Softboxes and umbrellas are great lighting modifiers but they can easily become airborne from a gust of wind when shooting outdoors. Don't risk damaging your flashes or studio lights.
The Phottix Stay-Put Sandbags II are available in three sized and hold 3 kg/6.5 lbs, 6 Kg/13.2 lbs and 10 Kg, 22 lbs of weight and can keep a light stand firmly in place when shooting outdoors or in the studio. These are indispensable accessories.
Shoot safely - always use a sandbag.