Soaking in the Sun

2016-04-17 17:43:40.000 – Michael Dorfman, Weather Observer/IT Specialist

 

The last few days have been wonderful! An Omega Blocking Pattern has set up for the last few days, giving us ample sunshine and not-so-frigid temperatures. It’s incredible how a 20 degree, sunny day in mid-April feels so much warmer than a 20 degree, sunny day in mid-January. Even though the temperatures are identical, the sun is at a much higher angle in the sky. This allows us to see much more solar gain from the sun this time of the year, and that can make the sun “feel” warmer (and can even melt snow when temperatures are still below freezing, as was apparent on our Observation Deck the last few days).

As many of you know, the angle of the sun is also what drives our seasons. The steeper the sun’s angle, the more energy the surface (and atmosphere) of the earth gains. June 21st is the Summer Solstice, meaning the sun will be at its highest point in the sky that day. That’s roughly 2 months away, which means roughly 2 months after the Solstice (late August) is the equivalent sun angle to right now.

So, if we have the same sun angle as late August, why aren’t we seeing balmy temperatures in the 80’s (in the valley) as in late August? You can imagine the earth as a frying pan. If you put it on simmer, your pan will rise to a certain temperature, then level off. While you’re still applying heat to this pan, it’s not actually warming the pan up any more. The pan is losing this heat as it radiates off energy and heats up your food or the air around it. When net gain in heat is equal to net loss in heat, your object is in what is called thermal equilibrium.

As the earth’s surface and lower atmosphere gains more energy from the sun the closer we get to the equinox, the seasonal temperature of the atmosphere increases. It’s gaining this energy faster than its releasing the energy, so temperatures begin to go up. Even after the equinox, while we weren’t gaining as much energy as when we were close to the equinox, the earth’s surface and lower atmosphere are still gaining more energy than they are releasing. Eventually, solar gain drops below the total energy loss in the earth system, and that’s when we actually start to cool off.

Another way to think about it (I’m on a metaphor spree) is like pushing a shopping cart. You accelerate the shopping cart to a certain speed by pushing it a certain amount. In comparison, a constant solar gain hitting the earth’s surface would gradually increase the surface’s temperature. Even if you slightly decrease the force you’re pushing the shopping cart with, you’ll still end up accelerating and pushing the cart faster. Just in this way, even if the amount of solar gain decreases slightly, you’re still putting in more energy than you’re getting out; your net energy gain is positive and you will still see an increase in temperature. This tendency for objects to remain the same temperature is called thermal inertia (similar to heat capacity), and, as seen above, has comparisons to kinetic inertia (the tendency for a massive object to avoid changes in movement).

Just like the seasons are affected by sun angle, so are daily temperatures. I’ve averaged our hourly temperatures over our whole 80 year record and have binned them into 24 hours of the day.

 

 

  

You can see a tendency for temperatures to be warmer during the day and cooler at night (called diurnal heating). This indicates that daily temperatures tend to change less in the wintertime when there is much less solar gain.  There are many possible drivers for temperature in the various seasons; these graphs aren’t isolating solely the effect of sun angle.

While temperature may seem like a straight-forward, easy concept to grasp, there are so many complicated details as to how and why we see daily and annual temperature changes. One thing is for sure; the sun is getting higher in the sky. So put on that sunscreen, get that bathing suit out and get ready for summer!

Note: The valleys may be feeling like summer, but the summit can feel like winter any time of the year. If you are planning an above-tree-line hike, be sure to check our Higher Summits Forecast and be prepared for colder-than-expected conditions!

 

Michael Dorfman, Weather Observer/IT Specialist

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