Shortwave propagation question

[KC2TAU]

One thing that I've noted in my time spent listening to the radio is that shortwave propagation will be fine in the morning with plenty of signals from Asia but by about 12 noon the band quiets down and doesn't pick back up again until about 4 in the afternoon which is when Europe starts to come in. Has anyone noticed a similar dearth in signals around the middle of the day? Is there any reason why this might be so?

[W3RSW]

If you mean recently, See the thread of only two days ago.
K3ZS, "radio blackout"

 Like the weather, every day, every year is different. Suggest you look at eleven/twenty two year sunspot cycles, the ionosphere, effects of normal day/night radio wave propagation related to frequency, effect of abnormal absorbtion during the height of solar cycle propagation, mass ejections from the sun, K indices, and so forth. Lately, especially the last several days have seen most all of the traditional high freqs. wiped out through mid-day.

See also , www.spaceweather.com

[Sam KS2AM]

One thing that I've noted in my time spent listening to the radio is that shortwave propagation will be fine in the morning with plenty of signals from Asia but by about 12 noon the band quiets down and doesn't pick back up again until about 4 in the afternoon which is when Europe starts to come in. Has anyone noticed a similar dearth in signals around the middle of the day? Is there any reason why this might be so?

Good propagation over long distances on the lower shortwave bands takes place when all or most of the path is in darkness.  i.e. if you are here on the east coast that will favor Asia-Pacific in the morning and Europe starting late afternoon.  Europe is a good example.  There are broadcasters on 40 meters all day long but we don't hear them here until late afternoon when the path of darkness is established and propagation improves over that path.

[KA0HCP]

It's related to the 'dark line' moving.
In our morning the line is between the US and Asia.  By mid afternoon it is crossing Europe.
 
The pattern exists year and year out.  Any number of the propagation or station management software show this graphically. 

[flintstone mop]

This link might help you understand how some of this shortwave stuff works

http://dx.qsl.net/propagation/greyline.html

Working the grey line in Ham radio can give you some exciting contacts that would not normally happen. 40M around 6:30 in the morning in Winter makes for some fun using SSB. You can make contacts to other Hams that are in between your dawn and their dusk for about an hour. Signals are usually pretty weak.

[Steve - K4HX]

It depends on the frequency. What you are seeing is probably happening on the 31, 25, 22 and 19 meter bands, and maybe to some extent the 41 meter band. I think you will find far more signals in the 49 and 41 meter bands during the nighttime hours and absolutely so in the "tropical" bands like 120, 90 and 60 meters.