This paper addresses how real-time weather information acquired through mobile technology can be leveraged to enhance the efficacy of mobile interventions for spurring users' healthier behaviors. Through a field experiment that each participant experience different weather conditions in two different treatment periods under the gain or loss interventions, we found that the effects of gain or loss interventions across sunny and cloudy weather are not uniformly distributed. Loss intervention induces higher levels of fulfillment of exercise goals than gain intervention in sunny weather, whereas gain interventions are more effective than loss interventions in cloudy weather. We also provided empirical evidence to uncover the underlying mechanisms and rules out alternative explanations. The follow-up experiment reveals that weather-based intervention can be used repeatedly over time without losing its effectiveness. Moreover, our result suggests that the observed effect is more evident for people with a lower exercise level and living in areas of lower income. Our study provides theoretical guidance and practical implications for academics, healthcare businesses, and policymakers on the strategy of using weather based messaging for enhancing physical activity levels.