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
STATION DESIGNATORS
- Sky cover
- Temp
- Wind
- Present wx
- Type of predominant cloud
- Sea level pressure
- Pressure change (past 3 hours)
- 6 hr precip
STATION PLOT FOR AVIATION
SURFACE ANALYSIS
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
SURFACE ANALYSIS
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
Chapter 4
4.2.2 Weather Depiction Chart
WEATHER DEPICTION CHART
- Being phased out in favor of the CVA
- Beginning at 0100z
- every 3 hours
- information for this chart taken from METAR
- computer generated
- ] to the right of the station circle means automated
WEATHER DEPICTION CHART
- Shows:
- 1. Total sky cover
- 2. Cloud height or ceiling
- 3. Wx and obstructions to vision
- 4. Visibility
WEATHER DEPICTION CHART
- Visibility
- Entry made when 5 SM or less
- Present Wx
- When automated ] is on the right
- Most significant wx or obstruction to vis is reported
- Sky cover
- Highest summation of cover is reported
- Ceiling
- First BKN or OVC layer reported in AGL
- Weather 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
WEATHER DEPICTION CHART
WEATHER DEPICTION CHART
WEATHER DEPICTION CHART
- Useful for:
- Choice place to start
- general wx conditions
- Great for locating areas of IFR, MVFR and VFR
WEATHER DEPICTION CHART
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