Time for Leap Year

It’s time for our quadrennial bonus day! This go-round I am in Hawaii, which means I will be among the last in the world to experience the rare glory of February 29^th. It also means that the lone drawback of this holiday, namely that it doesn’t extend June or October instead of gloomy February, is moot for me! Before I head to the beach, I have to take this once-in-four-years opportunity to talk about the time dimension of my weather modeling work.

Leap year is the perfect window for a discussion of time. Of course, the reason that we have an extra day every four years is that the Earth completes its orbit of the sun in roughly 365.2422 days.  Since it’s not an even 365.25 days, we skip leap year every 100 years, except if the year is divisible by 400, then we observe it (as we did in 2000). Because Earth’s solar orbit is affected by the moon and other planetary bodies, the length of a solar year can, in fact, vary slightly.  Complicating the math further, the average day now lasts for 24.0000006 hours since the second is now defined atomically rather than astronomically. The calendar is oh-so-gradually creeping forward (one could argue that this would be solved with universal adoption of the controversial-but-perhaps-overly-maligned leap second), but this won’t become a noticeable issue for another couple thousand years.

Why do I care about any of this? The angle of the sun is an important variable in the heat balance of Earth’s surface, a core ingredient to all weather models. The leap year phenomenon causes the solar zenith (and corresponding sunrise/sunset times) to change a little bit from year to year.  The annual sinusoidal movements created by the gravity of the sun and moon are summed up by the solar zenith equation, below, which does not consider the leap year variance. For the sake of exactitude, I modify my N input with a leap year correction term; this basically sets the N+10 intercept to the exact time of day that the winter solstice occurred the previous year, either on December 20^th, 21^st, or 22^nd.

Once you account for leap year, these four equations describe the incident angle of solar radiation for any location at any time. Of course the amount of solar radiation depends heavily on other atmospheric factors, but these few lines of code are still pretty powerful.

An exact expression of dates and times is also important when integrating data from numerous agencies and historical records. Data calls generally return the observation time along with the corresponding time zone information for the polled location, which is helpful. I then use the datetime package in Python to do the conversions for me – this normalizes every data input to the computer’s internal Unix time, which counts seconds since 0:00 GMT, January 1, 1970. Beyond preventing Y2K fears from being realized, this standardization (along with UTC time expression) has become absolutely crucial to how the world operates today, from telecommunications to air traffic control. Certainly beats the alternative of mass confusion resulting from differing conceptions of time, like the 300+ years when the world ran on both Julian and Gregorian calendars.

For most of my readers, this is probably more than you ever wanted to know about tracking time. I sincerely apologize, with the caveat that this day would not exist without the observations of early astronomers and many concerted efforts to standardize the calendar.  It’s a bonus day, hopefully that means I get a pass!

Changing Currents

Two understated news stories crept onto my radar this week. First, and this is a fun one, a British Airways flight from New York to London set the subsonic flight record across the Atlantic, smashing the previous record by 17 minutes while clocking in under 5 hours. To cut down flight time by a quarter from around 6.5 hours, the Boeing 747 jetliner had to surf a wildly fast 200+ mph jet stream. This uncommonly intense tailwind was made possible by the pull of a strong low pressure system over the British Isles, with credit to the polar vortex oscillating over Canada.

Second, not only are air currents setting records, ocean currents are also speeding up. Increased winds and warming waters have caused a 5% acceleration per decade in mean oceanic flows, both at the surface and up to 2 km deep. Raising the average flow velocity from 1.0 mph to 1.1 mph since 2000 may seem like a minuscule change, but the sheer volume of water in the ocean makes this trend very eye-opening. We don’t know what sort of effects this flow increase has on regional climates or on the deep ocean, but it could become substantial if current trends continue.

These stories may not sound consequential on their own: two small, random scientific details on an enormous globe. But in my eyes, they are emblematic of what climate change really looks like. Individual hurricanes and winter storms may be the flash points for climate change dialogue today, but causation is impossible to prove with any certainty. Meteorology and climatology are statistical fields, after all: definitive proof is unattainable without a vast collection of data from many sources over a long time period. Though weather and climate experts agree that global warming/climate change is happening, the field shies away from predicting its impacts because of the variable nature of weather.

However, simultaneous worldwide observation is possible in the age of satellites and the internet, and a whole-earth view reveals several indicators of accelerating climate change. The polar vortex is growing more unstable by the decade; the jet stream is speeding up; mean winds have increased by about 1.9% globally in the last 10 years; ocean currents have sped up by 5% in the same time; while the ozone hole in the southern hemisphere has mostly healed, mean carbon dioxide concentrations hurdle past 400 ppm at a steady 3 ppm/year rate; and over 90% of the world’s glaciers are receding. The fact that big changes are happening slowly in the world’s climate should be indisputable, unless you reject hard scientific evidence. If the conversation focused on this type of evidence rather than post-disaster what-if conjecture, perhaps the topic wouldn’t be so controversial and we could plan for our future wisely.