A recent study by Chen et al, suggests that a slight decrease in the air temperature increases calorie burn.  See the abstract from the study here.

Brief background: Our bodies contain two types of fat - or adipose tissue - brown and white. Brown fat is the kind that keeps us warm and is found in much greater abundance in babies and children than in adults.  It is referred to as BAT sometimes.

Adults have small amounts of BAT and when it is activated - to keep us warm - BAT takes its energy from white fat.  White fat is the kind that most of us are trying to lose.

In their small experiment, the scientists were able to measure this 'under the skin' BAT activity with a PET scan.  They found that when the temperature was lowered by just a few degrees, the volunteers' brown fat cells went into action.  The volunteers burned more calories (energy) under the lowered temperature than the 'normal' temperature.

As a person who never enters a building without a sweater and one who cannot afford to lose any of her white fat, I was not so happy with the authors comment....
this may represent a novel environmental strategy in obesity treatment.

I stay amazed at the lengths people go to in order to lose weight without having to eat less or move more..