Download Meteorology – Unit 1: Introduction Notes 1 – Weather Maps The

Meteorology – Unit 1: Introduction
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.
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.
Fig. 2 - A typical station model report
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.
Meteorology – Unit 1: Introduction
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
isobar. The isobars are generated from
Fig. 4 – Table of Precipitation Intensity
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).
Meteorology – Unit 1: Introduction
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
Meteorology – Unit 1: Introduction
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
Units
English
Metric
inch, foot, yard, mile
millimeter, meter, kilometer
ounces, pounds
milligrams, grams, kilograms
Volume (dry)
cubic inches, cubic feet
cubic centimeters, cubic meters
Volume (fluid)
pints, quarts, gallons
milliliters, liters, kiloliters
pounds
newtons
Length (distance)
Mass
Force
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‖ = 1/100th 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.
Meteorology – Unit 1: Introduction
 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:
2.0 inches = 2.0 in x
2.54 cm
= 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.
Meteorology – Unit 1: Introduction
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/in2,
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 = 14.6956
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 = 750.062 torr =
0.9869 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 = 0.001 bar = 0.750 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/s2; this is actually quite logical for physicists and engineers, honest). 1
pascal = 10 dyne/cm2 = 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,
101.325 kPa = 1 atm = 760 torr and 100 kPa = 1 bar = 750 torr.
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 1643.
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
Meteorology – Unit 1: Introduction
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:
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.
Meteorology – Unit 1: Introduction
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.
Energy (increasing)
gas
liquid
solid
Temperature (increasing)
Notice, that as energy increases temperature is also increasing.
Meteorology – Unit 1: Introduction
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 0O on the Celsius scale
and 32O on the Fahrenheit scale, while the boiling point is marked at 100O Celsius (100OC) and
212O Fahrenheit (212OF).
Celsius
100
Fahrenheit
Boiling
point
O
212O
100O between
freezing and
boiling points
180O between
freezing and
boiling points
Freezing
point
0O
32O
1OC = 1.8OF
Converting between Celsius and Fahrenheit scales is relatively easy using the following
equations:
o
o
C=
F - 32
1.8
o
F= 1.8 o C + 32
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 = -273OC
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
Meteorology – Unit 1: Introduction
Notes 5 – Station Map Symbols
Meteorology – Unit 1: Introduction