Grey-Line Map
Where the band of twilight between day and night is right now, and when it will pass over Lethbridge and the part of the world you want to work.
Twice a day, at sunrise and sunset, every place on Earth slides through a band of twilight called the grey line. For a little while the lower HF bands behave differently there, and signals can travel a long way. This map shows the grey line for the current time and lets you look ahead 24 hours, so you can see when it reaches Lethbridge and when it reaches a target region such as Europe, Japan or Australia.
It’s a quick, standalone view. For the grey line together with band conditions, solar data and more, open the full propagation map.
Grey-line map
Where it’s day, night and twilight right now, and over the next 24 hours. Lethbridge is marked in red; pick a target region to see when the grey line reaches it. Times are Alberta time (UTC−6).
How to read it
The bands of shading
Clear areas are in daylight. The warm orange band is civil twilight, when the sun is between the horizon and 6° below it: that’s the grey line. Darker blue-grey is nautical twilight (6° to 12° below), and the darkest shading is night.
The lines and markers
The orange line is the day–night line itself. The sun symbol is the spot where the sun is straight overhead. Lethbridge is the red dot, your target is the blue dot, and the dashed line is the shortest (great-circle) path between them.
The time slider
The map starts at the current time and redraws every minute. Drag the slider, or use the −1 h and +1 h buttons, to look up to 24 hours ahead. Now brings you back. The “Show” buttons in the table jump straight to a window.
The windows table
For Lethbridge and your target, the table lists the next morning and evening grey-line windows in Alberta time, with UTC in brackets. Below it, the map says whether both ends are in twilight at the same time, and when both ends are dark.
Alberta time. All local times are shown in Alberta time, which stays on UTC−6 all year from November 2026. Net and contest schedules are usually in UTC, so both are shown.
Why the grey line matters
The lowest layer of the ionosphere, the D layer, is made by sunlight. During the day it soaks up HF signals, and it soaks up the lower frequencies most. That’s why 160, 80 and 40 m are short-range bands at midday and come alive after dark. NOAA’s Space Weather Prediction Center describes the same pattern for the extra absorption caused by the sun’s X-rays: it is strongest where the sun is overhead, fades to nothing at the day–night line, and hits lower frequencies hardest.
Around sunrise and sunset there’s a short time when the D layer has faded or hasn’t built up yet, while the higher E and F layers that bend signals back to Earth are still strong. The amateur exam question pool sums it up this way: grey-line propagation happens because “at twilight, D-layer absorption drops while E-layer and F-layer propagation remain strong.” It’s described as long-distance communication at twilight on frequencies below about 15 MHz.
Watch both ends
The classic grey-line contact is when your end and the far end are both in twilight at once. Many low-band operators find the best pairing is sunrise at one end and sunset at the other. Pick a region and check the “both ends in the grey line” line under the table. Some days there’s an overlap; often there isn’t.
Sunrise here, sunset there
Low-band DX often turns up when one station is near sunrise and the other near sunset, with darkness over the path between them. The map shows you the darkness along the dashed path at a glance.
Be on early
The windows are short: at Lethbridge civil twilight lasts about 30 to 45 minutes, depending on the season. Have the antenna tuned and be listening before the window opens.
A guide, not a guarantee. The map shows where the sun is, not what the ionosphere is doing. Solar activity, geomagnetic storms and the season all change what you’ll actually hear. Experienced operators also debate exactly how grey-line paths work: in a 2016 QEX article, Ed Callaway, N4II, argues that low-band signals don’t travel along the twilight band itself, but between a place at sunset and a place at sunrise. Either way, twilight at one or both ends is the time to listen.
For live band conditions, solar flux and the K index alongside the grey line, go to the full propagation map.
Sources
- Sun position, sunrise and sunset: equations of the NOAA Solar Calculator (from Jean Meeus, Astronomical Algorithms), sunrise and sunset with the sun’s centre 0.833° below the horizon: NOAA Global Monitoring Laboratory, Solar Calculation Details. Cross-checked against the U.S. Naval Observatory’s Approximate Solar Coordinates and USNO’s 2026 equinox and solstice times (published by the National Weather Service, Earth Seasons 2024–2027).
- Civil, nautical and astronomical twilight (sun 6°, 12° and 18° below the horizon): NOAA, Solar Calculation Details.
- D-region absorption from solar X-rays greatest under the overhead sun, zero at the day–night line, stronger at lower frequencies: NOAA Space Weather Prediction Center, Global D-Region Absorption Prediction Documentation.
- Grey-line propagation occurs at sunrise and sunset because D-layer absorption drops while E- and F-layer propagation remain strong; long-distance communication at twilight below 15 MHz: U.S. Amateur Extra question pool (questions E3B08 to E3B11), as explained by Dan Romanchik, KB6NU, Extra Class question of the day: trans-equatorial, long path and grey-line propagation.
- Low-band paths between sunset and sunrise rather than along the terminator: Ed Callaway, N4II, Gray Line Propagation, or Florida to Cocos (Keeling) on 80 m, QEX (ARRL), November/December 2016.
- Coastlines: Natural Earth 1:110m land, public domain.