1. Weather & Climate Investigation 9

2. Air Mass Formation  Stagnant air – sometimes a large mass of air stays over a portion of Earth’s surface for a long time and does not move.  Source regions – stagnant air can form over any part of the Earth, but usually forms over areas with uniform surfaces with few mountains, called source regions  As the air stagnates, it changes, The kinds of changes depend on the type of surface below the air.

3. Air Masses  Air mass – large body of air that stagnates over Earth’s surface. They are large, thick bodies of air that are uniform in temperature and humidity.

4. Types of Air Masses  Maritime (m) – forms over water  Continental (c) – forms over land  Polar (P) – forms in cold, polar latitudes  Tropical (T) – forms in warm, tropical latitudes  Example: A maritime tropical (mT) air mass forms over warm, tropical water.

5. Where are the warmest/coldest air masses?  The coldest air masses form in the polar latitudes (around 60 N or S) in the winter.  The warmest air masses form in the tropics (around 30 N or S) in the summer.

6. How do air masses move?  Air masses do not stay in one place forever. Air pressure differences between locations move air masses from high pressure areas to low pressure areas.  In North America, cold air masses originate in the north and move south & east across the U.S.A.  Warm air masses that originate in the tropics move north & east across the USA.  This movement allows air masses of different temperatures and humidity to collide.

7. Fronts  Front – when two air masses meet, a boundary forms between them and there is very little mixing. Each air mass keeps its identity. The boundary between the air masses is a front.  Fronts take the name of the air mass that is moving faster and overtakes a slower air mass.

8. Fronts  Cold front – when a cold air mass overtakes a warm air mass. The colder, more dense air moves under the less-dense, warm air, lifting it up.  Warm front – a warm air mass overtakes a cold air mass  Stationary Front – does not move; could turn into a warm or cold front depending on which way it turns  Occluded Front – when a faster moving cold front overtakes a slower-moving warm front

9. What is the difference between weather & climate?  Weather – the condition of the atmosphere over a short period of time and can change hour by hour, day by day  Climate – the average weather condition over a long period of time

10. Climate Change  Greenhouse effect – the ability of gases in Earth’s atmosphere to trap heat (ex: Carbon dioxide). –This is good in the right amounts! It is what keeps Earth warm enough for life!  Global Warming – the increase in the Earth’s overall temperature. Can cause large environmental changes.