Unhealthy traffic conditions have significant adverse implications for environment-friendly, sustainable, and energy-efficient development. As an real-time environmental factor contributing to travel, comparing the effect mechanisms of weather on traffic conditions in multiple cities will provide a deeper understanding of the weather-traffic relationship. This study conducts a contrastive analysis of travel time index (TTI) variations under weather conditions in four cities. Notably, this study will consider the potential benefits of replacing an absolute index of traffic conditions, like traffic volume and travel speed, with TTI, a relative index. The results identified a strong association between complex weather measurements and traffic conditions. Overall, the impact of meteorological variables on TTI on non-weekdays might result in significantly worse traffic conditions. On weekdays, the impact is relatively weak, due to the flexibility for travel mode decision-making, especially during the commute hour. Weather impacts on traffic conditions are not only interrelated, like higher temperature with less rainfall or slower wind speeds, but they also varied with urban spatial layout and geographical condition. Besides, the traffic congestion is certainly related to extreme weather, but not closely. This study could benefit transit agencies by adding meteorological monitoring into traffic condition real-time analysis.