NATS 101

Lecture 22Air Masses

What is an Air Mass?

• Air Mass

Large area (>1600 km by 1600 km) of air that contains relatively uniform, horizontal distributions of temperature and moisture.

How Air Masses Form

• If surface air resides in a region for a few days, it acquires the thermal and moisture characteristics of the underlying surface.

• Source regions for Air Masses are:

Big in area [ >>(1600 km)2 ]

Dominated by persistent high pressure and light winds

Air Mass Source Regions

• Contrasting source regions are

Continents versus Oceans

Tropics versus Poles

• An Air Mass is designated in terms of its Source Region

Air Mass Characteristics

Ahrens Table 8.1

Air Mass Source Regions for NA

Ahrens Fig 8.2

Creation of cP Air Mass

Williams p22

Creation of cP Air Mass

Williams p23

Creation of mP Air Mass

Lutgens & Tarbuck, p 230

Lake Effect Snows

As cP air flows over the warmer, open Great Lakes, it is warmed and moistened. When the modified cP air flows onshore, prodigious snows of several feet can result.

Lutgens & Tarbuck, p 230

Creation of mT and cT Air Masses

Williams p24

Contrasting Air Masses

Ahrens Fig 8.917 Apr 1976

Paths of cP Air Masses

Ahrens Fig 8.3

cP Air Mass

Ahrens Fig 3 p20324 Dec 1983

Modification of cP Air Mass

Ahrens Fig 8.4Ahrens Fig 8.4

cPcP

cPcP

mPmP

mPmP

warm oceanwarm ocean

warm oceanwarm ocean

mP Air Masses

Ahrens Fig 8.7Ahrens Fig 8.5

Air mass modified Air mass modified further as it crosses further as it crosses several mountain several mountain ranges of West U.S.ranges of West U.S.

cT Air Mass

Ahrens Fig 8.1029-30 Jun 1990

Winter mT Pacific Air Masses

mTmT

mPmP

Ahrens Fig 8.8Ahrens Fig 8.8

“Pineapple Express”

Weather Map with Air Masses

Ahrens Fig 8.11

Summary

• Air Masses

Large (>1000 miles) regions with “uniform” temperature and moisture characteristics

• Classified by Source Region Continental (c) or Maritime (m)Polar (P) or Tropical (T)

• Source Regions

Big in area (>>1600 km by 1600 km)

Dominated by light winds (long resident times)

Assignment for Next Lecture

• Topic - Fronts • Reading - Ahrens pg 212-219• Problems - 8.12, 8.13