Find the mean temperature of the first 10 days and of the last 10 days.

Step 1. Means. $\dfrac{311\ ^\circ\text{C}}{10} = 31.1\ ^\circ\text{C}$ and $\dfrac{300\ ^\circ\text{C}}{10} = 30\ ^\circ\text{C}$.

Step 2. The problem. The line graph shows waves, not a steady fall, and a different split gives a different gap. One month cannot tell us about the next.

Step 3. A better sentence. "In this month, the last 10 days were on average about $1\ ^\circ\text{C}$ cooler than the first 10 days."