I am following the tutorial Tiny WiFi Temperature Sensor with Arduino. everthing works fine, but I have a little problem.

When the serial monitor is opened the sensor is reading fine like 23.20, 20.25 etc., but when I closed the serial monitor window I got wrong values, for example, 289.34, 289.90.

What does the serial monitor do?

I am stuck with this problem for last two days. Please give me a hand.