April is often called a month for showers, and that doesn't just mean rain.
The Lyrid meteor shower peaks tomorrow night, though it starts tonight (April 21) between midnight and dawn. The time for the most activity will be during the day, at 7 p.m. Greenwich time on April 22, but for people watching from the U.S. the best time is probably the early morning hours of April 22 and 23.
While there are fewer meteors than with the more famous Perseid shower in August, they more often generate trails and fireballs. About 15 percent will generate the trails, while fireballs are relatively rare.
Sometimes the Lyrids can be especially intense, with more than 90 meteors per hour. That happened in 1982. But the only way to know if a shower will be particularly intense is to go out and see it.
Unfortunately, the moon will be out on the same nights that the Lyrids reach maximum intensity and that means they will be harder to see -- generally the meteors are about the same brightness as the stars in the Big Dipper. To find the point in the sky where the meteors appear to be coming from, it's best to look east as the constellation Lyra, for which the shower is named, rises after sunset in April and gets higher in the sky as the night wears on. Lyra can be identified by the star Vega, which is one of the brightest in the spring sky for people in the northern hemisphere. (To find it, make a line from the back side of the bowl of the Big Dipper, which will point to Deneb in the constellation Cygnus. Facing north, Vega is to the right and higher in the sky).
The Lyrids have been observed for the last 2,800 years and were noted by Chinese astronomers as far back as 687 B.C. The Lyrids are debris from the comet C/1861 Thatcher, discovered in 1861 by A. E. Thatcher. The comet is a long-period one, which returns to the neighborhood every 415 years, so it won't be back for centuries yet.
Meteors are bits of rock that hit the Earth's atmosphere and burn up as they enter it. They can range from the size of a mustard seed to that of a pebble, and the vast majority burn up at altitudes of 50 to 60 miles. Some meteors are large enough to penetrate to lower altitudes, though they still break up before they hit the ground. These objects are called bolides.
Most meteor showers, Like the Lyrids and the more famous Perseids, are particles of rock and ice from comets. That is why the showers tend to occur at certain times of the year - the Earth is crossing the orbit of a comet, and plowing through the dust trail it left behind.
When meteors hit the atmosphere they are moving between 25,000 and 160,000 miles per hour, so fast that the air compresses in front of them, heating up the gases and vaporizing the rock. This is called ram pressure. (Most people think it is friction from the air, this is not quite true). One interesting effect is that if a meteor is big enough to hit the ground (and become a meteorite) the energy removed by the air actually lowers the temperature and slows it down - making the meteorite cool to the touch.