The sun safely crossed the equator yesterday, and won’t return to the northern hemisphere until March 20 in the US and Europe (March 21 in Asia). What are the ramifications of the sun at a lower angle? It’s not as simple as you might think.
At first blush, it might seem that the seasons are symmetrical. But the sun heats the atmosphere from the bottom up, and that means there’s a lag in the warming or cooling of the layers of atmosphere above the ground.
The Diurnal Cycle
Everyone is familiar with the change of temperature between day and night. The sun does not heat the atmosphere directly; it heats the ground and the ground heats the air. That is why, even though the sun is highest in the sky at noon, the maximum temperature for the day usually occurs around four p.m.
The same thing happens on a larger scale on a yearly basis. In the spring, as the sun’s angle increases, the earth is warmed. But it takes some time for the heat to be distributed into the region a few miles from the ground. As a result, the temperature decreases more rapidly with height in the spring than it does later in the summer when the heat has been more evenly apportioned.
The rate of change of the temperature with height is called the lapse rate. The steeper the lapse rate (the more rapidly temperature decreases with height), the more likely is a lifted ‘parcel’ of air to find itself warmer than its surroundings, and thus continue to rise. This is a state of instability, a requisite for thunderstorms and tornadoes, and it occurs most often in spring in most places. The United States and South America both have their seasonal thunderstorm peak in spring: May in the United States; October in South America.
Fall: More Stable Than Spring
The reverse effect is applicable to the fall. The longer hours of darkness allow the earth to radiate heat to space and the earth cools. It takes a while for the cooler temperatures to reach higher levels, and the lapse rate is shallower: The air is more stable.
This week’s weather map for the United States shows just a couple of areas of rain and scattered thunderstorms: near an old front in the southeast; in the southern Rockies due to the remnants of Hurricane Odile; and in connection with an ocean storm in the northwest.
Even though the weather is relatively calm in the US, there is still interesting weather. Let’s go around the world.
The UN Conference On Climate Change — And Some Indications Thereof
The 120 heads of state are meeting in New York to discuss….well, it’s not clear what they will discuss — only that it will concern climate change and how the problem of an atmosphere that threatens to go out of control should be addressed at next December’s (2015) summit in Paris. There, it is hoped, legally binding greenhouse gas emissions targets will be set.
The hand-shakers in New York might be asking, “What global warming?” as temperatures are right around normal. Perhaps they should have met in Hilo, Hawaii, which set a high temperature record on Saturday, and where the temperature was above normal every day in September until Sunday.
As a strange coincidence, Oslo, Norway also recorded its first day this month below normal on Sunday. And if that isn’t enough of a coincidence, Auckland, New Zealand recorded its first day below normal on Sunday since the first of September. The only conclusion to be drawn is that some places are unusually warm.
Glasgow, United Kingdom (emphasis on United) has been even warmer than Hilo, Oslo, and Auckland (relatively speaking). Every day this month has been above normal, and the forecast is for a clean sweep in September.
Weather in The Tropics
Though it is still near the peak of hurricane season in both the Atlantic and Pacific Oceans, both are relatively quiet, the Atlantic eerily so. It’s been repeated many times, but there’s still a lesson in the fact that Hurricane Andrew in 1992 was the first hurricane of the Atlantic season — striking Miami on August 24, 85 days into the hurricane season. A quiet spell does not diminish the chance of the one big one.
Hurricane Odile’s remnants are still plaguing New Mexico with heavy rains and flash floods. On Odile’s heels was Polo, now just a remnant low over the ocean. Yet another storm is forming in the breeding ground south of Mexico, and it is certain to become Rachel in the next few days. Any slight nudge from the jet stream could push Rachel into Mexico.
A wildfire west of Sacramento, apparently set by an arsonist, is burning out of control, despite firefighters’ using a record amount of fire retardant. The weather in the next couple of days will not be conducive to getting this blaze under control; winds ahead of a Pacific storm will fan the flames and push temperatures into the 80s.
As the storm gets closer, some rain and cooler temperatures will help firefighters to douse the flames.
Look A Little Closer
There may not be any spectacular weather events this week, but the atmosphere always has something interesting to show us. What’s going on in your neighborhood?
Decoding Science. One article at a time.