What happens at the first Trumpet?
At the first trumpet, hail and fire mixed with blood are cast upon the earth, burning up all the green grass and a third of the trees.
When the first angel sounds his trumpet, Revelation 8:7 tells us that hail and fire mingled with blood are cast down upon the earth. The result is devastating: all the green grass is burned up, and a third part of the trees are burnt as well.
This judgment is described as a fire, hail, and blood storm falling from heaven, striking the vegetation of the earth. Every bit of green grass is consumed, and one third of all the trees on earth are destroyed in this burning.
This same kind of judgment is echoed by the prophet Joel, who speaks of fire devouring the pastures of the wilderness and flames burning up the trees of the field, with the beasts of the field groaning because they have no pasture. Ezekiel likewise speaks of the Lord raining hailstones, fire, and brimstone, and kindling a fire that devours every green tree, showing that this kind of judgment by fire and hail has long been testified to throughout Scripture.
The scriptures
The first angel sounded, and there followed hail and fire mingled with blood, and they were cast upon the earth: and the third part of trees was burnt up, and all green grass was burnt up.
How do the beasts groan! the herds of cattle are perplexed, because they have no pasture; yea, the flocks of sheep are made desolate. O LORD, to thee will I cry: for the fire hath devoured the pastures of the wilderness, and the flame hath burned all the trees of the field. The beasts of the field cry also unto thee: for the rivers of waters are dried up, and the fire hath devoured the pastures of the wilderness.
And I will shew wonders in the heavens and in the earth, blood, and fire, and pillars of smoke.
And I will plead against him with pestilence and with blood; and I will rain upon him, and upon his bands, and upon the many people that are with him, an overflowing rain, and great hailstones, fire, and brimstone.
Son of man, set thy face toward the south, and drop thy word toward the south, and prophesy against the forest of the south field; And say to the forest of the south, Hear the word of the LORD; Thus saith the Lord GOD; Behold, I will kindle a fire in thee, and it shall devour every green tree in thee, and every dry tree: the flaming flame shall not be quenched, and all faces from the south to the north shall be burned therein. And all flesh shall see that I the LORD have kindled it: it shall not be quenched.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.