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Surface Data Details

Surface data is reported hourly from places like airports and automated observing platforms. The types of plots using surface data include a composite surface map, weather depiction for aviation, regional plots for the US, Canada, Mexico and Alaska. Contoured analyses of various parameters including temperature and pressure. Finaly, there are meteograms which are time cross-sections for individual cities. These data are updated hourly at around 30 minutes past the hour.


Composite Surface Map

Sample_surface_map
Sample_surface_map

The composite surface map portrays as much information about surface weather conditions as possible. This composite map contain the following analyses:
Radar summary (color filled/stippled areas),
Surface data plot (composite station model),
Frontal locations (in various bold lines) and
Pressure contours (in thin blue lines).

Radar Summary Plot

The radar summary displays areas where precipitation is falling based on returns from a weather radar. The intensity is based on color where:
Radar Legend

Color Intensity Description
Blue Light light rain or snow
Cyan Moderate  
Green Heavy light thunderstorms/moderate rain showers
Yellow Very Heavy moderate thunderstorms
Red Intense potential flooding rains/severe thunderstorms
Magenta Extreme flooding rains

Surface Data Plot

The surface data plot gives the following information:
Legend

Data Description
temperature (F) plotted upper left
present weather symbol plotted center left (see below)
dewpoint (F) plotted lower left
pressure (.1 mb-coded) plotted upper right as last 3 digits (987=998.7mb, 024=1002.4mb)
cloud cover center circle. white fill indicates % cloud coverage (see below)
winds wind barb (see below)

Weather Symbols

Legend

Legend  Legend

Frontal Location Plot

Frontal locations are denoted by bold lines in the following colors:
Legend  Legend

High and Low pressure systems are plotted with H and L and the associated pressure in millibars is displayed below the letter.

NOTE: Frontal data are only available every 3 hours so fronts may not exactly match the weather conditions. A label at the bottom left denotes what time the fronts are valid.

Pressure/Isobar Contours

The pressure contours (or isobars) are plotted as blue lines every 4 millibars and indicate positioning and extent of various pressure systems.


Surface Weather Depiction

Sample depiction
Sample depiction

The weather depiction chart is a composite chart aimed at aviation. The data plotted are:

Flight rule depiction (color filled/hatched areas),
Depiction data plot (composite model),
Frontal locations (in various bold lines)

Flight Rule Depiction

Shaded and hatched areas represent current flight rules:

Plotted Rules Comments
gray solid IFR Ceiling < 1000 ft or Visibility < 3 miles
gray hatched MVFR Ceiling < 3000 ft or Visibility < 5 miles
clear/black VFR Not IFR or MVFR

Depiction Data Plot

Data Description
cloud cover center circle. white fill indicates % cloud coverage (see symbols above)
present weather symbol left center (see symbols above)
cloud ceiling below (in 100s feet)

Weather Symbols

Frontal Location

The frontal analysis is also added to aid in determining general weather conditions.
Fronts


Frontal Location

Plot of frontal locations for most of North America. Frontal locations are denoted by bold lines in the following colors:

Sample  Legend

High and Low pressure systems are plotted with H and L and the associated pressure in millibars is displayed below the letter.


Regional Data Plots

Sample regional plot
Sample regional plot

These are simple data plots for various regions of the US, Canada and Mexico. The information plotted is using the surface station plot.
Surface Data Plot


Contour Plots

US Temperature Contour
Contour plot of temperatures (F) for the contiguous US.

North America Temperature Contour
Contour plot of temperatures (F) for North America.

24 hour Temperature Change
Contour plot of the temperature change from current hour to the same hour the previous day for the contiguous US. Areas below 0 represent where the temperatures are currently colder and reflect the passage of a cold front. Areas above 0 represent where the temperatures are warmer and often reflect the passage of a warm front.

US Wind Chill Contour
Contour plot of wind chill temperatures (F) for the contiguous US. The wind chill is the effective temperature of the skin from heat loss due to winds at cold temperatures. Wind chill temperatures less than -20F are considered low, temperatures less than -40F are extreme and temperatures less than -60F are considered dangerous.

US Heat Index Contour
Contour plot of heat index temperatures (F) for the contiguous US. The heat index is the effective temperature of the skin due to the inhibiting effect of humidity to allow the body to cool through sweating and evaporation. Heat index values of greater than 90F are considered high, index values greater than 100F are extreme and index values greater than 110F are considered dangerous.

US Pressure Contour
Contour plot of sea level pressure (mb) for the contiguous US. The maximum and minimum values reflect the location of high and low pressure systems.

3 hour Pressure Change
Contour plot of the change in pressure from the current hour to 3 hours previous for the contiguous US. This is a good indicator of the movement and intensification of pressure systems. If the pressure change is negative in the region of a low pressure system (see above chart), the low is deepening. If the negative pressure change is out in front of the low pressure system, it indicates movement. The location of the maximum pressure drops indicate the possible direction of the low pressure system.

US Dewpoint Contour
Contour plot of the dewpoint temperature (F) for the contiguous US. The dewpoint temperature indicates the amount of moisture (or humidity) in the atmosphere. The amount of moisture in the atmosphere can affect how you feel and is used in the heat index computation (see above). Dewpoints above 65 reflect humid conditions. Summer dewpoints can reach the low 80s. Dewpoints above 50 mean that there is enough moisture in the atmosphere to generate thunderstorms. The higher the dewpoint, the easier storms form and the more potent they can become. An additional factor of low level wind convergence is also needed to help storms form (see below). Intense storms can form at dewpoints of 55 if the right conditions are in place including high convergence values.

US Moisture Convergence
Contour plot of the convergence of surface winds weighted by the moisture in the atmosphere represented by the surface specific humidity. Positive areas represent where winds are converging and thus forcing upward motion. Negative areas often reflect the presence of sinking motion. Since this is weighted by the moisture, high values of convergence can often represent areas where forced convection (possible thunderstorms) may occur.