made to investigate the magnitude of difference
on days with lapse temperature structure. Air
among these observations.
temperature differences among the bridges ex-
ceeded 1.0C on 3 of 9 days with inversion struc-
Air temperature was first measured on the east
bank, then a few seconds later at the same height
ture. It is interesting to note that the temperature
at the east portal, center, and west portal of the
differences do not seem systematic, and that the
bridge; about 200 s later the measurements were
maximum temperature difference occurs between
repeated in reverse order. The mean temperature
the two most distant bridges on only two occa-
and the maximum temperature difference were
sions.
calculated for each set of six observations along
We can measure the temperature over the river
the bridge. The temperature differences have been
at an elevation of a few meters above the river
separated according to presence of lapse or inver-
only on those bridges. At seven other points (noted
sion conditions and stratified into the snow depth
on Fig. 3), air temperatures can be measured at
categories defined in Table 1. An analysis of the
the same elevation along the bank, 10 to 100 m
frequency of greatest temperature difference ob-
from the river's edge. The northernmost of these
served on the bridge is tabulated in Table 4. More
points (h) is 100 m east of bridge P. A comparison
than two-thirds of all observations have extreme
of the extreme differences among six indepen-
dent temperature observations on the bridge P
differences of less than the one digit uncertainty
and two replicate temperature observations on
of the readout device used to measure the air
the bank 100 m east of the bridge portal has been
temperature. This poses some problems in the
frequency analysis. Three readable temperature
difference intervals (0.0, 0.1 0.2C) lay within
the digital repeatability of comparison of 0.2C.
We have used the observed temperature differ-
Table 4. Frequency of temperature difference on
ence in these analyses, rather than grouping all
(P) Piermont bridge.
differences of less than 0.2C in a single interval.
Difference among 141 sets of six observations (C)
The mean of these six measured temperatures
Stability
is compared to the mean of the two temperatures
Median
84th Percentile
Maximum
measured 100 m distant in Table 5, again sepa-
rated according to lapse or inversion and strati-
Lapse
<0.1
<0.1
0.4
fied by snow category. The difference in mean
temperatures rarely exceeds 0.1C in lapse con-
Inversion
<0.1
<0.2
1.4
Table 5. Frequency of temperature difference at a 100-m distance from bridge.
Snow cover
Relative frequency of occurrence in temperature difference class
0C
<0.1C
<0.2C
<0.4C
<0.8C
<1.6C
and stability
Number
<3 cm, lapse
0.73
0.27
0.0
0.0
0.0
0.0
15
310 cm
0.63
0.31
0.06
0.0
0.0
0.0
16
1030 cm
0.36
0.50
0.07
0.07
0.0
0.0
14
>30 cm
0.67
0.0
0.33
0.0
0.0
0.0
3
Cumulative
0.57
0.91
0.98
1.00
48
<3 cm, inversion
0.41
0.27
0.09
0.05
0.14
0.05
22
310 cm
0.29
0.50
0.13
0.04
0.04
0.0
24
1030 cm
0.31
0.23
0.23
0.10
0.07
0.05
39
>30 cm
0.13
0.00
0.13
0.13
0.38
0.25
08
Cumulative
0.31
0.71
0.76
0.84
0.95
1.00
93
Cumulative all
0.40
0.49
0.86
0.88
0.96
1.00
141
9
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