Category: Weather whys

For weather forecasters, air quality remains one of the trickier things to predict. Several factors go into making good air quality forecasts, because so many weather events influence air pollution. As Houston heats up and air quality becomes a greater concern this summer, we wanted to talk a little about what air pollution is, and the part weather plays in it.

Primary vs. Secondary pollutants

Air pollution consists of two types: primary and secondary. Primary pollution comes directly from a source, like car and ship exhaust, power plant emissions, fires, etc. Secondary pollutants, on the other hand, aren’t emitted directly into the air. Instead, chemical reactions in the atmosphere create these pollutants. Ozone, for example, forms when sunlight interacts with nitrous oxide, carbon dioxide, and other “volatile organic compounds”, or VOCs. Ozone in the stratosphere (think of the ‘ozone hole’) acts as an invisible sunscreen layer for earth, preventing UV rays from reaching the surface. However, ozone near the surface, or ‘smog’, acts as a respiratory irritant. The more sunlight we get during a day, the more potential there is for smog.

The many sources of air pollution. “Chemical transformations” are another way to describe secondary pollutant creation (Science Direct)

Read More…

Why do meteorologists use different dates for seasons?

Posted by Braniff Davis at 3:30 PM

Last week, many meteorologists celebrated March 1 as the beginning of spring. Other than the “wait, did we even have winter?” reaction from some, others wondered why we say spring begins then, and not on March 21 (also known as the Vernal Equinox). The confusion stems from the two ways meteorologists classify seasons: meteorological seasons, versus astronomical seasons. What’s the difference? Why do meteorologists use one instead of the other?

Astronomical Seasons

The earth’s rotation around the sun, as well as the earth’s tilt, create the astronomical seasons. For example, at the Vernal Equinox (March 20-22, depending on the year), both the northern and southern hemispheres face the sun at the same angle. Both hemispheres, therefore, get the same amount of solar energy.

As the earth revolves around the sun, we reach the Summer Solstice (June 20-22), the earth’s tilt causes the northern hemisphere to face the sun more directly. This means longer days, more energy from the sun, and therefore, warmer temperatures. The southern hemisphere receives less direct radiation from the sun, which means shorter days, less energy, and cooler temperatures.That’s why Australia experiences winter when we experience summer.

Three months later, at the Autumnal Equinox (September 21-23), the situation is similar to the Vernal Equinox. Both hemispheres receive equal amounts of energy, and face the sun at the same angle. Finally, at the Winter Solstice (December 20-22), the northern hemisphere faces away from the sun–so the days are shorter, the sun’s energy is less direct, and therefore, the temperatures are cooler. Meanwhile, Australia bakes (more so than usual) during their summer.

Diagram of the astronomical seasons (courtesy NASA)

Read More…

How does wind chill actually work?

Posted by Braniff Davis at 11:27 AM

Earlier this summer, I wrote one of our first ‘weather why’ pieces on the heat index. Now that Eric has written the obituary for Summer 2016 (RIP), we can discuss the heat index’s winter cousin—the wind chill. Since regional wind chill temperatures could fall into the upper teens tonight, it’s important to understand how wind chill works.

What is wind chill?

Wind chill describes how cold the air feels against your skin, not the actual air temperature. Our bodies radiate heat out at all times, which is why we wear layers to trap our body heat in the cold. Without any layers to trap that heat, it radiates into the surrounding air. However, if the air is calm, that heat will hang out near your body, creating a thin, warm insulating layer near your skin.

The wind chill effect (graphic courtesy BBC Weather)

The wind chill effect. (BBC Weather)

Read More…

How the jet stream is keeping Houston hot and dry

Posted by Braniff Davis at 12:03 PM

October ended as an usually hot and dry month. Eric covered this a bit yesterday, but weather data from Houston’s Bush Intercontinental Airport shows that 25 out of 31 days were warmer than average, with a measly 0.14 inches of rain recorded for the month. By the end of October, the heat and lack of rainfall produced drought conditions in the eastern third of Texas.

The jet stream

Figuring out why our weather has been so warm starts with analyzing the jet stream. The image below is an upper-air map, showing the state of the atmosphere at 250mb (about 30,000 feet above the ground, or a just above the peak of Mount Everest).

250 mb map showing the location of the jet stream on 30 October, 2016 (courtesy of The College of Dupage)

250 mb map showing the location of the jet stream on 30 October, 2016 (courtesy of The College of Dupage)

Read More…