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What is an air mass? What is a front? Weather associated with a cold front Cold fronts in Hong Kong Written by: KONG Wai December 2010 1. What is an air mass? An air mass is defined as a large mass of air with fairly uniform temperature and humidity, or in
meteorological term, small temperature and humidity gradients. The size of an air mass may
extend up to millions of square kilometres horizontally and throughout the troposphere
vertically. The properties of an air mass depend on the characteristic of its underlying
surface. For example, a warm surface would produce a warm air mass while a maritime surface
would increase the moisture content of an air mass. Fig. 1 Back to content 2. What is a front? A front is defined as the transition zone between two different air masses, where the horizontal
gradients of temperature and humidity are relatively large. Since cold air is denser, the two air
masses are separated by a front which slopes upward over the colder air. Fig. 2 According to the properties of air masses and their relative motion, fronts are divided into four
different types which are cold front, warm front, stationary front and occluded front. A front is a
cold front if it moves such that cold air replaces warm air, vice versa. Fig. 3 If neither of two different air masses is strong enough to replace the other and their front has no
appreciable movement, it is called a stationary front. Fig. 4 Finally, when a cold front moves faster and overtakes a warm front, the warm air mass will be
separated from the ground and an occluded front is formed. Fig. 5 Back to content
3. Weather associated with a cold front In both cold and warm fronts, the denser cold air will undercut the lighter warm air which will ride
over the denser cold air. This upward motion leads to the expansion and cooling of the warm air,
resulting the formation of cloud and precipitation near the frontal area. Since a cold front has a
steeper slope than a warm front, the rising motion is often stronger and hence, the weather
associated with a cold front is usually more severe than that of a warm front. Fig. 6 The arrival of a typical cold front is commonly associated with thick layers of stratiform cloud and
precipitation. In some active cold fronts, cumulonimbus could be embedded and associated
with severe thunderstorms, hailstorms or even tornadoes. The layers of clouds and precipitation
are usually confined in a narrow band around the frontal zone, the weather will improve gradually
and clouds are well­broken behind the cold front. However, there are also cases in which there
are no clouds or precipitation during the passage of cold fronts. If the atmosphere is rather dry,
the weather could be fine throughout the process. Under the influence of the cold air mass, temperature usually falls rapidly during and after the
passage of a cold front. Moreover, pressure will rise quickly upon the intrusion of the cold air. Back to content
4. Cold fronts in Hong Kong Cold fronts always affect Hong Kong in winter and their arrivals also represent the beginning of
cold surges. The following is a case of a cold front passage in Dec 2009. Before passage: Fig. 7 Daily weather map on 15 Dec 2009: The cold front crossing southern China. Fig. 8 Visible satellite image captured at 1130 HKT, 15 Dec 2009: Cloud bands associated with the cold front generally covered southern China After passage: Fig. 9 Daily weather map on 16 Dec 2009: The cold front crossed the coastal area of Guangdong and entered the northern part of the South China Sea. Fig. 10 Visible satellite image captured at 1130 HKT, 16 Dec 2009: Cloud bands associated with the cold front moving into the sea On 15 Dec 2009, a cold front crossed Hong Kong at around 8:00 p.m. (i.e. 12 UTC). Fig. 11
indicated rapid temperature fall, significant precipitation and pressure at the Observatory during
and after the passage of the cold front. Fig. 11 Back to content