Category: Data stories (page 1 of 3)

Bangalore’s temperature outliers in February 2015

by Pavan Srinath

In the few months that Bangalore does not get much rainfall, temperature is the most interesting (and useful) thing to look at. I take a quick look at how the temperature is varying across the city, thanks to five Citizen Weather Network stations that regularly report data.

Visually representing the data often throws up insights far better than conventional statistical analysis, and the same is true here.

Looking at the past 10 days of weather in Bangalore, it is immediately apparent that most of them follow trends similar to each other. With variation of less than 1°C, at least four out of five stations are quite similar to each other in both maximum and minimum daily temperatures.

KnowYourClimate-Feb2015Temperature-CitizenWeatherNetwork

Two outliers stand out. For at least the 10 day period in February, Electronics City’s maximum temperature was consistently and significantly lower than the remaining 4 locations in Bangalore. On February 19, Electronics City recorded a maximum of 35.7 °C, a full 2.5 °C higher than all other locations in town.

Coming to minimum temperatures, GKVK, the agricultural univeristy campus north of town, consistently recorded lower temperatures than the rest of the locations. On February 23, the minimum temperature was 13.7 °C, a full 4 °C lower all other areas.

As the Citizen Weather Network grows, we hope to understand the spatial variation in temperature across the city much better. With more data, we may even be able to attribute causes to temperature variation. We will also be able to tell which temperature effects persist through the year, and perhaps quantify heat island effects on the city of Bangalore.

Pavan Srinath is a weather enthusiast at Know Your Climate, and tweets at @zeusisdead.

Temperature data used for the chart can be downloaded here.

The Changing Local Climate of Bangalore: Part One

Bangalore is a rapidly changing city, both for the better and for worse. If somebody from the Bangalore of the ’80s or the ’90s were to visit the city today, it would be very difficult for them to recognize the city – and so it is to those who return from elsewhere after decades. The same is said of Bangalore’s weather, of how ‘it isn’t what it used to be‘. Cold winters, warm but never-too-hot summers and an extended season of rains populate the Bangalore weather of our dreams – dreams that appear to be increasingly disconnected from reality.

Many blame global climate change as at least part of the reason for a hotter Bangalore, along with local urbanization and related heat island effects, while others focus only on the latter. What does data from weather stations tell us? This post seeks to find out.

The question of whether Bangalore is heating up can be broken down into four parts. First, is the climate of Bangalore getting hotter in the long run? Second, is the change in temperatures consistent with global climate change? Third, is this warming related to the changes that we might be feeling? And fourth, could there be changes in temperature in the city above and beyond what gets recorded at a weather station?

The long term temperature trends of Bangalore tell an astonishing story.

Changing Local Climate of Bangalore 1901-2000

It turns out that over the last century, between 1901 and 2000, minimum temperatures have been consistently rising in Bangalore at a rapid rate. However, maximum temperatures don’t quite follow the same trend. Instead, it appears that maximum temperatures peaked in the 1920s and 1930s! Since then, they fell sharply by 1940 by about 1.5°C and have been rising steadily ever since.

Why was Bangalore facing such high maximum temperatures in the 1920s and ’30s? The answer is that we simply don’t know. It is important to note while there are prominent global trends at play like anthropogenic climate change, there are often distinct but important local trends in climate that need to be understood and dealt with locally. These trends may be anthropogenic or natural, but their impacts are often quite real. The wind systems around Bangalore could have been different then, or cloud cover was unusually low for a couple of decades, or it could have been entirely something else.

Apart from the mysterious hump in maximum temperatures, it is also pertinent to note that temperatures are rising faster in Bangalore than the global average! Minimum temperatures in the city have increased by about 1.32 degrees Celsius over the last hundred years (± 0.11°C). This is about double the global average. It is equally interesting to note that maximum temperatures are rising even faster since 1940.

Trends in minimum temperatures of individual months also indicate that December, February and March are warming faster than the other months. All months display the same peculiarities in maximum temperatures as seen in the graph above. It is also unfortunate that comparable data is not currently available to examine how temperatures in Bangalore have been in the last decade.

Do these changes confirm our perceptions of the changing climate of Bangalore? The answer to that is both yes and no. Climate data strongly supports the notion that winter nights are getting significantly warmer in the city – with summer nights joining in. Day time temperatures are not increasing (despite what people think!), at least at the century timescale. The difficulty when it comes to understanding perceptions of weather and climate is that people are experiencing weather in the present, and employing memory to remember weather from the past. Both of these are quite flawed, and that is not a slur on anyone’s judgement. Also, it is quite difficult to perceive a 1° change over a century (or even 2 decades!) when temperatures change by about 10°C every day, and an even larger number over the year.

Finally, the faster rise in minimum temperatures in Bangalore could indeed be because of urbanisation, land use change, and heat island effects. But one cannot conclude for certain by looking at this data alone. Also, the built up area near somebody’s home could have increased, with a corresponding loss of trees. Data from a weather station in the central business district of Bangalore can never be able to tell us the effect of that.

How has urbanisation affected ambient climate in Bangalore? Are there other factors that determine whether you feel that a summer is particularly hot? More on these questions in the second part of this article.

Written by Pavan Srinath

The data used in this article is available here from IMD. You can also download the CSV file here. This article is the first in a series being written in the run-up to the Open Data Camp Bangalore 2013 that is happening over the weekend of March 2-3.

Urban Weather Impacts – A Snapshot of Bangalore

Here is a graphic on how the weather affected the city of Bangalore over a 6 week period during the 2011 monsoon season. In most Indian cities, we have a very limited quantitative understanding of how rains, storms and various weather phenomena affect city life. This is a quick attempt to get a preliminary understanding of the relationship of between the quantity of daily rainfall in Bangalore and and the various impacts it has on the city – from traffic jams and fallen trees, to flooded houses and lost lives.

Weather impacts were obtained from newspaper articles and reports, and were overlaid on daily rainfall data to get a sense of what happens when. Not only do we need to understand what impact weather has on the physical infrastructure of a city or on the lives of its residents, but also about how everyone, including the city administration and media, respond to weather events.

If you wish to help us make something similar (or better!) for the year 2012 and its weather, contact us at know.your.climate (at) gmail (dot) com!

Bangalore – The Year so Far

A short post today to break the hiatus, where we take a look at the year’s rainfall so far.

A major limitation with the weather and climate discussions in popular media is that rainfall is discussed in terms of deficits and excess in terms of percentage points – which really conveys a lot lesser information than you might think. This gets condoned by the Met department as they rarely put out other kinds of information. Deficits and excesses should always be examined in the historical context – so that one can figure out how unusual or infrequent a certain event is. Monsoonal rainfall in India varies so much that few numbers actually end up breaking or making records – it’s a different and depressing story that even as a country we are not yet fully equipped to deal with even the likes of ‘normal variation’

With this in mind, given below is a graph of 57 years Bangalore’s rainfall data and calculated cumulative numbers – to examine the progress of the monsoon. The median line shows well, the median (50th percentile) progress of the monsoon – this means that at any point, 50% of the years got more rainfall than the black line, and 50% got lesser. Similarly, I’ve drawn lines for select percentiles and the maxima and minima. Overlaid on this is the current monsoon in red.

As you can see, the city was doing alright in terms of rainfall received, up until the monsoon started. Rainfall in June quite literally flatlined, taking Bangalore from an above-median rainfall regime to a below median one, quickly dropping below the 25th percentile line – what this means is that the city had received more rains in over 45 of the last 60 odd years. Things could have picked up if we had received good rains in July – but mediocre quantities of rain in July has kept us in the same low-rainfall regime.

A caveat here is that the historical data-set underestimates rainfall by a small amount – which means that the 2012 numbers may actually be worse in comparison, although not by much. As one can see, there is plenty of rainfall yet to be received in the year, on average. How much Bangalore will actually get in 2012, only time will tell.

Written by Pavan Srinath. Views are personal.

A Dry June in 2012 – Why Bangaloreans Shouldn’t Worry.

It’s the end of June, and Bangalore has received very little rain till date. The monsoon so far has turned out to be a not-very-damp squib. There is a lot of concern and worry among people as to what this portends for the rest of the season and year.

I maintain that now isn’t the time to worry, but instead it’s time to prepare for the monsoon.

Rainfall this June in Bangalore has been quite dreadful, with Bangalore getting only 13mm of rain till the 25th of June, against a monthly average of 80mm. and even that in small, scattered showers of 1-2mm. The lowest rainfall ever received in June was in 1945, when Bangalore got only 4.5mm. June 2012 does not fare much better. When June numbers are laid out in ascending order as below, you can tell quite clearly that 2012 is in the bottom 5 percentile when it comes to June rainfall. Pending copious rain in the next couple of days, the quantity is dreadfully low and there aren’t two ways to see it. Farmers close to town without irrigation facilities would have no doubt suffered. Lakes and the water table will also have to wait to get recharged.

However, what does this mean for the year? To begin with, June is only the sixth wettest month for Bangalore. We get more rainfall during all other months between May and October. June, on average, contributes less than 10 percent of annual rainfall. That being said, it’s useful to check if there is a correlation between rainfall in June, and rainfall annually. That is, it’s important to investigate whether deficit rains in June imply a drought year.

Turns out that the answer is a resounding NO. Below is a plot of Annual rainfall versus June rainfall. If the two numbers were linked positively (or ‘positively correlated’), an increasing trend would have been seen on the graph. Instead what is found are data points scattered all across the board. The three encircled points are from the three years that received lesser rain in June than 2012. Only one of them is a drought year, while the other two appear to have received over 1000mm, more than the annual average.

There you have it. June is not the most consequential of months for Bangalore. Bad rains in June alone are not sufficient cause for Bangaloreans to worry. What each of us could do on our own is prepare our houses and buildings to harvest the rains that we are certain to get over the next few months. The onset of the monsoon, if nothing else, cooled Bangalore down for good. Rejoice, for the summer is over, and await the rains to come.

Written by Pavan Srinath.

Data used in this post is from here and here. June rainfall for the year 2001 is missing. The number used for June rainfall in 2012 is the sum of what was received between June 1 and 26, as collected from IMD’s automated weather station in Bangalore.

Update: June 27, 2012. 10:50 AM.
Karthik has an excellent graph of June rainfall versus rain from July-October that underscores what was said on this post. Click on it to view it in full size.

Update #2: June 27, 2012. 1:47 PM.

Karthik also has a similar graph for all of India, which tells the same story: that rainfall in June has no bearing on how much it would rain across the country during the monsoon. While scanty rain in June across much of India (except the north east and the western coast) has caused stress for farmers and others, it does not imply that the rest of the monsoon will necessarily be bad. What it does, however, is place a lot of importance on rains over the next few weeks. The IMD has released coarse-grained forecasts for the season which says that the monsoon this year will be normal or a little below normal for all parts of the country other than the North East. The accuracy and the limitations of this forecast will be tested over the coming months.

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