fredmet

FRONTS FOUR

In Fronts Four, a description of satellite imaging will be presented. This description is needed in order that an understanding of how the use of satellite images to locate fronts is possible. There are three types of satellite images. One uses infra-red, the second uses visible light, and the third uses a special infrared band that illustrates water vapor concentrations in the upper troposphere. All substances that are warmer than absolute zero radiate infrared. The warmer the substance is, the greater the amount of infrared energy is radiated per unit time. Therefore, the rate infrared energy a satellite receives from a substance is a measure of its temperature. Some infrared satellite images are in black and white plus shades of gray. White indicates the coldest areas and black indicates the warmest. The lighter shades of gray indicate lower temperatures than the darker shades. If these white areas are in bands, a jet stream indicated along with a possible front. These cloud tops are usually higher than 20,000ft or are cirrus clouds. Thunderstorms appear to be white blobs that are sometimes arranged in rows or bands. Tops of thunderstorms are often 30,000ft or higher. Since middle clouds are found between 6,000ft and 20,000ft, they are warmer and are usually in shades of gray. Low clouds are almost black since their tops are less than 6,000ft and they are usually the warmest clouds. The ground in warm areas is also almost black. At the higher latitudes during the winter, the ground or low clouds may be white because of very low temperatures. Some infrared images are colorized. There is usually a scale at the bottom or side of the image indicating the range of temperatures each color segment represents. The two advantages of the infrared images are that they are available day or night and an estimate of cloud heights can be obtained. Visible images provide the best resolution. The resolution can be down to a few feet. Visible images are in black and white, and show clouds as through you were in a satellite taking black and white photographs of clouds and transmitting the images to receivers on the ground. Unlike the infrared images, visible images can indicate whether or not clouds with high tops are thin or not. Visible images can indicate features on the ground through high thin cirrus that the infrared image suggest as thick. Bands of low warm clouds below cirrus clouds could be missed by infrared imaging. However, visible images are available only during day light. Water vapor is a substance which will emit only certain bands of infrared unique to water vapor like all gasses. By using a filter, only the infrared bands emitted by water vapor can be used for water vapor imaging. Since jet streams and their associated upper fronts induce upward and downward vertical motions in the atmosphere, the resulting zones or belts of greater and lesser concentrations of water vapor in the upper troposphere can be depicted by water vapor imaging. Since sinking air is associated with upper fronts and is found along the cold air side of jet streams, the air will be depicted as dry. Below and on the warm air side of the jet, rising air will bring moisture to the upper troposphere. Exactly how to use these images will be used in finding fronts and jets will be presented in front five by using examples. Fredmet

Rotation of Draining Water

    

   Which way would water spin as it drains out of a sink in the northern hemisphere?

    
   Some would argue that since a high pressure area is associated with downward motion in atmosphere and spins clockwise, water draining downward out of a sink would also spin clockwise in the northern hemisphere.  Others would ague that as the blobs of water moves towards the hole, they would be deflected to the right of their motion which is to the right of the hole causing the water to rotate counterclockwise.  The former belief is outright wrong while the later is only partly correct.  The correct answer is that the water could spin either clockwise or counterclockwise.  In fact, the water may not spin at all.  To explain, I will have to describe the Carioles acceleration.
    
   Assume that you are in a room that is rotating counterclockwise (same as the earth).  In the room, you are standing by one of the walls and friend is standing by the opposite wall.  You are holding a ball which you plan to pitch to your friend.  At the time you pitch the ball, the wall you are standing by happens to be on the west side of the building.  As the ball travels away from you, an observer standing on the ground will see the ball traveling in a straight line towards the east (the walls are transparent).  However, the wall you are standing by is becoming the southwest wall and the wall your friend is standing by is becoming the northeast wall due to the rotation.  Thus, you will observe the ball curving to your right and your friend will notice the ball curving to his left.  Your friend will have reach or run to his left to catch the ball.  Indeed, if the room is rotating fast enough, the ball will appear to travel in a clockwise circle back to you.  An observer outside of the room will see that you were carried counterclockwise by the rotating room fast enough to enable you to catch the ball you had thrown.   

  Now take water in a sink that is draining out through a hole.  The blobs of water that are moving towards the hole will be deflected to the right of their motion due to the rotation of the earth.  However, the pressure gradient force will push the blobs back towards the drain hole.  Thus, the blobs will tend to keep missing the hole to the right only to be pushed back to the left.  Thus, one would believe that this would cause the water to begin rotating counterclockwise around the hole.

   Problem: it takes 24hrs for the earth to complete one revolution.   Therefore, the few minutes that it takes for the sink to empty are not enough time for the earth to rotate more than a tiny fraction of a degree.  Thus, the deflection will be so small that is would be an insignificant factor in producing a rotation.  The rate of angular deflection is equal to 2 * W * sin La, where * means product or multiplied by, W is rotation rate of earth which is 360 degrees per 24hrs, and La is the degrees latitude.   Note that at latitude 30N it takes 6hrs for an eastward moving object to become a southward moving object if the Carioles acceleration were the only influence.  So what does determine the rotation and direction of revolution of water draining out of a sink through a hole?

   When water is in a sink, you most likely disturbed the water in some way.  You may have washed your dishes, hands, or whatever.  Thus, you may have induced an average slow rotation about the drain hole.  When you pull the plug, conservation of angular momentum will cause the rotation rate of the blobs of water to increase as they approach the drain hole.  To illustrate what I mean: fill your sink and stir the water clockwise.  Let the water set for a few minutes; then pull the plug.  Note that the water is rotating clockwise.  Repeat the experiment, except stir the water counter clockwise.  Note what happens.   However, one might wonder what would happen if the water took several hours to drain. 

  If large sink with a small hole took several hours to drain, then there would be enough time for the Carioles acceleration to take effect.  In that case, the water would spin counterclockwise about the hole.  Therefore, the answer to the question is that the draining water can spin either clockwise or counterclockwise depending on which way the water in the sink is already spinning before the plug is pulled.  However, if the sink takes long enough to drain, the rotation will eventually become counterclockwise.

Fredmet.

Pacific Moisture Myth

            

Abundant Moisture from the Pacific reaches east of the Rockies Myth.

     A widely held myth is that significant amounts of Pacific moisture reaches the areas east of the Rockies.  One source of this myth is from erroneous interpretation of infrared satellite images.  Often, when using these images, one sees dense bands of clouds extending from the Pacific across the Rockies to the Gulf States or the high plains.   It is tempting to interpret these bands as abundant moisture being carried from the Pacific into the central United States by the jet stream.  The problem is that these clouds are usually very high and therefore very cold.  Infrared satellite images make these clouds seem very thick.   Temperatures of the tops of these clouds are often lower than -35C.  Therefore, concentrations of water vapor and ice particles are low.  In fact, these clouds are often so thin that visible satellite images can depict features on the ground through the high clouds.  Thus,the IR images are sometimes misleading.   

     Another source of this myth is based of the fact that most weather systems move from west to east over North America.  If a weather system moves from the west, then some people conclude that its associated precipitation originates as water vapor from the Pacific.  This is largely true if one talks about precipitation in the Rockies and those regions west of the Rockies.   However, it should be noted that most of the water vapor is precipitated on the western slopes of the western mountain ranges.  This drop in precipitable water is especially noticeable when one compares precipitable water vapor along the west coast to the precipitable water vapor over the Great Basin  When the air is forced over the Rockies into Texas or the High Plains, the precipitable water from the Pacific drops further.   To observe precipitable water vapor distribution, the following address is available.  http://www.rap.ucar.edu/weather/.  To observe the precipitable water vapor field, click on Upper Air and on Forecast.   Here, you can see for yourself the low perceptible water values over the Great Basin and the Rockies as compared to that east of the Rockies or along the west coast.  Furthermore, there are times when some of the precipitable water vapor over the Great Basin and the Rockies came did not come from the Pacific.

     Most of the time during the summer, a large portion of the precipitable water vapor found over the Desert Southwest, the Great Basin, and the Rockies is from the Gulf of Mexico.   One can observe the moisture flux from the Gulf of Mexico by using the address given in the previous paragraph.  Click on Satellite then click on Visible.  Next, click on the Duration Loop.   During the summer afternoons, by using these satellite images, one can observe cumulus moving westward or northwestward up the Rio Grande, through the canyons between Monterrey, Mexico and Texas, and through a gap in the Sierra Madre Oriental to the west-northwest of Tampico, Mexico into the Chihuahuan desert of central and northern Mexico as well as into far west Texas.  One than can note the northwestward movement of cumulus into Arizona, New Mexico, southern Nevada, southeastern Utah, and Colorado.  One can go to the upper wind section and note the winds of the lower portions of the atmosphere support this cloud movement.  On most days during the summer, this is the pattern.  However, there are occasional surges of air from the tropical Pacific into the Sea of Cortes, western Mexico, and the Desert Southwest of the U. S.  These surges do contribute to precipitable moisture and are often associated with tropical cyclones that develop southwest of Mexico.  These cyclones can bring heavy rain to western Mexico and the Desert Southwest of the U. S. Yet, even in these cases, most or all of the precipitable moisture from these tropical cyclones fail to make to across the mountains of Mexico or the Rockies into the U. S. east of the Rockies

Fredmet

Myths in Precipitation Forecasts

Precipitation Probability Myths

   

    A common error made by TV weathermen comes while making precipitation probability statements.  For example; he may say that the probability for rain this afternoon is 30% or the probability is 30% for the period between noon and 6pm.  Then, in his graphics, he will indicate or say that the probability is 30% between noon and 3pm and 30% between 3pm and 6pm.  That is absurd. If the probability for precipitation is 30% from noon to 3pm and 30% from 3pm to 6pm, then the probability for precipitation from noon to 6pm is not 60%.

   In order to compute the probability of precipitation between noon and 6pm when the probability for precipitation between noon and 3pm is 30 percent and between 3pm and 6pm is 30 percent, one has to avoid counting the occurrence precipitation during the first three hours and the occurrence of precipitation during the second three hours twice.  The fact that precipitation may occur twice or more during the 6 hour period should not be counted more than once.  Therefore, the possibility that precipitation may occur during both 3 hour periods should not be counted twice.  How is this done? This is done by computing the probability that it will not rain during the 6 hours period and subtract that number from one.  In the example given above, the probability that there will be no precipitation during both of the three hour periods is 70% or .7.  The probability that there will be no precipitation during the 6 hour period is .7 X .7 = .49 or 49%.  Thus, the probability that precipitation may occur during the 6 hour period is 1-.49 = .51 or 51%.   The principal is the same even if the probabilities of the 3 hour periods are not equal.  For example, if during the first 3 hours the rain probability is 60% and the second 3 hours is 30%, then the probability it won’t rain is .4 X .7 = .28 =.28 or 28%.  The probability that it will rain is 72% not 90%.

   That is not the only error.  Some people believe that the precipitation probability is the chance that one will see precipitation falling when a door is opened or when one looks out of a window.   If one looks out of the window enough times and counts the number of times during an interval of time that precipitation is observed.  The number is divided by the total number of times that one looks out of the window gives approximately the fraction of time precipitation falls.  For example, if during a three hour interval of time, precipitation is observed 1/3^rd of the total number of observations, it is likely that precipitation fell 1/3^rd of the time.  This is not the probability precipitation may fall.  Instead this is the percentage of time that precipitation falls.   Another example is a narrow, solid, and fast moving band of precipitation may bring a high probability that precipitation may fall during an interval of time, but it may last only a tiny fraction of the same interval. 

    The probabilities for precipitation issued by the National Weather Service are point probabilities.  Point probabilities are probabilities that precipitation may occur during an interval of time at some airport.  It is not the percentage of time precipitation may occur; nor is it the percentage of some area precipitation may occur.

Fredmet.

The Green Clouds Hail Myth

   It is commonly believed that if clouds or precipitation falling out of the clouds are greenish, then hail is falling.   How could that be?   Ice is not green.  Indeed, if one looked into crevices in glaciers, he would note that the color is blue rather than green.  However, the greenish color is sometimes observed in heavy clouds.  So where does the green color come from? 

    The explanation is the same as the reason the sky is blue, sunrises or sunsets are red, and glacier ice is blue.   The white light from the sun contains all colors.   When the white light from the sun enters the earths atmosphere, air molecules, dust particles, etc, scatters some of the light.  It turns out that blue light is scattered the most, green light is scattered less, and red light is scattered the least.   The fact that the earth’s atmosphere scatters blue light the most causes the sky to be blue.  When the sun is near the horizon, the light will have a long enough trajectory through the earth’s atmosphere so that most of the blue and green light gets scattered out.   Thus, it is mostly the red light that comes directly from the sun giving the sun its red color at sunrise or sunset.  This is only the first part of the reason green clouds and precipitation sometimes exist.

     When white light travels through water, red light is more quickly absorbed than green light and green light is more quickly absorbed than blue.   Thus, blue light penetrates farther into water than both the green and red light.   Therefore, clouds producing heavy precipitation are usually bluish black.   So where do green clouds come from?  During the late afternoon when the occurrence of heavy to severe thunderstorms are most likely, sunlight will have a long enough trajectory through the earths atmosphere so that most of the blue light has been scattered out leaving mainly green and especially the red.  When this sunlight enters a cumulonimbus cloud with a large amount of liquid water and ice, most of the red light is absorbed leaving mainly the green.  Thus, the greenish clouds and precipitation are indicators of large concentrations of liquid water and/or ice in the atmosphere.   A heavy rain can produce green clouds in the absence of hail.  I have seen such clouds in south Texas with no hail.  However, hail storms often have strong updrafts which suspend large concentration of liquid water and ice.  Therefore, green clouds often have hail, but not always.   If a hail storm or heavy rain occurs when the sun is almost over head, greenish clouds will not be observed.

Fredmat

A Myth on wind speeds associated with tropical cyclones.

                                                      

A Tropical Cyclone Wind Speed Myth

     There is a myth commonly stated by TV weather men and women.  This myth is even stated by some who have a PhD in meteorology.  The myth is that you can add tropical cyclone wind speed to the motion of the storm to obtain maximum sustained winds.  It is usually true that looking in the direction of storm movement; the strongest wind is mainly on the right hand side of the cyclone.  However, some argue that; for example, if a tropical cyclone is moving NW at 16mph and has a wind speed of 100mph, the maximum sustain wind would be from the southeast at 116mph on the northeast side of the storm.   By the same logic, maximum winds on of the southwest side of the storm would be northwest at 84mph.  This is false logic. 

     This belief is based on the wheel analogy.  This is a false analogy.  The wheel is a solid body.  For example, the speed of a point on a spinning wheel is given by Wr.  W is the angular velocity in radians per unit of time, and r is the distance the point is from the axis of rotation.  If this spinning wheel is moved along the ground, then to compute the maximum speed reached by a point on the spinning wheel relative to the ground, you can add the speed of the point relative to the axis of rotation to the motion of the wheel.  However, a tropical cyclone is not a solid body.

     Parcels of air move from outside of the tropical cyclone into the cyclone within the lowest 1,500ft then back out in higher levels.   This happens within a few hours to about 1/2 day.  As a parcel (blob) of air moves into a tropical cyclone, it is subjected to a pressure gradient force which causes the air to accelerate.  The acceleration is caused only by the forces acting on the parcel.  This is Newton’s second law of motion.  The motion of the system causing the force is not relevant.  Therefore, the side of the storm with the strongest pressure gradient will have the strongest wind.

     A tropical cyclone in the tropical Atlantic or the Caribbean will often have a high pressure ridge to its north.  Therefore, the north side of the storm will have the strongest winds.  A storm in the Gulf of Mexico or along the east coast of the U.S. usually will have a high pressure center or ridge to its east or southeast.  In this case, the storm will have its strongest wind on its east or southeast side.   There are rare storms that will have the strongest winds on their left side.  In most of these cases, a cold front penetrates into the core of these storms.  A high on the cold air side of the front can cause the strongest pressure gradient to develop on the left side of the cyclone.  In this case, the strongest winds will be on the left side of the cyclone.  There are a few other rare exceptions.  Otherwise, most tropical cyclones in the northern hemisphere will have their strongest winds on their right side.   But these winds are not the result of the storm motion being added to the storm winds.

Fredmet    

      

   

    

FRONTS THREE

 

    In Fronts 1 and 2, methods of locating and defining fronts by use of temperature fields and jet stream positions were discussed.  In this discussion locating and defining fronts by use of sea level isobar and isallobar pattern will be discussed.  Before proceeding, some definitions must be made.  If you understand the reduction of station pressure to sea level pressure skip the next paragraph.

   An isobar is a line of constant pressure.  When analyzing an isobaric pattern on a sea level weather map, sea level reduced pressures are used.  This is done because of altitude differences between Stations such as Denver, Colorado will always have much lower pressures than Houston, Texas or New York, New York.  The actual pressure at Denver or any other station is called the station pressure.  At Denver the station pressure is usually near 840MB while at Houston, it averages near 1016MB.  To reduce station pressure to sea level pressure, one adds the previous 24hr high temperature to the 24hr low temperature and divides by two.  Then using the station pressure and the average between the high and low temperatures, a factitious atmosphere with an average temperature is assumed to exist below ground level.  The actual hydro-static equation used is complicated.  However, using graphs or special slide rules based on the hydrostatic equation, a sea level pressure is found.  Errors made in sea level reduction can be large for stations higher than 1 mile above sea level.  However, for stations 1 mile or less, the method usually works fine. 

  Isobar analysis using the reduced sea level pressures reveal the existence of low pressure areas, high pressure areas, low pressure troughs, and high pressure ridges.  A low pressure area is an area with lower pressures than the surrounding areas.  A high pressure area is an area with higher pressures than the surrounding areas.  A low pressure trough is a belt or zone where pressures are lower than on either side.  A high pressure ridge is a belt or zone where pressures are higher than on either side.  Ridges never contain fronts.  Most fronts are found in troughs, but not all troughs contain fronts.  Most low pressure areas will have fronts extending from them and fronts are found in low pressure troughs that extend from the lows.   

  Isallobars are lines of constant pressure changes.  Most analysis use three hour pressure changes.  Rising pressures often indicate cold frontal passage.  Falling pressures are associated with warm fronts.   The isallobar pattern is useful in locating fronts in mountainous regions.  Zones separating rapidly falling pressures from areas of more slowly falling pressures are likely to be warm frontal zones.   Likewise, zones separating areas with rapidly rising pressure from areas with more slowly rising pressures or falling pressures are likely to be cold frontal zones.

   To determine if a trough contains a front, methods described in FRONTS ONE and FRONTS TWO are used.  The locations of troughs as well as isallobar patterns are used to refine the actual frontal positions.   In FRONTS FOUR, the use of satellite images to locate fronts will be described.

Fredmet

      

FRONTS TWO

    In fronts one, there was a discussion on how to locate fronts using isotherm patters, especially the temperature patterns on constant pressure charts.  There was also a description of what fronts are not.  In this discussion, locating fronts by the use of jet stream positions and vertical wind shears will be described.  Of course, density patterns are implicit in these methods.

     Upper level jet streams can be used to locate fronts.  Pressures will decrease more rapidly with height in cold air than in warm air because at a given pressure, cold air is denser that warm air.  If you take a column of air with a pressure P at sea level, to reach pressure 1/2 P, you must ascend to a certain height.  In a colder column of air with the same pressure P at sea level, the height required to reach 1/2 P will be less.  After all, a column of air will shrink when chilled.  Therefore, the upper level constant pressure surfaces will be lower in cold air than in warm air. 

View this photo  This photo is a vertical cross section.   P0, P1, P2, etc. are constant pressure surfaces where P0 > P1 > P2, etc.  Cold air is to the left.

    

     Implicit in the above paragraph is that upper highs will be located above warm air and upper lows will be located above cold air.  (See figure 1 by clicking on View this photo).  It follows that the strongest horizontal pressure gradients in the upper levels are found between regions with warm air and regions with cold air.   Stronger pressure gradients mean stronger winds.  Therefore, the jet stream extends along areas with the greatest horizontal temperature contrasts.   Looking downwind, to the left of the jet will be cold air and to the right will be warm air.  The surface front will be to the right of the jet.  To get the idea, look at a 700MB chart and a 500MB chart.  Note where the air is the cold and where the air is warm.  Next, look at a 250MB chart.  Note that the jet at 250MB neatly divides the cold air from the warm air indicated at 700MB and 500MB.  What should be noted is that the upper level jets will not intersect a front.  This is a common mistake in map analysis.  However, there is an exception.  The subtropical jet which is often found over the Gulf of Mexico or the Gulf states during the winter can intersect a cold front penetrating into the subtropics or tropics.   In this case, the cold air layer has become so shallow, about 3,000ft or less, that it has little influence on the heights of the upper level pressure surfaces.   

  Because pressures decrease more rapidly with height in cold air than in warm air, the slopes of the constant pressure surfaces will chance with height between cold and warm air.  This change of the slopes of the pressure surfaces with height will cause winds to change speed and direction with height.   That is, a horizontal temperature gradient will cause a vertical wind shear.  Vertical wind shears can be used to identify fronts.  However, vertical wind shear in the lowest 2,000ft or so may be due more to friction with land or sea than due to temperature contrasts. 

Fredmet

FRONTS ONE

    

   Almost every one who owns a TV set believes he knows what a weather front is.  Or do they really know?  Almost every one who has watched a TV weatherman has formed an opinion what a weather front is.  There are some who believe that a cold air surge out of Canada is a cold front. They would be wrong.  The front is the leading edge of the cold air surge.  The rest of the cold air surge is a mass of cold air on the move.

     Some will tell you that a weather front is a boundary between two dissimilar air masses.  That definition is not bad.  Indeed! All fronts are boundaries between two dissimilar air masses.  However, there are boundaries between two dissimilar air masses that are not weather fronts.  An example is commonly observed in the Mississippi valley during the spring when hot dry air from the desert southwest over runs warm moist air from the Gulf of Mexico. The two air masses are often separated by a horizontal inversion layer.  An inversion layer is a layer where the temperatures increase with heights.  Clearly this type of an inversion layer is an air mass boundary; but because the layer just described is horizontal, it is not a front.   

     The last sentence gives a clue to what a front is.   A weather front is a zone in the atmosphere where the horizontal temperature gradient is the greatest.  On level ground, finding fronts by observing surface temperature patterns is usually easy.  However, in mountainous regions, surface temperature patterns are often governed more by altitude differences rather than air mass contrasts.  Therefore, a better definition is needed.

     The best definition of a front is that a front is a strongly baroclinic zone.  A baroclinic zone?  What is that?  A baroclinic zone is a zone where constant density surfaces are not parallel to constant pressure surfaces.  I will try to illustrate what a baroclinic zone is and how to locate the zones.

     At this point, I will refer you to the NCAR site:  http://www.rap.ucar.edu/weather/  Click on upper air then on the 850mb on the second row.  If you know what a constant pressure chart is, skip the next paragraph.

     A constant pressure chart must be defined.   A constant pressure chart is an analysis where pressures every where are the same.  It is an analysis that includes the heights of the pressure surface, temperatures, humidity, and winds.   A millibar (mb) is equals 1,000dynes.  A dyne is the force that causes one gram mass to accelerate 1cm/sec/sec if there are no other forces acting on the mass.  Gravity applies approximately 980dynes force on one gram mass.  Constant pressure charts commonly used are the 1,000mb, the 850mb, the 700mb, the 500mb, the 300mb, the 250mb, and the 200mb maps.    The heights of a constant pressure surface depend mainly on temperatures.   Except for near the ground, the colder the air is, the lower the pressure surfaces are.  This is because a column of air will shrink when chilled.  (Click on View the photo below)

View this photo    This figure is a vertical cross section.  P0, P1, P2, etc are pressure surfaces where P0> P1> P2, ETC.  Cold air is to the left.

      

On the 850mb chart, above freezing isotherms are in solid orange or red.   Freezing or below isotherms are in solid or dash blue.  Note those areas where the isotherms are close together forming isotherm ribbons.  Those isotherm ribbons are frontal zones or baroclinic zones.  Where the isotherms are farther apart are areas that are less baroclinic and probably no front exists.  The front at the ground or at sea level will be along or near the warm air side of the 850mb isotherm ribbon.   Note that the 850mb chart is no good for locating fronts in the Rockies.  This is because the 850mb constant pressure surface averages about 1,500m above mean sea level or just under a mile while the Rockies average more than 1mile high.  To find fronts in the Rockies, the 700mb and/or 500mb charts can be used.  The 700mb surface averages about 3,000M or 10,000ft above mean sea level and the 500mb surface averages about 5,670M or near 18,000ft above mean sea level.  This completes Fronts One.  The next part will discuss ways to define and find fronts by ways other than using isotherm patterns.

Fredmet