Transcript Chapter 20.2 Fronts and Lows

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
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
Fronts con’t
Less dense air is forced to rise over denser
air resulting in a wedge shape
Types of Fronts
Cold Front
Warm Front
Stationary Front
Occluded Front
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
Cold Front:
Types of Fronts – Warm Front
Boundary between advancing warm air
mass and cold air mass it is displacing
First signs are wispy cirrus clouds
Advancement of a Warm Front
Types of Fronts – Occluded
Front
Occurs when cold front catches up to warm
front, forcing warm air to rise.
Results in clouds and precipitation
Advancement of an Occluded
Front
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
Stationary Front
Front Symbols
Mid-Latitude Low
Mid-Latitude Cyclone
Upper-Air Flow
Low Pressure systems are controlled (path
and intensity) by upper air flow
See Diagram on pg 443
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
Anticyclones and Cyclones
 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