Exodus 10:21

And the Lord said unto Moses
About the eleventh day of the month Abib: stretch out thine hand toward heaven;
where the luminaries are, and from whence light comes: that there may be darkness over the land of Egypt, even darkness which
may be felt;
that is, what caused it, the gross vapours and thick fogs; for otherwise darkness itself, being a privation of light, cannot be felt: Onkelos paraphrases it,

``after that the darkness of the night is removed;''

so Jonathan; that it might appear to be different from that, and be much grosser.