1. Chapter 20.2 Fronts and Lows  Objectives:  -Describe the weather conditions associated with different types of fronts  Describe the life cycle of a mid-latitude low

2. What is a Front?  The boundary that separates opposing air masses  Can be 5 km high and 2000 long  Air masses on either side of a front may differ in temperature and humidity

3. Fronts con’t  Less dense air is forced to rise over denser air resulting in a wedge shape

4. Types of Fronts  Cold Front  Warm Front  Stationary Front  Occluded Front

5. Types of Fronts – Cold Front  Boundary between an advancing cold air mass and the warmer air mass it is displacing  Cold air slides below Warm air  Thunderstorms often accompany cold fronts if they replace humid mT air

6. Cold Front:

7. Types of Fronts – Warm Front  Boundary between advancing warm air mass and cold air mass it is displacing  First signs are wispy cirrus clouds

8. Advancement of a Warm Front

9. Types of Fronts – Occluded Front  Occurs when cold front catches up to warm front, forcing warm air to rise.  Results in clouds and precipitation

10. Advancement of an Occluded Front

11. Types of Fronts – Stationary Front  Front that does not move or moves very slowly  Can result in flooding if precipitation results from warm air rising

12. Stationary Front

13. Front Symbols

14. Mid-Latitude Low Mid-Latitude Cyclone

16. Upper-Air Flow  Low Pressure systems are controlled (path and intensity) by upper air flow  See Diagram on pg 443

18. Troughs and Ridges  Troughs – Upper level winds that slow, forcing air downward creating high pressure at the surface  Ridges – Upper level winds that spread out (diverge) causing surface air to rise, creating low pressure at the surface  Strengths of highs and lows are dependent upon how sharp the bend in upper air flow is

19.  High pressure areas (anticyclones)  Clockwise airflow in the Northern Hemisphere (opposite flow direction in S. Hemisphere) –Characterized by descending air which warms creating clear skies  Low pressure areas (cyclones)  Counterclockwise airflow in N. Hemisphere (opposite flow in S. Hemisphere) –Air converges toward low pressure centers, cyclones are characterized by ascending air which cools to form clouds and possibly precipitation Anticyclones and Cyclones