Chapter 9
Extratropical Northern Hemisphere Climates
2. What role does the low-level jet play in the climate of North America?
Answer: The low-level jet (LLJ) is a fast moving river of air in the low levels of the atmosphere. The LLJ
affects the climate of North America by supplying copious amounts of moisture from the Gulf of Mexico
into the southern Great Plains. This low-level moisture enhances the development of midlatitude cyclones
and increases the probability of severe weather occurrence.
4. Discuss the oceanic circulation regime responsible for the creation of the North Atlantic Drift.
Answer: The North Atlantic Drift is a warm surface ocean current that flows across the North Atlantic
Ocean as an extension of the Gulf Stream. The North Atlantic Drift is created from the clockwise rotation
of the Bermuda-Azores high. This circulation pattern on the east side of the high-pressure system
transports warm tropical waters poleward from the North Atlantic basin to the Arctic Ocean.
6. Compare/contrast the role of continentality in North America and Asia.
Answer: While continentality is an important climatic factor for North America and Asia, it has a
stronger influence on Asia’s climate. The climate of the eastern half of North America is affected by the
maritime influences from the Gulf of Mexico and the North Atlantic Ocean. The north-south alignment of
the Rocky Mountains in North America allows maritime tropical (mT) air masses to penetrate deep into
the continental interior east of the Rockies. Continentality is the primary climatic attribute of Asia. The
high-latitude interior of the continent is far removed from the moderating influence of major water
sources. This continentality produces an extreme range of temperatures throughout the year. The lowest
temperatures recorded in the northern hemisphere have occurred in Asia.
8. What is a monsoon?
Answer: A monsoon is a seasonal reversal of wind that occurs due to seasonal shifts in atmospheric
circulation. During the winter months the land surface cools off quickly and a high-pressure system
becomes established over the Asian interior while low-pressure is situated over the Indian Ocean. Winds
flow offshore responding to the pressure gradient force and create cold and dry conditions over much of
Asia. The Asian interior heats considerably during the summer months due to continentality. The intense
surface heating causes a thermal low to develop and results in onshore flow from the relatively cool
Indian and North Pacific Oceans. The persistent airflow over the continent supports the development of
clouds and precipitation over Asia. It is important to remember that the monsoon also induces a distinct
dry period that lasts for several months.
10. Discuss the similarities and differences between Desert and Steppe Climates.
Answer: Most of the True Desert and Steppe climates are classified as “cold” in the extratropical part of
the world. These dry conditions are associated with sinking air on the poleward edge of the Hadley cell,
along with continentality and rain shadow effects. Steppe climates receive more precipitation and
generally surround the True Desert climates. Steppe climates are regarded as transition regions between
an arid and semiarid climate. Precipitation totals tend to increase during the summer season when
convective thunderstorm activity is greater and the ITCZ approaches the region. The higher precipitation
totals in a Steppe climate result in the proliferation of grasslands that become taller and thicker with
increasing distance from the True Desert core.
12. Compare and contrast Mediterranean and Marine West Coast climate regions.
Answer: Mediterranean climate regions are noted by warm to hot summer temperatures and mild winter
temperatures. This climate type is characterized by a wintertime peak in precipitation and distinctly dry
summer conditions. In general, precipitation increases with latitude because poleward areas are more
affected by midlatitude wave cyclones. Marine West Coast temperatures are milder throughout the year
compared to a Mediterranean Climate. Unlike Mediterranean climates, annual precipitation totals in
Marine West Coast climates are higher and evenly distributed throughout the year due to the strong
influence of midlatitude cyclones. Marine West Coast climate have a high percentage of cloudy days and
light precipitation.
14. Why are Marine West Coast climates so warm for their latitudinal locations?
Answer: Marine West Coast climates are warm for their latitudinal locations due to the moderating
effects of mild oceanic air that is transported inland from large bodies of water without restriction from
topographic barriers. These climate types are influenced by the westerlies throughout the year, with
persistent onshore flow suppressing temperature ranges. This moderating effect on temperatures is
especially evident when observing the daily and seasonal temperature extremes. Growing seasons are
abnormally long for this climate type despite their latitudinal position.
16. Discuss the characteristics of the transition from the Subarctic climate to the Tundra and then Ice Cap
climates.
Answer: Extreme seasonality and frequent subfreezing average monthly temperatures help to characterize
a Subarctic climate. Annual precipitation totals are low due to the cold temperatures with a summertime
maximum of rainfall. The transition from a Subarctic to a Polar climate is noted by the treeline. The
tundra climatic type is noted by tundra vegetation that is specially adapted to low temperatures, low sun
angles and short day lengths. The growing season is nonexistent in a Tundra climate with summer
temperatures only slightly above freezing. Annual precipitation totals are also low in the Tundra climate
although snow cover can remain on the ground for long periods due to the low temperatures. Ice covers
the ground throughout the year in an Ice Cap climate as temperatures remain below freezing in every
month. Ice Cap climates receive very little precipitation due to the extremely cold conditions.
18. Discuss the relationships between vertical zonation and increases in latitude relative to resultant
climatic features.
Answer: Local-scale elevation differences termed vertical zonation create vast differences in climate. The
progression of climate zones with height largely mimics the progression of latitudinal climate zones. A
high mountain near the equator would likely be similar to a tropical climate regime near the base of the
mountain. As elevation increases the climate zones would progress through C and D climate zones with
the associated vegetation. Finally the E climate zone and associated vegetation would be located toward
the top of the ice capped mountain summit.<p>
20. Detail the katabatic and regional mountain winds of each of the continents discussed in this chapter.
Answer: Highland climates are noted for the development of a katabatic wind or mountain wind due to
wind channeling and abundant radiational cooling. During winter, the downslope-moving katabatic winds
are especially common during the early morning hours. In the Adriatic region of Europe, a bora wind
forms in the Balkan highlands and spills toward lower elevations through passages in the Alps toward the
Adriatic Sea. The meltemi wind is another significant European katabatic wind that develops over the
Balkan region. This offshore wind blows over the Aegean Sea due to a strong pressure gradient between
high-pressure over the Balkan region near Hungary and low-pressure to the southeast near or over Turkey.
The mistral wind that affects the Rhone River valley of France is perhaps the best-known katabatic wind
system in Europe. This wind flows directly from the Alps and spreads along the Mediterranean coast. The
most prominent Asian katabatic wind system is the oroshi of Japan although the winter monsoon is
technically considered a katabatic wind.