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Short Skip Radar

The latest foF2 (short skip) map with a brief explanation of what you're looking at below. If the image is more than an hour or two old, reload the page to be sure you're seeing the latest.


This is the latest foF2 or critical frequency plot from the Australian Space Weather Agency. You can get the latest image by bookmarking this page and returning to it or access it directly at this link. The map is updated about every 1 to 2 hours.

This is the submap for North America which is sometimes updated more often than the North Atlantic map. *Direct link to this map

foF2 is a symbol that denotes the highest frequency at which "vertical incidence" can occur. Vertical incidence is radio speak for "straight up and down skip" or really short skip. If Bill, WØQNI, happens to be on 75 meters and chatting with Bart, WA6HZN, since they are around 90 miles apart, they are reaching each other via skip that is almost straight up and bouncing straight back down.

foF2 is also called "the critical frequency". It is at its highest around noon local time and generally starts to fall throughout the rest of the day until sunrise on the next day. This isn't exactly true, but it's approximately what occurs. The critical frequency tends to be at its lowest in the predawn hours.

If it's in the early evening our time and the west coast is green, tan, yellow, or red, stations within about 300 miles are going (green) or gone. You can hear the long skip arriving, if you listen to a busy 75 meter frequency like our net and you can often see it coming by clicking up the latest map in the afternoon and early evening. If someone made a movie of a series of maps, you'd see something like wiggling colorful snakes slithering from right to left across your screen. The lowest critical frequencies move from east to west.

This is the map for September 24th, 2006 at 01 GMT (6pm PDT)...

...and this is the map for 02H, 60 minutes later.

As you can see, the critical frequencies have moved west by around 1000 to 1500 miles. In all probability, the critical frequency over the whole of the USA will be less than 4 mhz. in another hour or two.

There is a simple relationship between the critical frequency and the maximum usable frequency, the highest frequency at which skip can occur.

maximum usuable frequency = 3.3 X critical frequency (approximately).

If you have a general coverage receiver, you can get an idea of the maximum usuable frequency simply by tuning upwards in frequency until you hear few or no stations, coming from the direction of the darkness at your location. The critical frequency roughly this frequency divided by 3.3. If you're in California and the critical frequency falls below 3 mhz. over the western USA, you aren't going to hear WWV at 10 mhz. very well, if at all. On the other hand, WWV at 5 mhz. will probably knock your radio off the table.

You can explore the world of propagation at http://www.ips.gov.au/ - Their Space Weather section can be reached at http://www.ips.gov.au/Space_Weather. Their Space Weather section includes a link to the North America map and maps for other parts of the world.

Bart, WA6HZN, the web rat, webrat@warfa.org

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