Jenny Chen (810)
4/3/14
Lab 15: Climate Investigation
Investigating Climates of Mountain Ranges
Introduction:
Climate is the characteristic weather of a region measured over many years. One factor that
affects climate is mountain ranges in which the rain shadow effect takes place. On the windward side of
the mountain range, incoming humid air is force to rise when it hits the obstacle (the mountain). Higher
up, due to less gravitational pull, the air is less dense (colder), causing the moisture from the humid air
mass to condense along the windward side of the mountain range. When the warm air mass descends
down the leeward side, it had lost most of its moisture, thus, resulting in a drier climate on the leeward
side compared to the windward side with the mountain range under a “rain shadow”.
The purpose of this lab was to determine how mountain ranges affect the precipitation on the
leeward and windward sides. If there were a mountain range, then the windward side would get more
precipitation because when humid air is pushed up the windward side, it condenses causing
precipitation, so when the air mass reach the leeward side.
Method
Go to http://www.mysciencelearning.org/experiments/backbone-port/index.php and click on the
icon labeled “Climate Graphs” located on the black bar towards the top of the webpage. Next, click on
the blue “Open Map” button. This will take you to a map focused on the United States and data on
various pinpointed locations. Drag to view the western part of the United States and select the location
Spokane, WA and Seattle, WA since they are the leeward and windward side of a mountain range. Make
sure that no other locations are selected. To view the graph, click on “submit”. Now the graph will show
the data for the two selected locations in a double line graph. The data was collected by cooperative
observers and ROSA through GOES (Geostationary Operational Environmental Satellites) which
collected wide ranges of data ranging from river stages, precipitation, temperature, wind reports, and
other useful information in keeping track of a region’s climate. Now go under “Pick Parameter” to
specify the information needed by changing “Average Monthly Max” to “Total Monthly Precipitation”.
Set the time period into From January 1st, 2000 to December 1st, 2001. Finally observe the graph and
record your findings.
Results:
The data comes from the monthly precipitation of Spokane, WA and Seattle, WA between
January 1st, 2000 and December 1st, 2001. The amount of monthly precipitation in Seattle was overall
higher than the amount of monthly precipitation in Spokane. Both locations’ lowest monthly
precipitation was in August while their highest was in November. However, their lowest monthly
precipitation was about the same but for the highest monthly precipitation, Seattle had 2-3 time more
(about 200 millimeters more than Spokane’s). Furthermore, most of the monthly precipitation results in
Spokane are less than 50 mm while most of Seattle’s monthly precipitation data are greater than 50 mm
for the majority of the year. The precipitation changes pretty predictably though the seasons and there
are only a few points in which Seattle’s monthly precipitation was same/less than Spokane’s; April
2000, July 2000, September 2000, and July 2001. It is occasionally in the spring or summer of the ~3
years period that the total monthly precipitation of Spokane becomes slightly higher than that of Seattle.
During the winter, the total monthly precipitation in Seattle was always higher than that of Spokane’s.
This shows that for the majority of the time, the precipitation of the windward side (Seattle) is generally
more than that of the leeward side of the mountain range (Spokane).
Scientific Discussion
If there were a mountain range, then the windward side would get more precipitation because the
rain shadow effect causes leeward side of mountain ranges to be drier because most of the moisture in
the warm and humid wind/air mass was released when it condenses on the windward side. After this
investigation, the hypothesis was correct, because the total monthly precipitation in Seattle (located on
the windward side of the mountain range) was generally higher than the total monthly precipitation of
Spokane, WA which was on the leeward side. The mountain range causes the windward side to have
more precipitation than the leeward side. This occurred because when incoming warm and humid air
reaches a mountain range, it is forced up where it condenses (air mass becomes denser so it cools) and
moisture falls as precipitation. By the time it descends down the leeward side, it has lost most of its
moisture. This effect results in a rather humid climate on the windward side and arid on the leeward
side.
One source of possible error might be that we didn’t control the elevation of the two locations. If
they were at different elevations on either side of the mountain range, this may affect the temperature
and how much of the moisture from the humid air mass precipitates to that region. For this investigation,
we only observed the climate of the two regions for 23 months. If we analyzed the data of the two
regions for a longer period of time, the fact that the hypothesis is the truth may be more convincing. For
an extension, we can collect data on average the monthly precipitation on the leeward and windward
sides of many regions with corresponding elevations in mountain ranges.