Thrips are small (0.5 to 12.0 mm long, though most are less than 5 mm long) thin yellow-brown or black insects commonly found on flowers in summer. Though many are winged they are often reluctant fliers with a predisposition towards thunder storms as a suitable time of flight, hence they are called 'Thunder flies' or 'Thunder Bugs' in some places. Now you know! I do agree they are horrible little creatures - especially on picnics!!
And they are attracted to settling on white or yellow cloth and clothing, which information may help you a little in advance of a picnic. They are not as noticeable in the late afternoon either; it is popularly thought that they die but it seems just as likely that they simply lack the thermal uplift when the air cools; either way they're not such a pest then.
were you weiring a yellow top cause if you were then there attracted to you i got them yesterday and they itch dont they i think they might fly round in thunder storms