Sydney's main weather radar at Terrey Hills will be offline for more than a month for an upgrade ahead of this year's thunderstorm season.

The Bureau of Meteorology is updating equipment at sites near Adelaide, Melbourne, Brisbane and Sydney to improve forecasters' ability to differentiate types of precipitation and improve early-warning services.

Existing Doppler radar sites in those cities now only issue horizontal pulses that are reflected off rainfall droplets in clouds.

The upgrade will add a vertical detection capability, providing the bureau with a clearer view of the size and shape of the rain droplets - and whether it's falling as hail - or if the reflections are from smoke particles or even insects.