coldest is January, with 1 °C. Average annual rainfall is . The rainiest month is February, with an average of of precipitation, and the driest is July, with 1 mm of precipitation. This mountain had been referred in some historical book such as shahnameh in Davazdah Rokhwar as the Zibad mountain and its eastern part is called black mountain or kouh Gonabad. the long-range mountain called Qohestan and It extends from Bajestan to Birjand near the border with Afghanistan this rang mountain separate south Khorasan from Razavi Khorasan,part of this rang mountain near Kakhk is called black mountain or kouh e gonabad Zibad Castle Zibad Castle is one of the four historical monuments of Zibad. It is located in Gonabad and is believed to be the last shelter of the last Sasanian emperor Yazdegerd III. It was registered in 2001 as a national heritage property in Iran, as it is related to the history of pre-Islamic Iran. In addition to the castle of Zibad, there is also the royal castle of Zibad, nationally registered under the name of Shahab Castle in 2002. Sources conflict as to the exact manner of the emperor's final moments. One suggests that after his defeat, he sought refuge at a millers near Marv, who killed Yazdegerd in order to obtain his jewelry, whilst the Cambridge History of Iran states that the miller was sent by Mahoe Suri. According to the history of Belazari, Yazdegerd III was defeated in a war in the city of Gonabad. According to the excavations of Zibd-Gonabad Fortress and according to oral narrations and the report of Blazeri and Habib Abdolhai,Dr Ajam and Abas Zmani (historical study magazine 1974), it seems that the narration related to the murder of Yazdgerd in Merw Mill is more mythical than the truth. What is closer to the truth is the narration of Al-Baladhuri in Kitab Futuh al-Buldan, that he was killed in Gonabad The main text of Blazeri's book The Fate of Yazdgerd III: Then he went to Khorasan, then he went to Janabad (Gonabad) he was welcomed and ruler of merv send Nizak Trkhan to welcom him in gonabad. but after weeks he asked the emperor to marry his daughter and the emperor refused and this caused a war and at the end Nizak attacked and defeated the king army and Destroyed and captured the remnants of the imperial forces. the battle was