Chapter 12 WEATHER

Section 1 – causes of weather Short term variation in atmospheric conditions are called weather. Climate is the long-term average of variations in weather for a particular place. Climate takes into account the average of the past 30 years.

The constant movement of air on Earth’s surface along with the oceanic currents, redistribute thermal energy around the Earth. Weather is part of the constant redistribution of Earth’s thermal energy.

An air mass is a large volume of air that has the same characteristics as its source region. The source region is the area over which the air mass formed.

There are 5 types of air masses. 1. ARCTIC – source region above 60N latitude, results in cold, dry air.

2. Continental Polar – (continental means its occurs over land) origins are Canada and Alaska. Results in cold, dry weather.

3. Maritime Polar – (maritime means ocean) develops over the Northern Pacific or Atlantic. Results in cool to mild, humid weather.

4. Continental Tropical – develops over Southwestern U.S. and Mexico. Results in hot, dry weather.

5. Maritime Tropical – develops over tropical seas, results in warm, humid weather.

Air mass modification – when an air mass travels over land or water that has different characteristics from where it developed, it can take on some of those characteristics.

When an air mass undergoes modification it exchanges thermal energy and /or moisture with the surface over which it travels.

Section 2 – Weather Systems The direction of Earth’s winds are influenced by Earth’s rotation. This is called the Coriolis effect and results in fluids and objects moving in a curved path rather than a straight path.

Global wind systems help equalize the thermal energy on Earth.

There are 3 basic wind systems. 1. Polar Easterlies – begins as cold air sinks and turns east, usually cold, weak and sporadic.

2. Prevailing westerlies – moves west toward the poles. Steady winds that move most of the weather across the U.S.

3. Trade Winds – air sinks and moves east toward the equator. The horse latitudes are a band of weak winds resulting from the sinking of the trade winds.

The areas where the wind systems meet influence weather because of how air masses of different temp and pressure interact.

Jet Stream – a narrow band of fast moving wind in the upper atmosphere. The speed of the jet stream can reach 250 miles per hour.

Fronts – when two air masses collide Cold fronts – when cold, dense air displaces warm air which is less dense. Heavy precipitation and storms are associated with cold fronts.

Warm front – when advancing warm air displaces cold air.

Stationary front – when 2 air masses meet but neither advances.

Occluded front – when a fast moving cold air mass pushes a warm air mass up because there is another cold air mass in front of it. Heavy rains and storms can occur.

Pressure Systems Low Pressure systems – associated with rising air being replaced by air rushing in to take its place. Characterized by cloudy weather and rain. Air spins counter clockwise.

High pressure systems – sinking air pushes surface air out of the way. They are associated with fair weather. Air spins clockwise.

Section 3 – Gathering Weather Data Thermometer – measures temperature Barometer – measures air pressure Anemometer – measures wind speed Hygrometer – measures humididty

A radiosonde is used to gather information from the upper atmosphere. It is a package of sensors and a battery powered radio transmitter suspended from a balloon.

Doppler radar – measures the speed at which precipitation moves toward or away from a radar station. Good at detecting rotation associated with tornados.

Weather satellites – Use infrared imagery to detect differences in thermal energy. Also used to measure water vapor that help define and identify air currents.

Section 4 – Weather Analysis and Prediction A station model is a record of weather data for a particular site at a particular time.

Lines of equal pressure are called isobars. Lines of equal temperature are called isotherms.

Types of forecasting Digital – created by applying physical principles and mathematics to the atmospheric variables and making predictions about how they will change over time. This is the main method used today.

Analog – based on comparison of current weather patterns to similar patterns from the past. It is called analog because forecasters look for analogous patterns between past and current conditions.

How good are forecasts? A one to three day forecast is usually accurate for expected temperatures and for when and how much precipitation can be expected.