(1) Remember that the moving ray of sandstorm is BO, point A is the vertical line of BO, and point C intersects with BO, then △ABC is a right triangle with an angle of 30. According to AB=240 km and AC= 1/2AB= 120 km, this is the shortest distance from the sandstorm center to point A, because according to sandstorm,

(2) Take A as the center, make a circle with a side length of 150km, and intersect with the thunder wave at m, n, then this length of MN is the influence area.

AM= 150, AC= 120, so we can get MC=90 according to Pythagorean theorem, so MN= 180 km.

The time is 180/ 12= 15 hours.