Thursday, June 21, 2012

Meteorological Fun: Progressive Heat Impact

A couple of us have been working to develop a new way to quantify the impact of heat based on multiple factors.  Given heat wave as a term, to our opinion, is overused during the midst of the summer season, we thought how there could be a better scale or system to measure the true impact of heat in a given day besides mere daily high temperature.

We created a "new" scale to measure not just the impact of heat in one day but also have taken that to scale out to measure heat waves compared to each other.  That 2nd formula will be highlighted tomorrow.  First, the one we'll highlight today is the PHI.

PHI, or Progressive Heat Impact, is our way to measure the combination of heat duration and intensity, in order to come up with a "nasty" factor in comparing heat events in Philadelphia to each other over time.    It's a four part formula that combines the daily high temperature, low temperature, temperature differential from the prior day, and consecutive days of 90 degree weather.  These four ingredients are mixed into a formula that measures the progressive impact of heat in our region.  It is not to be confused with heat index, which factors in humidity to the equation.  Heat index does a great job of providing an apparent, or how it feels, temperature at a given hour or timeframe.  However, given dew point can vary based on wind trajectory (west winds can lower dew points thanks to a downslope effect) using it as one of the factors leads to a much more complicated formula than we wished to create.  Does one use the max heat index in a given day?  At a certain time?  There are a lot of factors we could interject.  After a fair bit of consideration, we chose to roll with high, low, and consecutive 90 degree days as our three primary factors, with an added consideration to large jumps in temperature as a heat wave gets going...the "shock to the system" value.

The third of those four, consecutive 90 degree days, is a way to reflect the way heat waves that exacerbate impact on life and society over time...not just the increase in heat deaths as heat waves continue but also the increased risk of power outages that can occur due to increased demand.  Cooling degree days, a measurement used quite a bit in the energy field to measure the number of degrees needed to cool to 65 (based on the average temperature for a day subtracted from 65 degrees), is one such measurement...however, it doesn't measure the compound factor over time as effectively.  Our formula does that.

PHI is a simplified way of looking at societal impacts that heat would produce -- not just on humans but to power supplies and society at large.  The greater the number, the greater the impact.

In our research, we've found 12 days where the PHI was 35 or higher.  We considered these the "worst of the worst" in Philadelphia hot weather days in history.  Eleven of the days featured highs of 100 or higher...the 12th day featured a high of 98 on the 18th day of a heat wave.  However, because that 98 degree high was part of a significant heat wave, the PHI is ratcheted higher since there's a progressive, or taxing impact of heat on human life.

You'll note that our two most recent 100 degree days in Philadelphia -- July 22nd and 23rd -- are ranked amongst the seven highest PHI on record, with July 22nd's PHI the highest in nearly 100 years.   For comparison's sake, Wednesday's 97 degree high generated a PHI of 18 and Thursday's 17.  The difference was that Wednesday also had a 15 degree jump in temperatures from the prior day...the shock to the system  in a large temperature jump gets factored in this equation.

The statistic isn't meant to be a be-all and end-all statistic.  It is our internal way of looking at how bad a specific day in a heat wave is compared to the others...and a way for us to develop a metric that will help better gauge how intense a heat wave is and can be compared to.

From here, we'll take this data and spell it out in what is known as our Heat Wave Index...which we'll talk about tomorrow!