וַיֹּ֤אמֶר אֵלָיו֙ מֹשֶׁ֔ה כְּצֵאתִי֙ אֶת־הָעִ֔יר אֶפְרֹ֥שׂ אֶת־כַּפַּ֖י אֶל־יְהוָ֑ה הַקֹּל֣וֹת יֶחְדָּל֗וּן וְהַבָּרָד֙ לֹ֣א יִֽהְיֶה־ע֔וֹד לְמַ֣עַן תֵּדַ֔ע כִּ֥י לַיהוָ֖ה הָאָֽרֶץ׃
I rzekł doń Mojżesz: "Gdy tylko wyjdę z miasta, wzniosę ręce moje do Wiekuistego: gromy ustaną, a gradu nie będzie więcej, abyś poznał, że Wiekuistego jest ziemia.
Rashi
כצאתי את העיר — This is the same as מן העיר [AS SOON AS I AM GONE OUT] FROM THE CITY. But he did not pray within the city because it was full of idols (Exodus Rabbah 12:5).
Chizkuni
כצאתי את העיר, “as soon as I leave the boundaries of the city;” seeing that earlier the Torah had written (verse 19) that anyone who will still be found outdoors will become a target for the hail, now, at the time when the plague was in full swing, the people might say that Moses was afraid to venture outside on account of that hail, Moses announces that he will open his hands in prayer as soon as he is beyond the borders of the city, and that the hail will not stop him from leaving the city. An alternate explanation for this phrase: Moses announces that as soon as he leaves the palace to go into the city, he will spread out his hands in prayer while inside the city. This is also what the Torah reports in verse 33: ויצא משה מעם פרעה את העיר וגו', “as soon as Moses left the presence of Pharaoh and went into the city, etc;”Yet another explanation: according to the plain meaning of the text the plague of hail was far stronger in the fields than in the city, [as there were no trees and grain fields inside the city to destroy, Ed.]. Moses said that he would leave the city to observe the damage caused by the hail and will then begin his prayer as necessary.
Da'at Zekenim
כצאתי את העיר, “as soon as I leave the capital, etc.” The reason why Moses had to pray more in order to stop the plague of hail than any of the other plagues, that the Torah had written that the G–d fearing people had taken his advice (verse 20) and had been saved. The Torah had stated that the capital was full of beasts which served the Egyptians as deities. Had Moses raised his hands in prayer in the capital this might have been misunderstood as his praying