BulldogLowell:
you will need to make some of those variables persistent if you want to maintain their values each time loop() terminates.so read up on scope and the static keyword
Thank you for your fast answer! 
I found the scope and it works fine for the Max Temperature, but not for the minimal temperature, because, i think the default value of the variable is 0, and i actually can't take it under 0°C.
But this is not what i want, i want to display the min temp. even when it's above 0 