One table contains the average temperature for each day of the year measured in oK. The table is small, as data values are stored only once per month (roughly). Interpolation is used to interpolate between these storage data points.
The other table contains the average temperature variation during one day. Also this table is small, as data values are stored once per hour. The temperature variation is stored as a multiplicative factor of the average temperature of the day.
This model is good enough for our purposes.