var swa0 = weather.swa.a[0].t;
var swa1 = weather.swa.a[1].t;
var swa2 = weather.swa.a[2].t;

OR

var i;
var swa_t = [];
if (weather && weather.swa && weather.swa.a)
{
for (i=0; i < weather.swa.a.length; i++)
swa_t[i] = weather.swa.a[i].t;
}

ECMA 357 PDF (E4X) should provide some relevant examples as well.
Other stuff regarding for loop can be found in Flanagan.

:-)