Chapter 20.2 Fronts and Lows

Objectives:-Describe the weather conditions

associated with different types of frontsDescribe 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 longAir 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 FrontWarm FrontStationary FrontOccluded 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 airThunderstorms 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

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