Chennai’s joy ride on a wave of easterlies may end; temperatures to flare up

Chennai might just have been enjoying a benign spring and an early summer this year, but all good things have to come to an end sooner than later. Especially when they relate to the Elements.

It turns out that the southern metropolis has been riding a wave of unusually strong and comforting easterly winds in the lower levels of the atmosphere, resulting in stronger-than-normal sea breeze.

Extended easterly bursts

The land heats up more quickly than the sea because the former absorbs heat faster than water. Hence, air over the land surface becomes more heated than air above the sea.

The hot air over the land being lighter, moves up, to be replaced by cooler air from the sea. Thus, a cool breeze carrying moisture blows from the sea to the land. This is what constitutes the sea breeze.

DS Pai, Head, Climate Research and Services, India Meteorological Department, Pune, told BusinessLine that the easterly burst has ensured that maximum temperatures in Chennai remain close to normal while the minimum (night) temperatures are slightly below normal.

According to S Balachandran, Deputy Director-General, IMD, Chennai, the current weather in Chennai is normal. The departure in maximum temperature is at best 1-2°C from normal.

Thunderstorms on the horizon

Pai told BusinessLine that in the next three to four days, the easterlies will weaken and the maximum temperature will start rising. “Higher temperatures will trigger formation of some convective (rain-bearing) clouds,” he added.

Convection is simply the process of warmer air rising since it is less dense than the surrounding atmosphere. The more convective energy available, the higher the cloud will reach into the skies.

If convective clouds reach above 6 km height, it is called deep convection. If they do not reach as high, it is called shallow convection. Only deep convection produces strong rain showers and thunderstorms.

According to private forecaster Skymet Weather, February and March are the least rainy months for Chennai with average rainfall of 3.4 mm and 3.5 mm, respectively. Day temperature is the main trigger actor for weather activity during this phase. Presence of moisture in the atmosphere and rise in temperatures set up convective clouds, rain and thundershowers during the latter part of the day.

No lockdown impact

Thomas Prasad, leading city-based data analyst and a weather aficionado, says the next couple of weeks could see the city transitioning into the summer, with temperatures escalating. A summer-time weather system (low-pressure area) cannot be ruled out in the Bay of Bengal along the Tamil Nadu coast, followed by the usual refreshing thunderstorm season.

Meanwhile, has the lockdown and restrictions on industrial activity, steep dip in power and oil consumption and the generally clean air had any impact on the pre-monsoon thunderstorm season?

Mrutyunjay Mohapatra, Director-General, IMD, says there has been normal thunderstorm activity in the country so far. “To my knowledge, there has been no such experiment that has probed the impact of the lockdown on thunderstorm activity worldwide,” he informed BusinessLine .

Less aerosol concentration

The meteorological factors leading to thunderstorm are intense heating, moisture availability in lower levels, instability in the atmosphere, and triggering mechanism such as troughs, cyclonic circulations, low-pressure areas.

Mohapatra agreed that the lockdown has resulted in reduction in aerosol concentration in the atmosphere, which may have a bearing on the concentration of hygroscopic nuclei needed for rainfall. “However, what could be the quantitative contribution cannot be told without research and experiment.”

Pai says: “I don’t think there is any lockdown impact on the dynamics of the weather. Yes, there is an impact on air quality for temporarily. The lockdown also might have reduced emissions of greenhouse gases. But it’s impact on weather will be felt after several years, and only if lockdown-type situation persists for many years.”

Lower April temperatures

Leading Chennai-based weather blogger K Srikanth of @ChennaiRains (COMK) says the city has seen lower temperatures in April, during 2005, 2007 and 2008 (see table). The numbers pertain to Chennai airport, which tends to be warmer during the day and cooler compared to the city observatory. Airport data come in handy because of long-term availability, Srikanth told BusinessLine .

In fact, he had written in a blog as early as on March 31 that many people have started to wonder if the lockdown could play any role in modulating weather particularly from the perspective of influencing the ‘urban heat island’ effect due to reduced vehicle traffic and pollution.

One of the earlier studies which COMK did had indicated that the winters are getting warmer in Chennai and it may not present an accurate picture of whether the lockdown is influencing the weather pattern.

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