Thursday, 16 January 2014

Why the rain pours down and no longer drizzles in São Paulo

It’s been a rainy week in São Paulo with good bit of storms and lightning bolts. A contradiction one might think in what is known as the cidade da garoa (drizzle city), but the reality is storms are occurring with increasing frequency and rainfall has increased by 30% or 425 mm in São Paulo in 80 years. And whilst the city of sporadic storms and frequent heavy rains is not a catchy name, there is no longer much justification for calling it the drizzle city. On the contrary, the number of days when there is only light rain (less than 5 mm) has reduced.

If we were to calculate this increased rainfall linearly we would be looking at an extra 5.5mm per year and predictions suggest this will continue. If that wasn’t scary enough the rain has not just intensified, it has also become more extreme. There are now storms in winter time too and these days when it rains, it really rains which often also means flooding. And when it doesn’t there can be extended periods with no rainfall at all. In summary, it’s a worst of both worlds scenario. A lose-lose situation.

So what is causing the weather to change in São Paulo? We may not have the solutions but we definitely know the reasons and there are two standout factors.

1 - Urbanisation

São Paulo is a stand out case of uncontrolled urbanisation and therefore a prime example not only of atmospheric pollution but also of the so-called heat island effect whereby temperatures fluctuate across the densely built up areas. If 25% of the territory of the Greater São Paulo area were covered by trees, the average temperature would fall by as much as 2-3ºC. Currently only 10% is covered by trees hence the plans for more green spaces and innovative solutions such as hanging gardens.

2 - Climate change

Amazingly there are still climate change deniers out there and they would do well to spend some time in São Paulo. The increase in greenhouse gas concentrations directly impacts air temperature which increases rainfall.

Worse still the surface temperature of the Atlantic ocean have risen by 1 ºC in the last 60 years from 21.5°C to 22.5°C. And whilst São Paulo is much derided for not having a beach it is a coastal state which means the increase in the rate of water evaporation from the Atlantic ocean has a direct impact on the city even if it lies many kilometres away. The sea breeze typically humid becomes even more moist with the higher temperatures and when it reaches over the hills and on to the plain on which the city sits, the outcome is often rain.

So if you ever wondered why it always rains at the same time in the afternoon during the summer this is because that’s when the sea breeze from Santos hits us. And if you have friends in the south of the city who know it is raining before you do, that is because the sea breeze usually hits the southern end of the city first.


testanto said...

Even though the total rainfall went up many mm over the years, the days with drizzle are the same as ever, according to USP. The report can be read pg. 46 here: (in Portuguese)

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