Difference between Lake Track & Treadmill Tempo

I noticed a very obvious perceived effort difference when
doing tempo run between treadmill and on the lake track.
The effort perceived on the mill is always easier
compared to lake track for the same pace and distance,
and this is confirmed by the HR reading. I think the
HR on the lake track is at least 10 beat higher than
those on the mill.

After some thinking, I think below are the reason:

(1)Turns
On the lake track, there are about 11 turns,
which 4 of the turns are quite sharp. I
estimate the sharp turn will cost me 2 seconds
each, while the gentle ones take
a second off each assumeing I am running on a
5:00m/Km pace. This totals to about 15
seconds per lap of slowing down.
Andthe 15 second extra for the 1.3K lap
translate into 5:12m/Km pace.
So, the turns slow the running for about
12 seconds per km.

Without realizing this previously, I always
tried to run a 6:30 lap and wonder why it is
soooooo much harder than the mill's.

If my estimation is right, then I am actually
runing 4:45min/Km for the 6:30 lap, and it has to
be some difference, right?


(2)Heat
While I run on the mill, I always ON the aircond
of the room and have another fan blowing towards
me to do the colling.
From the Polar watch's log, I can see my skin temperature
on the mill averages about 27/28 degree Celsius,
while those of the late evening lake track running
can rise up to 31 degree Celsius.

Of course, we know higher temperature translates into
higher effort needed to sustain the same pace.
I am not sure how much time it will cost me, but
I am sure it causes the higher HR recorded on the
lake track.

(3)Biomechanical Difference
I used to think that biomechanical difference of running
between overground and treadmill is the most
important contributor to the perceived effort difference.
I no longer think so. In fact, I think this is the most
negligible factor for the difference.

Taking into all these considerations, I am now happier
for my lake track tempo result.

I concluded that I just have to accept the fact
that the lake track tempo will always
give a lower VDOT then that on the mill for a
certain effort. I shouldn't compare them,
as it is like comparing an orange with an apple.

Crank on.