What are the Conditions Required to Form Distrupting Fog?

Delhi is experiencing its usual winter disruptions from the Winter fog and today seems worse. See the news article from December, 2010, on the train and flight delays and cancellations due to the thick foggy conditions.



Where is this fog coming from? What are the conditions necessary to form and sustain fog in the winters.

The following is a list of the primary processes that produce fog. A combination of several of these factors increases the likelihood of fog:

1. Saturated air at surface: Fog forms once evaporation into the air results in supersaturation. If the dewpoint depression is small after sunset, clear skies will cause radiational cooling of the air quickly toward saturation. The dewpoint can increase due to a rain shower, previously saturated soils and irrigation. Since vegetation evapotranspirates moisture, fog first forms over grassy and vegetated areas. Fog is common in situations where a daytime shower saturates the soil, vegetation and boundary layer and then skies clear in the evening into the night hours.

2. Overnight clear skies: Clear skies allow the surface temperature to cool off at a higher rate. If dewpoints are high or the dewpoint depression is low, saturation of the air will occur over night. Fog is not as likely if the ground is bone dry, vegetation is sparse or the dewpoint depression is too large.

3. Wet soils and rain dampened vegetation: Wet soils and dampened vegetation continuously evaporates water vapor. This allows the temperature and the dewpoint to converge more rapidly than would normally be the case. An afternoon shower can cause the dewpoint depression to be near zero in the evening. If skies then clear and wind is light, fog is very likely.

4. Light wind: If the surface is near saturation, a light wind will allow for the layer of air near the surface to remain near saturation. High wind speeds cause a mixing of air at the surface and higher into the atmosphere. Since air higher in the atmosphere is generally drier, high wind dries the air and prevents fog from forming. (click on the animation to access today's wind patterns over Delhi)

5. Slight warm air advection from maritime polar/ tropical air: Warm air advection causes rising air. Even a slight warm air advection can cause just enough uplift to make fog more likely, especially if the warm air advection is from a moisture source such as the Gulf of Mexico, Great lakes or Gulf Stream. Polar air and continental air tend to have larger dewpoint depressions than maritime air. Fog that occurs in polar and continental air is primarily due to saturation from above or large radiational cooling.

6. High dewpoint: Warm air has a higher capacity to contain and evaporate water vapor than cold air. Because of this, fog associated with maritime air tends to be thicker than fogs that form at very low temperatures.

7. Light drizzle and precipitation saturating PBL from above: Warm moist air overrunning a shallow air mass can saturate the shallow air from above and eventually to the surface. The contact cooling between the two air masses causes clouds and fog since the moisture in the warm air is beyond the carrying capacity of moisture relative to the cooler air. Precipitation evaporating into the air can saturate the atmosphere.

8. Wind direction from a moisture source: Since moisture is a key component for fog, advection from a moisture source is much more favorable than advection from a dry source. Widespread fog is more common with warm fronts than cold fronts. Part of the reason is that cold fronts tend to bring continental air (central and eastern US) while a south wind brings maritime warm air.