| Here
is an explanation of a few of the weather terms used frequently
throughout the book.
Air:
The mixture of gases forming the atmosphere. Dry
air is composed mainly of nitrogen and oxygen with small amounts
of carbon dioxide, hydrogen, ozone, and the inert gases. Moist
air contains, in addition, varying amounts of water vapor.
Air
mass: A body of air with similar properties
of density, heat and moisture content throughout. An air mass
will usually cover many thousands of square miles. It is at
the interface or boundaries of differing air masses that stormy,
unsettled weather occurs.
Anemometer:
An instrument designed to measure wind speed.
Atmospheric
pressure: The weight per unit area of the
total mass of air above a given point; also called barometric
pressure.
Blizzard:
A snowstorm accompanied by winds in excess of 35 mph and visibilities
of a ¼ mile or less for an extended period of time.
Circulation:
The flow of air occurring within a somewhat circular wind
system. There is the large scale flow that is the general
circulation around semi-permanent features covering the entire
earth. Secondary circulations occur around more transient,
migratory systems moving through the general circulation.
Deepening:
The decrease of pressure at the center of a low-pressure
area or storm system.
Frost:
Ice crystals formed on grass or other objects by the sublimation
of water vapor from the air.
Gale:
A wind speed from 32 to 63 mph or 28 to 55 knots. It is classified
as a storm if wind speed is from 64 to 73 mph or 56 to 64
knots.
Gust:
A sudden, brief increase in wind speed.
Jet
stream: A band of strong winds concentrated
in a narrow stream high in our weather-producing atmosphere.
In the middle latitudes, this stream of high wind is associated
with the main frontal boundary that separates cold air of
the arctic from warm air of the tropics. In general, the stronger
the temperature contrast across a frontal boundary, the stronger
the winds will be aloft. As mentioned below, low pressure,
or storm systems develop along these frontal boundaries. These
systems are then pushed along by the jet-stream winds.
Front:
A transition zone between air masses of differing
density. Temperature is the most important determinant of
density, so a front usually separates cold and warm air masses.
There are different kinds of fronts, all with distinguishing
characteristics. The main frontal boundaries are warm, cold,
occluded and stationary fronts.
Low
Pressure: An area of minimum atmospheric pressure.
In the Northern Hemisphere, wind flows counter-clockwise around
a low. These systems distribute heat from the tropics northward
in the southerly flow ahead of them and cold air from the
arctic southward in northerly flow behind them. On any given
day, there are a number of low-pressure areas on the weather
map across North America. The strongest of these systems turn
into memorable storms that dump heavy rain and snow and bring
gale-force winds to Upper Michigan.
A
low often develops along a stalled frontal boundary. As the
system matures, the boundary begins pin wheeling around the
low. This action develops the familiar warm and cold fronts
seen on weather maps. The structure of a low is such that
the heaviest, steadiest precipitation falls to the north and
northwest of the system in the cold air. A typical low will
produce the heaviest snow from 100 to as much as 250 miles
northwest of its track. The advancing cold front will usually
produce more scattered, showery precipitation.
High
Pressure: An area of maximum atmospheric pressure.
In the Northern Hemisphere, air flows clockwise around a high.
The largest of these systems sits over a region for days and
takes on the characteristic heat and moisture content of the
region. The high then becomes an air mass that can cover many
thousand square miles. Highs often develop over the arctic
regions of Canada, Alaska and Siberia. These house the bitterest
air masses that drop southward into Upper Michigan during
a typical winter. Another familiar spot for high-pressure
development is the eastern Atlantic Ocean. This high becomes
the “Bermuda High” that pumps heat and humidity
toward Upper Michigan in the summer.
The air flow around a high promotes stable, fair weather.
In Upper Michigan, the approach of a high-pressure system
in the cold season often does just the opposite—the
northerly component to the wind ahead of it blows across Lake
Superior and results in cloud development and lake-effect
snow.
Lake
effect: In general, the effect of a lake in
modifying the weather about it shores and for some distance
downwind. Since Upper Michigan is a peninsula surrounded by
water on all sides except its border with Wisconsin, lake
effect plays a large role in the climate during all seasons.
The U.P.’s most famous aspect of lake effect is snow.
During the cold season, air flowing over the relatively warm
water of Lake Superior causes evaporation, which leads to
snow showers downwind over the Peninsula. Snow off Lake Michigan
occurs less frequently because the predominant wind direction
during the winter is northwest. The largest seasonal snowfalls
in the United States outside of mountain locations occur over
the snow belts of Upper Michigan and in some down-wind areas
of upstate New York.
In the
warm season, the Great Lakes are huge, natural air conditioners.
Water warms much more slowly than land. This property of water
means that the Great Lakes and the air just above them retains
winter chill well into the warm season. This chilly air over
the water is denser than the warmer air over land. This means
that the cold, lake-chilled air regularly moves inland as
a lake breeze. This phenomenon gives rise to the familiar
forecast phrase “cooler near the lake.”
While
evaporation occurs in the cold season, lake effect in the
warm time of the year leads to condensation. Warm air with
relatively high moisture content flows across Lakes Michigan,
Superior and Huron and quickly saturates, forming great fog
banks that roll inland along the shore. This effect is most
pronounced in the spring and early summer when the water is
the coldest. By late summer and early fall, the Great Lakes
have finally reached their warmest levels of the season. This
lingering warmth keeps the growing season going longer along
the shoreline than in areas just a few miles inland. It also
means an early inland snowstorm will likely be rain or a mixture
of rain and sloppy snow within a few miles of the Great Lakes.
Millibar
(mb): Is the metric unit of pressure. One
millibar is 0.02953 inches of mercury (Hg). Average sea-level
pressure is 1013.2 mb or 29.92 inches of mercury.
Precipitation:
Falling products of condensation or sublimation such
as rain, snow, sleet or drizzle. Precipitation elements are
usually larger than 0.02 centimeters—particles smaller
than this usually remain suspended in the air.
Radiosonde:
A device attached to a weather balloon used to measure various
constituents of the atmosphere. The most common constituents
are wind, pressure, moisture and temperature measured at various
altitudes and geographical positions. The radiosonde then
transmits the information to a fixed receiver.
Severe
Thunderstorm: A thunderstorm with winds of
50 knots (58 mph) or greater and/or hail of ¾ inches
in diameter or greater.
Squall:
A strong wind characterized by a sudden onset with duration
on the order of minutes, and an equally sudden decrease in
speed. An event is considered a squall only if the wind speed
of 16 knots or higher is attained for at least two minutes.
A squall line is a well-marked line of strong, gusty winds,
turbulence and often heavy showers and/or thundershowers.
Straight-line
winds: Winds blowing from the same direction.
This term is usually used in distinguishing winds blowing
in a straight line out of a thunderstorm from the circular
winds of a tornado.
Tornado:
A localized and violently destructive windstorm occurring
over land characterized by a funnel-shaped cloud extending
toward the ground.
Trough:
An elongated area of low barometric pressure.
Upper
Air: In this book, the term refers to the
weather-producing atmosphere more than a mile above the ground.
Upper-air
trough: A trough
of low pressure in the upper atmosphere. Looking at a map,
it appears to be a “dip” in the upper-level flow
or jet stream. Surface low pressure often forms in the lower-right
or southeast quadrant of an upper-air trough.
Upper-air
ridge: This feature
appears, literally, as a ridge or hill in the upper-level
flow. Surface high pressure develops just downwind of the
axis or peak in the ridge.
Troughs
and ridges take up residence in any one location for a number
of days, weeks and even a season. Their position in relation
to Upper Michigan strongly influences the type of weather
we receive.
Warm
sector: The portion of a low-pressure area
containing warm air, lying in advance of a cold front and
to the rear of a warm front. The usual location of the warm
sector in a low over the Northern Hemisphere is in its southeastern
quadrant.
Wind:
Is air in motion. This phenomenon occurs naturally
in the atmosphere and is usually considered to be air moving
parallel to the ground. |
