Viscosity of Water and Air: Opposite Behavior as Temperature Rises

Unlike density, which decreases with heating for most substances, the viscosity of liquids and gases behaves very differently as temperature changes.

This contrast becomes clear when comparing how the viscosity of water and air changes with increasing temperature.

At the same time, the absolute viscosity values differ by about two orders of magnitude: water is far more viscous than air.

How Liquids Behave

For liquids, dynamic viscosity decreases as temperature increases. Molecules are packed closely together, and viscosity is governed by intermolecular forces that hold adjacent layers of liquid in place.

As the liquid warms up, thermal motion intensifies, these forces weaken, and the layers can slide past each other more easily — the liquid becomes less viscous and more fluid.

Example (water):

• at 0 °C: μ ≈ 1.8×10⁻³ Pa·s;
• at 20 °C: μ ≈ 1.0×10⁻³ Pa·s;
• at 100 °C: μ ≈ 0.28×10⁻³ Pa·s;

Between 0 and 100 °C, the viscosity of water decreases by nearly a factor of four.

How Gases Behave

For gases, the situation is the opposite: dynamic viscosity increases as temperature rises. Gas molecules are far apart, and viscosity is determined not by cohesion but by how efficiently molecules transfer momentum during collisions.

As a gas is heated, molecular speed increases, collisions become more frequent, momentum exchange between layers intensifies, and the gas offers greater resistance to shear.

Example (air at normal pressure):

• at 0 °C: μ ≈ 1.7×10⁻⁵ Pa·s;
• at 100 °C: μ ≈ 2.1×10⁻⁵ Pa·s;

The increase is modest (about 20–30%), but the trend is opposite to that of liquids.

Why This Matters in Engineering

For HVAC and building systems engineering, this difference is important:

• when water is heated in piping, friction losses decrease → hydraulic resistance drops at the same water velocity.;
• when air is heated in ductwork, viscosity increases slightly → pressure losses rise at the same airflow velocity.