the following quote is taken only for convenience from a website whose creedal leanings are not known to me except that they rely on those who don't belong to the jam'ah and hence I won't post the link: Alamah Ibn Qudamah Hanbali رحمھالله writes in Al Mughni (vol.1, p.382): ‘There is no diference in the scholars that he time of Isha commences when Shafaq disapears. There is only a minor diference betwen two sayings as to exactly when Shafaq is said to have ocured. Some say Shafaq disapears when the red light after Magrib disapears, others say it is when the white light disapears. 1. Ibn Umar and Ibn Abas , Ata’, Mujahid, Saed Ibn Jubair, Imam Zuhri, Imam Thawri, Ibn Abi Layla, Ishaq, Imam Malik, Imam Ahmed, Imam Shafi and Imam Abu Yusuf and Muhammad Shaybani رحمھمالله, all state that Shafaq disapears with the disapearance of the red light (أحمر ) after Magrib. 2. Anas bin Malik , Abu Hurairah , Umar bin Abdul Aziz, Imam Awzai’, Ibn Munzir and Imam Abu Hanifa الله رحمھم al states that Shafaq disappears when both the red (أحمر ) and white light (أبیض ) have disapeared after Magrib and night commences.' ----- This clearly shows that in relating the aftermath of Karbala the ulema are not referring to the redness caused by particles present in atmosphere (the astronomical twilight أحمر) but a different redness which was blood red in colour and not the pinkish or orange hues which are naturally seen at sunset. Consider the following: During sunset hours, the light passing through our atmosphere to our eyes tends to be most concentrated with red and orange frequencies of light. For this reason, the sunsets have a reddish-orange hue. The effect of a red sunset becomes more pronounced if the atmosphere contains more and more particles. The presence of sulfur aerosols (emitted as an industrial pollutant and by volcanic activity) in our atmosphere contributes to some magnificent sunsets (and some very serious environmental problems). Which proves that it is quite possible for the local horizon in a particular region to appear redder than usual and for a limited time period. ----------- "The circumambient medium which we call the atmosphere is to the earth only as so much ground-glass globe to a lamp—something that breaks, checks, and diffuses the light. We have never known, never shall know, direct sunlight—that is, sunlight in its purity undisturbed by atmospheric conditions. It is a blue shaft falling perfectly straight, not a diffused white or yellow light; and probably the life of the earth would not endure for an hour if submitted to its unchecked intensity. The white or yellow light, known to us as sunlight, is produced by the ground-glass globe of air, and it follows readily enough that its intensity is absolutely dependent upon the density of the atmosphere—the thickness of the globe. The thickening of the aërial envelope lies in the particles of dust, soot, smoke, salt, and vapor which are found floating in larger or smaller proportions in all atmospheres. .... Dust is always present in the desert air in some degree, and when it is at its maximum with the heat and winds of July, we see the air as a blue, yellow, or pink haze. .... This air-blue is seen at its best in the gorges of the Alps, and in the mountain distances of Scotland; but it is not so apparent on the desert. The coloring of the atmosphere on the Colorado and the Mojave is oftener pink, yellow, lilac, rose-color, sometimes fire-red. And to understand that we must take up the ground-glass globe again. .... So it is that in dry countries like Spain and Morocco or on the deserts of Africa and America, you will find the sky rose-hued or yellow, and the air lilac, pink, red, or yellow........I mean now that the air itself is colored....... I am not now speaking of the color of objects on the earth, but of the color of the air. A thing too intangible for color you think? But what of the sky overhead? It is only tinted atmosphere. And what of the bright-hued horizon skies at sunrise and sunset, the rosy-yellow skies of Indian summer! .... The truth is all air is colored, and that of the desert is deeper dyed and warmer hued than any other for the reasons just given. It takes on many tints at different times, dependent upon the thickening of the envelope by heat and dust-diffusing winds. I do not know if it is possible for fine dust to radiate with heat alone; but certain it is that, without the aid of the wind, there is more dust in the air on hot days than at any other time. When the thermometer rises above 100° F., the atmosphere is heavy with it, and the lower strata are dancing and trembling with phantoms of the mirage at every point of the compass. It would seem as though the rising heat took up with it countless small dust-particles and that these were responsible for the rosy or golden quality of the air-coloring. .... The stronger the wind, and the more of dust and sand, the brighter the coloring. The climax is reached in the dramatic sand-storm—a veritable sand-fog which often turns half the heavens into a luminous red, and makes the sun look like a round ball of fire. ....... The yellow haze of the desert is seen at its best when there is a yellow sunset, and the pink haze when there is a red sunset, indicating that at least the sky has some part in coloring by reflection the lower layers of desert air. Whatever the cause, there can be no doubt about the effect. The desert air is practically colored air. By that I do not mean that one looks through it as through a highly colored glass. The impression should not be gained that this air is so rose-colored or saffron-hued that one has to rub his eyes and wonder if he is awake. The average unobservant traveller looks through it and thinks it not different from any other air. But it is different. In itself, and in its effect upon the landscape, it is perhaps responsible for the greater part of what everyone calls "the wonderful color" of the desert. ...... But granted the quantity and the quality of local colors in the desert, and the fact still remains that the air is the medium that influences if it does not radically change them all. The local hue of a sierra may be gray, dark red, iron-hued, or lead-colored; but at a distance, seen through dust-laden air, it may appear topaz-yellow, sapphire-blue, bright lilac, rose-red—yes, fire-red. During the heated months of summer such colors are not exceptional. They appear almost every evening. I have seen at sunset, looking north from Sonora some twenty miles, the whole tower-like shaft of Baboquivari change from blue to topaz and from topaz to glowing red in the course of half an hour. I do not mean edgings or rims or spots of these colors upon the peak, but the whole upper half of the mountain completely changed by them. The red color gave the peak the appearance of hot iron, and when it finally died out the dark dull hue that came after was like that of a clouded garnet. ..... Through the long desert day the sunbeams are weaving skeins of color across the sands, along the sides of the canyons, and about the tops of the mountains. They stain the ledges of copper with turquoise, they burn the buttes to a terra-cotta red, they paint the sands with rose and violet, and they key the air to the hue of the opal. The reek of color that splashes the western sky at sunset is but the climax of the sun's endeavor. If there are clouds stretched across the west the ending is usually one of exceptional brilliancy. The reds are all scarlet, the yellows are like burnished brass, the oranges like shining gold." SOURCE. The above quote further shows that in a desert the redness of the sky can be of several shades and due to several influences. In this is a lesson for the nay-sayers that if the ulema have accepted the reports, it's not because of their credulity or the superstitious tendencies of the 'uneducated' folk but because of their faith that the same Almighty who is capable of changing the draperies of the skies everyday and even every hour is also capable of fore-warning the enemies of His Rasul (peace be upon him) and his noble household (ridhwanullahi'alayihim ajmayin) of His ('azzawajal) imminent wrath by painting the skies with blood red colors. And Allah knows best. wassalaam.