Forecasting basics
How to read a weather map
Weather maps look intimidating. Once you know the symbols, they tell a clear story. Here is what every mark means.
The three main map types
Surface map
Weather at ground level. Fronts, pressure systems, station observations.
Upper air map
Weather at higher altitudes. Usually 500 hPa (about 18,000 ft).
Model forecast
What weather is expected. Same conventions.
Radar composite
Precipitation returns.
Satellite loop
Cloud animation.
Pressure systems
H — High pressure
Cool, dry, sinking air. Usually fair weather. Winds clockwise (Northern Hemisphere).
L — Low pressure
Warm, moist, rising air. Storms. Winds counterclockwise.
Central pressure number
Given in millibars (e.g., 1010 mb).
mb / hPa
Same unit. hPa is the SI version.
30.0 / 29.92 / 30.5
Inches of mercury, older US convention.
Isobars
- Lines connecting equal pressure.
- Typically drawn every 4 mb (1000, 1004, 1008, 1012...).
- Closer together = tighter pressure gradient = stronger winds.
- Wider apart = light winds.
- Winds flow along isobars, slightly across toward low pressure.
- Circular isobars = well-defined high or low.
- Ridge = elongated high.
- Trough = elongated low.
Fronts
Cold front (blue triangles)
Cold air pushing under warm air. Line of thunderstorms possible. Wind shift + temperature drop.
Warm front (red half-circles)
Warm air rising over cold air. Overrunning precipitation. Widespread rain.
Stationary front (alternating)
Neither moves. Rain persists on cold side.
Occluded front (purple)
Cold front catches warm front. Complex, often mature low.
Dryline (orange)
Moisture boundary. Not officially a front. Common springtime severe setup.
Trough of low pressure
Elongated low without frontal character.
Squall line
Narrow line of severe thunderstorms. Not a front.
Outflow boundary
Cool downdraft air from a dying storm complex.
Station observations
Each weather station has a "station model" showing multiple parameters:
- Temperature (upper left).
- Dewpoint (lower left).
- Pressure or altimeter (upper right).
- Wind arrow (barb from station).
- Cloud cover (circle: empty=clear, filled=overcast).
- Current weather (icon).
- Visibility (may be given).
- Present pressure change (small tag).
Wind barbs
- Line points UPWIND (wind comes from that direction).
- Small feather = 5 knots.
- Full feather = 10 knots.
- Pennant (triangle) = 50 knots.
- Multiple feathers add up.
- North = up, East = right (standard orientation).
Radar interpretation
- Reflectivity — precipitation intensity.
- Green: light rain. Yellow: moderate. Red: heavy. Purple/pink: hail or intense.
- dBZ scale: 20 dBZ = light rain, 40 dBZ = heavy, 60 dBZ+ = severe / hail.
- Velocity — winds toward and away.
- Green: winds toward radar. Red: away from radar.
- Couplet: adjacent green+red = rotation (mesocyclone).
Satellite imagery
Visible (VIS)
Cloud reflection. Only during daylight.
Infrared (IR)
Temperature. Colder = higher clouds = often more severe.
Water vapor
Upper-atmosphere moisture. Jet stream visible.
Enhanced IR
Color-coded to show cloud top temperature intensities.
Cloud microphysics
Multi-spectral. Identifies fog, ice, snow.
True color
Composited to look like human eye view.
Common upper-air maps
850 hPa (~5,000 ft)
Low-level warm air advection. Where moisture flows.
700 hPa (~10,000 ft)
Mid-level flow. Storm steering.
500 hPa (~18,000 ft)
Mid-level. THE map for weather patterns.
300 hPa (~30,000 ft)
Jet stream. Divergence patterns.
250 hPa (~35,000 ft)
Peak jet stream height.
200 hPa (~40,000 ft)
Upper divergence.
The severe weather map to watch
SPC Day 1 Outlook is the master weather map for severe risk.
- Categorical risk levels: General Thunder → Marginal → Slight → Enhanced → Moderate → High.
- Individual hazard maps for tornado, hail, wind.
- Percentage probabilities.
- Updated multiple times daily.
- Read left to right, top to bottom.
- Bold hazards get boxes around them.
The everyday weather map to watch
- Weather.gov: home page.
- Weather Underground: PWS integration.
- AccuWeather Ultimate: subscription features.
- Windy: elegant global visualization.
- Pivotal Weather: chase-oriented model views.
- RAP mesoanalysis: SPC operational tool.