The average high or low temperature is the normalized high or low temperature for at least a 30 year period. Thus, an average high or low is a statistical average. Records need to be kept at least 30 years for an average high or low temperature to be statistically meaningful.hus, not always will 50% of the high or low temperatures for a particular date be above or below the average high or low temperature. When the high temperature is 75 on a day the average high is 75, this day may not be typical. The high temperature just happened to fall between the average of the extremes.