Wind motion and precipitation

It is common for meteorologists to use terms such as a low pressure trough, depression, well-marked low, deep depression, cyclonic storm, and severe cyclonic storm. These are systems that are generally associated with inclement weather patterns. So how do meteorologists label these systems? In this blog, I would try and explain them in as simple terms as possible. 

First, let us talk about winds and their motion. The two most important aspects of wind are speed (or intensity) and direction. In meteorological charts (one shown on the top right figure), a solid line with a slanted tail at the end indicates wind. The position of the slanted tail denotes the wind direction. If the slanted tail is to the right, then the wind direction is from right to left, if it is up, then north to south and so on.  The speed of the wind is denoted by the number of slanted tail(s). One tail indicates weak wind, two means moderate wind, and three would mean strong wind. 

I thought it would be good to show a schematic representation of wind that we discussed so far. See the figure below that is taken from internet.

Generally, the winds are measured at different vertical locations. Three wind locations are important for meteorological studies, namely 10 m above ground (corresponding to 950 millibar (mb) pressure), 1000 m above ground (850 mb pressure) and 4500 m above ground (500 mb pressure). As we move away from the ground, the intensity of wind increases, primarily because of the absence of any obstruction such as buildings, mountains etc. The wind motion on land and ocean has two preferential paths, namely, clockwise and anticlockwise. It is important to note here that the motion of the wind is relative to the earth, which itself is in rotational motion, such that the northern hemisphere (above the equator) feels an anticlockwise motion and the southern hemisphere (below the equator) feels a clockwise motion. In the northern hemisphere, which has an anticlockwise motion, if a wind moves anticlockwise, then the motion of wind is called cyclonic, and if it moves clockwise, then the motion is called anti-cyclonic. This is the basis for genesis of the words ‘cyclones’ and ‘anticyclones’. A cyclonic wind will have the same motion as that of the earth in that particular hemisphere. Therefore, a cyclonic wind will have anti-clockwise motion in northern hemisphere and clockwise motion in the southern hemisphere. Thus, the word cyclonic means that the motion of the earth is going to aid the wind motion. On the other hand, for anticyclonic wind, the motion of earth is going to suppress the wind motion (since the motion of the earth and wind are opposite).

I believe it is now clear that the wind motion can either be cyclonic or anticyclonic. In the meteorological chart on the right, I have pointed these two motions, where cyclonic wind is shown in blue color and anticyclonic wind is in red color. This chart is at 850 mb level. Out of the two motions, cyclonic winds are crucial, since they lead to updraft of air from the surface to above. In contrast, anticyclonic winds lead to downdraft. Why is updraft important? To understand this, now let us further assume that the winds contain moisture (water vapor), which they have picked up while moving over a water body. The moisture (measured using relative humidity) is important for formation of clouds and hence precipitation. As the moisture-laden wind tracks a cyclonic pattern, it is advected upwards and in the process starts cooling and eventually condenses leading to formation of rain-bearing clouds. The intensity of updraft, amount of moisture, rate of cooling, and the height of a cloud determine the propensity of a cloud to precipitate. Anticyclonic winds do not form clouds, but rather bring warm air to the surface and hence are associated with hot and humid weather. 

The cyclonic wind is normally associated with low pressure, and anticyclonic wind is associated with high pressure. Therefore, a low-pressure trough basically refers to a cyclonic wind pattern with moisture in it. The trough does not necessarily have to be local; it could extend from one region to another (as shown by the dotted line in the figure on right). In the figure, the letter ‘C’ marks the cyclonic wind pattern. The dotted line shows the extension of this cyclonic trough. 

Here, another image is shown where the cyclonic circulation (C) is concentrated in the west coast of India. In the image, the cyclonic circulation has formed near Gujarat, with the trough extending all the way north and south. As the intensity of the cyclonic motion increases and as it becomes more and more prominent (leading to further drop in the pressure), we transition to a depression or a well-marked low (WML). The moisture content in the wind also plays an important role. As the intensity keeps increasing and the cyclonic motion grows larger in span and moisture is more, then we move towards deep depression and cyclonic storms. The image on the bottom shows a depression (D), where the winds have intensified.

Depression and associated trough

Therefore, meteorologists are always on the lookout for cyclonic wind motions, their intensity, how big they are (i.e. the span of the cyclonic system), and how much moisture is present in this system. This helps them in making predictions about low-pressure troughs, depression etc. and hence predict the precipitation over land and ocean.






Disclaimer: The content given here are all personal views and the author will not be held responsible or held liable for any oversight or misinterpretation.