The average high and low temperatures are common information on a weather TV cast. What exactly do these average high and low temperatures mean?
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.
Often high and low temperature information is misinterpreted as being the TYPICAL high and low temperature. Average temperatures are NOT typical temperatures. If the average temperature is not the typical temperature than what is the difference between the two?
Typical temperatures are the MOST COMMON temperatures a location experiences. While an average temperature is the average of the extremes and everything in between, the typical temperature is the temperature you would most expect for a location. Here is an example for Mississippi:
The typical afternoon high temperature in Starkville, MS for the middle of August is 95 degrees. However, the average high is about 92 degrees. What happens is that in some years the high temperature is much cooler than typical. In a 5 years period, the temperature may be 95 on 4 of those days and then 80 on another due to a rainy overcast day. When averaged, the average high is 95+95+95+95+80 / 5 = 92 degrees. Depending on location and season the typical high or low could be more or less than the average high or low. Thus, 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. If you ever wonder why the weather is not being average, it is due to the average actually being the more rare occurrence.
For your location and season, keep track of how the actual highs and lows differ from the average highs and lows. This will give you insight into the typical high and low temperatures that your location experiences.
Cloud cover, precipitation, anomalous air masses, unusual weather events (such as hurricanes, blocking weather patterns) are major factors which cause a day to not have typical temperatures.