Written by Chris Webster, Meteorologist
You are probably familiar with seeing lows and highs on our weather maps around New Zealand. See, for example, previous blog posts on a mid-July northern low, How Lows and Highs move and the satellite loop in Winds Aloft.
The lows or depressions that affect us in the mid-latitudes are accompanied by warm and cold fronts with marked contrasts between the warm/moist air and cold/dry air that follow behind these two types of front, respectively. See our Learning Centre for more.
Occasionally, at favourable times of the year, lows form in the tropics that, on the face of it, may seem to be quite similar to the lows around New Zealand. But they are actually very different. If you looked at only 2-D surface weather maps you might think that tropical lows and cyclones are like our mid-latitude lows, but their structures differ markedly.
In different parts of the world these tropical cyclones are given other names. Here is a satellite image of a typhoon affecting the Philippines (in middle-right of picture).
This typhoon, called "Parma", was following hot on the heels of Typhoon Ketsana that caused serious flooding in the Philippines and then Vietnam in late September.
The image is specially enhanced to highlight the deepest and most active cloud, shaded white and blue in this case. The temperature of these coldest cloud tops is -90C. This is perhaps an example of a paradox: In the global troposphere the coldest air is at upper levels in the tropics, not in the mid-latitudes and not at the poles.
Also note that with this enhancement there is a double entry for black: cloud tops between -20 and -30C appear similar to warm tropical sea (high 20s) between the clouds.
As I implied above, these tropical lows don't have fronts associated with them. They normally have a warm core with a much more symmetric structure than mid-latitude lows. This core of warm air near the centre also means that their strongest winds are near the ground.
Another difference is a region of light winds at the centre, called an eye. The eye can't always be detected in satellite pictures, but does show in the recent example below. Typhoon Melor (upper right of picture, encircled by purple and blue) was curving from westwards to northwards over the North Pacific Ocean. Parma was just north of the Philippines at this time.
When the tropical lows fully develop they become the most damaging of tropical weather features. I will explain the reasons for this in my next blog post, with a focus on how they can affect our weather in New Zealand.