Water vapor must have something, such as a dust particle, to condense. For this reason, dust is necessary in clouds and snow.

While an earlier Roman emperor, Nero, made it illegal to be a Christian, Constantine made it illegal not to be a Christian.

If air has a humidity of 100%, we say that the air is saturated.

Air always flow away from a region of high pressure to fill in a region of low pressure.

Augustus Caesar was the first emperor of the Roman Empire.

Only about 47% of the sun's radiation gets through the atmospheric barrier. About 35% is reflected into space and the other 18% is absorbed by the atmosphere.

A message on the much written-on walls of Pompeii read, "It is a wonder, O Wall, that thou has not yet crumbled into ruin under the weight of so much written nonsense."

In older times, modern France was called Gaul.

The Senate of ancient Rome was a gathering of the oldest and wisest Roman men.

The Alps are the largest mountain system in Europe.

The timber line of a mountain is the point beyond which trees cannot grow.

The auroras are caused by solar winds made up of subatomic particles colliding with air molecules in the thermosphere at the North and South poles.

Air is 78% nitrogen, 21% oxygen, and 1% carbon dioxide and other gases.

The city of Alexandria in Egypt was the most important city of its time period.

The dividing line between the moon's lighted and dark regions is called the terminator.

Sirius, one of the stars in the Canis Major constellation, is the brightest star in the sky.

The sun is about 93,000,000 miles away from the Earth, yet its rays only take eight minutes to reach our planet.

Greece is on the Balkan Peninsula, surrounded by the Aegean Sea, the Ionian Sea, and the Mediterranean Sea. The Gulf of Corinth almost cuts Greece in half.

Finnish sailors once believed in "windropes," which, when their three knots were untied, supposedly caused the wind to blow.

Europe is home to three of the world's smallest countries: Andorra, Liechtenstein, and San Marino.