"Even if you had foam covers that were stripped off seconds before launch, the rate of ice build-up would be dangerous and in the several minutes of ascent through a moist atmosphere, thick ice could build up then drop off and hit the shuttle."

"External Tank: Each external tank is 154 feet long and 28.6 feet in diameter."

The maximun amount water that could form ice must come a 28.6' cylinder of air going from sea level to space. How much quanity of moisture is in that column? Or a slightly different measure could how much ice could form on exposed Shuttle ET sitting on the pad in brisk wind in a time period of around 7 minutes?
Going back to first case the moisture from this column will be distributed around the circumference of the tank and the topmost section of cold ET surface will draw the most amount of moisture from the air and as this air moved down the tank it will be drier therefore the colder section below the topmost section will have less moisture to draw from the air.
If for instance there is 6" of water in the column, you have the maximun water which could freeze on the tank as 321 cu ft of water. The circumference of ET is 89.8', therefore each 10' of length has 898 sq ft of surface area. 50' length has 4492 sq ft, therefore if all the water was deposit on just this 50' section it would just less than a inch thick. And also 321 cu ft of water weigh 20,544 lbs. Though having 6" of water in this column is a lot of moisture in the air- not sure air can hold this much- 1 to 2" might more reasonable. And probably half of this moisture is going to be in the first mile up. And finally what percentage of this total moisture will be drawn from air and stick to ET.