Radiation Fog
Radiation fog is so-called as the formation process depends upon a balance of “radiative heat fluxes” (I’ll explain what this means below). Radiation fog usually forms overnight or early morning during the coldest hours of the day, and then dissipates after the sun comes up. It primarily forms over land but has been observed to form over shallow inlets and harbours as well. It is the most common type of fog we see in New Zealand.
For radiation fog to form, we need several ingredients:
Clear skies,
Light winds,
Sufficient moisture in the lowest layer of the atmosphere, near the ground.
These conditions are most commonly met when we have high pressure over the country, but there are other situations where these conditions can be met too. In short, on a clear night with light winds, the air can cool down enough to reach its “dew point” (100% relative humidity) and water vapour in the air will start to condense into fog. Let’s have a look at this process in a little more detail.
Radiation Fog in Christchurch, photographed by Julienne Nacion.
Radiative Heat Fluxes
The process of radiation fog formation starts at sunset, when the balance of radiative heat fluxes changes. Everything radiates heat – with warmer objects emitting more radiation than cooler objects. On a sunny day, incoming solar radiation is greater than the radiation emitted by the Earth back to space – this causes the ground and the air directly above it to warm up. On a cloud-less night, the Earth continues to radiate heat away to space – but with no incoming solar radiation, this causes the ground, and the air directly above it, to cool quickly.
Clear Skies
Cloudy skies change the radiative heat balance described above. If there is cloud in the sky, it will absorb the heat emitted by the Earth’s surface and re-radiate it back towards the ground. This prevents the land surface from cooling as much as it would on a cloudless night. Clear skies allow the ground to cool quickly by emitting heat to space via radiation, allowing a ‘temperature inversion’ to form (this is when the temperature increases with height, rather than decreasing with height like it usually does – more on inversions here.) Radiative cooling is an essential part of the formation of radiation fog, so the less cloud around, the greater the chances that fog will form.
Light winds
If the wind is too strong, then this can cause warmer, drier air from aloft to be mixed up with the air near the ground, meaning that the land surface cannot cool as quickly.
Conversely, if there is no wind at all, you are more likely to get a very heavy dew instead of fog – this is because very light winds help to mix the cool air through a shallow layer of air near the land surface. If there isn’t enough wind, the cooling will be restricted to the lowest few centimetres of the atmosphere, allowing dew but not fog.
Sufficient moisture
The relative humidity in the lowest layers of the atmosphere needs to be high enough that radiative cooling overnight will allow the air to reach its “dew point”. The dew point is a measure of how much water vapour is in the air – when the dew point is equal to the air temperature, this is equivalent to 100% relative humidity, and the air cannot hold any more water vapour. In addition, warm air can hold more water vapour than cool air. If the air cools to its dew point, and then continues to cool, the water vapour needs to go somewhere – initially it condenses on the ground as dew, but if we have sufficient radiative cooling and light winds to mix the cool air through a shallow layer as described above, the water vapour will begin to condense into fog as well.
While having lots of water vapour in the lowest layer of the atmosphere helps fog to form, it is in fact beneficial for the air aloft to be quite dry. This is because water vapour in the air absorbs infra-red radiation, and, much like cloud, can re-radiate some of this heat back to Earth. This means that if the air aloft is dry, the land surface can cool much more efficiently.
Radiation Fog in New Zealand
Many of our airports in New Zealand are prone to radiation fog, and our aviation forecasters spend a lot of time thinking about whether or not radiation fog is likely to affect each airport when night falls.
Hamilton Airport is our foggiest airport in New Zealand, with an average of 92.4 nights a year when fog is reported. This is followed by Dunedin Airport, with 64 nights a year.
Since it needs to be cool and damp for radiation fog to form, it is much more common during winter than summer. Auckland Airport shows a particularly strong seasonal trend – with only a couple of cases of fog occurring during the warmer months between September 20th and March 20th over the past ten years (all these cases were advection fog rather than radiation fog – you can find an explanation of advection fog later).
Radiation fog can be particularly disruptive when it occurs at Auckland Airport, which has an average of 19.3 foggy nights a year. While some aircraft are equipped to land in low visibility, taxiing around the runway is still dangerous in fog, and fewer aircraft can land per hour during low visibility operations. Another type of fog that can affect Auckland Airport is post-frontal fog, which is a special case of radiation fog.
Shallow radiation fog by Addington Brook in Hagley Park, photographed by Don Gracia. The brook serves to provide a source of moisture in the lowest layer of the atmosphere, aiding the formation of fog.
