Surface Analysis Maps

Transcription

1 Notes 1 Weather Maps The purpose of a weather map is to give a graphical or pictorial image of weather to a meteorologist. As a forecasting tool, weather maps allow a meteorologist to see what is happening in the atmosphere at virtually any location on earth. Complex three-dimensional models of weather systems can be made by collecting weather data at multiple levels in the atmosphere. Computers then compile that information to produce the pictures that weather scientists analyze. In the early days of meteorology, these pictures were all drawn by hand! The key to understanding a weather map is to understand the weather symbols that are used on the map. Weather map symbols were created as a method of reporting meteorological data in mass quantities to weather analyzing agencies such as the National Weather Service. The symbols are a form of shorthand used when writing out the information longhand becomes difficult. How do meteorologists use weather symbols? The types of symbols used depend on the type of map being constructed. The two most common weather maps are the surface analysis map and the station model map. The surface analysis map uses symbols to indicate locations of fronts, areas of high and low pressure, and lines, called isobars to connect areas of equal pressure. The surface analysis map can also use color-coded swatches to denote type and intensity of precipitation. Fig. 1 - A typical surface analysis map The station model map summarizes local weather conditions at sites maintained by the National Weather Service. The station model uses symbols and numbers to indicate amount and type of cloud cover, wind speed and direction, local barometric pressure, current temperature, dew point temperature, and information regarding recent changes in weather conditions. Typically, station models are used to construct surface analysis maps. Surface Analysis Maps The surface analysis map is the type of weather map with which we are most familiar. If you watch the weather report on a local news show, the surface map is the one from which the weatherman does most of his reporting. Fig. 2 - A typical station model report As mentioned earlier, the surface analysis map indicates the location of weather fronts (figure 3), areas of high and low air pressure, and connect lines of equal barometric pressure. The surface analysis map allows a person to determine the direction of wind, location and intensity of precipitation (figure 4), and can help to predict future weather conditions. Weather fronts are the boundaries between air masses with different characteristics such as temperature, humidity, and air pressure. An air mass is a large parcel of air with roughly the same temperature, humidity, or pressure throughout. Different air masses tend not to mix. The clashing of two or more air masses is what causes severe storms. The front is literally the front end of an air mass.

2 Movement of a front will depend largely on the conditions inside the air mass. Air masses tend to be either moist or dry in humidity content. Temperatures are either cold (polar) or warm (tropical). On the surface analysis map, areas of low barometric pressure are represented by a large, red L, while areas of high pressure are indicated by a large, blue H. The locations of high and low pressure will tell an observer something about wind direction air moves clockwise around a high pressure center and counter-clockwise around a low pressure center. In addition, the locations of pressure centers can indicate amount of clouds in an area. Low pressure tends to bring clouds to the region, while high pressure pushes clouds out of a region. Fig. 3 Types of weather fronts The presence of precipitation can be included on the surface map through the use of Doppler radar. Radar intensity is a way to "see" through rain. A pulse of energy is beamed through a cloud and the amount of echo returned will give the intensity of precipitation. The echo is actually a reflection of the energy and a computer will generate a color code to indicate the amount of precipitation. The DBZ on the Image is Decibels (a measure of intensity), when the radar echo reaches the precipitation, the larger the raindrops, or the more raindrops there are, the greater the DBZ value and the heavier the precipitation. Surface analysis maps can also include information about barometric pressure. On the map points of equal pressure are joined by a line, this line is called an Fig. 4 Table of Precipitation Intensity isobar. The isobars are generated from mean sea level pressure reports and the pressure values are given in millibars. Sea level pressure reports are available every hour, which means that maps of isobars are likewise available every hour. The solid blue contours are isobars and the numbers along particular contours indicate the pressure value of the isobar (figure 5).

3 Surface maps of isobars are useful for locating areas of high and low pressure, which correspond to the positions of surface cyclones and anticyclones. A map of isobars is also useful for locating strong pressure gradients which are identifiable by a tight packing of the isobars. Stronger winds are associated with larger gradients in pressure. Station Model Map Fig. 5 Surface map with isobars plotted Weather Stations often report weather in a station model. The central part of a station model is the cloud cover. The amount of cloudiness is indicated by a series of symbols and numbers. The cloud types are reported using universal symbols. When reporting rain to a local area, meteorologists need a universal way to indicate the conditions they observe. A meteorologist will use these to communicate the information nationally. Wind barbs are used to show wind intensity with a series of small pennants. The information for wind speed is always given in knots. All you have to do is add up the numbers on the wind barb and you get the wind speed. To get the wind direction, all you have to do is imagine a compass placed right over top the wind diagram or station model and it will point to the wind direction. Sea level pressure reports are available every hour, which means that maps of isobars are likewise available every hour. The solid blue contours are isobars and the numbers along particular contours indicate the pressure value of the isobar. Fig. 6 Explanation of a station-model map

4 Notes 2 Measuring Systems Measuring systems are used to quantify objects and phenomena in the physical world. This process of quantification is useful for describing, comparing, and assessing change in the world. Physical properties or characteristics that are measured include distance (length), mass, volume, weight, time, speed, acceleration. force, energy, and power. Measurement Systems and Units There are two measurement systems in use in science and industry: the English (Standard) system and the Metric system. The English system is based on standards that were agreed to and taken from the everyday experiences of people. The Metric system is also based upon agreed to standards, but these standards have specific origins and definitions. The table below summarizes the commonly used units in the English and Metric systems: Dimension English Units Metric Length (distance) inch, foot, yard, mile millimeter, meter, kilometer Mass ounces, pounds milligrams, grams, kilograms Volume (dry) cubic inches, cubic feet cubic centimeters, cubic meters Volume (fluid) pints, quarts, gallons milliliters, liters, kiloliters Force pounds newtons Converting between units and systems Converting between units in the English system can be difficult as it requires memorization of specific values and then multiplying and dividing by those values. Example: 2 pints = 1 quart, 4 quarts = 1 gallon. How many pints in 1.5 gallons? Converting between units in the Metric system is relatively easy as the system is based on base units (meter, gram, liter) and prefixes that describe fractions or multiples of the base unit Fractions and multiples are factors of ten, so dividing and multiplying is simple Example: the prefix centi = 1/100 th of a base unit, so there are 100-cm in 1-m. The prefix kilo = 1,000 base units, so 1-km = 1,000-m.

5 Converting between comparable units in the English and Metric systems is possible, but knowledge of specific equalities is required. Refer to Appendix A, page 471 in the textbook Example: 1 inch = 2.54 centimeters, so: 2.54 cm 2.0 inches = 2.0 in x = 5.08 cm in Meteorology and Measuring Systems The collection and comparing of data is an essential component the study of meteorology and in the practice of weather forecasting. Throughout the school year we will be collecting, collating, comparing, and archiving weather data. It is critical that all of us are familiar with the two measurement systems and how to convert data within and between those systems. Notes 3 Air Pressure and Barometers Pressure is a measure of the force against a surface. Pressure is usually expressed as a force per unit area. Pressure Units Unit Pounds per square inch (psi, PSI, lb/in 2, lb/sq in) Atmosphere (atm) Torr (torr) Bar (bar) Millibar (mb or mbar) Pascal (Pa) Kilopascal (kpa) Equivalent measurements, comments Commonly used in the U.S., but not elsewhere. Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds. Normal atmospheric pressure is defined as 1 atmosphere. 1 atm = psi = 760 torr. Based on the original Torricelli barometer design, one atmosphere of pressure will force the column of mercury (Hg) in a mercury barometer to a height of 760 millimeters. A pressure that causes the Hg column to rise 1 millimeter is called a torr (you may still see the term 1 mm Hg used; this has been replaced by the torr. "mm Hg" is commonly used for blood pressure measurements). 1 atm = 760 torr = 14.7 psi. The bar nearly identical to the atmosphere unit. One bar = torr = atm = 100,000 Pa. There are 1,000 millibar in one bar. This unit is used by meteorologists who find it easier to refer to atmospheric pressures without using decimals. One millibar = bar = torr = 100 Pa. 1 pascal = a force of 1 Newton per square meter (1 Newton = the force required to accelerate 1 kilogram one meter per second per second = 1 kg. m/s 2 ; this is actually quite logical for physicists and engineers, honest). 1 pascal = 10 dyne/cm 2 = 0.01 mbar. 1 atm = 101,325 Pascals = 760 mm Hg = 760 torr = 14.7 psi. The prefix "kilo" means "1,000", so one kilopascal = 1,000 Pa. Therefore, kpa = 1 atm = 760 torr and 100 kpa = 1 bar = 750 torr.

6 What Is a Barometer? A barometer is a widely used weather instrument that measures atmospheric pressure (also known as air pressure or barometric pressure) - the weight of the air in the atmosphere. There are two main types of barometers the most widely available and reliable Mercury Barometers, or the newer digital friendly Aneroid Barometer. How does a Barometer Work? The classic mercury barometer is typically a glass tube about 3 feet high with one end open and the other end sealed. The tube is filled with mercury. This glass tube sits upside down in a container, called the reservoir, which also contains mercury. The mercury level in the glass tube falls, creating a vacuum at the top. The first barometer of this type was devised by Evangelista Torricelli in The barometer works by balancing the weight of mercury in the glass tube against the atmospheric pressure just like a set of scales. If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises. If the weight of mercury is more than the atmospheric pressure, the mercury level falls. Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. In areas of low pressure, air is rising away from the surface of the earth more quickly than it can be replaced by air flowing in from surrounding areas. This reduces the weight of air above the reservoir so the mercury level drops to a lower level. In contrast, in areas of high pressure, air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas. There is more air above the reservoir, so the weight of air is higher and the mercury rises to a higher level to balance things out. Modern Technology and Barometric Pressure The majority of modern weather instruments such as weather stations can measure barometric pressure, but use a digital barometer that uses electrical charges to measure air pressure. This enables them to take multiple accurate recordings of the pressure and produce more accurate weather forecasts. Using Atmospheric Pressure to Forecast the Weather Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather because weather patterns are carried around in regions of high and low pressure. Weather maps use lines of equal pressure called isobars to indicate areas of equal pressure. A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will not last long, with heavy rain and strong winds. By carefully watching the pressure on a barometer, you can forecast local weather using these simple guidelines:

7 Decreasing barometric pressure indicates storms, rain and windy weather. Rising barometric pressure indicates good, dry, and colder weather. Slow, regular and moderate falls in pressure suggest a low pressure area is passing in a nearby region. Marked changes in the weather where you are located are unlikely. Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by brief spells of wind and showers. A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours. Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop. A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather. Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely. A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way. Notes 4 Temperature and Energy In our daily lives we think of temperature as a measure of the hotness or coldness of something; we say that a summer day is hot and a winter day is cold. To scientists, however, hot and cold are relative terms that depend on the perception of the observer, so temperature has a specific meaning for them, and is defined by the behavior of matter. The Kinetic Theory Scientists use the Kinetic Theory to explain the behavior of matter as it absorbs or releases energy. The Theory is based on three main ideas: 1. All matter is composed of tiny particles atoms or molecules. 2. These particles are in constant, random motion. As the particles in a substance or object absorb energy, the velocity of the particles increase, and as the particles lose energy their velocity decreases. 3. The particles are colliding with each other and the walls of their container. Based on the Kinetic Theory, scientists define temperature as the average kinetic energy of all the particles in a substance or object. Because the particles in a substance or object have (very little) mass and are in motion they have kinetic energy. When the substance or object absorbs heat (energy) the velocity of the particles increase, their kinetic energy increases, and consequently, the temperature of the substance or object increases. When the substance or object releases heat (energy) to the surroundings the velocity of the particles decrease, the average kinetic energy decreases, and consequently, the temperature of the substance or object decreases. The diagram below illustrates the relationship between energy and temperature.:

8 Energy (increasing) Meteorology Unit 1: Introduction gas liquid solid Temperature (increasing) Notice, that as energy increases temperature is also increasing. Temperature Scales When we discuss temperature we never talk in terms of joules of energy, rather we talk about temperature in degrees as measured on a temperature scale. The Celsius and Fahrenheit Scales The Celsius and Fahrenheit temperature scales are based on the freezing and boiling points of pure water as measured at sea-level. The freezing point is defined as 0 O on the Celsius scale and 32 O on the Fahrenheit scale, while the boiling point is marked at 100 O Celsius (100 O C) and 212 O Fahrenheit (212 O F). Celsius Fahrenheit 100 O Boiling point 212 O 100 O between freezing and boiling points 180 O between freezing and boiling points Freezing 0 O point 32 O 1 O C = 1.8 O F

9 Converting between Celsius and Fahrenheit scales is relatively easy using the following equations: o o F - 32 o o C = F= 1.8 C The Kelvin Scale The Kelvin scale is based on the concept of absolute zero. Absolute zero is defined as the temperature at which the random motion of all the particles in a substance or object ceases. Absolute zero = 0K = -273 O C To convert temperature expressed in Kelvin to degrees Celsius just add 273 to the Kelvin temperature. There is no direct conversion between Kelvin and Fahrenheit, so the conversion is a two-step process: Kelvin Celsius Fahrenheit

10 Notes 5 Station Map Symbols

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