Transcript Weather Fronts - SCHOOLinSITES

Weather Fronts
CScope Unit 9
Did you know?
 Today’s
weather is influenced by air from
thousands of kilometers away, perhaps
from Canada or the Caribbean Sea?
What is an air mass?
 An
air mass is a huge body of air that has
similar temperature, humidity, and air
pressure.
 Once the air mass enters the U.S., they
are blown from west to east (known as the
jet stream).
What is a front?

As air masses travel across land and oceans,
two masses may bump into each other. If they
have different temperatures and densities, they
form a front.
 Because different densities do not mix (like oil
and water!), this collision of fronts may cause
storms and change the weather.
 A front may be 15-200 km wide and reach up to
10 km in the troposphere!
There are 4 fronts. . .create a grid to write the descriptions.
We will also draw a diagram for the warm and cold fronts.
Cold Front
Stationary Front
Warm Front
Occluded Front
Cold Front
A
cold front forms when denser cold air
moves under warm air.
 The warm air is pushed upward,
condenses and forms clouds or
precipitation.
 Cold fronts move quickly so they can
cause violent storms at times.
 After a cold front, the air will be cool and
dry.
Warm Front
A
warm front forms when warm air moves
over a slow moving cold air mass.
 If the air is humid, light showers, rain, or
snow in the winter will fall. If the air is dry,
clouds may form.
 Warm fronts move slowly so it can be
rainy, foggy, snowy, or humid for several
days!
Stationary Fronts
 Sometimes
a cold and warm air mass will
meet, but neither has enough force to
move the other so they face each other in
a “standoff”!
 Water vapor in the warm air condenses
into precipitation and if the stalled area
remains, it may bring many days of clouds
and precipitation.
Occluded Fronts

At an occluded front, a warm air mass is caught
between two cooler air masses.
 The denser cool air masses move underneath
the less dense warm air mass and push it
upward.
 The cooler masses mix and the ground
becomes cooler so the warm air mass is
occluded, or cut off, cools, condenses, and the
weather turns cloudy and rainy or snowy.
The Highs and Lows
 Atmospheric
pressure at the Earth’s
surface is one of the keys to the weather.
 Weather maps feature H’s for high
pressure and L’s for low pressure.
High Pressure
 High
pressure occurs when air pressure is
higher than the surrounding air pressure.
 The air is slowly descending. As it lowers,
it warms so clouds do not form. This is
why this pressure is associated with fair
weather.
 The sinking air spirals outward, or
clockwise, and is steered west to east by
upper level winds.
Low Pressure

Air rises near low pressure areas.
 When a weather forecaster says a low pressure
area or storm is moving toward your region, this
usually means cloudy weather and precipitation
are on the way because as air rises, it
condenses and forms into clouds.
 Counterclockwise circulation of air surrounds
this area with the system being steered from
west to east by upper level winds.
Reading A Weather Map, Page 526
Homework Project
 Copy
the Lows, Highs, Warm and Cold
Fronts for each day on the U.S. map.
 Shade in precipitation.
 Record the weather facts for each day.
 You can use weather.com to find the
information.
 We will summarize the information on
Thursday.