On skyvector you see "World high", "world low" and the VFR map.
VFR is always under the clouds, correct? But how high do you have to be to follow the high/upper and how high for the low/lower airways?
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Skyvector uses official charts from the US National Aeronautical Chart Office (NACO) where available. For the rest of the world, it synthesizes maps using the NACO style based on data from each country's Aeronautical Information Publication (AIP). This will not match what local charts look like and is not valid for navigation. Within the US, you can select individual charts by name or the "World" layers that do a clever panoramic blend of all available charts into one seamless worldwide chart.
The "Lo" and "Hi" charts refer to the NACO "IFR En-Route Low" and "IFR En-Route High" charts. These are roughly equivalent to NACO VFR sectional charts. NACO "IFR Area" charts (only available in a few areas) are roughly equivalent to NACO VFR Terminal charts.
You should look at the NACO chart legends for a full explanation of what all the symbols and numbers mean, but for example, the number above the airway label (without a * or G prefix) is the Minimum En-Route Altitude (MEA), which provides both terrain and obstacle clearance and navaid radio reception. The number below is the length between waypoints in nautical miles.
The NACO "Low" charts show airways with an MEA below FL180, which have labels beginning with V or T, and NACO "High" charts show airways with an MEA of FL180 or higher, which have labels beginning with J or Q. Most other countries use the ICAO naming scheme where all "Upper" airways have a "U" prefix plus some other letter, though what "Upper" means varies by country. Skyvector tends to put "U" airways on the "World Hi" chart and all others on the "World Lo" chart, but it's not consistent across countries, so they may be using other data as well.
This much depends on where you are. Generally speaking, airways are no longer VFR, but IFR. For most sectors, there is a minimum height in the charts - generally speaking, they must be in controlled airspace, but also terrain will have its impact... Plus on all airways, there are limits to maximum and minimum altitude - and this will result in what charts they are shown...