Heat, as mentioned in Revelation 16:9, signifies the lust of evils which are contained in self-love and the delights of it.

(Odkazy: Apocalypse Revealed 692)

692. 16:9 And people were scorched with great heat, and they blasphemed the name of God who has power over these plagues. This symbolically means that owing to the delight of the love of self arising from their grievous lusts for evils, they did not acknowledge the Divinity of the Lord's humanity, even though from it flows every goodness of love and truth of faith.

Heat symbolizes the lusts for evils present in self-love and its accompanying delight (nos. 382, 691). Therefore to scorch with great heat means, symbolically, to be caught up in grievous lusts and so in the delight accompanying the love. To blaspheme the name of God means, symbolically, to deny or to refuse to acknowledge the Divinity of the Lord's humanity and the holiness of the Word (nos. 571, 582). To blaspheme is to deny or to refuse to acknowledge, and the name of God is the Lord's Divine humanity and at the same time the Word (no. 584). To have power over the plagues means symbolically that from the Lord flow every goodness of love and truth of faith by which evils and falsities are removed (nos. 673, 680, 690). And because the seven angels having the seven plagues came out of the temple of the tabernacle of the testimony (Revelation 15:5-6), and the temple of the tabernacle of the testimony symbolizes the inmost of heaven where the Lord is present in His holiness in the Word and in the Law contained in the Ten Commandments (no. 669), and because the influx symbolized by pouring out the plagues came from there (no. 676), it follows that God's having power over the plagues means the Lord from whom springs all else.

[2] We will briefly describe the nature of self-love. Its accompanying delight surpasses every other delight in the world, for it consists of nothing but lusts for evils, and every one of the lusts produces its own delight. Every person is born into this delight, and because it impels the person's mind to focus its thought continually on itself, it draws the mind away from thinking about God and the neighbor except from the perspective of self and with a focus on self. As a consequence, if God does not favor the person's lusts, he becomes angry with God, as he becomes angry with the neighbor who does not favor them.

As this delight grows, it causes the person to be unable to elevate his thought above himself, but instead draws it down to a level beneath him, for it immerses the mind in the inherent nature of his body. As a result the person becomes gradually sensual, and a sensual person speaks in a lofty and elevated tone about worldly and civic matters, but he can speak about God and Divine matters only from memory. If he is a politician, he acknowledges nature as responsible for creation, his own prudence as his guide, and denies God's existence. If he is a priest, he speaks about God and Divine matters from memory, even in a lofty and elevated tone; but at heart he little believes them.