The Lines on the Map
Professional forecasts use maps covered in curved lines. These are "Isobars." They connect areas of equal air pressure.
The word "isobar" comes from Greek: "iso" meaning equal, and "baros" meaning weight. Each line represents a constant atmospheric pressure, typically measured in millibars (mb) or hectopascals (hPa). Standard spacing is 4mb between lines.
The Physics
Wind is driven by pressure gradient force. Air flows from high pressure to low pressure. The closer the isobars, the steeper the pressure gradient, and the faster the wind.
Buys-Ballot's Law: In Northern Hemisphere, stand with back to wind—low pressure is to your left
The Slope Concept
Think of air pressure like a hill. High pressure is the top of the hill. Low pressure is the bottom. Wind is a ball rolling down the hill.
The steeper the hill, the faster the ball rolls. On a weather map, tightly packed isobars represent a steep pressure hill—this means strong wind. Widely spaced isobars represent a gentle slope—this means light wind.
A pressure difference of just 10mb across 100km can generate 25 knots of wind. The same 10mb spread over 500km might only produce 8 knots.
Spacing Tells the Speed
You do not need to be a meteorologist. You only need to look at the gap between the lines.
- Lines Close Together: The hill is steep. The "ball" rolls fast. This means strong wind.
- Lines Far Apart: The hill is flat. The "ball" rolls slowly. This means calm or light wind.
Isobar Spacing vs Wind Strength
Tight Packing
Barcode pattern
25-35 knots, rig 3-4m wing
Moderate Spacing
Clear gaps visible
12-20 knots, ideal conditions
Wide Spacing
Huge gaps between lines
<10 knots, marginal or dead
The Geostrophic Wind
Isobars don't just tell you wind speed—they tell you wind direction. Wind flows parallel to the isobars, not perpendicular to them.
This is because of the Coriolis effect. In the Northern Hemisphere, wind is deflected to the right of the pressure gradient. In the Southern Hemisphere, it is deflected to the left. This creates a balance where wind flows along the isobars rather than directly from high to low pressure.
At the surface, friction slows the wind and allows it to cross isobars at about a 15-30° angle toward lower pressure. Aloft, where friction is minimal, wind flows almost perfectly parallel to isobars.
Reading Isobar Direction
Northern Hemisphere: Wind flows counterclockwise around low pressure, clockwise around high
Southern Hemisphere: Wind flows clockwise around low pressure, counterclockwise around high
Surface winds: Cross isobars at 15-30° angle toward low pressure due to friction
Upper-level winds: Flow nearly parallel to isobars with minimal crossing
Pressure Systems and Patterns
Recognizing common pressure patterns helps predict weather and wind changes:
- Closed Low: Circular isobars around a low pressure center. Indicates storm system with strongest winds near the center.
- Closed High: Circular isobars around a high pressure center. Light winds near center, calm conditions.
- Trough: Elongated area of low pressure. Often brings wind shifts and unsettled weather.
- Ridge: Elongated area of high pressure. Typically stable conditions with light winds.
- Col: Saddle point between two highs and two lows. Variable, unpredictable winds.
Calculating Wind Speed from Isobars
You can estimate wind speed using isobar spacing. The pressure gradient force is proportional to the pressure change over distance:
Wind Speed (kts) ≈ 30 × (ΔP in mb) / (Distance in km)
Practical examples:
- Tight spacing: 12mb pressure change across 150km = 30 × (12/150) = 2.4 → ~25 knots
- Moderate spacing: 8mb across 200km = 30 × (8/200) = 1.2 → ~15 knots
- Wide spacing: 4mb across 300km = 30 × (4/300) = 0.4 → ~5 knots
This formula is approximate but remarkably useful for quick wind estimation from weather maps. Friction and local effects can modify the result by ±30%.
Frontal Zones and Kinks
When isobars develop sharp bends or kinks, you're looking at a frontal boundary. These zones feature:
- Wind direction shifts: Can veer or back 45-90 degrees within an hour
- Enhanced convergence: Wind speed often 1.3-1.8× higher than gradient wind
- Gustiness increase: Expect gust factors of 1.4-1.8 near fronts
- Duration: Frontal passage typically takes 2-6 hours with peak winds lasting 30-90 minutes
Practical Isobar Reading Tips
Count the lines: 3+ isobars crossing your area in tight spacing = strong wind day
Track the low: Watch where low pressure centers move for upcoming wind shifts
Look for kinks: Bent or kinked isobars indicate frontal zones with gusty wind
Compare daily: Watch how isobar patterns evolve to predict multi-day trends
Check pressure tendency: Rapidly falling pressure (tight isobars approaching) = increasing wind
Isobar Spacing Guide:
Lines Packed Tightly (Like a Barcode)
High Wind - Rig small wing
Lines Widely Spaced (Big Gaps)
Light Wind - Might be too calm
Real-World Application
When checking the morning forecast, pull up the isobar map first. Look for the pattern over your region:
- Tight lines approaching from the west? Wind will increase in 6-12 hours.
- Lines spreading apart? Wind will die down.
- Low pressure center passing north of you? Wind will veer (shift clockwise) as it passes.
- High pressure settling in? Expect light, variable winds for several days.
Summary
When looking at a weather map, look for the "tight" areas. If the lines are packed together near your location, rig a small wing. If the lines are widely spaced, expect light or no wind. Learn to read the flow direction along the isobars to predict wind direction. Understanding isobars gives you a 24-48 hour preview of wind conditions before they arrive.