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Night temperatures over parts of north-west India are expected to dip over the next couple of days raising threat of ground frost in Haryana, Punjab and north Rajasthan.
These areas are already witnessing thick fog during morning hours as the annual weather trend began to settle over the plains of north-west India.
Ground frostDense fog has combined with ground frost after a western disturbance left behind oodles of moisture – which can either drift as thick fog or settle on ground as frost.
This is because temperatures take a dip after the western disturbance yields space to colder westerly to north-westerly winds from across the international border with Pakistan.
Low temperatures are particularly good for the Rabi wheat crop, but fog that persists for a week to 10 days could prevent radiative heating of the plant.
This can make it vulnerable to pest attack and diseases. Farmers need to be wary of these conditions, more so if the low temperatures lead to ground frost.
No changeForecasts rule out any drastic change to the scenario until the arrival of the next western disturbance by Sunday/Monday.
On Wednesday, the India Met Department warned of fog to dense fog conditions and ground frost for parts of northwest India until the weekend.
Fog to dense fog conditions develop after wind, temperature and moisture values freeze to ‘critical’ thresholds. Winds decelerate to ‘calm’ state ahead of the arrival of a western disturbance, the principal weather maker for the region during this time of the year.
Ensuing clear nights, below-normal temperatures and humidity (ranging between 80 to 90 per cent) conspire to set off bouts of fog/dense fog/very dense fog.
Dominant wind‘Advection fog’ (advection refers to horizontal transfer of any atmospheric property by the wind and typical in northwest India) has a tendency to move with the dominant wind of the season (west to east). This is how fog to dense fog move along with the wind from Afghanistan/Pakistan to cross the international borders into north-west India.
Advection fog is a type of fog caused by the horizontal movement of moist air over a cold surface and the consequent cooling of that air to below its dew point.
Intervening small gusts of wind can trigger what is known as shallow fog.
This is a low-lying fog that does not obstruct horizontal visibility at a level two metres (6 feet) or more above the surface.
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