct seasons of winter spring summer and autumn in the sub-arctic regions we mainly associate with canada and russia large land masses causing large seasonal variations in the temperature very hot summers very long winters they last a long time with a quick spring and autumn season in between and in the polar regions we get the most extreme seasonal variation we get very very cold winters and relatively warm summers for the polar regions and we also get a consistent day and night in the summer and winter because we are in those parts of the world where within the arctic circle as well as the latitude variations you get local variations one of the big ones is the monsoon we get a large scale of heating effect of the land compared to the sea which causes a large flow of air in land in the summer and in the winter the process has reversed and it's a large flow out to the sea in the winter you can think of it as a large scale sea breeze or a large scale land breeze you also get local winds some of the ones featured in europe are the shiroco the mistral the boar on the phone schrocco hot dry from the mediterranean mistral down the rhone valley borah over the balkan mountains and a phone wind is a hot wind caused after the peak of a mountain