Category: Ask Dr. Bob

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I understood that last year was a typical El Nino winter and someone said following that we would have a dry summer and fall.

Dr. Bob, I have followed your forecasts and comments for years and appreciate your good work. My question: I understood that last year was a typical El Nino winter and someone said following that we would have a dry summer and fall. That has been very correct and I am wondering if we follow the “normal” patterns, when would the rains return? Not an official forecast but just what is the normal coming off the El Nino? I live between W’house and Troup and we are probably 7 or 8 inches below Tyler which is not so good itself. Thank you for your time.

ANSWER:
This won’t be encouraging; the SST (sea-surface temperatures) and low ocean heat content (down to 300M) are quite certainly in a La Nina phase in the East Central Equatorial Pacific; those values are forecast to continue at present levels for several months, yet. I’d think it will probably be some time well into 2011 before we start getting significant rainfall again.
RKP

What’s the possibility of snow this year (2010), and how soon?

ANSWER

This year does not look promising for snow. Last Winter was a textbook example of an El Nino event, which usually results in cool and wet weather for us during the cool season. The current Winter is the exact opposite–La Nina–in other words cool waters off the North American Pacific Coast. This usually results in above normal cool season temperatures and below normal precipitation.

Dr. Bob you mentioned the other day you agreed with the Farmers Almanac that we are going to have above normal temperatures this winter.

Dr. Bob you mentioned the other day you agreed with the Farmers Almanac that we are going to have above normal temperatures this winter. What now is considered normal winter temperatures for us? Does that mean we are going to have spring weather from November to May?

ANSWER

No, ma’am, it does not mean Spring temperatures. What it means is
a) a small number of very cold outbreaks, and
b) little, if any, frozen precipitation.
The November average high is about 66 and average low about 46, December 58 and 38, January 57 and 37., February 60 and 40, and March 67 and 47.
RKP

Have there been any predictions made yet for the winter of 2010?

ANSWER

Yes, Sir, there have. This will be a La Nina Winter–in other words, waters off North and Central America’s Pacific Coast will be cool. From Central Texas eastward, this usually translates to a mild and dry cool season. Last year was a textbook of an El Nino Winter, with warm waters off the west coast of the Americas. This translated to a cold and wet cool season.
A cool season with above normal temperatures and below normal precipitation is what the long-range forecasts are predicting.
The next one comes out on August 19; if you’d care to write back after that time, I’ll check it.
However, sea-surface temperatures and sub-surface heat content out to the 170th merridian were still below normal last week, and I doubt there will be any change.
RKP

Why do high pressure ridges form so easily during the summer?

I believe high pressure systems provide sinking air, so I understand why clouds do not form but I do not understand why the air becomes so hot compared to the winter when high pressure provide cold weather. Thanks as always.

ANSWER:

Sir,
Would that it were so simple.
High pressure means subsiding air. These air masses have four possible origins: Tropical Continental (to our south or west over land areas; they are usually dry and hot); Tropical Maritime (they originate over ocean waters, are usually moist and warm); Polar Continental (they originate over land in extreme northern latitudes, and are cold and dry); and Polar Maritime (they originate over the North Pacific, are cool, and usually not moist–since the moisture was lost in crossing the Pacific coastal mountains).
Another factor in determining the ground temperature is the altitude at which the pressure falls to 500 millibars. If above about 18,000 feet, then we see above normal temperatures–our summertime heat machine. During the cool season, this altitude is quite a bit lower.
So, whether we are hot or not under high pressure depends on a) the air mass’s origin, and b) the altitude at which the air pressure drops to 500 mbs–about 15 inches of mercury. To make the latter value a bit more confusing, the 500-mb. level is reported in decameters; earlier last month, the 500-mb. level was 594 dm–about 19,600 feet.
Subsiding air warms as it moves downward through the atmosphere, explaining why the higher that level is–the warmer we are.
Another factor explaining how hot we get is the state of vegetation and soil moisture. A protracted dry period during the warm seasons dries soils and vegetation. This causes higher temperatures; some weeks in late-May and early-June were 5-7 degrees warmer than normal. Moist soils and green vegetation acts to lower temperatures, since moist air cannot hold as much heat as can dry air. One of the fringe benefits of our 11.55 inches of rainfall in June is that it is unlikely that we will see excessive heat this Summer.
The Spring and Summer of 1998 were extremely dry–there was very little rainfall after March 13. In 1998, we had over 40 days with the temperature at or above 100, including about 19 consecutive.
Temperatures did not begin to cool until late in September. The temperature exceeded 90 on 22 days in September 1998, and 100 on 2 days.
The Summer of 2000 was similar–though not quite as hot.
RKP

After reading most weather stations RAW/METAR there is a set of numbers at the end of their remarks. This set of numbers read like this. T02610244. Can you help me interpret these.

ANSWER
It is the Celsius temperature and dew point written in such a way as to accurately convert it to Fahrenheit; T02610233 is 79 temperature and 74 deg. dew point. If the first digit of either group of four numbers is a ‘1’, then it indicates below zero Celsius. In other words, T10331050 would be a Fahrenheit temperature of 26 and dew point of 23.
RKP

I don’t totally understand relative humidity, is it 100% when its raining?

ANSWER:

Sir,
I wish relative humidity was that simple. The actual definition is this: relative humidity is the percentage of moisture vapor in the air compared with the maximum amount of moisture vapor which the air could hold at that temperature. In other words, if the air temperature is 94 deg. (which is it as I am writing this), and the relative humidity was 50 percent, then the air contains one-half of the amount of moisture vapor which it could hold at the 94-degree temperature.
Relative humidity can be as low as 40 or 50 percent with rain. This happens because at the altitude where the raindrops form, the relative humidity is 100 percent–actually a little more than that because the moisture vapor condenses into droplets; they collide, form raindrops, which eventually get so heavy that they fall to Earth. If those drops fall through a dry layer of air, then much–but not all–of the water in the drop evaporates. The drops which reach the ground are small–and we call it light rain. But remember, the lower atmosphere is dry–otherwise the drops wouldn’t evaporate and shrink. So, the humidity on the ground can be under 50 percent.
This is most likely to occur during the cool season at the beginning of rain events when the ground temperature is cool.

By the way, thanks for the compliment. I still enjoy (after nearly 48 years) doing the weather.
RKP

Dr. Bob, It seems to me over the past several years that our seasons seem to be “shifting” on the calendar.

Dr. Bob, It seems to me over the past several years that our seasons seem to be “shifting” on the calendar. By this I mean it seems that we are entering our seasons later in the year and exiting the season later. The length of the season seems the be the same. I have no research to support this. It just seems that the rainy/dry or cold/wet times of the year are shifting slightly on our calendar. Is there any validity to this?

ANSWER
After thinking about this, I would agree to the point that, when our seasons have been extreme, then there has been something of a forward shift. In other words, extremely warm Summers tend to commence earlier–as occurred this year, in 2005, 2000, and 1998.

Some say tropical storms will be enhanced by the El Nino we’ve had this 09-10 season. Your take on this?

El Nino just doesn’t want to call it quits just yet. System in the gulf is just one of many we’ve had these past 6 months. Some say tropical storms will be enhanced by the El Nino we\’ve had this season. Your take on this?

ANSWER

I think there are factors more significant than El Nino which affect the Atlantic Basin tropical season: the most important of which is the North Atlantic Oscillation–which rotates warm water around the basin in about 40-year intervals. The oscillation is currently warming along the North American east coast; about five years ago, the peak warmth and ocean heat content was in the Central North Atlantic, and we had an El Nino event the previous cool season–and we got Katrina and Rita.

Last year was an unusually mild tropical season; I’d expect something of an uptick this year–but I wouldn’t expect anything along the lines of 2005.
RKP

Could you please tell me what is the heaviest one day rainfall total ever recorded for Tyler.

ANSWER: There are two days which nearly tied: 9.07 inches on September 13, 1913–during a major hurricane, and 9.06 inches on October 19, 1985. With that one, we had an upper air low to our northeast and a strong low-level jet which continually fed moisture into the low for about six hours. There was a drowning fatality in that one on East Erwin Street when a woman was drowned in her car.

When the % chance of precipitation is given, does it mean the % coverage of precipitation or the actual CHANCE of precipitation?

ANSWER

“Probability for measurable precipitation” means the mathematical likelihood that 0.01 inch of precipitation or greater will fall at any given location within the forecasting group–usually three or four counties and amounting to about 3,000 square miles. I do not like the term, but I’ve been persuaded to use it for pops of 50 percent or less. 60 and 70 percent are termed “likely,” 80 and 90 percent are “widespread”, and near 100 percent is “general”.
RKP

Since we have received snow today on 3/21/10 is this the lastest snow fall that has been recorded for the East Texas area?

ANSWER
It is not; the latest trace snowfall was in April 2008; the latest measurable snowfall was March 29, 1937.
RKP

What effect will El Nino have on our usual spring storms. Should we look for more intense systems?

ANSWER: The determining factor is how long it persists; we’re expecting this one to persist into May or June. That would translate, in my judgement, to an active Spring severe weather period–beginning Next Monday. The January 20 event was an indirect result of ENSO.

RKP

Does a El Nino weather pattern in winter foretell what summer weather will be?

ANSWER: No, Sir, it does not very well. The best predictor is when the warm water in the East Central Equatorial Pacific and the high heat content of the top 300 meters of water gives way to a more normal or cooler value.

Though we were able to predict accurately last October that we’d have frozen precipitation at some time this Winter–we’ve had four such events, I’m not ready yet to make a warm season prediction. El Nino looks to hang in through late-Spring. It showed a moderation in January, than a re-intensification this month.

The 90-day forecasting models are showing normal temperatures and normal precipitation; that actually means equal chances of above, normal, or below–in other words there are no clear signals yet, either way.

I think we can say this: I would be very surprised to see significantly above normal temperatures before mid-June.
RKP

How does the snow fall enter into the precipitation amount? Is it one inch of snow equals one inch of rain ?

ANSWER:

Sir,
The precipitation event of February 11/12 brought 1.00 inch exactly of liquid yield. About a third of that fell as a rain/sleet/snow mix through 6 p.m. of the 11th; the remainder fell as snow, and we reported a 6.0 inch snowfall. The usual snow-to-rain formula is 0.1 inch of snow equals 1.0 inch of liquid. However, this was a very wet snow–why so many power lines failed because of the weight of the snow. So, the 1/10 conversion was not accurate–it was more like 1/8 or 1/9.

In the precipitation event of February 23, it was all snow. We measured 0.17 inch of precipitation; though only a trace of snow accumulated in Tyler, we reported that as 1.7 inch snowfall.

To capture the precipitation, we dismantle the rain gauge–leaving a tube two feet high and 8 inches in diameter. The melting is pretty low-tech: I put it in the sink and fill the sink with hot water–that melts the snow. We then pour the water into the measuring tube–and voila.
RKP

What does the term dew point mean?

ANSWER
The dew point is the temperature, to which if you lowered the air temperature, the water vapor in the air would condense and form dew.

In other words, an air temperature of 45 and a dew point temperature of 36 means that, if you lowered the air temperature to 36, then the moisture in the air would condense and form dew.
RKP

I know you’ve probably had a hundred emails about the winter storm but please indulge me for a few questions.

I know you’ve probably had a hundred emails about the winter storm but please indulge me for a few questions.
Posted: Monday, 15th February 2010 3:27AM
I know you’ve probably had a hundred emails about the winter storm but please indulge me for a few questions.

1. What was the highest total snow amount observed in East Texas? Like most, we had right at 6 inches.

2. How does this event compare record wise for the East Texas area?

3. It began as snow Wednesday morning, then changed to mix during the day Wednesday, then changed back to snow Wednesday at dark. If it would have been cold enough to snow continuously, how much total snow could we have gotten?

4. This was a rare event. How perfectly did the elements align for this to occur? Please be as technical as possible. If I\’m unsure of the vocabulary, I\’ll look it up.

5. Finally, have we been a warm winter pattern since the 1980s? I\’m 31 years old and most family and friends who can remember tell me this was much more common pre 1980s.

ANSWER
a) Total snowfall could have been about 9 or 10 inches had the entire precipitation been snow;
b) the highest snow totals I could find were in a band from northwestern Smith County into Titus and Bowie Counties–between 6.0 and 8.0 inches.
c) the reason that the entire precipitation event was snow was because of the temperatures between ground level and 10,000 feet, and the soil temperatures; both were above freezing at one time or another during the precipitation event;
d) This is our heaviest snow since January 13, 1982. And, yes, there were significant snow events between about 1951 and 1978–which with the exception of 1982 and 1985 have not been repeated. Climate is cyclical, and we were in a cooling trend between about 1961 and 1979; since that time, there has been significant warming through 2008.
I hope this answers your questions.
RKP

When was the last time that we had a winter with an El Nino in control of the weather.

ANSWER
There was a weak El Nino in 2007-2008, and a strong one in 2003-2004.
RKP

What is the determination for how large a tropical cyclone is?

ANSWER
It is the wind speed, wave heights, and lowest central pressure.
RKP

Why do many in East Texas use the term bitter cold?

ANSWER
The best answer I can give you is that we don’t have all that much extreme cold down here; when it happens, folks tend to mob the grocery stores and prepare for an apocalyptic event. The term “bitterly cold” is one which NWS used in its forecasting language until a few years ago.
RKP

What is the difference between “mostly” and “partly” cloudy?

What is the difference between “mostly” and “partly” in terms of cloudiness or sunniness? And what actually causes wind?

ANSWER
a) the distinction in “mostly” vs. “partly” cloudy is the percentage of sky cover. “Mostly cloudy” is 50% or greater sky cover; “partly cloudy” is 12.5% to 50%; “sunny” is under 12.5%; “fair” is cloudiness above 12,000 feet, and under 12.5%.

b) Wind results from air moving from an area of higher atmospheric pressure into an area of lower atmospheric pressure.

The tighter the “pressure gradient”, the stronger is the wind velocity. The “pressure gradient” is lines of constant barometric pressure drawn on a map. If the lines are close together, than the velocity is higher than if those lines are farther apart. Vertical air mixing can produce wind also. This means that winds are blowing at a considerable velocity a few thousand feet above ground level, and that air is brought down to the earth’s surface by heating. I hope this was clear.

RKP

What is the difference between base/composite reflectivity?

ANSWER
In answer to your question, base reflectivity is the return in one-half degree slices in the vertical plane; composite reflectivity is an interpolation of the highest base returns in the entire 90-degree vertical plane. Thus, composite reflectivity usually appears more impressive than does base reflectivity, because with base you are getting only one narrow slice of the return, while composite puts the entire echo return together. I hope this helps

RKP

I observed a large gray circle around the sun, outlined by a rainbow.

ANSWER
What I think you saw was the combination of two tricks of optics:

a) moisture vapor in the air can refract light to create a rainbow effect, and

b) the lenses in your sunglasses acted as a prism to project this rainbow on the retinas of your eyes. There really was not a rainbow; but the moisture vapor in the air and the sunglass lenses made your brain think there was one. Thanks for your question.

RKP

Why do you see your breath in cold weather?

ANSWER
The air which emerges from your mouth when you breathe has an approximate temperature of 98.6 deg. F. It is also fairly moist. When that air comes into contact with cold air outside the body, the moisture condenses on tiny particles which are always present in the air, and some of which emerge when you breathe. It is these particles, with a coating of condensed moisture, which you see. Without moisture vapor, it could not rain. There is a rainfall cycle which includes: a) moisture vapor cooling to the point where the vapor condenses on those tiny particles, and form a cloud, and b) sufficient up and down motion as well as sidewise motion of the droplets in the cloud to the point where they bang together, form drops, and these drops are too heavy to remain airborne. They fall to earth as rain. I hope this helps.

RKP

What are the relationships are between: Air pressure and cloud cover…?

ANSWER
a) – Generally speaking, when air pressure is high, skies are clear to fair; when pressure is low, then there will be variable degrees of cloudiness. Today {March 16, 2003,} the pressure was low–10043 mbs. at 6 p.m.; we had multiple cloud layers.

b) – Air flows out of high pressure and into low pressure. Because of the friction exerted by the earth’s surface, and something called the Coreolus Effect, when you stand with your back to the wind, the area of low pressure will be on your left hand, and the center of high pressure on your right hand. This is called the “Law of Storms”.

Wind velocity is determined by the rate of rise or fall of the pressure; a rise of 3.0 mbs. in three hours would mean winds of 15-20 mph; a pressure rise {or fall} of 1 mb. in three hours would mean a wind of 10 mph or less. Other factors can affect velocity, but the rate of pressure rise/fall is the strongest.

c) – The dew point is the temperature where the moisture vapor in the atmosphere would condense if the air temperature were reduced to that temperature. Relative humidity is the percentage of water vapor which the atmosphere actually contains compared with the maximum amount of water vapor it could contain at that temperature and pressure. A temperature of 56F and a dew point of 56F would yield a relative humidity of 100 percent.
Hope this helps!

RKP

Explain evaporation causing lack of rain reaching ground level?

ANSWER
Sir, Here is what can happen. Assume the atmosphere is moist above 5,000 feet, and dry below. If the air mass is saturated above 5,000 feet, and there is vertical air motion, then precipitation can form above that altitude. If the atmosphere is dry below that altitude, then the precipitation will evaporate when it goes below 5,000 feet. This is not uncommon, and such precipitation is called “virga”, and cools temperatures at the surface.

RKP

Expain how “lows” are calculated?

ANSWER
Low temperatures are those between midnight GMT and noon GMT, and high
temperatures are between noon GMT and midnight GMT. If you are in the
Eastern time zone–as I think you may be, the low temperature is that
between 8 p.m. EDT (7 p.m. EST) and 8 a.m. EDT (7 a.m. EST.) The high
temperatures are for the following twelve hours. This is a bit confusing,
as they are prepared from a product called the Coded Cities Forecast which
the National Weather Service releases twice daily. For a time zone other
than Eastern, add or subtract the appropriate numbers.
RKP

Does air pressure effect the temperature?

ANSWER
The relationship between air temperature and air pressure is indirect; that between air temperature and relative humidity is more direct.

As an example of the temperature/pressure relationship: take today. Until mid-afternoon, the pressure was falling because of a crossing low pressure center through the Red River Valley. This caused a strong southwest wind. Winds from that direction flow down slope before reaching here, and this causes warming. The pressure has been rising rapidly since about 3 p.m., and the temperature has been falling. Arctic high pressure is building into the area, with a cold air flow. However, during the warm season, an Atlantic/Gulf high pressure ridge controls our weather, and we are warm. So, the relationship is indirect.

The temperature/humidity relationship is stronger. High humidities retard nighttime cooling and daytime warming. Other things being equal, the temperature will not drop as far at night nor rise as far during the daytime with higher humidities as would be the case with lower humidities.

RKP

Who do you credit with having the most influence in your radio career?

ANSWER
In answer to your question, I would say two individuals and one group.
a) News readers on the British Broadcasting Corporation: they were objective, absolutely trustworthy, and regardless of how bad things were–they were calm and unemotional. In reporting severe weather events, I tried to emulate them.
b) Kern Tipps: he was a Southwest Conference football broadcaster in the 1940s and 1950s; I took from him that you can be on the air, and did not have to mask your Texas accent. c) Edwin Smith: he was on the air in Tyler between 1942 and about 1976. I came very near to giving it up in 1973; he asked me to remain on the air two weeks longer; I still am. Your use of the word “career” brought me up short. Since I’ve been on for forty years–I guess it is a “career” instead of just something I do every day. I hope this helps.

RKP

How are snow levels are reported?

ANSWER
Snow levels are reported as the elevation above mean sea-level. An example: in the Seattle area, when snow levels are reported at 500 feet, this is 500 feet above sea-level. This is somewhat above the elevation of the city, which is under 100 feet. I hope this helps.

RKP

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