Evaporation increases the viscosity

5.4.3 Evaporation

The spreading oil has a large surface from which a strong evaporation of the volatile oil components can take place. The evaporation rate depends on:

§ type of oil (boiling behavior, vapor pressure),

§ weather conditions (wind speed, temperature / solar radiation),

§ Water conditions (sea state, water temperature).

As a general rule, evaporation is greatest in the first few hours after an oil spill.

The higher:

§ the water and ambient temperature,

§ the solar radiation and

§ is the movement of water,

the higher the evaporation will be.


With evaporation, the volume, the flammability and also the toxicity of the oil on the water decrease over time. At the same time, density and viscosity are increased, so that the spread of the oil is hindered.

Very lightly refined products often evaporate completely within a few hours. This creates a risk of explosion and fire in narrow, enclosed areas (ports).

Relationship between changes in flash point, density and viscosity as evaporation progresses

The following examples show how different the evaporation rates are:

Evaporation of spilled oil, conditions: 100 barrels (15897 liters), wind: 10 knots, 20 ° C water temperature

% evaporates

after ... hours




Lagomedio (crude oil)






Prudhoe Bay (crude oil)



In the Amoco Cadiz accident, around 40% of the oil that had leaked evaporated in the first 3 days.
This percentage of evaporation was already reached on the first day of the Ekofisk accident.