<?xml version="1.0" encoding="UTF-8"?>
<document>
  <sNone>
    <October>3.21</October>
    <November>-5.41</November>
    <December>-15.81</December>
    <January>-21.69</January>
    <February>-21.70</February>
    <March>-12.60</March>
    <April>-6.41</April>
    <May>-0.06</May>
    <June>5.42</June>
    <July>13.32</July>
    <August>14.12</August>
    <September>7.55</September>
  </sNone>

  <sLichen>
    <October>1.99</October>
    ...
</sLichen>
</document>

XDocument doc = XDocument.Parse();

var doc = "<?xml version="1.0" encoding="UTF-8"?....";
XDocument xdoc = XDocument.Parse(doc);
var items = xdoc.Root.Elements("sNone").Descendants().Select(d => d.Value).ToArray();
Console.WriteLine(items);

var items2 = xdoc.Root.Elements("sNone").Descendants().Select(d => decimal.Parse(d.Value)).ToArray();