I. Introduction to Meteorology

3. Weather nouns and jargon

During wintertime, every once in a while we hear the weatherman saying: "A continental cold air mass is coming down from the north ..." What exactly does this mean? This is an air mass which is formed in the polar region and enters mainland China as a result of the movement of upper air flow, and then moves to the East Sea and the Pacific Ocean through central China. Since it originates in the polar region, it is categorized as a "cold air mass" and since it passes across the continent, it is called a "continental cold air mass". In the following section we will introduce you to a few types of weather phenomena caused by the continental cold air mass.

a. Cold surge

Wow! It's cold! When you see everybody outdoors wrapped up like a sleeping bag, and you really would like to cover yourself as well, you know that a "cold surge" has arrived. When a continental cold air mass moves south, it will drastically reduce the temperature. And, what drop in temperature constitutes a "cold surge"? Usually in weather forecasting, when the predicted temperature in Taipei is 10 °C or lower, then we would say a cold surge is in town!

b. Strong continental cold air mass, continental cold air mass, strengthened northeast monsoon

Can you tell the difference in when to use one of the three terms listed in the heading? In weather forecasts, when an approaching cold air mass is predicted to lower the temperature in Taipei to between 12 °C~14 °C, then the forecast calls for a "continental air mass". If the temperature is predicted to decrease to 12 °C or below, then the forecast calls for a "strong continental air mass". Finally, if the minimum temperature is not lower than 14 °C, then it can only be called a "strengthened northeast monsoon". Now, one can appreciate the different situations described by these three terms with regard to the weather!

c. Transformed air mass, moving high pressure, high pressure reflux

When we feel frozen during cold weather, we might wonder: 'When will it get warmer?" All we can look forward to is the "transformation" of the continental air mass. When a continental cold air mass moves out of its originating area, it will be influenced by the local environment of the regions it passes through. Combining this with the internal movement of the air mass itself results in a change of nature to the air mass. It will become a "transformed air mass" which in plain terms means that the weather will begin to get warmer as the temperature ceases to drop.

A continental cold air mass is a high pressure center, and it is represented on the weather map by the capital letter "H". It will change as it moves southward, and will become a "transformed continental air mass" when it reaches east China. After the change, the high pressure center will shift its course to the east and enter the East China Sea. At this time, the cold front before the air mass has already passed Taiwan and the gloomy weather will begin to change. The cloud cover will decrease and the temperature will rise; we will sometimes have sunny days. The change of weather is caused by "moving high pressure".

If the moving high pressure center continues to move east over the East China Sea, it will bring Taiwan warm and wet air. Temperatures throughout the island will substantially increase, even to the point of becoming comfortably warm. This weather phenomenon is known as "high pressure reflux".

d. Fronts

Imagine what would happen if a cold air mass met a warm air mass? In meteorology, when a cold air mass comes in contact with a warm one, the two will create a forward moving band of interface called a "front". The length of a front can vary from several hundred kilometers to several thousand kilometers. Its width increases as the altitude increases and forms an inclined shape. Cold air is at the lower part of the front while warmer air will be lifted to the upper part.

A front will move as influenced by the movement of the cold and warm air and is divided into 4 types based on the differences in movement: cold front, warm front, stationary front, and occluded front. These four different types of fronts create different types of weather situations. The most common fronts in Taiwan are cold fronts and stationary fronts.

When cold air pushes the front toward the direction of warm air, the front is called a "cold front". When this kind of cold front passes through an area, the surface temperature will drop and the weather will become colder. Cold fronts move with different speeds. In a slow moving cold front, the warm air will rise in a slow and steady fashion, and there will be stratiform clouds and mild rain. In a fast moving cold front, the warm air collides with the approaching cold air and is quickly lifted up, creating thick cumulonimbus clouds. In this situation, there will be heavy thunderstorms on the ground.

If the strength of the warm air is stronger than the cold air, the direction will be reversed and warm air will push toward the cold air; a "warm front" is formed. The regions through which this warm front passes will experience a rise in temperature. The front edge of a warm front is an extensive cloudy raining area, which can measure up to several hundred kilometers in length, creating steady rainfall in the affected regions. The nature of the rainfall, however, is different from that of rainfall resulting from a cold front.

Warm fronts and cold fronts are formed when the strength of warm air is stronger than cold air or vice versa. However, if the difference in strength is small, then the front will not be able to move quickly. It will hover and stay at the location, resulting in cloudy and rainy weather that includes the characteristics of both types of front.

Finally, we have to explain the nature of an "occluded front". When different fronts are moving in the same region, if a cold front catches up with a warm front or if the two fronts overlap, a tug-of-war of sorts will take place between them. Ultimately, the two fronts will combine and become an "occluded front"; the nature of the clouds and rain under an occluded front will share the characteristics of both fronts, thereby creating a complex weather situation.

A "maritime tropical air mass" is a high pressure system located in the atmosphere above the Pacific Ocean. It is also called a "subtropical high". Since it stays in the ocean throughout the year, it is also known as a "Pacific high" or a "Pacific air mass".

The strength and range of the maritime tropical air mass varies according to seasonal change and temperature. During a warmer season, it will be intensified drastically both in strength and range. When this happens, areas in China and Taiwan will experience warm and wet southwesterly and southeasterly winds. This is the major cause of seasonal rainstorms. Since the location and range of the maritime tropical air mass varies; the weather situation it generates also changes. Along the edge of the air mass is a rain band and the land in this region will experience frequent and heavy rainfall. In weather forecasts the effect is called a "southwesterly flow" or "southerly flow". If afternoon thundershowers are caused by the air mass, the forecast will call it being affected by a "maritime tropical air mass". However, if the center of the air mass moves above Taiwan, resulting in sunny and hot weather, the forecaster will describe the condition as "enveloped by a high pressure system".