Section 3

Chapter 25
Section 25.2 Surface Analysis

SURFACE ANALYSIS CHART
Every 3 hours
Shows past wx history
Station data comes from surface observations
Commonly referred to as the weather map

SURFACE ANALYSIS CHART
Shows pressure systems and fronts
Troughs and ridges
Drylines, outflow boundaries (gust front), sea-breeze fronts, convergence lines, squall lines
Isobar lines of equal pressure are plotted usually spaced at intervals of 4 millibars

SURFACE ANALYSIS CHART
Charts may be color or black and white
They may include station data or not
They are available from 5 different NWS offices

SURFACE ANALYSIS CHART
Weather Prediction Center (WPC) http://www.wpc.ncep.noaa.gov/

SURFACE ANALYSIS
Chart symbols

STATION DESIGNATORS
1. Sky cover
2. Temp
3. Wind (where it’s from)
4. Present wx
5. Type of predominant cloud
6. Sea level pressure
7. Pressure change (past 3 hours)
8. 6 hr precip

STATION PLOT FOR AVIATION

SURFACE ANALYSIS
Temp in F°
Weather

SURFACE ANALYSIS
Wind in 5kt increments
Direction in true
Shows which way the wind is coming from
If calm then only a circle

SURFACE ANALYSIS
Ceiling
Ceiling is plotted in hundreds of feet above ground level (AGL)
Visibility
Surface visibility is plotted in whole statute miles (sm)

SURFACE ANALYSIS
Pressure in millibars
Add a 9 or a 10 which ever will bring the number closer to 1000
For example:
410 = 1041.0
987 = 998.7

SURFACE ANALYSIS
Station pressure vs Sea Level pressure
All official barometers are calibrated to standard sea level conditions so they all read to the sea level reference datum
Station pressure is the actual pressure the station records
This number has to be corrected for elevation
So to provide a common reference for wx maps all station pressure is corrected to standard sea level conditions
That’s why there is a difference between SLP and Altimeter setting

SURFACE ANALYSIS
Pressure trend

SURFACE ANALYSIS
Sky cover

SURFACE ANALYSIS
Pressure systems L or H
Trough is denoted by dashed lines and TROF
Ridges are usually not depicted but may be a sawtooth line
A change in type of front is depicted by 2 hash marks

UNIFIED SURFACE ANALYSIS
WPC, OPC, TPC, NHC and WFO Honolulu puts out a unified SA
Issued 4 times daily
The AAWU puts out it’s own

SURFACE ANALYSIS

CHAPTER 25
Section 25.3 CONSTANT PRESSURE CHART

CONSTANT PRESSURE CHART
Covers a surface of equal pressure
Gives a 3 dimensional view of upper air wx
twice daily
Valid at 00 and 12Z
CONSTANT PRESSURE CHART
Being phased out however the SPC still puts out this version here
https://www.spc.noaa.gov/obswx/maps/

Chapter 25
25.3.2 Radiosonde Observation Analysis

Radiosonde observations
The skew T is one of the primary analysis tools used to decipher radiosonde data
The skew T can be used to
Determine the freezing level (or levels);
Determine the stability of the atmosphere;
Determine the potential for severe weather;
Determine the height and depth of inversions;
Infer cloud bases, tops, and cloud layers; and
Determine soaring conditions

The Skew-T
The Skew-T shows
Temperature
Drawn at a 45° angle (hence the name)
Pressure
These are horizontal in millibars
Dry Adiabats
Red curved lines starting at the bottom
Represent the rate unsaturated air cools
This is the dry adiabatic lapse rate
Moist Adiabats
Green curved lines starting at the bottom
Represent the rate saturated air cools
This is the moist adiabatic lapse rate
Mixing Ratio
Mass of water vapor compared to mass of dry air
Dew point
Wind speed/direction

Lifting Condensation Level
The height at which a parcel of air becomes saturated when lifted dry adiabatically.
To obtain the LCL, start with the surface temperature and follow up the dry adiabat until it cross the saturation mixing ratio of the surface dew point.
Convection Condensation Level
The height at which a parcel of air, if heated from below, will rise adiabatically until it is saturated (humidity equals 100%).
This is the level of the flat bases of cumulus clouds.
To obtain the CCL, start with the surface dew point and follow the saturation mixing ration line up until it crosses the temperature curve.

Convection Temperature
The surface temperature at which convective clouds will begin to form from heating of the ground.
To obtain, begin at the convective condensation level (CCL) and follow the dry adiabat down to the surface level and read the temperature at that point.
Level of Free Convection
The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely.
To obtain, begin at the Lifting Condensation Level (LCL) and follow the moist adiabat up to where it intersects the temperature line.
There are times when no LFC will occur as parcel will always remain cooler than the surrounding atmosphere and therefore will never rise freely.
When this occurs, the atmosphere is absolutely stable.

Skew t
Issued hourly
Horizontal axis is temperature in degrees Celsius, skewed to the right, labeled (minus) -20, 0, and 20 (Celsius).
Vertical axis is pressure levels in millibars, labeled 1,000 (near sea level) to 100 (approximately 53,000 ft MSL).
Bold solid red line represents the temperature profile over the station taken from the radiosonde observation (weather balloon).
Bold solid green line represents the dewpoint profile.
Wind aloft is shown on the far right side.

Skew t
https://www.spc.noaa.gov/exper/soundings/

Usefulness of the Skew-T

Usefulness of the Skew-T

Chapter 25
Section 25.4 Freezing Level Graphics

FREEZING LEVEL GRAPHICS
This chart shows current freezing level and forecast with the slider
Forecast comes out hourly
Forecast is good out to specified times in the future
Keep in mind inversions may create more than 1 freezing level

FREEZING LEVEL GRAPHICS
Where does the data come from?
4 different sources
Commercial aircraft
Profiler related:
Wind profilers (404 and boundary layer 915 MHz)
VAD (Velocity Azimuth Display) winds from WSR-88D radars
RASS (Radio Acoustic Sounding System)
Rawinsondes and special dropwinsondes
(a rawinsonde is a radiosonde that can measure wind)
Surface:
GFS total precipitable water estimates
GOES cloud-top data (pressure and temperature)
GOES total precipitable water estimates
SSM/I total precipitable water estimates
GOES high-density visible and infrared (IR) cloud drift winds
Experimental:
Radar reflectivity (3-d)
Lightning
Regional aircraft data with moisture (TAMDAR)

FREEZING LEVEL GRAPHICS
Colors are 100s of feet above sea level
White regions are below freezing
Hatched areas indicate surface below freezing and multiple freezing layers aloft
When the scale jumps by more than 1 color, the surface is above freezing with multiple freezing layers aloft

FREEZING LEVEL GRAPHICS
Case 1 represents the condition where temperature is below freezing at the surface and all levels above the surface (represented in the graphic above by white-colored pixels).

FREEZING LEVEL GRAPHICS
Case 2 represents the condition where the temperature goes above and below freezing two or more times vertically through the atmosphere while the surface temperature is less than 0°C.
These regions are hatched with white.
The underlying color represents the lowest height where the temperature crosses the 0°C line as shown by the blue arrow on the vertical temperature graphic.

FREEZING LEVEL GRAPHICS
Case 3 represents the condition where the temperature goes above and below freezing three or more times vertically through the atmosphere while the surface temperature is higher than 0°C.
These regions are located in areas where adjacent pixels change by more than one color when compared against the color scale.

FREEZING LEVEL GRAPHICS
Case 4 is relatively simple and represents the condition where the temperature at the surface is above freezing and the air generally cools with height crossing the 0°C line once.

FREEZING LEVEL GRAPHICS
Let’s take a look at a real one:
https://aviationweather-cprk.ncep.noaa.gov/icing/frzlvl

Chapter 25
25.5 Icing Analysis

CURRENT ICING PRODUCT (CIP)
Ready for this? 10 graphics provide a 3 dimensional picture of icing
Uses sensor and numerical data from:
WSR-88D
Satellite
PIREPS
Surface Wx reports
Lightning and computer models
This product should be used in conjunction with AIRMETs and SIGMETs

CIP
Included in the suite are:
Icing probability
Icing probability max
Icing severity
Icing severity max
Icing severity probability >25%
Icing severity probability >25% max
Icing severity probability >50%
Icing severity probability >50% max
Icing severity plus supercooled large droplets
Icing severity plus supercooled large droplets max

CIP
Generated every 2,000 feet from 1,000 to FL300
Max products cover all altitudes
PIREPs up to 75 minutes old are included
Probabilities range from 0% to 85% (nearly certain icing)
Probabilities indicate % chance (either 26% or 51%) to 100%
Let’s take a look at them

CIP
Max covers probability for all altitudes

CIP
Icing severity
Icing intensity categories are
Trace
Light
Moderate
Heavy
Note severe is used instead of heavy when giving a PIREP
Brown indicates terrain above the selected altitude

CIP
Icing severity max

CIP
Icing severity probability >25%
This means your probability for ice is greater than 25% up to 100%
Gray color is used to denote areas less than 25%

CIP
Icing severity probability >50%
This means your probability for ice is greater than 50% up to 100%

CIP
Icing severity probability >50% max

CIP
Icing severity plus supercooled large droplets
Supercooled water droplets are defined as being larger than 50 micrometers
A micrometer or micron is 1 millionth of a meter
The average human hair is 80 microns in diameter
Freezing drizzle is down in the small micron range

CIP
Icing severity plus supercooled large droplets max

CIP
Issuance
Updated hourly + 15
0,1,2,3,6,9,12 and 18 hours
Some aircraft are ice magnets others are not
Icing experienced by a Cessna will be different than for a 787
Pilot perceptions of icing differ as well
Assessing the size of supercooled water is difficult your results may vary
This product assesses the meteorological environment not the resultant icing that may occur
Results not typical, your mileage may vary
Does not include any exclusions, exemptions or innuendos
Not meant to address your level of macho or sexual orientation

FORECAST ICING POTENTIAL (FIP)
The FIP provides the same suite of products
The CIP is the latest analysis of icing data
The FIP is the forecast of anticipated icing
The FIP uses National Weather Prediction (NWP) data only whereas the CIP combines sensor data with the NWP
The CIP/FIP are shown as a conjunctive product on the AWC site

FIP
Displays the relative potential for icing

FIP
Good for determining the likelihood of icing

Chapter 25
Section 25.6 Graphical Turbulence Guidance (GTG)

GRAPHICAL TURBULENCE GUIDANCE (GTG)
4 dimensional diagnosis and forecast of CAT
Comes out hourly, however
0,1,2,3,6,9,12 and 18hr forecasts comes out every hour
Depicts the location and intensity potential of CAT and mountain wave turbulence
Issued every 1000 feet between 10,000 and FL450
Should be used in conjunction with AIRMET and SIGMET
Accuracy is dependent on the computer models
Turbulence PIREPS are overlayed
Only displays info for upper level fronts and the jet stream, no convective info

GTG
Just like the CIP has a max setting which covers all the altitudes

CTG
Here are 4 examples

Chapter 25
Section 25.7 Real-Time Mesoscale Analysis

Real-Time Mesoscale Analysis
Issued hourly
Shows surface temperature