Tag: monthly rainfall

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.

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.

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