After Makkas Barad, Paroh threw up his hands in defeat and said: ה' הַצַדִּ יק Hashem is right! The Meforshim note that this is the only Makka where Paroh said such things. Why is Barad special? Mitzrayim is a country that never has rain or hail. (Rashi).
Chazal say that Seder Zera’im is called Emuna, since a farmer always looks up to Shamayim for the rain he needs for his crops. But Mitzrayim lives off the Nile River. Instead of looking up, they look down. This explains why the Mitzri'im became:
תנחומא (בובר וארא אות כ) הִּנְנִּי מַמְטִּיר כָּעֵת מָּחָּר בָּרָּד כָּבֵד מְאֹד וגו', וְעַתָּה שְלַח הָּעֵז אֶת מִּקְנְךָ וגו' (ט יח -יט ). אמרו רבותינו בכל המכות לא אמר פרעה ה' הצדיק אלא במכת ברד בלבד, למה , אדם שהוא מבקש להלחם עם חבירו ולנצח אותו, פתאום הוא בא עליו, והורגו ונטל כל מה שיש לו, אבל הקב"ה אמר לפרעה, וְעַתָּה שְלַח הָּעֵז אֶת מִּקְנְךָ וגו' , באותה שעה אמר ה' הצדיק.
Rashi in Parshas Vayigash explains that Mitzrayim does not drink rainwater, but rather the Nile rises and waters the land. From the blessing of Yaakov and onward, Paroh would come to the Nile and it would rise to greet him and water the land (Tanchuma).
In Yeshaya, it is explained that all of Egypt is made of canals, and rain does not fall there, but the Nile rises and waters it. Therefore, the destruction of Egypt is compared to the drying up of the canals.
Ramban on the verse הִּנְנִּי מַמְטִּיר כָּעֵת מָּחָּר בָּרָּד כָּבֵד מְאֹד אֲשֶר לֹא הָּיָּה כָּמֹהוּ בְמִּצְרַיִּם לְמִּן הַיּוֹם הִּוָּּסְדָּה וְעַד עָּתָּה (ט יח) notes that it was a great wonder for hail to fall in Egypt, where rain never falls.
Chazal in Shabbos say that Seder Zera’im is called Emuna because a person believes in the Creator and sows. Even so, יִּרְ אַת ה' הִּיא אוֹצָּרוֹ – Yiras Hashem is the treasure.
In Taanis, it is said that rain falls only for בעלי אמונה – people of faith. (Tehillim: אֱמֶת מֵאֶרֶץ תִּצְמָח וְצֶדֶק מִּשָמַיִּם נִּשְקָף).
Without Yiras Shamayim, people become שְ טוּפֵי זִּימָּה – immoral. They are strictly Olam HaZeh’niks. In Mitzrayim, we fell into מ"ט שַעֲרֵי טוּמְאָּה – the 49 levels of Tum'ah. Mitzrayim also means Meitzar, a boundary, i.e., they are M’tzum’tzam (confined) and cannot pull out of their Yetzer Hara which they are addicted to. They are in the jail of their Yetzer Hara.
