Rain - Precipitation that falls to earth in liquid drops that are greater than 0.5 mm in diameter. Liquid drops that are smaller than .5 mm are categorized as drizzle. The largest raindrop that has ever been measured occurred in September of 1995 in Brazil as well as in July of 1999 in the Marshall Islands. At both these sites the raindrops were measured at a whopping 8.6 mm (.338 in) diameter!

Winter Storm Warning - It is issued by the National Weather Service when severe and life threatening winter weather conditions are occurring or are imminent. Most are issued for heavy snow and wind, but may also be issued for ice and sleet storms. The amount of snowfall needed to warrant a winter storm warning varies around the country.

Tropical Depression - A tropical cyclone with sustained winds lower than 30 knots/35 mph/55kph. The main impacts from a tropical depression would be heavy rain, gusty winds, and a modest storm surge where they make landfall.

Hygrometer - An instrument designed to measure the water vapor content and relative humidity in the atmosphere. Leonardo da Vinci built the first version of a hygrometer back in the 15th century.

Smog - The first definition of smog was a mixture of smoke and fog in the atmosphere which reduces visibility. Over time smog has now changed its meaning to restricted visibility due to pollution emitted from human activities. A type of smog known as photochemical smog, common to heavily populated areas, forms when UV light from the sun reacts with low level pollutants such as nitrogen oxides. On hot days this reaction can create ozone gas at the surface which is dangerous to humans.

Mesosphere - The mesosphere is a layer of the Earth's atmosphere located at an altitude of 30 to 50 miles above the Earth's surface. It is the layer where most meteors burn up upon entering the atmosphere. Because of this, there is a high concentration of iron and other metals in the mesosphere.

Perihelion - The point in the orbit of a planet in which it is closest to the sun. For Earth, the time of year when it's orbital path comes closest to the sun occurs in early January. This can be surprising to learn for folks in the Northern Hemisphere who experience winter during that time of the year. This just proves how much more impactful the Earth's tilt is compared to its distance from the sun during its orbit.