Weather

Weather is what the atmosphere is doing right now, in one place: temperature, wind, clouds, rain, snow. It can change in minutes. Anybody who's watched a sunny Idaho spring morning turn into a snow squall by lunch knows exactly what that means.

Weather is NOT the same as climate. Weather is your mood today. Climate is your personality over years. (More on climate in Chapter 22.)

Humidity is the amount of water vapor in the air. Warm air can hold more water vapor than cold air.

• Relative humidity compares how much water vapor the air has to the most it could hold at that temperature. 100% means the air is full, or saturated.
• The dew point is the temperature air has to cool to before it becomes saturated and water starts condensing. When the grass is wet on a clear morning, it cooled to the dew point overnight.

Idaho's air is usually dry, which is why your lips crack and static shocks are everywhere in winter.

Clouds form when air rises and cools to its dew point. The water vapor condenses onto tiny specks of dust, smoke, or salt called condensation nuclei, forming billions of tiny droplets or ice crystals.

Air rises for a few reasons: warm air rising by convection, air pushed up over mountains (called orographic lifting), and air shoved up along weather fronts.

Clouds are named by shape and height:

• Cirrus: thin, wispy, and high, made of ice crystals. They often mean a change in weather is coming.
• Cumulus: puffy, cotton-ball clouds with flat bottoms. Usually fair weather.
• Stratus: flat, gray sheets that cover the sky like a blanket. Can bring drizzle.
• Nimbus means rain. A cumulonimbus is a towering thunderstorm cloud that can reach 12 km high with a flat "anvil" top. A nimbostratus is a thick gray layer that brings long, steady rain or snow.

Fog is just a cloud at ground level. Idaho valleys often get winter fog when cold air gets trapped under warmer air above, called an inversion. Inversions also trap smoke and pollution.

Precipitation is any water that falls from clouds. Droplets bump together and grow until they're too heavy to stay up.

• Rain: liquid drops.
• Snow: ice crystals that form when it's below freezing all the way down. Every flake has six sides.
• Sleet: raindrops that freeze into ice pellets as they fall through a cold layer.
• Freezing rain: rain that freezes the instant it hits cold surfaces, coating roads and power lines in ice. Dangerous.
• Hail: balls of ice built up in layers as they get tossed up and down inside a thunderstorm. The bigger the storm, the bigger the hail.

The Rocky Mountains catch a lot of precipitation as air is lifted over them. On the far side, sinking air dries out, creating a rain shadow. It's one reason southern Idaho is high desert.

An air mass is a huge body of air with about the same temperature and humidity throughout. It takes on the traits of where it forms:

• Continental (c): dry, formed over land.
• Maritime (m): humid, formed over oceans.
• Polar (P): cold. Tropical (T): warm.

So a continental polar (cP) air mass from Canada brings Idaho its frigid, dry winter cold snaps. A maritime polar (mP) air mass from the Pacific brings the mountains their snow.

A front is the boundary where two different air masses meet. That's where weather gets interesting.

• Cold front: cold air plows under warm air and shoves it up fast. Brings quick, intense storms, then cooler, clearer weather.
• Warm front: warm air slides gently up over cold air. Brings long, steady rain or snow and warmer temps afterward.
• Stationary front: neither air mass wins, so the front stalls. Can mean days of gloomy weather.
• Occluded front: a fast cold front catches up to a warm front.

• Low-pressure systems (cyclones) have rising air, which makes clouds and storms. On a map, a red L.
• High-pressure systems (anticyclones) have sinking air, which brings clear, calm weather. On a map, a blue H.

In the Northern Hemisphere, winds spin counterclockwise around a low and clockwise around a high.

Meteorologists collect data from weather stations, balloons, satellites, and Doppler radar, which can see rain and the wind inside storms. They feed it into powerful computer models to predict what's next. A 3-day forecast is right most of the time. Beyond about 10 days, the atmosphere is too chaotic to predict reliably.

Weather tools: thermometer (temperature), barometer (pressure), anemometer (wind speed), wind vane (direction), rain gauge (precipitation), hygrometer (humidity).

On a weather map, lines connecting equal pressure are called isobars. Isobars packed close together mean strong winds.