Ask USA TODAY Weather

Doyle Rice, the USA TODAY weather editor, is available to answer your weather and climate questions. Send your questions to askweather@usatoday.com. If your question is selected, you can look for the answer below. Due to the volume of questions received, not all can be answered, either individually or on this page. ***********************************************

Q: Why is Texas so humid?— D.A. Marquez

A: Eastern Texas and the entire southeastern part of the USA receive plenty of moisture blown in from the nearby Gulf of Mexico, along with evaporated moisture from lakes, rivers, the ground and vegetation. With extensive heating from the spring, summer and fall sun high in the sky at these low latitudes, the evaporation potential is high, and the combination of warm temperatures and high-moisture content makes it really feel muggy.

However, while eastern Texas can be very humid and muggy, much of western Texas is quite dry, thanks to mountain air that dries out as it descends into Texas from Mexico and New Mexico. The dividing line is most pronounced in summer and is known as a dryline, which separates the different air masses.

Our state weather snapshot of Texas has more about the climate of the Lone Star State. — Doyle Rice

******************************************************************************************

Q: My 8-year old son asked me the other day, "At what temperature can you see your breath?" Can you help us? — John Robinette

A: That was a tough one, so I asked for help from an expert at the National Weather Service, meteorologist James Peronto. Here is his answer:

You can see your breath at a variety of temperatures (even as high as 70 degrees). It really depends on how much moisture is in the air (relative humidity). When you exhale, the air has water vapor in it. That exhaled water vapor is at a certain temperature (typically warmer than the surrounding air you are exhaling into). Warmer air can hold more water vapor.

Therefore, as your exhaled air hits the cooler surrounding air, the water vapor condenses into tiny water droplets (the same way clouds form). The higher the relative humidity of the surrounding air you exhale into, the better chance of creating water droplets from your breath.

So, getting back to your original question, the temperature at which you see your breath can really vary, depending upon the relative humidity of the surrounding air. I've seen my own breath at temperatures around 60 degrees in very moist air! Hope this helps. — James Peronto

******************************************************************************************

Q: Can it be too cold too snow?— James Hoffman

A: No, it can snow even at very cold temperatures if there's a source of moisture and some way to lift or cool the air. But when temperatures drop into the single digits or below zero, the air's capacity for water vapor becomes very small, making heavy snow unlikely. Most heavy snowfalls happen with relatively warm air temperatures near the ground — usually 15 degrees or above.

One example of a cold place that doesn't get much snow is Barrow, Alaska, where an average of only two inches of snow falls in January, when the average high temperature is only 2 degrees and the average low is -20 degrees.

A little snow even falls at the South Pole, where the temperature hardly ever rises above zero – the highest ever there is 7 degrees. But, the colder the air, the less moisture it has to make snow, which is why a year's worth of South Pole snow melts down to less than two or three inches of water.

The National Snow and Ice Data Center has an FAQ with much more info about snow. — Doyle Rice

******************************************************************************************

Q: I would like to know the direction of the sunset, when facing the horizon, over several weeks, in the months of September and October. Would the sunset move to the right or to the left on the horizon over 5-day intervals?— Sophia Deno

A: In the Northern Hemisphere, between the summer solstice (roughly June 21) and the winter solstice (roughly Dec. 21), the location at which the sun sets, relative to the observer, moves a fraction toward the left each day. The process reverses between the winter and summer solstices, as the sunset location would move a bit to the right each day.

This USA TODAY resource page and this FAQ page have much more about the seasons, solstices, equinoxes, and the length of days. — Doyle Rice

******************************************************************************************

Q: I often wonder why the sky, which starts-out brilliant clear on October mornings, usually clouds-up around 10 a.m.? I am a photographer and the clouds dull the colors of the foliage. What's even stranger is that folks just 40 miles to the south, at about 1,000 feet lower altitude, say the sky was sunny all day. Any explanation? This has happened quite a few times. Around 3 p.m., suddenly the clouds dissipate but it's too late for me by then. — Myron Shulman

A: First, when the air is unstable, despite a clear start, clouds tend to increase in the late-morning and into the afternoon.

According to the excellent weather textbook A World of Weather: Fundamentals of Meteorology, these clouds are known as fair-weather cumulus clouds, those shorter heaps of puffy clouds that often develop on a tranquil, sunny day as parcels of surface air are heated by the ground and buoyed upward. Sometimes enough clouds form to entirely cover the sky and rain will fall. The clouds will diminish later in the day as the sun's heating fades away.

This blog entry from Nick Borelli at FoxWeather details more about this.

The air's stability is important for our weather, as it helps determine what types of clouds form and what kinds of precipitation fall from them. This USA TODAY resource page explains much more about atmospheric stability and instability.

Second, as for why more clouds tend to form at higher elevations, it has to do with orographic lift, when air is forced up and over mountains. This air rises, cools, and forms clouds.

For more info, this Google page has lots of photos and diagrams about orographic lift. — Doyle Rice

******************************************************************************************

Q: Does anyone track the number of daily record high temperatures or daily low temperatures that are set across the USA? It would be interesting to see if there are more record high temperatures than record low temperatures, in view of global warming.— Jack Lucero Fleck

A: Yes. Meteorologist Guy Walton of the Weather Channel has kept track of this since 2000. Through last week, since Jan. 1, 2000, he says there have been 310,437 daily record highs and 152,064 daily record lows. The numbers for all-time record highs and lows are even more striking: Since Jan. 1, 2000, there have been 1,470 all-time record highs and only 40 all-time record lows.

Walton uses this U.S. records database from the National Climatic Data Center to track this.

Walton co-authored a 2009 study in the journal Geophysical Research Letters about this. According to the article abstract, the current ratio of daily record high maximum temperatures to record low minimum temperatures, when averaged across the USA, is about two to one.... Based on one of the global warming scenarios from the Intergovernmental Panel on Climate Change, this ratio is projected to continue to increase, with ratios of about 20 to 1 by mid-century, and roughly 50 to 1 by the end of the century. — Doyle Rice

******************************************************************************************

Q: With more carbon dioxide and other gases being released into the atmosphere as a result of climate change, is the standard atmospheric pressure at sea level increasing? And, are lighter gases being displaced into outer space because of the increase in CO2, etc.?—David Glass

A: I turned to an expert, Andrew Gettelman, a scientist at the National Center for Atmospheric Research, for help with these questions. He determined that the answer to both questions is no:

"The gases that are being released add only a little bit of mass to the atmosphere. Carbon dioxide comprises oxygen that's already in the air, as well as carbon stored in the ground. However, the amounts are small and many orders of magnitude less than the changes in mass and pressure caused by regular surface pressure variations due to storms.

The atmosphere, below 60 miles, has a mixture of major long-lived gases (such as oxygen, nitrogen, argon and carbon dioxide), which are not displaced by the gases that are being released by human activities. The gases mix with the other gases, the mixture we call 'air.'" — Andrew Gettelman

******************************************************************************************

Q: Which presses down on Earth's surface more, a high- or low-pressure area?— Allen Langley

A: High pressure presses down on the Earth more than low pressure, since air sinks in high pressure and rises in low pressure.

As the name says, an area of "high" pressure is an area where the air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count.

The pressure is high at the surface where air is slowly descending — much too slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is usually associated with nice weather.

The air that descends in high-pressure areas has to get to high altitudes in some way, and its done by rising in areas where the pressure at the surface is low. As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there's enough water or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.

Incidentally, I wrote an article last year about new research into how high and low pressure might be a trigger for earthquakes. — Doyle Rice

******************************************************************************************

Q: What kind of winter are we expecting in the Middle Atlantic states?— Sue Lasater

A: The Climate Prediction Center, the best source for long-term climate forecasts, shows that for the months of December 2010, January 2011 and February 2011, most of the southern tier of states should be warmer than average. The areas that are expected to be colder-than-average are the northern Plains, the Northwest, and much of Alaska. The rest of the nation — including the Mid-Atlantic — should see temperatures close to average. This map shows this.

As for rain and snow, most of the southern tier should be drier-than-average, while the western Great Lakes, Ohio Valley, and Northwest should be wetter than average. Most of the Mid-Atlantic should see a typical amount of rain and snow, with southern Virginia and the Carolinas seeing less precipitation than normal. Winter precipitation map.

These forecasts will continue to be refined and updated over the next several months, with their "official" winter forecast coming out in November.

This forecast is based partially on the fact that the USA has entered a La Nina climate pattern. During typical La Nina winters, significant cold-air outbreaks can be more frequent across the northern tier of the USA, while the southern states experience less storminess and precipitation. In the eastern USA, during a La Nina winter, there are generally fewer coastal storms and more Alberta Clippers than normal.

This USA TODAY graphic shows more about La Nina and El Nino.

AccuWeather also came out with its winter forecast earlier this month. — Doyle Rice

******************************************************************************************

Q: In a 365-day cycle, does every point on Earth have an equal number of daylight/night minutes?— G. Kiner

A: That's an excellent question, one that stumped me. For help, I contacted Travis Metcalfe, an astrophysicist at the National Center for Atmospheric Research in Boulder, Colo. Here is his answer:

"For questions like this, I find it helpful to think of the extreme cases (with fictional flat horizons). Someone on the equator gets 12 hours of daylight and 12 hours of darkness every day, 365 days a year (no seasons). Someone at one of the poles gets 24 hours of daylight for half the year, and 24 hours of darkness for the other half. This means that at the poles, there is only one sunrise and one sunset per year, as the sun just spirals upwards in the sky for a few months, and then spirals back down.

A slight subtlety is that because Earth is not on a perfectly circular orbit around the sun, the number of days between the spring and fall equinox (186.32 days in 2010) differs from the number of days between fall and spring equinox (178.84 days in 2009-10).

The Earth moves slightly faster when it is closer to the sun (with the peak speed around the New Year), so someone at the North Pole gets 2% less than six months of darkness, while someone at the South Pole gets 2% more than six months. At the mid-latitudes here in the USA, the asymmetry is slightly smaller.

So the final answer would be no, every point on Earth does not get an equal amount of darkness and light in a given year." — Travis Metcalfe

******************************************************************************************

View the original article here