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Understand the Monsoon: Interview with a Meteorologist
Category Guides To Choose Treks On Himalayan Treks Expert Opinion
By Swathi Chatrapathy
We’re entering the first week of June. Standing on my balcony, I see heavy clouds floating above the Bangalore skyline. The air is cooler than it was three days ago. An earthy scent hangs in the air. Monsoon is almost here. In fact, every afternoon it has been raining! 🙂
Given that the monsoon is officially in Kerala, today, I thought I’d deep dive a bit into the monsoon season, especially how it affects our Himalayan treks.
To get my answers, I again got in touch with our expert meteorologist, who you may have seen recently on the news, sharing his views on the cyclones that affected the country.
His name is Akshay Deoras. He has been a forecaster for the past 11 years. He is also a researcher and a public speaker, pursuing his doctoral study at the Department of Meteorology, University of Reading, UK. (You may remember him from this interview I did with him on How snowfall occurs in India).
There were some deeper questions in my mind.
How does the monsoon appear during the same week every year without fail? Was there more to it than the low and high pressure difference between the land and the sea?
How come we can comfortably trek in J&K and Himachal, when its neighbour at Uttarakhand gets heavy rains? Was it just the natural progression of the monsoon, or was there something else at play here?
Why did Uttarakhand get such devastating floods during the monsoon of 2013? Was it just a cloud burst or was there something else behind this?
Over an hour-long conversation, Akshay Deoras gave me more insight about the Monsoon than what I could get from any reading.
That conversation is what I want to share with you today.
I’m laying it out as questions and answers so it’s easier for you. I start out with the basics, to help lay the foundation, and then deep dive a bit.
I’ve also added my personal notes in this colour to relate it more to the trekking world and give you some interesting perspectives. Don’t miss out on those!
1. What brings monsoon to our country?
Meteorologist Akshay Deoras: The explanation to this is fairly simple. During summer (March to May), in a tropical zone like India, where you have soaring temperatures, the land gets very heated. The heated air above the land rises upwards to the troposphere (around 10-15 km above the land).
This creates a low pressure area just above the land. You’ll see this happening more so in the hottest parts of the country, like Rajasthan.
However, there is one important difference when it comes to land and water. Water has what is called a higher ‘specific heat capacity.’ It takes much longer to heat than land. This is why you notice that water at a beach is always cool, even if the sand at its shores is blazing hot.
So you have very high temperatures above land. In contrast, you have lower temperatures above the ocean.
This creates a difference between land and sea.
The natural state of wind is to blow from a high pressure area to a low pressure area. So moisture-laden winds above the sea or otherwise called monsoon winds come in from the oceans over the land.
Monsoon winds carry moisture/water vapour with them when they blow from the Indian Ocean towards India. When they move over the land, these winds flow in air parcels, like packages of courier. Over land, some of these air parcels rise higher in the atmosphere.
Up in the atmosphere where the temperature is cooler, the parcel starts cooling down, resulting in condensation of the water vapour coming down as rain.
Which is why during the monsoon it is not everyday that it rains, though it may be cloudy over your head. Air parcels that are not moist don’t result in rain. At the same time large moisture laden air parcels can give you a deluge for two days.
Meanwhile, the warm and moist air that rises up from the land to the troposphere cools down because of lower temperatures. When that happens, the cool air, which is heavy and dense, floats towards the ocean and sinks. This forms a circulation or wind pattern between the land and sea, and brings monsoon to the country.
Essentially, this pressure gradient is what drives the monsoon circulation in the country.
Of course, there are other technicalities that involve slightly larger factors like the movement of the Intertropical Convergence Zone (ITCZ) and the earth’s rotation. But we can leave that for a later time.
2. How is monsoon so punctual, always appearing around the first week of June?
Monsoon is a natural phenomenon. There may be slight deviations by around a week or two, but it never happens that monsoon arrives as early as May or even as late as July in a city like Mumbai.
No matter the effects of climate change, be it anthropogenic or natural, seasons still remain unchanged. You still have March, April, May as summer. Summer is, of course, the big factor that drives the monsoon circulation, bringing monsoon winds from the ocean to the land. These seasons don’t change. That’s why the arrival of the monsoon is so stable.
3. I notice that you can predict monsoon up to the exact date when it may occur. How is there so much accuracy in the prediction of monsoon? This is not the case with most other weather systems (like snow) in our country.
Monsoon is one of the most studied weather systems in our country, given the huge impact it has on the economy. In fact, this is one of the topics I’m investigating here at Reading as well, about how much in advance we can predict monsoon low pressure systems.
As per today’s scenario, it appears that the #monsoon‘s arrival over most of eastern & northeastern parts of #India will not happen before 8 June. The monsoon normally reaches #Kolkata around 10 June, but this year it can reach during 10–15 June.
— Akshay Deoras (@akshaydeoras) May 31, 2021
Having said that, it’s important to know one thing. When it comes to tropical meteorology, some things can be predicted quite well — by “well” I mean that there isn’t much deviation from forecast and observation. The large-scale monsoon wind patterns and low-pressure systems are one of them.
To give you an understanding, when I talk about the onset of the monsoon over Kerala or its further progression, what I’m doing is basically predicting the wind patterns.
By studying the wind speed and directions at different levels, we can tell how the monsoon is going to progress about 10-15 days in advance or even further. We can even predict an active-phase in the monsoon or a break-phase in the monsoon.
The real challenge comes when you have to predict more local features of monsoon. For example, it’s very hard to predict what is going to happen in Mumbai on June 13th during the monsoon.
It’s also hard to predict the amount of rain very accurately. These are very local features. Most weather models struggle to predict these localised features as compared to large scale wind patterns.
4. If you have to compare it to the prediction of snow?
Snow, as such, is not a weather system. It is a by-product of different processes that happen in the atmosphere. What would come down as rain turns into snow because of low temperatures.
However, we do know that the Western Disturbances bring snow to the northern parts of our country. Being another large-scale wind pattern, the WDs are also fairly predictable. They don’t have that much variation in their track and intensity.
But very similar to monsoon, it’s hard to predict the more localised features of the WDs, especially because of the varying elevation and local conditions. There’s also a lack of high-grade meteorological observations available to calculate these predictions.
To summarise, we have good forecasting systems to predict large-scale wind patterns that impact the weather — like monsoon wind patterns, WDs or even cyclonic winds. But it’s very hard to predict small-scale weather patterns.
Personal Notes: A while ago, I had written about how snowfall occurs in our country. You should read that for a better understanding of Western Disturbances and snowfall. You’ll find it here.
5. There is a pattern in the South-West monsoon movement. Over the entire season, do you see many branches of it?
We don’t really have too many branches of the monsoon, just two. The first branch that we commonly know is the Arabian Sea Branch. This branch usually brings the first rains to the mainland, arriving first in Kerala and proceeding further towards the Western Ghats.
The other one is the Bay of Bengal Branch, which is important for the North-Eastern parts of the country. Outside of these two, there are no other branches.
In terms of the progression of branches, in an ideal scenario, both branches are expected to progress uniformly. But that’s not always the case.
For instance, right now in 2021, the Bay branch is a bit sluggish, whereas the Arabian one is gaining momentum (monsoon has reached Kerala but yet to reach Myanmar). Quantification of the rate depends on meteorological conditions in a particular year, such as any cyclone or a low-pressure system to put it very simply.
This GIF gives you an idea of the normal progress dates for monsoon. This is not the forecast for this year, but a general idea of when and how the monsoon progresses. Source: IMD
Personal Notes: Most of the rain that we experience on Himalayan treks is brought in by moisture-laden winds from the Bay of Bengal. Of course, those who trek in the Western Ghats of the country would be well-acquainted with the monsoon brought in from the Arabian sea.
6. Do the Himalayas have a role to play in the monsoon?
Indirectly, yes. The Himalayas play an important role in the monsoon.
The Himalayas shield India from cold air sweeping in from mid-latitudes and higher latitudes. That blockade helps us have warm summers, and thereby good monsoon.
Some researchers have been doing some experiments, where they hypothetically remove the Himalayas and the Tibetan plateau and see how it affects the monsoon. What would happen is that you would still get something like the monsoon circulation, but the system isn’t going to be robust enough to bring the required rainfall to the country.
More than anything, the Himalayas act as a very natural source of convergence in the lower troposphere, which helps in bringing rainfall in the foothills and further in the interiors of the country.
7. Does the effect of the monsoon change as you move along the Himalayas from the East? Say, from West Bengal and Sikkim towards J&K?
The typical June to September rainfall during the monsoon season is seen more in states like West Bengal, Jharkhand and Bihar. These states see around 1,000 mm – 1,400 mm rainfall. There’s a gradual reduction in the amount of rainfall as you proceed further North. Uttar Pradesh may get around 800 mm rainfall.
Uttarakhand is slightly exceptional because it gets around 1,100 mm rainfall because of the location of the state.
But Himachal, J&K and Ladakh get much less rainfall. Himachal gets around 763.5 mm and J&K (including Ladakh) gets around 567.5 mm rainfall. They’re far away from the monsoon winds as compared to say, the proximity of Maharashtra and Karnataka.
Personal Notes: As trekkers, we thank this particular feature of the monsoon, because while rain washes out trekking trails in most other parts of the country, Himachal and J&K open up to trekkers. With mild rainfall, they make for a few of the best treks in the country during monsoon.
8. What about some floods that we have seen in the past in Uttarakhand (2013) and J&K (2014). Why did those occur and could they have been predicted?
These catastrophic floods were likely caused by an interaction between a Western Disturbance and a monsoon low-pressure system.
When the monsoon winds come in, the wind patterns change. The Western Disturbances migrate northwards. In some events, the monsoon winds coming from central or eastern India collide with the WDs in the northern parts of the country. Especially during the months of June and September, when the weather is transitioning, is when an event like this could occur.
Both of them are heavily moisture-laden winds and can come down as torrential rain. (Of course, these kinds of floods don’t happen every time the WDs meet the monsoon winds, but sometimes they can).
About their predictability, the researchers have done a lot of research on whether these floods could have been predicted. While there is some consensus that they could have predicted heavy rains, there’s no way of predicting the amount of rain.
Personal Notes: To explain this further, I’m attaching a voice note of Akshay explaining this below. Listen to it:
Indiahikes · What Caused Floods in Uttarakhand and J&K in 2013 and 2014? – Meteorologist Akshay Deoras Explains
9. We often see heavy rainfall in Sikkim and West Bengal as early as April and May. What brings that rainfall?
These rains we see in March, April and early May do not have anything to do with the monsoon. During these months, the Western Disturbances are usually still active. We know that WDs impact the northern parts of our country. If they are strong enough, they carry on even to the eastern parts of the country.
In addition to this, we also get tropical cyclones over the Bay of Bengal. They usually hit the land somewhere near Orissa or West Bengal or even Myanmar to some extent. Tropical cyclones are big weather systems with heavy moisture laden-winds and can have a big impact on the weather.
Personal Notes: If you’ve seen foggy weather or rains when you’re trekking to Sandakphu or Goechala during April / May, this is what causes that.
The monsoon is a 27 million-year-old weather system. It takes time to deep dive and understand them. Being trekkers, for us, it’s exciting to see how weather patterns interact with the environment, causing impacts that we experience first hand.
If you have any observations or questions about monsoon, and especially how it affects the Himalayas, drop in a comment below. We’ll write back to you!
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