# Atmospheric Pressure and Wx Charts 5&6

CHAPTER 5
Atmospheric Pressure and Altimetry

ATMOSPHERIC PRESSURE

• Evangelista Toricelli who was a student of Galileo invented the barometer.
• The Italian discovered in 1643 a way to measure the atmospheric air pressure. He placed a tube fully filled with mercury upside down in an open mercury container. The mercury level in the tube balanced around a height of 30 inches.

ATMOSPHERIC PRESSURE

• There are 2 common types of Barometers:
• Mercurial

ATMOSPHERIC PRESSURE

• Aneroid Wafer type.

ATMOSPHERIC PRESSURE

• There is a third type
• The water barometer

ATMOSPHERIC PRESSURE

• Atmospheric pressure defined as the force per unit area
• –For example 14.7 pounds per square inch
• –Or force per square centimeter (millibar)
• –Or the force of 1 newton per square meter (pascal)
• 1 hectopascal is = to 1 millibar

STANDARD ATMOSPHERE

• 1013.2 Millibars, or Hectopascals
• 29.92 inches of mercury
• 14.7 psi
• 59° F
• 15° C

ATMOSPHERIC PRESSURE

• Station pressure is simply the pressure measured at a particular airport
• –Higher elevation airports have lower pressure than low elevation airports
• Pressure drops at an average of 1 inch/ 1000’ as we go up in the atmosphere

PRESSURE & ALTITUDE

• As altitude goes up pressure goes down, Thus the air pressure on Mt Denali is much lower than that at MWH
• So there’s got to be a method to make station pressure relevant to other station pressures taken at different elevations
• First we must understand how temperature effects pressure

TEMPERATURE

• Causes an airmass to expand or contract
• –Warm temps cause expansion
• –Cold temps cause contraction
• Therefor a given pressure line will be higher when warmer
• The pressure line will be lower when colder
• So when it’s colder than standard at your altitude, you are closer to the terrain than your altimeter indicates
• CFIT

TEMPERATURE

• As illustrated in the diagram, the atmosphere shrinks due to colder temps, our true altitude becomes less
• –This can become hazardous in mountainous terrain

TEMPERATURE

• We can account for non-standard temps by using our E6b
• We will need:
• –OAT
• –Pressure altitude
• –Indicated altitude
• PA = 10,000
• Temp = -40
• True Alt. = 8689
• So because the temperature at 10,000 is far below standard, you are 1,311 feet closer to the ground than your altimeter says!

PRESSURE

• The same idea applies to non-standard pressure
• However the altimeter has the Kollesman window for us to input the correct pressure
• It does not however have a mechanism for compensating for non-standard temps

PRESSURE

• If station pressure were used on pressure charts what we’d see would be a contour map of the terrain
• –Lower pressures at higher altitudes and vise versa
• So there are two methods for leveling the pressures across elevations
• Reduction to SLP involves using an empirical approach to essentially predict what the temp would be if you dug a hole all the way to sea level and measured the pressure
• Altimeter Setting uses a different math formula to guess what the pressure would be at that station if it were located at sea level

Altitude Correction

• At the surface pressure changes 1”/1000 or 10mb/100m
• If station elevation is 300m then 30mb need to be added to get sea-level pressure
• Once this correction is applied isobars can be drawn

PRESSURE

• METAR KMWH 142352Z 36013KT 10SM OVC100 M08/M14 A3010 RMK AO2 SLP217 T10781139
• Station Pressure is the actual pressure reading at the station
• Altimeter Setting is the value of atmospheric pressure used to adjust the altimeter so that it indicates the height of the aircraft above a known reference surface
• Sea Level Pressure (SLP) is empirically determined from station pressure accounting for temp and elevation
• –1021.7 millbars would be the pressure if MWH was at 0’ elevation
• If we convert 30.10 we get 1019.30 millibars at station elevation of 1189
• –However SLP is reported at 1021.7
• To convert inches to millibars multiply by 33.8637526
• To convert millibars to inches multiply by .0295301

Density

• Density ρ(rho) is the degree of compactness of a substance, in this case air molecules
• The book gives us Boyle’s law
• –M is molar mass (found by adding the atomic mass of the elements that make up air)
• –P is pressure
• –R is the universal gas constant 8.3144621(75)JoulesKelvin-1 mol-1
• –T is temperature in Kelvin
• But we can look at it a simpler way
• –P=T x ρ x constant
• –Reworking the equation for ρ we get
• If the temp is constant and pressure increases the density will increase
• If the pressure is constant and temp increases density decreases
• So if we work a problem:
• –288K is 279+15°C
• –1.226 kg/m3 sea level density
• –2.87 is the constant for air

Density

• Density is also affected by humidity
• Dry air has a molecular weight of about 29 g/mol
• Water vapor has a molecular weight of about 18 g/mol
• Adding water vapor then decreases density in a given air volume
• A 500’ error in density altitude is possible for temps above 90 and RH above 80%

ALTIMETRY

• Indicated altitude
• –read off a correctly set altimeter
• –Can be found by using your eyeballs
• Pressure altitude
• –altitude of the 29.92” line or read off altimeter when set to 29.92
• –Can be found using your calculator
• Density altitude
• –pressure altitude corrected for nonstandard temp.
• –Can be found using your E6B
• –Rule Of Thumb: 120’ for each degree above or below standard
• Absolute altitude
• –the height above the surface (AGL)
• –Can be found by dropping your mother in law out the door and timing the drop
• True altitude
• –actual altitude above sea level
• –Can be found using your E6B
• –Could be inaccurate if the temp doesn’t follow a standard lapse rate

ALTIMETRY

• High Density altitude refers to height not density
• –Reduced power
• –Reduced thrust
• –Reduced lift
• Causal factors behind high density altitude
• –High temp
• –High altitude
• –High humidity
• –Low pressure
• Use the same indicated airspeeds but TAS is higher ground speed is higher in no wind conditions
• Effects of high density altitude
• –Longer takeoff roll
• –Longer landing roll
• –Lower climb rate
• –Lower service ceiling

PRESSURE or TEMPERATURE CHANGES in FLIGHT

• When flying from High to Low “Look out below”
• When flying from Low to High “High in the sky”
• Above 18,000 feet the altimeter is set to 29.92 and only pressure altitudes are flown

LOW PRESSURE

• Cyclonic
• Rotates counterclockwise
• Area of rising air
• Usually clouds present
• No Kid’n Serious Low Pressure

HIGH PRESSURE

• Anticyclonic
• Rotates clockwise
• Area of descending air
• Usually no clouds
• Good weather

Chapter 6

Weather Charts

Weather Charts

• A weather chart is a map on which data and analyses are presented that describe the state of the atmosphere over a large area at a given moment in time.
• Historically the following charts have become more or less the standard
• Surface Charts
• Constant Pressure of the upper atmosphere
• Because weather systems are three-dimensional (3-D), both surface and upper air charts are needed.
• Surface weather charts depict weather on a constant-altitude (usually sea level) surface, while upper air charts depict weather on constant-pressure surfaces.

Weather Observation Sources

• Land surface (e.g., automated surface observing system (ASOS), Automated Weather Observing System (AWOS), and mesonet)
• Marine surface (e.g., ship, buoy, Coastal-Marine Automated Network (C-MAN), and tide gauge)
• Sounding (e.g., radiosonde, dropsonde, pibal, profiler, and Doppler weather radar Velocity Azimuth Display (VAD) wind profile)
• Aircraft (e.g., Aircraft Reports (AIREP), Pilot Weather Reports (PIREP), Aircraft Meteorological Data Relay (AMDAR), and Aircraft Communications Addressing and Reporting System (ACARS))
• Satellite. Figure 6-1. Weather Observation Sources

Analysis

• Analysis is the drawing and interpretation of the patterns of various elements on a weather chart.
• Computers have been able to analyze weather charts for many years and are commonly used in the process.
• However, computers cannot interpret what they analyze. Thus, many meteorologists still perform a subjective analysis of weather charts when needed.

Analysis Procedure

• The analysis procedure is similar to drawing in a dot-to-dot coloring book
• Isopleths are drawn between dots representing various elements of the atmosphere.
• An isopleth is a broad term for any line on a weather map connecting points with equal values of a particular atmospheric variable.

Analysis Procedure (Cont.)

• Step 1: Determine the Optimal Contour Interval and Values to be Analyzed
• The first step in the weather chart analysis procedure is to identify the maxima and minima data values and their ranges to determine the optimal contour interval and values to be analyzed

Analysis Procedure (Cont.)

• Step 2: Draw the Isopleths and Extrema.
• The second step is to draw the isopleths and extrema (maxima and minima) using the beginning contour value and contour interval chosen in the first step.
• It is usually easiest to begin drawing an isopleth either at the edge of the data domain (edge of the chart), or at a data point that matches the isopleth value being drawn.
• Interpolation must often be used to draw isopleths between data points and determine the extrema.

Drawing Isopleths and Extrema

• When drawing isopleths and extrema on a weather chart, certain rules must be followed:
• The analysis must remain within the data domain.
• Analysis must never be drawn beyond the edge of the chart where there are no data points.
• That would be guessing.
• Isopleths must not contain waves and kinks between two data points.
• This would indicate a feature too small to be supported by the data.
• Isopleths should be smooth and drawn generally parallel to each other.

Drawing Isopleths and Extrema

• When an isopleth is complete, all data values must be higher than the isopleth’s value on one side of the line and lower on the other.
• A closed loop isopleth must contain an embedded extremum (maximum or minimum). •
• When a maximum (minimum) is identified, data values must decrease (increase) in all directions away from it.
• Isopleths can never overlap, intersect, or cross over extrema.
• It is impossible for one location to have more than one data value simultaneously.

Drawing Isopleths and Extrema

• Each isopleth must be labeled.
• A label must be drawn wherever an isopleth exits the data domain. For closed loop isopleths, a break in the loop must be created where a label can be drawn.
• For very long and/or complex isopleths, breaks should be created where additional labels can be drawn, as necessary.
• Extrema must be labeled.
• Extrema are often denoted by an “x” embedded within a circle.
• Beneath the label, the analyzed value of the field must be written and underlined.
• Isopleths and labels should not be drawn over the data point values.
• If necessary, breaks in the isopleths should be created so that the data point values can still be read.

Drawing Isopleths and Extrema

Analysis Procedure

• Step 3: Identify Significant Weather Features
• The third (and final) step is to interpret significant weather features.

Surface Chart

• A surface chart (also called surface map or sea level pressure chart) is an analyzed chart of surface weather observations

Constant Pressure Chart

• A constant pressure chart (also called an isobaric chart) is a weather map representing conditions on a surface of equal atmospheric pressure.
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