Unit 12: Precipitation, Air Masses, and Fronts
• Air masses
• Lifting Mechanisms for
Precipitation
• Thunderstorms and
related phenomena
(tornadoes)
A front in the making over the N Pacific Ocean.
OBJECTIVES
• Develop the concept of air masses—their
character, origin, movement patterns, and
influence on precipitation.
• Explain lifting mechanisms behind
precipitation.
• Discuss the four basic mechanisms for
producing precipitation: convergent lifting,
frontal, convectional, and orographic
precipitation.
Air Masses
An air mass is a large body of air with relatively homogeneous
character of temperature and humidity.
Classification by latitude, moisture
mT, mP, cA, cP, cT
Movement and transitions-character
of air masses can change as moves over
different land surfaces, or crosses
mountains, ie mP can become cP after
crossing the Rockies, cP can become mT
when moving over warm water.
Source:
https://www.meted.ucar.edu/sign_in.php?go
_back_to=http%253A%252F%252Fwww.met
ed.ucar.edu%252Ffire%252Fs290%252Funit7
%252Fprint_3.htm
North American Air Masses & Sources
Source regions and common paths of the principal air masses that affect
the continental United States.
Lifting Mechanisms That Produce Precipitation
• Convergence-when similar air masses
converge, they are forced to rise,
forming clouds
• Convection-surface heating causes
warm air parcel to rise and form clouds
• Orographic-mountains and large
obstacles force air to rise, forming clouds
• Frontal-when warm and cold air masses
meet, cold air lifts warmer air forming clouds
Source:
http://keithrogershome.com/Chap7CldsPcpnFog.html
Lifting by Convergence
Along the ITCZ warm trades meet and rise. Precipitation along
the ITCZ shifts with season, into the summer hemisphere.
Average daily rainfall rates (mm/day) for January and July based upon
measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite for
the years 1998-2007.
Frontal Precipitation
Vertical cross-section through a warm front (top) and cold front (bottom). The
vertical scale is greatly exaggerated. Warm fronts typically have a 1:200 verticalto-horizontal ratio; the ratio for cold fronts is approximately 1:70.
Convectional Precipitation
Convection often leads to isolated showers from building cumulus clouds.
Occasionally, these clouds grow large enough to form thunderstorms.
Each thunderstorm cloud, cumulonimbus, has a life cycle of 3 stages.
The three stages of the life cycle of an air mass thunderstorm: cumulus, mature,
and dissipating. Updrafts dominate the developing stage, both updrafts and
downdrafts are found during the mature stage, while only downdrafts are found
during the dissipating stage. The approximate width at each stage of the
thunderstorm is shown at the bottom.
Cumulonimbus grows near the Canadian Rockies in
an afternoon thunderstorm.
Source: http://www.physicalgeography.net/fundamentals/7t.html
Severe Thunderstorms
Conceptual model of a severe thunderstorm producing
sizeable hailstones. The red dashed lines represent the warm
updrafts, the blue lines show the cold downdrafts, and the
green lines represent the movement of hailstones. The
thunderstorm is moving from left to right.
Global Lightning Distribution
Average yearly number of lightning flashes observed by satellites between 1995-2003. Widespread
convergence and convection in the tropics and along the Equator produces air mass thunderstorms
that are present year round. Air mass thunderstorms and severe thunderstorms along frontal
boundaries occur during the warm season in the middle latitudes. Thunderstorms are much less
common in the high latitudes due to the weak surface heating and more limited air mass contrasts.
Source: http://www.lbagroup.com/international/tower-lightning-protection.php
Thunderstorms
Figure 5.36
Tornadoes and Their Impacts
• Tornadoes occur in most mid-latitude continents, where warm
and cold air masses collide and strong polar jet streams add
windshear.
• Tornadoes are extremely low pressure vortices that descend
from powerful thunderstorms to the ground. They may be
smaller than a city block, but have winds of up to 130 m/sec
(300 mph). Strong winds and large pressure gradients can pick
up or move huge objects or destroy houses.
• The U.S. has more than 1000/yr with most in tornado alley (the
south central Plains)
• Tornadoes occur in all 50 states
• and in all months, with most
• occurring in spring-early summer.
Source:
http://www.spc.noaa.gov/faq/tornado/
Damage path of EF-5 tornado that ripped through
Joplin, Missouri on May 22, 2011.
April 27, 2011 Tuscaloosa, Alabama in the most deadly
tornado day since 1925 Tri-state outbreak. In the 3-day
outbreak across the SE, 358 tornadoes struck with 324
fatalities.
Tornadoes
Figure 5.39
2011 Tornado Reports-Nearly 1700 confirmed
April 2011 set records with 875 tornadoes, 226 on April 27th.
Tornadoes occur in all 50 states, with tornado alley (south central
Plains) having the highest frequency in the world.
Source: http://www.spc.noaa.gov/wcm/ustormaps/1981-2010-stateavgtornadoes.png
Southern states have less storm shelters, basements;
more mobile homes.
Source: http://www.spc.noaa.gov/wcm/ustormaps/1981-2010-stateavgfatals.png
Source: http://www1.ncdc.noaa.gov/pub/data/cmb/images/tornado/clim/tornadoes_bymonth.png
Orographic Precipitation
Fig 13.7
Orographic precipitation on the upper windward slope of the Cascade Mountains in west-central
Oregon. Note the significant temperature and moisture differences between the windward and
leeward sides of this major mountain barrier. (Vertical scale is greatly exaggerated.)
Fig 13.8
Oregon’s precipitation pattern, with the distribution of isohyets exhibiting the results of
the orographic effect as westerly winds off the Pacific are forced across the northsouth-trending Cascade Mountains. The transect across the Cascades between Eugene
and Bend, diagrammed in Fig. 12.10, is marked by a red line.