A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 9:3MURRAIN OF BEASTS. (Exo 9:1-7)
Behold, the hand of the Lord is upon thy cattle--A fifth application was made to Pharaoh in behalf of the Israelites by Moses, who was instructed to tell him that, if he persisted in opposing their departure, a pestilence would be sent among all the flocks and herds of the Egyptians, while those of the Israelites would be spared. As he showed no intention of keeping his promise, he was still a mark for the arrows of the Almighty's quiver, and the threatened plague of which he was forewarned was executed. But it is observable that in this instance it was not inflicted through the instrumentality or waving of Aaron's rod, but directly by the hand of the Lord, and the fixing of the precise time tended still further to determine the true character of the calamity (Jer 12:4).
exo 9:6all the cattle of Egypt died--not absolutely every beast, for we find (Exo 9:19, Exo 9:21) that there were still some left; but a great many died of each herd--the mortality was frequent and widespread. The adaptation of this judgment consisted in Egyptians venerating the more useful animals such as the ox, the cow, and the ram; in all parts of the country temples were reared and divine honors paid to these domesticated beasts, and thus while the pestilence caused a great loss in money, it also struck a heavy blow at their superstition.
exo 9:7Pharaoh sent . . . there was not one of the cattle of the Israelites dead--The despatch of confidential messengers indicates that he would not give credit to vague reports, and we may conclude that some impression had been made on his mind by that extraordinary exemption, but it was neither a good nor a permanent impression. His pride and obstinacy were in no degree subdued.
exo 9:8PLAGUE OF BOILS. (Exo 9:8-17)
Take to you handfuls of ashes, &c.--The next plague assailed the persons of the Egyptians, and it appeared in the form of ulcerous eruptions upon the skin and flesh (Lev 13:20; Kg2 20:7; Job 2:7). That this epidemic did not arise from natural causes was evident from its taking effect from the particular action of Moses done in the sight of Pharaoh. The attitude he assumed was similar to that of Eastern magicians, who, "when they pronounce an imprecation on an individual, a village, or a country, take the ashes of cows' dung (that is, from a common fire) and throw them in the air, saying to the objects of their displeasure, such a sickness or such a curse shall come upon you" [ROBERTS].
exo 9:10Moses took ashes from the furnace--Hebrew, "brick-kiln." The magicians, being sufferers in their own persons, could do nothing, though they had been called; and as the brick-kiln was one of the principal instruments of oppression to the Israelites [Deu 4:20; Kg1 8:51; Jer 11:4], it was now converted into a means of chastisement to the Egyptians, who were made to read their sin in their punishment.
exo 9:18PLAGUE OF HAIL. (Exo. 9:18-35)
I will cause it to rain a very grievous hail, &c.--The seventh plague which Pharaoh's hardened heart provoked was that of hail, a phenomenon which must have produced the greatest astonishment and consternation in Egypt as rain and hailstones, accompanied by thunder and lightning, were very rare occurrences.
such as hath not been in Egypt--In the Delta, or lower Egypt, where the scene is laid, rain occasionally falls between January and March--hail is not unknown, and thunder sometimes heard. But a storm, not only exhibiting all these elements, but so terrific that hailstones of immense size fell, thunder pealed in awful volleys, and lightning swept the ground like fire, was an unexampled calamity.
exo 9:20He that feared the word of the Lord . . . regarded not, &c.--Due premonition, it appears, had been publicly given of the impending tempest--the cattle seem to have been sent out to graze, which is from January to April, when alone pasturage can be obtained, and accordingly the cattle were in the fields. This storm occurring at that season, not only struck universal terror into the minds of the people, but occasioned the destruction of all--people and cattle--which, in neglect of the warning, had been left in the fields, as well as of all vegetation [Exo 9:25]. It was the more appalling because hailstones in Egypt are small and of little force; lightning also is scarcely ever known to produce fatal effects; and to enhance the wonder, not a trace of any storm was found in Goshen [Exo 9:26].
exo 9:27Pharaoh sent, and called for Moses and Aaron, and said unto them, I have sinned--This awful display of divine displeasure did seriously impress the mind of Pharaoh, and, under the weight of his convictions, he humbles himself to confess he has done wrong in opposing the divine will. At the same time he calls for Moses to intercede for cessation of the calamity. Moses accedes to his earnest wishes, and this most awful visitation ended. But his repentance proved a transient feeling, and his obduracy soon became as great as before.
exo 9:31the flax and the barley was smitten, &c.--The peculiarities that are mentioned in these cereal products arise from the climate and physical constitution of Egypt. In that country flax and barley are almost ripe when wheat and rye (spelt) are green. And hence the flax must have been "bolled"--that is, risen in stalk or podded in February, thus fixing the particular month when the event took place. Barley ripens about a month earlier than wheat. Flax and barley are generally ripe in March, wheat and rye (properly, spelt) in April.