Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The time of Zuhr prayer begins when the sun passes its meridian and it continues until an object`s shadow is approximately the same length as the object itself.
In his famous work, Fiqh As-Sunnah, Sheikh Sayyid Sabiq, elaborates on the subject as follows:
Times of the five daily prayers
`Abdullah ibn `Amr reported that the Messenger of Allah (peace be upon him) said, “The time of the noon prayer is when the sun passes the meridian and a man`s shadow is the same length as his height. It lasts until the time of the afternoon prayer. The time of the afternoon prayer is until the yellowing of the sun (during its setting). The time of the evening prayer is as long as twilight. The time of the night prayer is to the middle of a night of medium duration. And the time of the morning prayer is from the appearance of the dawn until the time of sunrise. When the sun rises, abstain from praying, as it rises between the horns of Satan.” (Muslim)
Jarir ibn `Abdullah narrated that the angel Gabriel came to the Messenger of Allah and said to him, “Stand and pray,” and they prayed the noon prayer when the sun had passed its meridian. He then came to him for the afternoon prayer and said,”Stand and pray,” and they prayed the afternoon prayer while the length of a shadow of something was similar to the length of the object. Then he came a t sunset and said, “Stand and pray,” and they prayed the sunset prayer when the sun had just disappeared. Then he came at night and said, “Stand and pray,” and they prayed the night prayer when the twilight had disappeared. He came again when dawn broke (and they prayed the morning prayer). Then Gabriel came on the next day at noon and said (to the Messenger of Allah), “Stand and pray,” and they prayed the noon prayer when the length of the shadow of something was close to the length of the object. Then he came for the afternoon prayer and said, “Stand and pray,” and they prayed when the shadow of something was twice as long as the length of the object. Then he came at the same time (as the previous day) for the sunset prayer, without any change. Then he came for the night prayer after half of the night had passed (“or,” he said, “one-third of the night”). Then he came when the sky was very yellow and said, “Stand and pray,” and they prayed the morning prayer. Then Gabriel said, “Between these times are the times for the prayers.” (Reported by Ahmad, an-Nasa`i and at-Tirmidhi) Al-Bukhari observes, “It is the most authentic report concerning the prayer times.
Time of Zuhr prayer
The previous two hadith make it clear that the noon (Zuhr) prayer begins when the sun passes its meridian and it continues until an object`s shadow is approximately the same length as the object itself. If it is extremely hot, it is preferred to delay the noon prayer until it is cooler. This is done in order to retain the humility and awe of the prayer. If this is not the case, it should be prayed early in its time. This opinion is based on the following hadith: Reported Anas, “If it was extremely cold, the Prophet, upon whom be peace, would pray early. If it was extremely hot, he would wait for it to cool down.” (Related by al-Bukhari.) Abu Zharr relates, “We were with the Prophet, upon whom be peace, on a journey. When the caller to prayer wanted to give the azhan, the Prophet said `Let it cool down.` This happened two or three times, until we saw the shadows of the hills. Then the Prophet said, `The extreme heat is from the fragrance of Hell. If the heat becomes extreme, delay the prayer until it becomes cool.” (Related by al-Bukhari and Muslim.) However, this delay does have a limit. According to Ibn Hajr`s Fath al-Bari, “The scholars differ over how long one may wait to let the temperature cool. Some say, `Until the shadow of an object becomes an arm`s length,` or `Until the shadow becomes one-fourth of one`s height.` Others say one-third or one-half, and so on. Its ruling is according to its basic principle, and it changes with different circumstances, provided that the prayer is not delayed until the end of its time.”
Almighty Allah knows best.