Quick quiz

Science World, Feb 7, 2005 by Jeanna Bryner

You're taking a ski trip to Denver the next day. But what should you pack? Try your hand at meteorology, the study and prediction of weather to decide.

Rumbling thunder, or even a snowstorm, results as air pockets with differing temperatures trek across a region. The air's journey balances out Earth's heat. Weather maps are snapshots of the marching air masses. By observing the map's current weather patterns, Edwin Danaher, a meteorologist with the National Oceanic and Atmospheric Administration, can forecast approaching weather.

Use the map above and weather-science basics from Danaher below to forecast the weather for your trip.

MERCURY RISING?

Check Denver's air temperature, or the measure of the speed of air molecules (particles of two or more atoms joined together). Chilly air is tightly packed with molecules, meaning it has a high density (amount of mass per volume). Should you pack a parka?--

MOVERS

When two air masses nudge each other, a front (boundary) forms between them. Look out: Fronts can bring a change in weather.

A warm front happens when hot air replaces cooler air. To replace the cold heavyweight, hot air rises. As it climbs, the balmy vapor chills and condenses (changes from vapor to liquid) into clouds and rain.

A cold front forms when chilly air swoops beneath warm air and replaces it. Once the front passes, temperatures drop and skies clear.

When two fronts collide, the hotter air settles above the frostier air. That forms an occluded front. Expect cold-front weather. Is Denver in for a weather change?--

UNDER PRESSURE

Air pressure (force over an area) is one factor that determines whether skies are clear or cloudy.

As a cold front passes, a high-pressure system can follow. The "H" signals that the air is jam-packed with molecules. The heavy air sinks, holding back the rain. Expect sunny skies.

A lingering low-pressure system means less push on the ground-hugging air. That shallow air can rise easily, turning skies rainy.

Will the passing front bring a high or low pressure system to Denver? Should you pack rain gear or sunglasses?--

ANSWERS

1. Yes, pack a parka. The high in Denver is just 39[degrees]F.

2. Yes, temperatures will drop and skies will clear.

3. The passing cold front will bring sunny skies. Pack sunglasses.

Did You Know?

* Relative humidity is the amount of water in the air compared with the maximum amount of water that air can hold. And the warmer the air, the more water droplets it can hold.

* The coldest temperature ever recorded on Earth was -89[degrees]C (-129[degrees]F) at Vostok Station in Antarctica on July 21, 1983.

* Snowflakes form when water vapor in clouds freezes around grains of dust to form ice crystals. The crystals fall from the clouds. During the descent, the crystals bump into each other, and clump together into flakes.

Resources

* Try your hand at changing sunny skies to dreary with this Scholastic interactive weather maker: www.scholastic.com/kids/weather/

* This USA Today site is packed with weather information and interactive graphics: www.usatoday.com/weather/wresources.htm