What speed indicates the transition from a tropical depression to a tropical storm?

The transition from a tropical depression to a tropical storm is defined by a sustained wind speed of 39 miles per hour (MPH). This classification is important for meteorologists and emergency management, as it signifies an increase in the potential impact of a storm. A tropical depression, which has wind speeds below this threshold, is considered the initial stage of a storm system, while any increase to 39 MPH or more indicates a tropical storm, which carries a higher risk of severe weather events such as heavy rains, strong winds, and the potential for further intensification. Understanding this distinction is crucial for forecasting and preparing for storm impacts, making the identification of this specific wind speed a key element in tropical weather classification.