Yosemite Nature Notes - Episode 8 - Snow Line


Uploaded by yosemitenationalpark on 28.02.2010

Transcript:
[Music]
Wet weather to return to the Central California
Interior by this weekend.
A pair of Pacific weather systems will arrive
late this week.
The first will move into Central California Thursday night,
with periods of rain developing,
continuing into Friday.
A typical winter pattern in California is that
we'll see anywhere from four to seven major storms,
that's a typical winter weather pattern.
You know, we've been under a ridge of high pressure here
for the past, you know, few days and it has been warm
and almost borderline spring like conditions.
And so now the cold air is reloading out
over the Eastern Pacific
and it's going to bring another big system,
so we'll see temperatures coming back down
and snow levels dropping.
If you're going to be out there Friday,
you're going to be getting wet.
Now, the question is, is it going to be rain
or is it going to be snow?
So we come back over here and we take a look
and it says here around 4,800 feet,
we're expecting snow, for points 4,800 feet and above,
we're expecting snow.
So here we are standing at the boundary
of Yosemite National Park.
We've got the Merced River flowing down here next to us,
and on the other side, Highway 140,
and up the road is the Arch Rock Entrance Station.
We've got some rain, it's the beginning of a big storm,
it's supposed to rain and snow all day,
and we've got a temperature of 46 degrees
and we are here at 2,126 feet.
We've got a benchmark nearby that verifies that.
So we're going to head up the road
and see where we can find rain turning to snow.
In some strict term, snow line is actually
the elevation at which snow stays year round.
And then you have a seasonal snow line
and that's the line at which
snow can be expected to be in the wintertime.
And then you have the snow line
that can happen in a certain storm,
which is extremely variable over time
and space in each individual storm,
that transition is what we are chasing.
We're trying to find out, in this storm,
on this day, in this hour,
where on this mountain is that transition happening.
So here we are at Tunnel View Lookout,
a very popular place to come
and see an excellent view of the Valley,
which you can see here.
It's about 4,400 feet in elevation
and our thermometer said 37 degrees.
So 37 degrees, it's getting colder and still raining.
We seemed to have found the
lower end of the transition zone here,
it's a very first sign of snow happening at 5,000 feet,
and it continues to transition just all around us right here.
Here we are on Glacier Point Road,
headed up towards Badger Pass,
we're at about 6,200 feet in elevation
and it's 30 degrees out, and it's definitely snowing.
It was an interesting experience driving up
the road from 5,000 feet,
maybe at 5,800 feet was when we had like snow on the road
and we needed to drive much differently,
and people were putting on chains,
so it's an interesting transition
from 5,000 to 5,800 feet I would say,
the transition from where
snow began to where it was both snowstorm.
So the average snow line to the best of our ability
to estimate is about 5,700 feet in Yosemite.
If the snow line is at 5,700 feet,
that means snow is falling on approximately 90%
of the park and about 10% of the park is receiving rain.
The park's topography is such that
the majority of the landscape lies between
7,000 and 9,000 feet, and right now
all of that area receives snow during a typical storm.
The snow line in the Sierra Nevada
is really important for us to understand,
is it shifting, we need to know.
There is some variability from year-to-year,
just as there is variability from storm-to-storm.
What we want to know is, is there a trend happening
over many years where we're seeing it a creep upward?
And one of the concerns is, as the climate warms
and that snow line goes up,
a significant portion of the park will begin to receive
rain rather than snow and that has a number of effects.
If you get snow, then it has a tendency to run off
more slowly in the springtime,
as opposed to if you get a very heavy rain event
at the higher elevations,
it's going to run off immediately.
An extreme example are what folks refer to as
rain on snow events,
when the snow line will go all the way up to 10,000 feet
and above and for a landscape like Yosemite that means
95% of the park is actually receiving rain.
And when that much area is receiving rain,
at the intensity that we normally get,
which can be three to five inches in 24 hours,
that's a significant amount of water,
and it's moving rapidly into the streams
and down into the main channels,
such as the Merced.
On January 1, 1997, Yosemite National Park
experienced its worst flood in 42 years.
A forceful tropical storm dumped rain onto a deep snow pack,
melting snow and sending torrents of water
to the Park's lower elevations.
Roads were undercut by erosive currents,
leaving asphalt surfaces with no support underneath.
Thousands of feet of rock retaining wall
and road shoulder were ripped away.
In some places entire sections of road were washed out.
So if you look behind us, you see Pohono Bridge.
In the 1997 flood, Pohono Bridge was completely covered in water.
The Merced River here is currently at about
180 cubic feet per second,
that's how much water is moving past any given point,
that's the volume.
In the 1997 flood, it was over 2,400 CFS.
So for reference this is 180, that was 2,400,
it was the biggest flood on record
in Yosemite National Park, on the Merced River.
So here we are at Badger Pass, 7,200 feet,
and it is 29 degrees is what our thermometer said.
And it is snowing, there is no question,
constant steady snowstorm happening all around us.
This experiment can help us understand
one of the main reasons
why the ecosystems in the Sierra Nevada are so diverse,
because we have variable temperatures,
we have this variation that's created
just by a change in elevation,
just by availability of water, just by temperature regime;
how warm is it, how cold is it.
It's interesting, because I have studied snow and rain
and the transition zone and tried to understand
how those things are significant hydrologically,
but I have never chased the snow line this way,
where we're actually taking a journey up the mountain
to find out where is it actually happening today.
So quantifying that has been really exciting,
it's been very, very interesting.