In most of the northern hemisphere, summer comes in June, July, and August. Not so in India. Here, summer begins in April and ends with the start of the monsoon wind shift– late May or June.
Deadly summer heat waves are not unusual in India; hundreds died in the events of 2002, 2003, and 2010. But this May has been especially brutal, with temperatures soaring to near 120 degrees Fahrenheit in parts of southeast India.
Following a cool (relatively) April, the extreme heat caught meteorologists, and most Indians, by surprise.
With much of India lacking air conditioning, and with power outages preventing those who have it from using it, residents were overwhelmed by the heat, and many (up to 1,800 by some reports) succumbed to it.
According to the International Disaster Database, this is the second deadliest heat wave in India, and the eighth deadliest in the world, going back to 1900.
Loo Delivers The Hot Air
A local phenomenon of the Indian subcontinent known as Loo, a hot wind from the northwest, is responsible for the high temperatures. Loo brings air from Pakistan and Afghanistan across India to the southeast coast. The dry, cloudless flow heats under the high spring sun as it moves southeast. Temperatures normally reach 100 degrees and higher over much of India in May.
Loo is actually one phase of the Indian monsoon, and the country will enter a different phase — the one people are most familiar with — in the next few weeks, as somewhat cooler, humid, and rainy conditions overspread the country.
We normally associate the word monsoon with this rainy phase, but the complete monsoon is a yearlong sloshing of air back and forth from land to sea.
Sea Breeze: A Mini-Monsoon
Coastal dwellers and frequent beach-goers are familiar with the sea breeze, a cooling wind off the water on almost every summer afternoon.
The physics is simple: Solid ground does not conduct heat very well, and when the sun beats down during a summer day, the ground becomes very hot; some of the heat is transferred to the atmosphere. Warm air tends to rise, and the rising air onshore initiates the sea breeze circulation.
The ocean absorbs the sun’s heat and conducts it downward. The surface of the ocean may rise a couple of degrees on a sunny day, but the ground heats many tens of degrees.
The rising air over the land begins a vertical circulation, with offshore wind aloft, sinking over the ocean, and onshore wind at the surface.
The sea breeze is what keeps cities like Boston and Los Angeles relatively cool. It is a shallow, local system, rarely reaching beyond ten miles inland of the shore.
Monsoon: Sea Breeze Writ Large
Sea breezes occur on a daily basis, reaching a maximum in late afternoon and subsiding after sunset. Sea breezes occur in a 24-hour cycle because that is the period of the earth’s rotation and the period of diurnal heating.
A monsoon is simply a sea breeze on a larger scale, both temporally and geographically. The cycle is yearly — following the period of revolution of the earth around the sun.
Just as it takes until mid-afternoon for the sea breeze to kick in, there is a delay in the flow of air from sea to land from the time the sun becomes intense in April to the start of the onshore flow in late May or June.
The Rest Of The Monsoon Cycle
The sea breeze is more accurately known as a land-and-sea breeze. At night, the air that sloshed ashore during the afternoon returns to the sea via a land breeze. A monsoon has a similar land-to-sea component. In India, the winter part of the land-to-sea phase of the monsoon is a cool northwest wind.
As the sun heats the land in spring, this cool breeze becomes the hot Loo. In the last couple of days, the Loo has weakened slightly, and the first breath of monsoon sea breeze, with just a few showers, has reached the coast.
How Hot Did It Get?
Here are some of the temperature highlights of this summer’s Indian heat wave, and some of the results:
- Nearly 2,000 have died.
- The high temperature hit 117 degrees in Andhra Pradesh and Telangana Provinces.
- Streets melted in New Delhi.
- Hyderabad reached 112 on May 21.
- Delhi topped out at 114 on May 25 and has been above 105 for twelve consecutive days. The streak will likely extend to 15.
- Mumbai has had only one day below normal this month.
The worst of the heat is over in central and southern India, as the first whiffs of sea breeze come ashore. A few sprinkles have already arrived along with increasing clouds in the southeast.
As the monsoon moisture and wind move north and west, the heat wave will break. Once the rain begins to fall in earnest, temperatures will mostly be in the mid-80s. Even in the far north, there will be no repeat of the 100-plus degree readings after mid-June.
The Nagging Question: Is It Global Warming?
Considering that worldwide air and sea temperatures have set records in six of the last nine months, and in any period longer than three months ending with the most recent month, it is obvious that most places are getting hotter. It’s no surprise, then, that India is getting hotter.
Furthermore, extreme weather events are on the increase planet-wide. This particular heat wave cannot be proven to be connected to global warming. But there’s a good chance it is.