today we will talk about the isa atmosphere also known as standard conditions in aviation first of all let's see what isis stands for isa is the acronym for international standard atmosphere this is a model of atmospheric conditions adopted as a universal frame of reference the atmospheric conditions described in this model refer mainly to air pressure and temperature values at different altitudes this model was created by the international civil aviation organization in the 1950s and that's the reason why the acronym isa is sometimes interpreted as icao standard atmosphere once the isa model was created it was published in icao document 7488 where all the conditions and parameters used in the model are specified but now the question is why is the iso model needed in aviation well first of all we must say that the atmospheric conditions of a certain place are changing constantly depending on the season and local weather patterns for example here we have two people one of them is on a coast with a tropical climate while the other is on a coast with a colder climate in this case the person on the tropical coast would say that the usual temperature at sea level is 30 degrees celsius while the person on the cold coast would say that the normal temperature at sea level is 5 degrees celsius as we can see in this example for a certain altitude in this case sea level we have two different reference temperatures depending on the conditions of each location this is why it is necessary to have a worldwide standard reference against which different conditions can be compared with this in mind let's look at the principles and characteristics of the isa model this model assumes that air behaves as an ideal gas so it is assumed to be free of moisture and suspended solid particles such as dust or sand obviously this is not the case in practice but these assumptions help in the development of the model so based on this the icow determines certain reference values for air pressure and temperature at different altitudes however before going into detail with these we must say that in general terms pressure and temperature decrease with increasing altitude in other words at sea level we will find a higher pressure and temperature than at higher altitudes with this being said let's now see how the pressure behaves in the iso atmosphere here it is assumed that the pressure at sea level is 29.92 inches of mercury or 10 13 hectopascals depending on the unit of measurement used this pressure decreases with altitude at a variable rate however we can say that the pressure reduction with altitude in the first levels corresponds to approximately one inch of mercury per 1000 feet or one hectopascal per 30 feet this means that under standard conditions at sea level we would have an atmospheric pressure of 29.92 inches of mercury at 1 000 feet we would have approximately 28.92 at 2 000 feet we would have 27.92 and so on this results in a graph like this as we can see the line is not completely straight since as we said previously the rate of change of pressure with altitude is variable however up to about ten thousand feet we can experience a reduction of one inch of mercury per 1000 feet now that we have seen how pressure behaves let's take a look at the temperature in the iso model the temperature is assumed to be 15 degrees celsius at sea level and it is reduced by 1.98 degrees celsius for each 1 000 feet of altitude increase within the troposphere which can be rounded up to 2 degrees celsius per 1000 feet this means that under standard conditions at sea level we would have a temperature of 15 degrees celsius at 1000 feet 13 degrees celsius at 2000 feet 11 degrees celsius and so on now this rate of temperature reduction with altitude applies only within the troposphere as we know the earth's atmosphere is divided into several layers that have different characteristics the first layer we find right above the surface is known as the troposphere under standard conditions within this layer the temperature decreases by 2 degrees celsius per 1000 feet until reaching an altitude of 36 089 feet at this altitude we find the tropopause which is the transition layer between the troposphere and the stratosphere from this point on the temperature is assumed to remain constant with altitude until reaching 65 617 feet where the temperature starts increasing with altitude however considering that most aircraft operate below 50 000 feet we will not take into account that temperature increase with altitude in the stratosphere so with this in mind the temperature would be 15 degrees celsius at sea level and it would be reduced by 2 degrees for every 1000 feet so at 15 000 feet we would have minus 15 degrees celsius at 30 000 feet we would have minus 45 degrees celsius until 36 089 feet is reached where the temperature would be minus 56.5 degrees celsius from this point the temperature remains constant at minus 56.5 degrees celsius this means that if we measure the temperature at 40 000 or 50 000 feet we would still have minus 56.5 degrees celsius now we must keep in mind that these values apply only within the iso model since in practice the altitude at which we find the tropopause varies depending on several factors for example near the equator the tropopause will be much higher than at the poles however since we are talking about a universal standard reference model the values are fixed all this we have discussed about the temperature in the isa model results in a graph like this one as we can see the temperature decreases from sea level to 36 089 feet and then it remains constant now in many occasions it is necessary to determine the standard temperature at a certain altitude and although we could calculate it mentally we can also use the following formula i said temperature is equal to 15 minus 2 times altitude divided by one thousand a side note here if the altitude is higher than thirty six thousand feet the standard temperature is directly assumed to be minus fifty six point five degrees celsius let's see an example of how to use it let's say we want to determine the standard temperature at 23 000 feet well in this case we just have to replace the altitude in the formula with 23 000 feet and then after doing the math we obtain minus 31 degrees celsius it is important to note that all the values we are calculating are approximate since the actual and exact values are published in this table found in the icao document 7488 another consideration is that in practice the actual atmospheric conditions are almost never standard so the iso model is more used as a reference rather than to represent the average conditions of the atmosphere with this in mind the iso model is widely used in aviation mainly for instrument calibration and determining the nominal performance of an aircraft now as the isa model is used as a reference there is another important concept which is the iso deviation this is a term used to express how different the actual atmospheric conditions are compared to the iso model normally in terms of temperature let's see an example suppose a person at sea level measures the air temperature theoretically according to the iso model it should be 15 degrees celsius however under real conditions let's say the person actually measures 25 degrees celsius now in this situation if we compare the actual conditions with the standard model we can see that there is a deviation of 10 degrees celsius and since the actual conditions are 10 degrees warmer than the standard we say that the iso deviation is isoplus 10. in other words the iso deviation is obtained measuring the difference between the actual temperature and the standard temperature according to the iso model for example under isa conditions at sea level we would have 15 degrees celsius at 2000 feet we would have 11 degrees at 4007 degrees at 6003 degrees and so on now if we look a model with a deviation of minus 20 degrees it would be known as iso minus 20. in this particular model at sea level we would have a temperature of minus 5 degrees since it is 20 degrees colder than the standard at 2 000 feet we would have minus 9 degrees at 4 000 minus 13 degrees at 6 000 minus 17 degrees and so on on the other hand if we look a model with a deviation of plus 20 degrees it would be known as isoplus 20. in this model at sea level we would have a temperature of 35 degrees since it is 20 degrees warmer than the standard at 2000 feet we would have 31 degrees at 4027 degrees at 6000 feet 23 degrees and so on as we can see the different conditions are expressed as a deviation in relation to the standard iso model in this case we analyzed the iso minus 20 and iso plus 20 models but we can really have any other deviation value positive or negative for example iso plus 3 iso plus 8 or iso minus 12. now something to keep in mind is that at a certain airport or area the iso deviation is not always the same as it will vary throughout the day and with the weather conditions let's see an example here we have an airport at sea level in this case the standard temperature according to the isa model would be 15 degrees celsius this means that theoretically the temperature at the airport should always remain at 15 degrees celsius which simply does not happen in real life for example let's say that at 6 a.m the temperature recorded at the airport is 8 degrees celsius if we then compare the actual temperature with the standard we would obtain an ice a deviation of minus seven degrees in other words the conditions at the airport are seven degrees colder than standard and therefore they are expressed as iso minus seven now this is true only at six a.m since as the day progresses the temperature will gradually increase let's say for example that at 1 pm the temperature recorded is 18 degrees celsius if now we compare the actual temperature with the standard we would obtain a deviation of plus three degrees in other words the conditions are three degrees warmer than standard and therefore they are expressed as isoplus 3. this was an easy example since we already know that the standard temperature at sea level is 15 degrees celsius now let's look at an example with a different elevation say four thousand feet now in order to determine the standard temperature at that airport we need to know first what is the standard temperature at four thousand feet to do so we can use the formula we saw previously here we just replace the altitude with 4000 and we find that the standard temperature at that level is 7 degrees celsius now let's suppose that the actual temperature at the airport at 6 am is 11 degrees in this case the deviation would be plus 4 degrees and as the day progresses the temperature increases gradually let's say that at 1 pm the temperature recorded is 21 degrees now the iso deviation would be plus 14 degrees now this concept does not only apply to airport elevations but also to any other altitude or level for example let's say an aircraft is flying at six thousand five hundred feet and the pilot reads on his instruments that the outside air temperature is eight degrees celsius in this case what is the current iso deviation well first of all we must determine what is the standard temperature at 6500 feet by applying the formula we obtain 2 degrees celsius if we then compare the actual conditions of 8 degrees with the standard of 2 degrees we obtain an iso deviation of plus 6 degrees this information can be useful to the pilot as it allows him or her to make accurate aircraft performance calculations let's see an example of how this information can be used here we have a typical cruise performance table for a light aircraft it takes into account different conditions and parameters in order to show the expected aircraft performance one of these is the air temperature but as we can see in the upper row the temperature is expressed in terms of iso deviation for example the middle column corresponds to iso conditions so this should be used only when the iso deviation is zero in the left column we can see iso minus 20 and in the right side iso 20 in other words in order to properly use the table the pilot must first determine the iso deviation and then use the corresponding column now in some aircraft it is not necessary for the pilot to manually calculate the isa deviation since these aircraft have systems that automatically calculate the deviation in real time as we can see in this example let's see a short summary of everything we talked about in the video isa is an international standard model of atmospheric conditions mainly pressure and temperature at different altitudes in terms of temperature it is assumed that at sea level the temperature is 15 degrees and it decreases by 2 degrees for each 1000 feet increase in altitude until reaching 36 089 feet from where it is considered constant at 56.5 degrees in terms of pressure it is assumed that at sea level the pressure is 29.92 inches of mercury or 1013 hectopascals and it is reduced by approximately one inch of mercury for every 1000 feet or one hectopascal for every 30 feet at the first levels finally the ice a deviation is a value obtained by comparing the current conditions in relation to the standard conditions i hope the information presented in this video has been useful if so don't forget to share like subscribe and leave a comment down below thanks for watching [Music] you