Katabatic wind is a wind which has its source in the high mountains where the temperatures are quite low. It’s etymological origin is the Greek word Catabasis that means descending. That is the reason that another name for this wind is the fall wind. The temperature of the katabatic wind depends on the temperature of the source region and the amount of descent. The speed of the winds is generally 5-10 knots.
Mountain wind is a cool, high density, descending wind which flows from the highlands towards the stations located along the foot of the hills. As it happens, the rarer atmosphere of the high mountains radiates heats back to the atmosphere on a dry, clear night at a much faster rate than the plains. The mountain slopes, under the influence of radiation cooling, get much cooler quickly and the air in close contact with the mountains slopes gets much cooler as a consequence. As it cools, it gets denser and heavier and starts sliding down along the mountains slopes, under the action of the force of gravity, at a speed normally in the range of 5-10 knots or 9-18 km/hr. towards the nearby located deep valley or the foot of the hill plateau location and reaches there in the early hours of the morning, causing quite a chill there even in the relatively warmer months.
Here we discuss a typical Katabatic wind scenario in Pakistan taking into account the temperatures of Hill station of Murree, the foothill station of Islamabad and the Plain station of Lahore
The Murree Islamabad Scenario
During the cool dry fall months, the atmosphere at Murree is usually dry at nights. The skies are quite clear and the relative humidity is very low. When the sun sets, due to the clear atmosphere and dry air, the mountains slopes of Murree cool at a rapid rate due to re-radiation of the heat accumulated during the day back to the atmosphere. As the mountains slopes get cooler, so does the air in close contact to it. Now it is known that the temperature is inversely proportional to the density of the air. Therefore, the air becomes dense and heavy and under the influence of the force of gravity starts descending towards to foot of the hills at an average rate of 5-10 knots. It reaches Islamabad in the early hours of the morning and creates quite a chill there which is not otherwise possible had there been no katabatic phenomenon. The following temperature comparison will further illustrate the point.
Let us compare typical late October temperatures of Murree (a hill station), Islamabad (a plateau station located close to the hills) and Lahore (a plain station located far-off from the hills). As described above the necessary conditions for a katabatic effect to fully establish are clear atmosphere, little or no wind at the source location and low relative humidity. Incidentally all these factors are available at Murree in late October to Early November. The temperature at Murree falls sharply at night and the air along the slopes becoming cooler, descends down towards Islamabad, cooling it more than 5-6 degrees as compared to the plain stations. At times, the minimum temperature at Islamabad falls below the minimum of Murree, the source location of katabatic wind. The typical day time high and nighttime low for Murree, Islamabad and Lahore for late October are as follows:
Murree 19 8
Islamabad 29 7
Lahore 31 15
Note the effect of katabatic wind on the minimum temperature of Islamabad. It is 8 degrees Celsius cooler than that of Lahore although the maximum is 10 degrees hotter than Murree. Also note the diurnal range of temperature at Islamabad that is an enormous 22 C. This is typical of Katabatic Regions.
In short, Katabatic wind is a cool, dense wind that dramatically alters the temperature regime of the are over which it descends.