Early versions of Netatmo Weather Station will connect to Wi-Fi only at regular intervals to communicate their data to Netatmo's servers. This interval is approximately 5 minutes.
Netatmo Weather Stations hardware versions including this behavior:
Later models, like Netatmo Weather Station with hardware version V3, will always stay connected. This model also supports Apple HomeKit.
Note: To check your Netatmo Weather Station hardware version, open the Netatmo Weather app on your device, go to Settings > Station Name > Indoor Module > Hardware version.