The viscosity of a liquid decreases as the temperature is raised, while the viscosity of a gas increases as the temperature is raised. In a liquid, the increased temperature causes the molecules to move faster, which means that they spend less time pressing against each other and holding each other down. The same heat slows the motion of gas molecules by making them bump into each other more, which makes them move more slowly overall, increasing the viscosity.
When measuring or testing the level of viscosity in a material, it is important to note and maintain a constant temperature. Allowing the temperature to change or fluctuate will allow for the material's viscosity to change without warning. These variations can occur with a change of just a few degrees in temperature and can result in more than doubled viscosity. Reducing the temperature of a liquid or gas predictably has the opposite effect on the materials. Liquids move slower, up to the point of freezing where they cease to move altogether. Gases, on the other hand, move faster and flow better at the colder temperatures because they are colliding and slowing each other down less often. The viscosity of gases is approximately equal to the square root of the temperature at which the gas is heated.