Is it possible to pull in far away digital stations these day's? I remember a few nights when the atmosphere was just right pulling in analog stations that were 3, sometimes 4 states away with a good rooftop antenna. Does anybody stay up late at night playing with there antenna trying to get far away stations these day's?
The truth is yes and no.
Yes during days of high tropospheric propagation such as in the summertime when you have a cloudless day with a high pressure over your area, you will sometimes enjoy days of good reception.
Usually the reception follows the boundaries of the high and low pressure system, anti cyclonic weather and will come in from different angles then a direct line of sight, but again, it is not a reliable form of communications.
Your best bet for reception is to use a antenna with the highest amount of gain and the smallest amount of Front to back. The proper way to receive a signal is with the antenna pointed directly towards the strongest part of the signal. To do that, it requires that you use a antenna rotor.
A pre amplifier that will overcome loss in the wire due to long runs will help your cause as much as anything else. But you do not use a amplifier as a substitute for a good antenna.
Tropo is formed when there is a inversion layer in the atmosphere where one temperature is on top of another temperature and a inversion layer forms. Kind of like a electric blanket.
In the early morning when the earth is cool and the atmosphere heats up faster then the surface of the earth, a inversion layer forms.
Again in the late evening when the atmosphere cools faster then the surface of the earth, a second inversion layer will occur.
Usually by 11 Am the atmosphere equalizes and the tropo disappears.
Sometimes tropo will last all night.
Moisture in the atmosphere - fog - will help reception, but a heavy rain will block reception. Swarms of bugs, flocks of birds, a airplane between you and the signal will all block the signal.
Most all UHF signals are line of sight. If there is trees, a building more then 3 stories high, or a hill or mountain between you and the transmitter, you will not have any reception - due to the fact that without something for the signal to bounce off - it just travels out into space.
The curvature of the earth plays a big role in how this all works.
VHF antenna's were mounted high as possible. You had something called height gain, where the higher you went, the less power it took to transmit a equal distance. The FCC takes this into account when they allocate how much transmit power you are allowed to have.
UHF - which is not ground following, will skip over the most local people and will transmit out to it's boundary. In some markets, you can actually see the light blinking on top of the tower, yet have no signal.
To combat that, they lower the transmitter stick down to say 150' about average terrain, not the height in feet above sea level or the height above ground, but the height above everything else around the tower. Be it trees or mountains or buildings or what ever.
When trying to calculate line of sight, you have to take into account the curvature of the earth and refraction. Light waves and radio waves behaves exactly the same.
If you built a 1000' tower on the shore of lake Erie and you built a second tower 1000' tall tower 48 miles away and you stood on top of the one tower, you could see the light blinking on the top of the other tower - about 48 miles away. Even though the natural horizon might only be 26 miles, the reason why you could see the light on the other tower is due to the light bending around the curvature of the earth.
When ever you have a clear field of view, and something for the signal to bounce off, like a lake. Your line of sight can increase up to about 90 miles for a radio wave. That is the reason why some people in Florida complains about Tropo reception in the summer time and no reception in the fall and winter, especially when there is hurricanes in the fall, wintertime and spring.
Most of Florida is at or just slightly above sea level. Surrounded by water and there is not much to block the signals. You can sometimes watch a VHF station 100 miles away, turn the antenna a different direction and watch a station 50 miles in the other direction on the same frequency. Since television numbers are both actual and virtual.
) from 100+ miles away. While the reception time span was limited, it can be done. 
