1 You are more likely to get injured
During cold temperatures muscles become rigid and stiff. A sudden movement can result in muscle damage or injury if warming up wasn't done properly.
2 Your body warms up slowly
In cold temperature the body warms up slowly thus extra energy and time are needed to get the body to warm up.
3 Your body cools down fast
The body cools down fast in cold temperatures. While this can help the body maintain its ideal temperature it can also result in a reduced performance as the body goes back to the pre-warming up state.
4 Less toxins leave your body
When a person sweats toxins and harmful materials leave his body. Exercising with Air conditioner on results in less sweating and as a result less toxins leave the body.
5 You feel sleepy
A very cold temperature can make a person feel sleepy and make it harder for him to warm up. When the body gets cold it reduces the activity level to help preserve energy.
6 A low temperature makes the body less active
A low temperature can make a person less active and sleepy. While a very hot temperature can affect performance in a bad way a cold temperature might prevent a person from exercising well.
7 Hot weather increases heart rate
Exercising in hot weather increases heart rate. When the heart rate increases the person becomes more active. Some Pre-workouts make people more active by increasing the heart rate. An effect that can be naturally achieved by turning off the air conditioner. (See Why Some People Never Exercise?)
8 High temperature lowers blood pressure
When the temperature gets high the blood pressure is lowered. The opposite happens when the temperature is lowered.
9 Sweating gives a feeling of satisfaction
Sweating gives a psychological feeling of satisfaction. While more sweat doesn't necessary mean that more calories are burned still sweating can psychologically make a person think that he exercised well. (See How to strengthen your core muscles?)
10 A sudden drop in body temperature signals sleeping time
If a person came from a hot area to a place where the air conditioner is adjusted to a cold temperature then the sudden drop in temperature might signal that body that sleeping time has came making the person less active. This is why a warm bath helps people sleep.