In a Cloud Computing scenario, cloud services are accessed via Internet and typically there is a communication latency between the service provider and its clients. As a consequence, QoS parameters noticed by the clients (such as service response time) have different values depending on their location. In this paper, we propose the relative QoS (RQoS) concept, which defines that the quality of a service is relative to its geographical location. We also introduce a monitoring mechanism that provides RQoS parameters of cloud services in the SaaS (Software as a Service) layer, so that clients can choose the services that better suit their needs. In order to validate this new concept and the monitoring mechanism, we present a proof of concept and some computational experiments to show that there are variations in the values of QoS depending on the geographic location where services are monitored. The performed experiments also aimed to evaluate the performance of the monitoring mechanism in terms of the assessment of RQoS parameters.