WFO SHV - ArkLaTex Meteorological Inquiries (Past Questions page two)

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ArkLaTex Meteorological Inquiries

COULD YOU EXPLAIN WHY RAINFALL OCCASIONALLY COMES FROM AN APPARENTLY CLEAR SKY?

Wind from the surface to the base of the cloud carries rain drops to the surface. Thus, if a shower/thunderstorm is nearby and if the winds above the surface is just right; the rain will be deflected with the wind to perhaps into an area where no clouds are present straight up (90 degrees). However, there will be a cloud somewhere in the sky.

This phenomenon is more pronounced in the summertime when "pop-up thunderstorms" occur with clear to mostly clear skies around the isolated thunderstorms.

Bill Murrell
Meteorologist

LIVING NEAR TOLEDO BEND ON THE NORTH SIDE, I SEE STORMS DIVIDE AND GO NORTH AND SOUTH OF OUR LOCATION. THERE ARE HIGH HILLS (CONSIDERING THE LOCALE} TO THE NORTH AND SOUTH OF OUR LOCATION AND I AM WONDERING IF THIS MAY CAUSE STORMS TO SPLIT?

THERE HAS BEEN STRANGE LIGHTED OBJECTS AT NIGHT LOOKING AS SMALL REDDISH DOTS THAT MOVE AROUND AS IF ENTERING OR LEAVING THE ATMOSPHERE. WHAT COULD THIS BE?

Although it may seem to us sometimes that thunderstorms avoid or dance around a certain area, this is not the case. Now our geography does play a part in the amount of weather we receive, but not according to individual storms.

We recognize severe weather when the following phenomena occur: hail of 3/4 inch (dime size) or larger is falling, wind gusts are 58 mph or greater, or a tornado is on the ground.

Now most severe weather events can be very isolated and localized, a storm may drop large hail in Zwolle, LA, but Many, LA may not receive anything. A squall line's winds may blow trees down in Benton, but not one tree will be uprooted in Plain Dealing. A tornado can pass one block from your house, but your house may not be damaged.

However, being close to a lake such as Toledo Bend does cause a few small scale features to be observed. One of these features is on a warm and humid day, clouds will first form over the land, while the sky over the lake will be cloudless. The reason for this is that the land warms up much faster than water does. Perhaps, this differential in heating could cause some small scale boundaries (similar to the sea breeze, but much weaker) to form, which in turn could create enough lift for storms to develop near Toledo Bend.

In regards to the small reddish dots, one of the meteorologist here just read an article in SCIENTIFIC AMERICAN PRESENTS that dealt with a new kind of lightning in the article "Big Sky, Hot Night, Red Sprites" in volume 11 number 1 issuance. The author explains about a new lightning called "sprite" which has a salmon red fading to purple or blue in lower part of the phenomenon. The "sprites" appear above thunderstorms at heights up to 55 miles, and are in shapes and sizes of blobs, columns and plumes. They only last between 10 and 100 milliseconds and is visible to the naked eye.

Bill Murrell
Meteorologist

WHAT IS THE DOMINATE AIR MASS OF TEXAS AND WHAT KIND OF WEATHER IS MOST COMMON?

Air masses are classified according to moisture content and source regions. Moisture content is categorized as either dry or moist. While source regions or categorized as polar which is cold air, tropical which is hot air. These air masses moderate as they leave their source region, thus Texas will usually have a modified air mass.

Cold air (Polar) and hot air (Tropical) can be experienced in the state of Texas depending on the time of the year. Now due to access to moisture from the Gulf of Mexico, the eastern part of the state normally is under the influence of a moist air mass. While the western part of the state is usually under the influence of a dry air mass, however sometimes moist air masses sometime work their way into west Texas via moisture from the Pacific.

Texans receive a variety of weather and one type would be hard to pin down as most common. However Texas is known for its severe weather, especially severe weather associated with the dry line.

Bill Parker
Public Outreach Meteorologist

HOW IS THE JET STREAM LOCATION DETERMINED ON A DAILY BASIS AND PREDICTED LONG RANGE?

The jet stream is determined by sending up weather balloons twice a day to get data from the atmosphere. In the continental US, the National Weather Service has around 70 upper-air sites (about one every 200 miles) including one here in Shreveport. Weather Balloons are launched not only at the same time across the entire nation, but in fact are launched worldwide at the same time. Balloon times are at 00 and 12 Greenwich Mean Time or Universal Coordinated Time. Currently in the central time zone, that translates into 7 am and 7 PM In standard time, it is one hour earlier.

These weather balloons are equipped with an instrument called a radiosonde. This instrument sends back temperature, relative humidity and pressure. By tracking the instrument's location, wind direction and speed can be determined throughout the atmosphere until the weather balloon bursts. This occurs around 100,000 feet. The typical jet stream will range somewhere between 25,000 and 40,000 feet.

From the upper-air observations made twice a day across the country, meteorologists can locate where the jet streams are, and interpolate between upper-air sites. Satellite imagery is another tool that aids in this process of finding where a jet stream is located. There are three main satellite images: Visible, Infrared and Water Vapor. Using the upper-air data in combination with the satellite imagery, meteorologists can locate the jet stream quite accurately. Looking ahead a couple of days to two weeks, models using quite complex formulas give meteorologists an idea the intensity and location of jet streams evolving through time.

Bill Murrell
Meteorologist

WHAT IS VIRTUAL TEMPERATURE?

The formal definition of Virtual Temperature is the temperature of dry air having the same density and pressure as the moist air (Glossary of Meteorology). So virtual temperature is the temperature that the moist air would have if it was completely dry with the same density and pressure as the moist air. Virtual temperature will always be warmer than the actual temperature. Thus the more moist the air is, the warmer the virtual temperature will be at a given dry bulb temperature (actual temperature) and pressure.

Virtual temperature is used daily on upper-air soundings.

Bill Murrell
Meteorologist

WHY DOES ROUGH WEATHER SEEM TO HIT EAST TEXAS AFTER NIGHTFALL, BUT USUALLY HITS THE DALLAS AREA IN THE AFTERNOON?

Severe weather can occur at anytime of the day. However, severe weather is more likely to occur during the afternoon and evening hours due to daytime heating. Since weather systems normally head from west to east across the mid-latitudes (including Texas), the weather in Dallas will usually spread eastward into east Texas, but the question remains why might the weather hit Dallas in the afternoon and east Texas during the night.

A boundary called a dry line is usually present across west Texas. The dry line is a boundary separating a warm, moist air mass and a warm, dry air mass. Daytime heating combined with this boundary will lead to thunderstorm development occasionally, especially during the spring and early summer when upper level dynamics are most favorable. This line of thunderstorms will generally form near the dry line - west of the Dallas/Fort Worth Metroplex - during the early to mid afternoon hours. The line of thunderstorms will generally track eastward and affect the Dallas area during the mid to late afternoon, and then affect east Texas during the evening hours if they hold together. This might explain why it seems that east Texas experiences severe weather after nightfall, but the Dallas area in the afternoon.

Other boundaries such as cold and warm fronts produce a significant amount of showers and thunderstorms for the region, but move through the region at various times of the day. Cold fronts usually will affect the Dallas area before east Texas, but move through the region at various times of the day. Thus, the precipitation will usually affect the Dallas area before east Texas, but will once again occur at various times of the day.

However, the best chance of showers and thunderstorms is during the afternoon and early evening as daytime heating aides the development of showers and thunderstorms with everything else in the atmosphere being equal.

Bill Murrell
Meteorologist

DOES LIGHTNING COME FROM THE GROUND UP OR FROM THE SKY DOWN?

Lightning actually goes both ways. First, the charge in a thunderstorm becomes separated with the negative charge at the base of the thunderstorm and the positive across the top (including the anvil). This negative charge at the bottom of the thunderstorm induces a positive charge at the ground. Once the charge becomes strong enough, electrons start to flow to the ground in about 50 meter intervals called stepped leaders. This continues until about 50 or so meters from the surface when the positive electrons flow up from the surface to meet the downward negative electrons. This positive charge flowing up is called the return stroke. The meeting of the negative and positive charge creates the illumination. The flashes of light is caused by subsequent negative electrons flowing down in the same stepped leaders, and the positive charge from the ground flowing up it.

Cloud to ground lightning that occurs in a thunderstorm's anvil (the overhang of a thunderstorm) will have positive charges flowing down the stepped leaders. The negative charge at the ground will flow up to meet the ionized step leaders as the electrons come close to the ground. The National Severe Storms Laboratory has a good web site about lightning
(http://www.nssl.noaa.gov/edu/ltg/ ).

Bill Murrell
Meteorologist

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