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Beyond Rain Delays: Weather's Role in America's Favorite Pastime

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One of the classic lines from the motion picture Bull Durham (1988) is, “A
good friend of mine used to say, ‘This is a very simple game. You throw the
ball, you catch the ball, you hit the ball. Sometimes you win, sometimes
you lose, sometimes it rains.’ Think about that for a while.” But from a
meteorological and physics point of view, is it really that simple of a game, even if it
doesn’t rain?
by Jan Null
WEATHER’S ROLE IN
AMERICA’S FAVORITE PASTIME
A classic microburst with 80-mph gusts approaches a minor league baseball stadium in Columbus, Ohio, on February 11, 2009.
24 W EATH ERW ISE JANUARY / FEBRUARY 2014
When most people think about weather and
America’s pastime, it’s probably about whether to
bring sunblock or if today’s game might be rained
out. And even that’s less of a factor when seven
out of the 30 major league ballparks have domes.
But meteorological factors play a large role in the
sport, whether it rains or shines, ranging from
the microscale physics that affect the ﬂight of a
thrown or batted ball to the macroscale impacts
of the shape and location of a ballpark.
The Physics of a Baseball in
Motion
On the face of it, it should be pretty simple,
as the motion of a smooth ball or other sphere
moving through the air obeys Newton’s Laws of
Motion. Once a ball is moving, under Newton’s
ﬁrst law, it will tend to move in a straight line
unless it is acted on by external forces. Those
forces are weight, drag, and lift. The weight of a
baseball is distributed throughout the ball, but it
can be thought of as single point, or its center of
gravity—that point about which it rotates. Also,
as a ball moves through the air, there is drag,
which is the resistance of the air to the motion of
the ball. This is a function of the properties of the
air such, as its density and viscosity. And the ﬁnal
basic force on a ball in motion is the aerodynamic
lift that is perpendicular to the ball’s direction of
ﬂight. The complex interaction of these forces
and a baseball’s motion are further inﬂuenced
by a number of other factors. First, a thrown or
batted ball is spinning, sometimes a lot and other
times, like with a knuckleball, very little. Second,
the stitches on a baseball, all 108 tiny raised red
stitches, further disturb the ﬂow of the air around
the ball. And third, the air through which it is
traveling may itself be moving and have a wide
range of temperature, humidity, and density.
The aerodynamics of air ﬂowing around
a spinning baseball is the same as that of air
ﬂowing over the wing of an airplane. Originally
postulated by Sir Isaac Newton in 1672 after
watching tennis being played at Cambridge,
England, this effect was further described by
German physicist Gustav Magnus in 1852 and is
known as the “Magnus Effect.” It says that the
ﬂow of air on the side of a sphere will be faster
due to the combined effect of its motion through
the air and any rotation of the ball. This side
with the faster ﬂow creates an envelope of lower
pressure, and the ball will move in that direction.
This is further enhanced by the raised seams on a
baseball, which help the ball’s ability to develop
a boundary layer and a greater pressure difference
between the upper and lower zones.
A typical baseball is thrown with varying
degrees of spin, be they backspin (the motion
of the stitches on the top of the ball is in the
opposite direction of travel) or off-axis spin (as
would be the case of a curveball). The amount
of spin can be in excess of 2,000 revolutions per
minute. It may seem intuitive, but the amount of
movement due to the spin of a ball is positively
correlated to the speed of the ball. It is the
magnitude of all of these lift forces combined
with the drag and weight that determine the
ultimate path of the ball.
When a baseball is thrown or batted its trajectory is affected by gravity (weight), the
density of the air (drag) and the aerodynamics forces due to its rotation (lift).
The backspin on a pitched baseball can be in excess of
2,000 rpm and significantly affects its aerodynamic
properties.
JAN NULL
JAN NULL
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Air Density and Humidity and the
Baseball in Flight
Importantly there is also the density of the
air through which a ball may be moving. The
density affects the amount of drag on the ball,
and so both the distance that the ball may
travel and how much it might curve. The
density is a function of the interactions between
temperature, atmospheric pressure, and the
amount of moisture in the air.
There is considerable confusion about the
effects of humidity on a baseball and its ﬂight.
“Conventional wisdom” is that moist air is heavy
and therefore dense, causing more drag on the
ball. Actually the opposite is true, because moist
air is lighter than the same volume of dry air.
The molecular weights of diatomic nitrogen
(N2) and diatomic oxygen, which make up
99% of the atmosphere, are 28 and 32 atomic
units, respectively. Compared to these, a water
molecule (H2O) only has an atomic weight of
18 atomic units. Consequently, at a constant
temperature, the more air molecules that are
replaced by water molecules, the less dense the
air. But the differences are slight.
For example, on a 70-degree day, the density
of air that has a relatively low dewpoint of 20
degrees (15% relative humidity, or RH) is
1.198 kilograms per cubic meter (kg/m3). If the
dewpoint is raised to 40°F (34% RH) the density
drops by only .002 kg/m3, and by raising the
dewpoint to 60°F, it only drops by another 00.4
kg/m3. With a 90-mph Major League curve ball,
these differences equate to less than one-tenth of
an inch displacement between the pitcher and
Table 1. Effects of temperature and
elevation on air density.
Air Density (k/m3)
Note: The atmospheric and aerodynamic calculations are
based upon a number of online tools from the NASA Glenn
Research Center “Aerodynamics of Baseball” Web site
(http://www.grc.nasa.gov/WWW/K-12/baseball/index.html).
homeplate, and would changes the distance of a
375-foot home run by less than a foot.
While the humidity of the air has a relatively
minor impact on the ﬂight of the ball, it may
actually have an inﬂuence on the ball itself. In
higher humidity areas, it is theorized that the
ball gains weight by absorbing moisture from
the air, and thus its resiliency and the distance it
might travel are decreased. At the high altitudes
of Denver, Colorado (elevation 5,280 feet), the
low air density and the usually very low humidity
contributed to one of the highest home run rates
in the Major Leagues after the Colorado Rockies
moved to town in 1993. To mitigate this, since
2002, baseballs for the Rockies games are kept
in a humidity-controlled room at 50% RH in an
The temperature has an even greater impact
on the density of the atmosphere, and thus the
ﬂight of the ball, than the humidity. The density,
with constant pressure and humidity, at 50°F is
50°F 70°F 90°F
Elevation sea level 1.241 1.194 1.151
Elevation 1000 feet 1.153 1.111 1.069
Elevation 5000 feet 1.032 0.993 0.956
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1.24 kg/m3, which is approximately 7% higher
than on a 90-degree day, when it’s 1.15 kg/m3.
In the ﬂight of a baseball, this translates to about
two inches of extra “break” on a curveball, or a
decrease of nearly 16 feet on a 356-foot ﬂy ball.
But the biggest differentiator in regard to
the density and thus the ﬂight of the ball is
atmospheric pressure due to the elevation above
sea level. On a 70-degree day, the air density at
sea level is 1.194 kg/m3, which drops by about
3% to 1.15 kg/m3 at 1,000 feet, then by another
4% to 1.11 kg/m3 at 2,000 feet, and all the way
down to 0.99 kg/m3 at 5,000 feet. For each 1,000
feet of elevation change, a curveball will break
three-quarters of an inch more in denser air and
three-quarters of an inch less in air that’s not as
dense. This means that a pitch that might break
as much as four inches at sea level would only
break one-quarter inch at Coors Field in Denver,
Colorado—a change that would mean signiﬁcant
adjustments for both batters and pitchers.
Thinner air has an even more dramatic effect
on batted balls. A well-hit long ﬂy ball at sea
level in San Francisco, California, or New York,
Table 2. Effects of temperature and
elevation on home run distance.
Home Run Distance (feet)
Note: The atmospheric and aerodynamic calculations are
based upon a number of online tools from the NASA Glenn
Research Center “Aerodynamics of Baseball” Web site
(http://www.grc.nasa.gov/WWW/K-12/baseball/index.html).
New York, might go 376 feet on an average
day with 70-degree temperatures and moderate
34% humidity. A similarly hit ball with the
same weather conditions in Atlanta, Georgia,
or Phoenix, Arizona, at about 1,000 feet above
sea level would go about 381 feet. But in mile-
high Denver, Colorado, a ball hit the same
would travel 405 feet! Again, this explains the
previously mentioned placement of baseballs for
Denver Rockies in a humidiﬁer to mitigate the
less dense air.
If baseball were played at the summit of Mount
Everest (elevation 29,029 feet), a similarly batted
ball would be a 505-foot homer. But this would
pale to the same well-hit ball on the atmosphere-
less and low-gravity moon, where it would travel
4,014 feet!
Wind in Play
Thus far, only the motion of pitched and
batted balls through still air has been considered.
However, the air is seldom calm, and the
prevailing winds can signiﬁcantly impact the
ﬂight of the ball, whether it is thrown or batted.
The winds in a particular city are even taken
into account with the orientation of a ballpark
or the type of hitters that a team might focus on
acquiring. And the velocity of wind does not
have to be very strong to make a big difference.
A well-hit baseball on a 60-degree day at
sea level will travel approximately 376 feet.
However if there is a tailwind of just 5 mph, the
ball will travel 415 feet, which is a home run in
most ballparks. And if that wind is increased to
10 mph, the ball will travel another 40 feet and
land about 455 feet from home plate. Conversely,
a 5-mph headwind would decrease that 376-foot
hit to just a 336-foot ﬂy ball out, and a 10-mph
headwind would keep the travel distance down
to just 296 feet.
A head or tail wind also impacts a thrown
ball. For example, a pitcher’s almost “unhittable”
95-mph fastball may become a somewhat more
50°F 70°F 90°F
Elevation sea level 371 376 382
Elevation 1000 feet 376 381 389
Elevation 5000 feet 398 405 410
THINKSTOCK/HEMERA
The thinner air at higher-elevation Coors Field in Denver, Colorado, affects how a ball flies
through the air.
WWW.WE ATHE RWI SE.O RGWE ATHE RWI SE 27
hittable 90-mph pitch when thrown into a
5-mph breeze. Or with a tailwind of 5 mph, that
pitch is a 100-mph blur! The bottom line is that
pitchers love winds blowing in toward home
plate because their pitches are faster, and if they
are hit, they do not travel as far. On the other
hand, batters want to see the winds blowing away
from the plate, which will take something off the
speed of the pitch and will also make the ball go
farther.
But the wind is seldom a true tailwind or
headwind, nor is it the same at every location
inside a stadium. Due to the typical construction
of a baseball stadium, the winds are anything but
a consistent, smooth, constant ﬂow. Instead the
winds are more likely to be swirling, with many
different speeds and directions, all affecting the
ﬂight of any given ball. The wind blowing from
an angle behind the pitcher not only adds some
velocity, but also pushes it sideways for more of
a curve. Or when a ball is hit, the calm winds at
ground level may be a 5-mph tailwind as it rises
to 50 feet in its arc and 10 mph near its peak. Or,
any and every combination of the above could be
happening.
Over the years there have been numerous
efforts to mitigate the effects of the prevailing
wind and other weather elements like rain,
snow, and even the heat through stadium design.
These attempts have been included orienting the
stadium itself to block the wind, changing the
playing ﬁeld dimensions to shorter or longer to
either decrease or increase the number of home
runs, and putting “roofs” over stadiums to keep
the rain out or cool in.
The gusty summer sea breezes and a constant
battle with the elements in San Francisco,
California, have been part and parcel of the
Giants’ history since moving to California in
1958. Candlestick Park opened in 1960 in
a location where the seabreezes swirled and
drained into the edge of the San Francisco Bay.
It only took a year before this was highlighted
on national television during the 1961 All-Star
Game. In the ninth inning, San Francisco relief
pitcher Stu Miller was blown off the mound by a
gust of wind with a ball being called, allowing the
tying run to score from third. In 2000, the Giants
moved to AT&T Park, which was purposefully
located in an area with considerably lighter
winds, and the grandstands were even oriented
in such a way as to block those breezes.
Currently, there are seven major league
baseball stadiums that have gone even further
to mitigate the weather by having either domes
or retractable roofs. The only ﬁxed roof is in
Tampa, while there are retractable roofs in
Houston, Texas; Tampa and Miami, Florida;
Milwaukee, Wisconsin; Phoenix, Arizona;
Seattle, Washington; and Toronto, Canada. The
WIKIMEDIA COMMONS/CAROL M. HIGHSMITH
During the 1961 All-Star Game at breezy Candlestick Park in San Francisco, California, the San Francisco relief pitcher Stu
Miller was blown off the mound by a gust of wind.
28 W EATH ERW ISE JANUARY / FEBRUARY 2014
purpose of the roofs varies in relation to the local
climate. In Phoenix, the retractable roof (and
air conditioning) is used to combat the average
99-degree maximum temperature during baseball
(Seattle, Milwaukee, and Toronto) control for
precipitation, plus early and late season cold
weather, while in in the southeast (Houston,
Tampa, and Miami), the intent is to mitigate
summer heat, humidity, and showers.
In locations without roofs, baseball teams
must deal with the possibilities of rain delays
and rainouts and the resulting negative impacts
on schedules and revenue. The highest number
of rainouts is in the northeast, with Boston,
Massachusetts; Pittsburgh, Pennsylvania;
Cleveland, Ohio; both New York teams; and
Philadelphia, Pennsylvania, all having at least
21 cancellations between 2000 and 2010. During
that same decade, the ﬁve California teams have
only seen about a half-dozen rainouts combined.
But having a roof doesn’t guarantee that you
won’t get rained out. In September 2004, rain
and ﬂooding from Hurricanes Frances and Jeanne
caused games to be canceled in Tampa, Florida,
despite the roof, and the same happened with
Hurricane Ike in Houston, Texas, in 2008. And
even stranger was a 2003 game that was rained
when the retractable roof got “stuck” open!
Freeze-Outs
Baseball is not only plagued by the rain, but at
times the cold has made it less than hospitable for
the “boys of summer.” Historically, the beginning
of the season in April will see some below-freezing
overnight temperatures, but it is rare that even
night games are sub-freezing. However, with
Denver, Colorado, joining the Major Leagues in
1993, there have been games played in weather
more suited for ice hockey. The coldest of these
has been documented in the night game played
between the Colorado Rockies and the Atlanta
Braves on April 23, 2013. The temperature at
the time of the ﬁrst pitch was a very unspring-
like 23°F, and snowmen could be seen scattered
Miller Park in the snow, Milwaukee, Wisconsin, taken April 7, 2007.
FLICKR/JERAMEY JANNENE
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in the stands among the over 15,000 fans. This
smashed the previous ﬁrst-pitch low temperature
record of 28°F.
The World Series, played in October, has also
has had issues with wintery weather. The third
game of the 1997 World Series in Cleveland
against the Florida Marlins was played with
wind chills down to 15°F and occasional snow
ﬂurries. Ironically, the warm weather Marlins
hung on to win 14 to 13! In 2008, the ﬁfth and
ﬁnal game of the World Series took four days to
play, as it was plagued with heavy rain one day,
snow the next, followed by ﬂurries and then rain
showers the third. It wasn’t until the fourth day
that the Phillies would ﬁnally beat the weather
and Tampa Bay Rays. And the seventh game of
the 2001 World Series in Phoenix, Arizona, got
a double whammy, with the game being delayed
ﬁrst by a dust storm and then rain, an even rarer
event in October in Arizona!
So, despite being a simple game, the weather
can indeed make playing a baseball game a battle
of not only the teams on the ﬁeld but also a battle
with the elements. W
JAN NULL, Certified Consulting Meteorologist, is founder of
Golden Gate Weather Services, a part-time faculty at San
Francisco State University, and a former Lead Forecaster with
the National Weather Service.
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