Temperature normally decreases with height through the lower atmosphere. On some mornings, however, the reverse occurs within a layer near the ground: the valley floor is cold and foggy while a higher slope is milder and clear. This is a temperature inversion, named because temperature increases with height inside that layer.
How it develops on a calm night
After sunset, the ground loses energy through radiation and cools the air touching it. With clear skies, light winds and a sufficiently long night, cooling can be strong. Colder air is denser, so it tends to drain down slopes and collect in low terrain, rather like invisible water filling a bowl.
Relatively milder air remains above. The boundary between them acts like a lid that suppresses vertical mixing. Persistent high pressure can preserve an inversion for hours or, in winter, several days. Strong wind and cloud usually reduce night-time cooling or mix the layers, weakening it.
Why nearby places can report very different temperatures
During an inversion, a few hundred metres of elevation or a different position within a valley may change the outcome substantially. A ventilated hillside station does not necessarily represent a sheltered valley floor, and an urban station cannot describe every rural hollow. A forecast temperature for one town should not simply be transferred to a nearby orchard, sunken road or campsite.
Topography, aspect, vegetation, ground surface, moisture and exposure to wind all create microclimates. An official station minimum is valid for that instrument's location and observing conditions; it is not a universal reading for the whole district.
Fog, frost and stagnant air
If pooled cold air reaches its dew point, water vapour condenses and fog can form. Our guide to relative humidity and dew point explains that threshold. Frost may develop where a surface falls to 0 °C or below and enough moisture is present, even while a nearby station higher up remains above freezing.
An inversion also restricts vertical dispersion. Smoke and other pollutants may become concentrated near the ground, especially in enclosed urban or industrial areas. Not every inversion produces dangerous pollution: emissions, duration and ventilation also matter.
How to recognise it and use a forecast well
- Look for clear skies, light winds and high pressure overnight, especially in autumn and winter.
- Compare observations from stations at different heights, not only one city value.
- Notice a “sea” of fog in the valley with clear summits above.
- For farming, driving or camping, consider the lowest point on the route as well as the start.
An inversion is not itself a weather warning. Its consequences depend on fog, frost, air quality and the planned activity. Check the forecast and recent AEMET observations before making a decision.