It’s been a slow climate news week, so today’s post will be about the weather. Every year in April, Dr. William Gray’s group at Colorado State University puts out an Atlantic hurricane forecast. The news media usually gobble this up like it’s actually news. I haven’t heard anything yet this year, but I think that’s because I’ve been watching less of the news on TV. This year is no different; they predict that there will be 15 named storms, 8 hurricanes, and 4 intense hurricanes.

On their website, they provide an archive of their forecasts from 2001-2008. The number of actual storms is also available, so it’s possible to assess the skill of their forecasts.
Continue Reading »

Precipitation in the Southwest United States comes in two flavors: winter and summer. During the winter, the precipitation is mostly from the passage of mid-latitude storms. And during the summer, the precipitation comes from what is colloquially called “the monsoon”.

What is a monsoon?

The word monsoon derives its name from the Arabic word ’season’. Sailers in the Indian Ocean would use the word to describe the change in wind direction throughout the year. They had noticed that during the summer the winds blow from a different direction than during the winter. The word monsoon refers to the seasonal reversal of winds, and not to changes in the precipitation (National Weather Service).
Continue Reading »

Fully understanding the effects of aerosols on stratocumulus clouds is important in the climate change discussion. Low-level clouds have a significant cooling effect on the planet. In the 1970s Twomey showed that, all things being equal, by adding pollution (aerosols) to these clouds, they have a higher optical depth and thus reflect more of the incoming sunlight. This cools the surface more than the original cloud. However, things aren’t that simple; they rarely are in science. There are other effects that must be taken into account when looking at the effects of aerosols.

The work done by Guillaume S. Mauger and Joel R. Norris in Stratocumulus Sensitivity to Aerosols and Dynamics [May be behind paywall] is such an example. They attempt to find the causation behind the correlation between aerosol optical depth and cloud cover. The do this by looking at the stability of the boundary layer.
Continue Reading »

When I was younger, I thought that Arizona was a desert. Not just an arid region, but a desert like the Sahara or the Gobi. As I grew older, and especially when I moved here, I realized that it isn’t. There’s green plants all around. There are a lot of Sahuaro cactus and other desert scrub which nicely green up the landscape.

The precipitation here comes in two seasons: summer and winter. We receive almost no rain during the spring or fall (autumn). The reason is that our winter rain comes from passing mid-latitude cyclones, and our summer rain comes from the monsoon. The AMS defines a monsoon as “A name for seasonal winds. It was first applied to the winds over the Arabian Sea, which blow for six months from northeast and for six months from southwest, but it has been extended to similar winds in other parts of the world.” During the summer months, the direction of the winds changes in the desert Southwest which brings us much-needed moisture.
Continue Reading »