A solar energy system can be controlled during periods of reduced insolation. For example, one or more environmental condition sensors can detect environmental properties indicating current or expected insolation levels and can generate at least one signal indicating a current or impending transient reduced-insolation event. The at least one signal can be received (for example, by a controller) from the sensors that indicates changes in insolation. Responsively to the at least one signal, characteristics of a current reduced insolation event or of an impending transient reduced-insolation event can be calculated. In response to the calculated characteristics, a quantity of available insolation can be calculated. An attemperation flow rate in the solar steam system can be controlled responsively to the calculated quantity of available insolation such that the temperature of steam entering the steam turbines is maintained within a predefined range.