automatically connect to an AP without ssid in wpa_supplicant.conf

Hi,

If I bring my wireless handset to the street, I don't know what is the ssid in an AP, then how can I specify an ssid in the wpa_supplicant.conf? Is it possible let wpa_supplicant to find an AP and to connect it without an ssid in wpa_supplicant.conf?

Thank you.

Jim