For the sun is no sooner risen with a burning heat - Isaiah Isaiah 40:7 employs the word "wind," referring to a burning wind that dries up the flowers. It is probable that the apostle also refers not so much to the sun itself, as to the hot and fiery wind called the simoom, which often rises with the sun, and which consumes the green herbage of the fields. So Rosenmuller and Bloomfield interpret it.
It withereth the grass - Isaiah 40:7. It withereth the stalk, or that which, when dried, produces hay or fodder - the word here used being commonly employed in the latter sense. The meaning is, that the effect of the hot wind is to wither the stalk or spire which supports the flower, and when that is dried up, the flower itself falls. This idea will give increased beauty and appropriateness to the figure - that man himself is blasted and withered, and then that all the external splendor which encircled him falls to the ground, like a flower whose support is gone.
And the grace of the fashion of it perisheth - Its beauty disappears.
So shall the rich man fade away in his ways - That is, his splendor, and all on which he prideth himself, shall vanish. The phrase "in his ways," according to Rosenmuller, refers to his counsels, his plans, his purposes; and the meaning is, that the rich man, with all by which he is known, shall vanish. A man's "ways," that is, his mode of life, or those things by which he appears before the world, may have somewhat the same relation to him which the flower has to the stalk on which it grows, and by which it is sustained. The idea of James seems to be, that as it was indisputable that the rich man must soon disappear, with all that he had of pomp and splendor in the view of the world, it was well for him to be reminded of it by every change of condition; and that he should therefore rejoice in the providential dispensation by which his property would be taken away, and by which the reality of his religion would be tested. We should rejoice in anything by which it can be shown whether we are prepared for heaven or not.

For the sun is no sooner risen - We need not pursue this metaphor, as St. James' meaning is sufficiently clear: All human things are transitory; rise and fall, or increase and decay, belong to all the productions of the earth, and to all its inhabitants. This is unavoidable, for in many cases the very cause of their growth becomes the cause of their decay and destruction. The sun by its genial heat nourishes and supports all plants and animals; but when it arises with a burning heat, the atmosphere not being tempered with a sufficiency of moist vapours, the juices are exhaled from the plants; the earth, for lack of moisture, cannot afford a sufficient supply; vegetation becomes checked; and the plants soon wither and die. Earthly possessions are subject to similar mutations. God gives and resumes them at his pleasure, and for reasons which he seldom explains to man. He shows them to be uncertain, that they may never become an object of confidence to his followers, and that they may put their whole trust in God. If for righteousness' sake any of those who were in affluence suffer loss, or spoiling of their goods, they should consider that, while they have gained that of infinite worth, they have lost what is but of little value, and which in the nature of things they must soon part with, though they should suffer nothing on account of religion.

For the sun is no sooner risen with a burning heat, but it withereth the grass, and the flower thereof falleth, and the grace of the fashion of it perisheth: so also shall the rich man fade away in his (k) ways.
(k) Whatever he purposes in his mind or does.

For the sun is no sooner risen with a burning heat,.... As it is about the middle of the day, when it shines in its full strength, and its heat is very great and scorching, especially in the summer season, and in hot climates:
but it withereth the grass; strikes it with heat, causes it to shrivel, and dries it up;
and the flower thereof falleth; drops off from it to the ground:
and the grace of the fashion of it perisheth; its form and colour, its glory and beauty, which were pleasant to the eye, are lost, and no more to be recovered. This shows, that earthly riches, like the flower of the field, have an outward show and glory in them, which attract the mind, and fix an attention to them for a while; they are gay and glittering, and look lovely, are pleasant to behold, and desirable to enjoy; but when the sun of persecution, or any other outward calamity arises, they are quickly destroyed, and are no more.
So also shall the rich man fade away in his ways; riches are uncertain things now, they often make themselves wings and flee away; they are things that are not, that are not solid and substantial they are a vain show; they sometimes fade away in a man's lifetime, before he dies; and he fades away, and comes to decay, amidst all the ways and means, designs and schemes, he forms and pursues, and all the actions and business he does; and if not, when he fades away, and dies amidst all his riches, his glory does not descend after him, but falls off from him, as the flower of the field before the heat of the sun.

Taken from Isaiah 40:6-8.
heat--rather, "the hot wind" from the (east or) south, which scorches vegetation (Luke 12:55). The "burning heat" of the sun is not at its rising, but rather at noon; whereas the scorching Kadim wind is often at sunrise (Jonah 4:8) [MIDDLETON, The Doctrine of the Greek Article]. Matthew 20:12 uses the Greek word for "heat." Isaiah 40:7, "bloweth upon it," seems to answer to "the hot wind" here.
grace of the fashion--that is of the external appearance.
in his ways--referring to the burdensome extent of the rich man's devices [BENGEL]. Compare "his ways," that is, his course of life, James 1:8.

For the sun arose and withered the grass - There is an unspeakable beauty and elegance, both in the comparison itself, and in the very manner of expressing it, intimating both the certainty and the suddenness of the event. So shall the rich fade away in his ways - In the midst of his various pleasures and employments.