If you have the sneaking suspicion that the climate where you live has been changing the past few years, calculating the mean annual temperature will help dispel the mystery. The mean annual temperature refers to the average of the maximum and minimum temperatures of a year, taking the mean average of the coldest month of the year and averaging it with the mean average of the hottest month of the year. The mean annual temperature is a valuable climatology tool that can assess an area's climate change.
Calculate the mean daily temperatures of an area, usually a town or city, for each day of the year. Take the temperature readings on the hour for a 24-hour period. Then divide that number by 24 to get the mean daily temperature. According to the American Meteorological Society glossary, a mean solar day is used, with the first measurement at midnight and the last at 11 p.m. of the same day.
Calculate the mean monthly temperature by adding up the daily mean temperatures for each day of a month and then dividing that sum by the number of days in the month. For example, for the month of January you would add up the 31 mean daily temperatures, and then you would divide by 31.
Calculate the average of the 12 mean monthly temperatures. Add the mean monthly temperatures for the months of the calendar year, January to December, together, and then divide by 12. This will be the mean annual temperature.
TL;DR (Too Long; Didn't Read)
Two areas with very different climates can have the same mean annual temperature. A climate that has temperate weather throughout the year can have the same mean annual temperature as an area with a highly variable winter and summer.
For more precision, you could add the daily means together and then divide them by the number of days of the year.
For less precision but greater ease, you can just take the average of the highest and lowest temperature of the day to get the mean daily average.