The commonly adopted trigger of zero degrees Celsius or lower for three consecutive nights, was proposed as a workable timescale in which to activate severe weather emergency provision (SWEP) following an informal consultation with local authorities and London voluntary agencies, as it provides enough time in which to open, staff and publicise the provision, as well as to encourage clients to take up spaces.
However, local authorities are free to determine what trigger they use to activate severe weather emergency provision within their area.