You will need
- - accurate thermometer;
- - a diary of observations;
- - the average temperature for each day of the month;
To calculate the average monthly temperature of air (average temperature index for any given month), add up all the daily average values.
When calculating the average daily air temperature have multiple dimensions. Type used for this thermometer would depend on your goal. An ordinary alcohol thermometer is better to check for a reference, since alcohol devices tend to become obsolete. Mercury heater in the winter is not recommended, when severe frosts it can fail.
The graduations of the scale will depend on the required accuracy. An ordinary alcohol thermometer has a precision of one degree. If placed high enough requirements to the definition of the temperature, use a device with smaller graduations with indicators to hundredths or thousandths of a degree.
Putting the experiment in warm months, make sure that the thermometer was in the shade. The conditions of observation must be the same, otherwise you will not avoid mistakes. Write down in a journal observations of the thermometer in the morning, afternoon, evening, midnight. Add them up, divide by 4 (number of observations).
Put positive and negative numbers exactly as you always do. For example, if the thermometer shows -2°C and in the daytime +4°C, divide by the number of observations is the value at +2°C.
Once you get and add up all daily mean values of air temperature for the month, divide the resulting amount by the number of days (30, 31, 28 or 29).
Round the resulting number up to the precision that you need (often quite a tenth, but sometimes can take hundredths and even thousandths). On the same principle it is possible to calculate the average day and night temperature separately from each other.
The average water temperature is determined according to the same principle as for air. The only difference is the applied measuring instruments.