Difference between the high and low temperature of Monday $=26°-(-3°)=29°$;

Difference between the high and low temperature of Tuesday $=32°-(-5°)=37°$;

Difference between the high and low temperature of Wednesday $=22°-(-7°)=29°$;

Difference between the high and low temperature of Thursday $=40°-3°=37°$;

Difference between the high and low temperature of Friday $=40°-2°=38°$;

Therefore, the mean of the differences $=\frac{29°+37°+29°+37°+38°}{5}=34°$