Category: Data stories (page 2 of 3)

Climate of Bangalore – A Prelude

The posts on this blog so far have focused mostly on weather and not really on climate. (Not counting the talk at Open Data Camp). Today’s post seeks to rectify that.

When people talk of climate, it usually refers to averages and numbers from long periods. “How long?” is a fairly nebulous question, but the convention today is that it ought to be for at least about 30 years.

Below you’ll find a monthly rainfall profile of Bangalore, which is a result of averaging monthly rainfall amounts from a 100 years of data, from 1901-2000. This monthly data is actually publicly available at an old IMD page which was discovered with the help of Anand, so feel free to use it yourselves as well. Unlike other sources, this is a 100 years of station data, not gridded, not massaged, so it’s a valuable resource. I know that I’ve made strong statements in favour of looking at daily data, but monthly numbers do have their uses, and climate information straight from the weather stations is ideal, if you can get your hands on them.

I’ve chosen the full 1901-2000 period above in order to capture whatever long-term patterns exist, and the graph that you see above can change a little when you take an average over say the more recent period.

The hundred year span of data, however, is quite a luxury most of the times. Bangalore was a seat of a British cantonment and also a prominent city in the Kingdom of Mysore and was fortunate enough to have a weather station that was set up late in the 19th century. Thus, for standardization, meteorological departments choose a common 30 year period during which they have continuous data for the most number of weather stations and rain gauges across the country. One standard followed by the Indian Met Department is the period 1951-80. The graph from then is below.

Note that there is only minimal deviation from the previous graph here. Rainfall in March, May, September, October, and November are a bit higher, the rest barely changed. What would happen if we instead try to look at the climate of the most recent 30 years for which we have data, 1971-2000?

Turns out that things change quite a bit! September rains are way higher at 233mm, small increases in June and August, losses in October, May and November. The line graph below has all three together.

Now what does this mean? Does it mean that Bangalore’s climate is changing?  The annual totals for the three periods (1901-2000; 51-80; 71-00) come up to 930mm, 963mm and 968mm. Since the annual totals are not very different, does it mean that the distribution of rainfall in Bangalore is changing rapidly?

The questions can be asked rather easily, but answers are harder to find. The short answer is: no, not quite. A lot of things change when you look at only averages, but these don’t necessarily imply underlying trends. Or even if they do, whatever is changing is not always obvious. Over the next few posts, I’ll be taking a closer look Bangalore’s rains and try to examine how it might be changing.

Written by Pavan Srinath. This post was a result of conversations on twitter and email with @zenrainman. The monthly rainfall data is taken from an old page on the IMD website.

Cooling down with some Summer Rains

A few weeks back, I had blogged about the hottest day in April in Bangalore, and were hoping that that worst of it was behind us. Since then, Bangalore has received a substantial amount of rain on several days, and in general, the weather seems a lot more pleasant. When you aren’t stuck in traffic during the rain, that is.

In today’s blog post, we ask a simple question: how much colder does Bangalore get, after it rains? We all know that it gets cooler, but by how much? A comprehensive answer would require a historical analysis and data from a number of years, but we can start by looking at the past couple of months.

Before diving into the numbers, it’s useful to think about the dynamics of temperature, and how the human body experiences it. Our bodies cannot directly measure temperature – we can only feel how our surroundings either strip the heat away from our body, or how they add to it. The rate at which this happens is determined by ambient temperature, humidity and wind. It’s the latter two factors that go into the “feels like” temperature indices where it feels hotter when there’s very high humidity, and it feels much colder when there is a wind blowing. The latter makes a very strong different when you are faced with sub-zero winds.

In Bangalore, we are fortunate enough to neither experience very high humidity nor winds that are freezing. This makes analyses much easier – the heat we feel depends largely on ambient temperature alone.

When nothing else operates, more incoming solar radiation means more heat. That means that the hottest day ought to be the day of the summer solstice, and the coldest on winter solstice. But once you bring in clouds and rain, all that starts changing. Clouds reduce the solar radiation that reaches the surface, and rains cool the earth by absorbing a lot of the heat. Apart from all this, cold or warm winds from other regions can also affect temperature. It’s usually the latter operating when a region experiences a cold wave or a heat wave.

So, how much cooler does Bangalore get thanks to rain? By my estimate, a good bout of rain can bring the temperature down in the city by about 5 degrees Celsius. Temperatures were hovering between 35 and 36 degrees during the third and fourth weeks of April, until the fateful 25th, where thanks to a depression over the Arabian sea and rain over the city, temperatures plummetted very quickly. From a high of 37.7 degrees on April 24th, temperatures did not cross 33 degrees celsius for the next 6 days. Data from http://www.imdaws.com/

As you can see from the graph, temperatures started climbing again from the 3rd of May, but thanks to some really heavy bouts of rain, maximum temperatures came back down to 31-32 degrees. IMD’s information page tells us that temperatures often touch 37 degrees in May as well, confirming the notion that rains cool the city by about 5 degrees. The graph below shows the temperature profile along with the rainy days in the 6 week period. Numbers in the graph below indicate the amount of rain that fell on those days, in mm.

Data from http://imdaws.com/

One needs to be very careful about drawing general conclusions from such data, but a few points stand out. Solitary bouts of 1-2 mm of rainfall are often insufficient to affect temperatures much, but larger quantities do have an effect. In the summer, rains can bring temperatures down by up to 5-6 degrees in Bangalore, but it takes a day or two for that to happen. Minimum temperatures usually drop a day after the rain as well, likely due to a drop in night-time humidity. 

Having said all of those things, the day of April 25th merits a closer examination. As I’m sure most of us in Bangalore will remember, that was the first day this year that Bangalore got some real rains. The skies grew dark and cloudy early in the afternoon and the temperatures plummeted rapidly. The graph below gives us a picture of just how significant that was. The hourly temperature profiles of the day before (24th – and the hottest day of the year) and the 25th show an almost-staggering difference.

Data from http://imdaws.com/

On the 25th at 3.30 in the afternoon, the temperature was a full ten degrees cooler than it was at the same time the previous day. After that, the difference only became more staggering! While bouts of summer afternoon rain can do wonders to cool a place down, such dramatic changes need a lot more. Such a rapid cooling took place because of a much larger weather system that was operating over South India and the Arabian sea, and had given Kerala and Tamil Nadu a lot of rain. Though not unheard of, such weather systems are a lot less common than convective summer rains.

Written by Pavan Srinath. Views are personal.

Talking about Climate Data

It’s been a quiet few days at the Know Your Climate blog, but there will be more content coming your way very soon.

In the mean time, I’m glad to share that the good folks at HasGeek (& @nigelbabu in particular) had taken it upon themselves to edit videos of all talks at the Open Data Camp that took place back in March and put them up on youtube.

I had shared the slides in the introductory post here, and you can access them on slideshare. Here’s the video.

This is also a good opportunity for me to correct some of the small mistakes I made while presenting.

First up, around 2:30, I talk about how the Indian Meteorological Department is 170-180 years old. Actually, the met department is a little bit younger, at 137 years old. The department was set up after a major cyclone and famines in the 1860s and 70s, to have one agency that looked at all the meteorological data. However, the instrumental record of climate data in India goes back to the late 1700s and early 1800s, around when the British started setting up weather stations around the country.

Second, around 3:20 I talk about rain gauges maintained by various state departments, which are completely manual. While this is mostly true, I’m happy to say that things are looking up. The Karnataka State Disaster Monitoring Cell, I’m delighted to learn, has set up several hundred fully automated weather stations across Karnataka over the past decade or so. This is a huge leap forward, and from what I gather, this hasn’t been sufficiently acknowledged by the IMD either. However, their rich data remains far from open.

After I start talking about the ‘Joy of daily rainfall data’ (~ 11:30), I’ve unfortunately used the words ‘drought period’ to describe what is essentially a 2-week dry period that occurs in Bangalore around the end of June. Unfortunately in India, we use the word ‘drought’ a little too much. A drought is an extended is an extended period of months or years when a region notes a deficiency in its water supply whether surface or underground water. As per the IMD, a region suffers from ‘meteorological drought’ when it receives 25% less rain than usual in a year (with 50% less rain for a severe drought). While that reduction in rainfall does significantly impact a region, it doesn’t necessarily mean that drought has occurred. We need deeper discussions on what drought is, and I’m afraid my loose usage of the words ‘drought period’ did not help. All I was referring to was a recurrent dry period that occurs in Bangalore after the monsoon has already started.

Written by Pavan Srinath.

Feeling April’s Heat in Bangalore

We still have a few days left to go in April, but I’m not patient enough to wait before taking a closer look at the month’s temperatures.

Bangalore, as everybody who doesn’t stay here knows, has an excellent climate. But you wouldn’t come to know of it if you spoke to the locals! Too hot, too cold, too windy, too much rain, or not enough. There’s always something to complain about. But I’ll maintain that there is a lot more that most Bangaloreans appreciate, perhaps in an unspoken manner.

Bangalore sizzles at 37.1 degrees celsius: 2012’s hottest day yet ran the Times of India on Tuesday morning (24th), only to be 1-upped by the same day’s temperatures!

Well, how hot was Bangalore this month, really? Here’s a comprehensive graph that shows you everything you need to know about April 2012’s temperatures. And perhaps a wee bit more. :-)

Is it hot in here, or what?

As you can see, yesterday, the 24th, was hotter than anything we’ve seen in 10 years, with a maximum temperature of 37.7 degrees Celsius*. It missed the all-time-record by less than a degree. While this is true, a closer look at the previous highs shows us that they weren’t all that different. April temperatures have touched 37 degree mark three times over the last ten years, and come close in most of the others. A preliminary analysis of humidity indicated that the ‘felt temperatures‘ were not particularly higher either.

The only thing a bit different about this April has been the near-complete lack of rain in the month. On average we get about 40 mm of rain in April, with many of the last few years getting very heavy rain on at least one day. As Bangalore’s resident meteorological director Mr. Puttanna said, “If it rains in the next few days, we can achieve the target in just one day…” And that could very well be today. :-)

So what’s the take home message, you ask? April is the hottest month in Bangalore. If the gathering storm clouds are any indication, the worst is behind us. Let us look forward to keeping our slums safe, harvesting rain water and listen to people elsewhere complain about their weather.

Plant some trees and paint your roof white while you’re at it. The temperatures shown above are at a weather station, how hot your house is depends on how much care has gone in to designing the built area.

Just for the record, this post shows absolutely no evidence that climate change is fake or fictitious. Bangalore, like the rest of the world, has indeed grown hotter by the year. We’ll throw some data up on that some other time.

Written by Pavan Srinath. Views are personal.

*The IMD Bangalore press release stated a high of 37.1 degrees on Monday, April 25. However, IMD’s own automated weather data shows a slightly different number of 37.3 degrees.

Portrait of a Rainy Day in Bangalore

Today the Know Your Climate blog features a small experiment in trying to map out an evening’s rain on the city of Bangalore.

It’s an understatement to say that climate and weather are extremely complex phenomena, and only a small part of the complexity gets captured in weather stations and rain gauges. The city of Bangalore is a great example in this regard – with varying elevation, land cover and wind systems, there is an enormous amount of variation in the rainfall the city receives. Not to mention the differences between the city and its surrounding areas.

An old adage in the city tells us that while it is raining at one end of the road, it remains completely dry at the other end. People on platforms like twitter come alive at the very prospect of rain, waiting eagerly for the rain gods to smile on them (or not).

This post details a very rough and preliminary effort at trying to map the complex patterns of rain that fell on the city of Bangalore on April 17, 2012.

Rain spell #1. The first spell of rain today started in the Rajajinagar-Malleswaram area between 3 and 4pm (highlighted in green). It should be noted that the official IMD automated weather station (just a few kilometres to the South East) registered absolutely no rainfall during the same time, while showing a mild, westerly breeze.

The rain spell appears to have weakened a bit while moving northward towards IISc and Jalhalli by around 5-5.30pm.

View Rains in Bangalore in a larger map

Rain spell #2. The second spell of rain in the city fell at around 6.10pm in the South, around Jayanagar, Koramangala and select parts of Banashankari (I stage). This barely lasted 10 minutes and appears to have been far lower in intensity than its northern counterpart. (Highlighted in Orange)

Rain spell #3. All seemed quiet on the Eastern front until around 7.45pm, when reports started flowing in of rainfall around Indiranagar, the Inner Ring Road and Koramangala. (Highlighted in Blue) This spell of rain appears to have been substantiative, and came just before:

Widespread Drizzles #1: Between 8 and 8.30pm, regions all across South Bangalore experienced brief and weak drizzles, from Kumaraswamy layout in the west to the Silk board in the east and Wilson garden in the north. The drizzles again stopped short of the IMD station and the cricket ground at Chinnaswamy stadium. (Information sources appear as cyan markers.)

Rain spell #4. Meanwhile up north, Yelahanka and its surrounds experienced a mdoerate amount of rainfall around 8.30pm, with Widespread Drizzles #2 being observed in Vidyaranyapura, Hebbal and elsewhere.

While these spells of rain were blooming across the city, darker clouds were gathering in the hills and valleys to the West of Bangalore. Reports started coming in from Hesaraghatta and Soldevanahalli (to the north west of the city) of thunder and lightning and strong winds blowing. 9.30pm onwards, the IMD station in the centre of town started showing winds blowing east instead of west, for the first time in the day. A thunderstorm grew in force, and started moving in slowly into the city. Probably the first Bangalore thunderstorm this year, it will remain a gentle cousin to many violent ones which are certain to follow as the summer lengthens.

By 11.30, most parts of northern Bangalore were receiving rainfall, including the Chinnaswamy cricket stadium where the local Royal Challengers won an exciting game and just about managed to escape the rain. Southern and eastern parts of Bangalore largely remained dry as the day drew to a close.

There you have it folks. Four separate spells of rain, lots of drizzles and a thunderstorm. All in an evening’s rain in Bangalore.

Your comments and feedback are most welcome. We hope to bring more such stories about the weather and climate, and better.

A Brief Note. I got back to my home in Jayanagar just seconds after a small shower started here, and I started wondering (after a couple of years of idle thoughts and some excellent conversations with @zenrainman) about how it was raining elsewhere in Bangalore. An evening’s twitter campaign followed. All information that has been shown on the map and described above has been taken from people who were gracious enough to reply and share their trysts with the evening’s rain on twitter.

PS. By no means am I claiming this picture of the evening’s rainfall to be very accurate. The ‘contours’ on the map were drawn by hand and are purely indicative. Not all inconsistencies have been ironed out, although an honest attempt has been made at doing so. With luck, some of the flavour of the mind-boggling, schizophrenic pattern of rainfall that Bangalore gets to work with, has been captured here. Cheers.

Written by Pavan Srinath. Views are personal.
[Update: Know Your Climate is now on Twitter @KnowYourClimate.]

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