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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

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

Version off the ADDS site

Chapter 4

4.2 Ceiling and Visibility Analysis

Ceiling and Visibility Analysis

  • This can be found on the ADDS site under OBSERVATIONS
  • http://www.aviationweather.gov/cva
  • Issued every 5 minutes
  • Most useful for VFR pilots trying to avoid IFR conditions
  • Shows current ceilings and visibilities from the METARs

Ceiling and Visibility Analysis

  • Between stations the CVA uses estimated ceiling values
  • If the GOES satellite data indicates no clouds, clear conditions are depicted
  • If the ceiling is less than 200’ the CVA indicates “Possible Terrain Obstruction”
  • Pale yellow = ceiling less than 1000’ vis less than 3 miles (IFR)
  • Orange = ceilings less than 200’ (LIFR) Possible Terrain Obstruction

Ceiling and Visibility Analysis

  • Since the chart is updated every 5 min SPECI METAR reports will show up
  • However this is not a forecast
  • Should be used conjunctively with the TAF and AIRMET
  • Between the stations there may be errors
  • The 5 mile grid system may lead to accuracy issues
  • Useful:
  • Determine where areas of IFR exist at a glance
  • Planning an escape route

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

  • žVisibility
  • —Entry made when 5 SM or less
  • žPresent Wx
  • —When automated ] is on the right
  • —Most significant wx or obstruction to vis is reported
  • žSky cover
  • —Highest summation of cover is reported
  • žCeiling
  • —First BKN or OVC layer reported in AGL
  • žWeather flying category analysis
  • —IFR less than 1000 and 3 mi (hatched)
  • —MVFR 1000 to 3000 3 to 5 mi (outlined)
  • —VFR greater than 3000 vis greater than 5 mi

žWEATHER DEPICTION CHART

žWEATHER DEPICTION CHART

žWEATHER DEPICTION CHART

  • žUseful for:
  • žChoice place to start
  • žgeneral wx conditions
  • žGreat for locating areas of IFR, MVFR and VFR

žWEATHER DEPICTION CHART

ž

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

http://www.ospo.noaa.gov/Products/atmosphere/soundings/sfov/skewt/index.html

Usefulness of the Skew-T

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