The annual shift of the Intertropical Convergence Zone (ITCZ) brings tropical moisture from the Gulf of Mexica, and, to a certain extend, the Sea of Cortez (Gulf of California) into the American Southwest, primarily New Mexico and Arizona. High and/or Low Pressure Systems located over these States determine whether enough moisture is present to allow cloud build-up by convectional heating that may result in isolated thunderstorms. Ideally, in Arizona to have a chance of rain during the summer months, the High Pressure Cell would be stationary over the Four Corners area. In the past, the onset of the monsoon in Arizona was defined by having three consecutive days with a dew point temperature of 55F or higher. This usually happens in late June to early July. Two years ago it was changed to 'Monsoon Season', essentially a double negative, and the season is now from 15 June until 30 September, regardless of the dew point temperature. In this picture, over the White Sands National Monument, located between Las Cruces and Alamogordo, New Mexico, there is enough moisture in the air for clouds to build that will result in isolated thunderstorms later in the day. This picture was taken in September 1997 and later scanned, Sig...