CHAPTER 4

Chapter 4

4.1 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

• 5 different offices chart the SA
• Weather Prediction Center (WPC) http://www.wpc.ncep.noaa.gov/
• Ocean Prediction Center (OPC) www.ocean.weather.gov
• National Hurricane Center (NHC) www.nhc.noaa.gov
• Alaskan Aviation Weather Unit (AAWU) https://www.weather.gov/aawu/
• Weather Forecast Office in Honolulu (WFO HNL) https://www.weather.gov/hfo/
• Each one offers links to different maps

SURFACE ANALYSIS

• Chart symbols

STATION DESIGNATORS

1. Sky cover
2. Temp
3. Wind
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

SURACE ANALYSIS

• ADDS Links
• http://www.aviationweather.gov/briefing

Version off the ADDS site

Chapter 4

4.2 Ceiling and Visibility Analysis

Ceiling and Visibility Analysis

• This product has been terminated since it was based on the older GOES Satellites
• http://www.aviationweather.gov/hemst
• We will cover this in chapter 6

žChapter 4

ž4.2.2 Weather Depiction Chart

ž

žWEATHER DEPICTION CHART

• žBeing phased out in favor of the CVA
• žBeginning at 0100z
• ževery 3 hours
• žinformation for this chart taken from METAR
• žcomputer generated
• ž] to the right of the station circle means automated

žWEATHER DEPICTION CHART

• žShows:
• ž1. Total sky cover
• ž2. Cloud height or ceiling
• ž3. Wx and obstructions to vision
• ž4. Visibility

žWEATHER DEPICTION CHART

• žHas been discontinuedž

CHAPTER 4

4.3.1.1 CONSTANT PRESSURE ANALYSIS CHART

CONSTANT PRESSURE ANALYSIS CHART

• Being phased out
• Covers a surface of equal pressure
• Gives a 3 dimensional view of upper air wx
• Twice daily
• Valid at 12Z and 00Z

Chapter 4

4.3.2 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