These echoes ('Polar Mesospheric Summer Echoes - PMSE), as their name suggests, occur mostly during midsummer, when they are often (but not always) associated with noctilucent cloud. They are also often associated with Es. Accordingly, when they occur during winter, they are termed 'non-summer', or sometimes just 'winter'.
It's not clear how these winter echoes come to exist. Infrasound from the sea surface and meteoric debris are both thought to play a part, though there is a lot of work to do to figure it all out.
If you have an interest, this Facebook page is quite interesting in relation to mesospheric echoes in general. A more challenging, academic read can be found here.
2 comments:
Hello John, I remember from my CB years there is always one or two days in winter that ES occurs reasonable strong. It probabely hasn't anything to do with PMSE? It is intertesting though. 73, Bas
Yes, Es is fairly common near mid-Winter, mainly due to meteoric inputs being fairly high then. The correlation between NLC and PMSE/PWSE is not very high, maybe around half the time or so. I guess the correlation between Es and PWSE is much the same.
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