Why do meteorologists use different dates for seasons?

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)

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Expect more showers Friday and Saturday before Houston clears next week

Let’s face it, Tuesday’s cool front hasn’t done a whole lot of cooling. While some parts of Houston have fallen into the upper 50s, most of the central and southern part of the region have only fallen into the low 60s. We’ll have to wait until this weekend for a somewhat stronger front.

Today

Mostly cloudy skies and an easterly wind should hold temperatures today down to about 70 degrees, but there should be no rain. Clouds and winds shifting to the southeast tonight will preclude overnight lows from falling further than into the mid-60s for all but far inland areas.

Thursday

While we can’t rule out the possibility of some scattered showers on Thursday, as moisture levels rise, it looks like most of the area will remain dry. Skies will again be mostly cloudy, with temperatures rising into the mid- to upper-70s.

Friday and Saturday

A combination of increased moisture levels, and lower pressure systems moving in from the west will lead to the likelihood of rain showers and potentially some thunderstorms on both Friday and Saturday. In terms of accumulations I don’t think the area will be looking at more than 1 to 2 inches of rain—with isolated totals a bit higher than that—during the end of the week.

NOAA rain accumulation forecast for now through Sunday morning. (Weather Bell)

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Storms possible today, and rain returns later this week to Houston

It’s another warm morning across Houston, with temperatures generally having fallen only to about 70 degrees. The main concern today will be the possibility of some storms and heavy rain along with the passage of a cold front.

Today

A cold front now in north Texas will move down toward the Houston area, likely pushing through the city during the mid-afternoon hours, and the off the coast by late afternoon. Due to decent moisture levels and and lift associated with the front, much of the area is likely to see some moderate to briefly heavy rain with the front’s passage. I’m expecting on the order of 0.5 inch to 1.0 inch or so, late this morning and during the afternoon hours. The storms may slow, or even stall along the cost, producing more rain there. Along with the front there will likely be some embedded thunderstorms, and a slight chance of some hail. After the front passes tonight look for temperatures to fall into the mid- to upper-50s.

(National Weather Service)

Wednesday

With northerly winds and partly cloudy skies the day will be moderately cooler, with highs in the low 70s. A very slight chance of rain returns during the late afternoon and evening hours. Most of the cooler air is gone by Wednesday night, allowing low temperatures to fall only into the 60s.

(Space City Weather is sponsored this month by an anonymous donor.)

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See lightning over Houston in an incredible new way

Back in November, meteorologists were euphoric as a satellite named GOES-R (now GOES-16) was launched into orbit. “GOES” stands for “Geostationary Operational Environmental Satellite.” For meteorologists, it represents the future of weather observation from space. The satellite is now sending back imagery and data in an experimental, non-operational capacity. GOES-16 will be declared operational and unleashed for use in forecasting and observations later this year.

There’s a lot to love in GOES-16. We’ll have higher resolution imagery from a satellite than we’ve ever had in North America. In fact, at times, the satellite will be capable of sending back pictures every 30 seconds. By comparison, that’s more frequent than what a land-based Doppler radar typically scans. It will have 16 bands of imagery compared to five bands on the current GOES fleet. These satellites will give us more to look at, more to study, and more high quality data that will be ingested in weather models to help with short and medium range forecasts. It will also have the ability to see lightning.

Geostationary Lightning Mapper

The geostationary lightning mapper, or GLM, attached to GOES-16 can detect changes in a scene that indicate there’s lightning present. It’s the first such operational lightning mapper in orbit. The GLM will give us a capability to see and study lightning in a brand new way. It will also allow us to build up a historical database of lightning across much of the Western Hemisphere, including the tropics and deep oceans, where we lack a lot of good observational data. Best of all, it will allow for some really cool visualizations of storms.

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