Predicting the heatwave
Heatwaves arrive like clockwork over many regions of the world and are getting even more attention due to the upward trends in many regions in terms of their duration, frequency, intensity, and scale.
They arrive at more or less expected times of the year because they are directly related to the natural seasonal arrival of spring. This pattern is called the climate.
Most people see heat waves on the horizon as March rolls in over the Indian subcontinent.
Fortunately, India is getting better at predicting extremes and managing the resulting disasters.
The challenge
The challenge of predicting heatwaves, or in fact any extremes, raises the question of how we should depict them for the people at large.
For better or for worse, a new trend has emerged where even reputable scientific organisations, such as NASA, publicise frightening total-temperature maps.
While these maps may communicate alarm well, they can also be misleading.
Scientific methodology says we need to show anomalies, not the absolute value.
In reality, heat waves are excess temperatures over the expected seasonal values.
Seasonally warm temperatures are expected and they don’t become heatwaves until we have anomalous temperatures riding on top of them.
This is a simple point, yet it requires strict adherence to avoid confusing the general public
The total temperature can be the average value over several days or the average of the daily minimum and daily maximum temperatures.
Temperatures averaged over several days show the warmth (which we expect) over the ocean and on land, with lower temperatures over the Himalayas and the Tibetan Plateau.
The daily minimum temperature map shows colder temperatures everywhere compared to the daily averages, again as one would expect.
The daily maximum temperatures are similar to those in NASA’s maps — and they now appear routinely in media reports.
And for good measure, some reports also use words like “frying pan” and “cauldron” to describe temperatures over large areas
Note that even if specific countries are identified with their maximum temperatures, these annotations are not so useful for real-world disaster management.
Total temperature maps hide many details of what we need from forecasts to guide disaster management.
This fact becomes clear in the anomaly temperature maps for the same days.
The contrasts with the total temperature maps couldn’t be starker than when we examine the anomalous quantities in the same temperatures.
We derive these anomalies by subtracting the long-term average temperatures for the same days from the recorded values.
For example, if the long-term average is 30 degrees C and the recorded value is 34 degrees C, the anomalous value is 4 degrees C.
The daily averages and minimum and maximum temperatures for the days considered here were in fact lower than normal over parts of India and Pakistan.
Anomalies in the maximum temperature are indeed much lower than anomalies in the minimum temperatures over these regions.
However, Pakistan’s heatwave season extends into July, so it isn’t out of the woods yet as far as heatwaves this year are concerned
Total Temperature map
Anomaly Maps
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