Moisture from Station Observations
Below are surface station models from four cities. Use this data to answer the questions.
City A
75
076
72
City C
City B
32
980
25
82
45
Which city has the:
206
City D
65
65
925
Look for the highest and lowest values on
the upper left corner of the station model
Look for the highest and lowest values on
the lower left corner of the station model
1. highest temperature?
C 82F
2. lowest temperature?
B 32F
3. highest dew point temperature?
A 72F
4. lowest dew point temperature?
B 25F
5. highest saturation vapor pressure? C
6. lowest saturation vapor pressure? B
7. highest vapor pressure?
A
8. lowest vapor pressure?
B
9. highest relative humidity?
D
10. lowest relative humidity?
C
***One exercise you can do to make sure you
understand these concepts is to rank each station
for each variable. That way you can see why you
chose each one.
Saturation vapor pressure tells us the maximum
amount of water the atmos can hold at given temp. As
T goes up, SVP also goes up (the higher the temp, the
more water the atmos can hold). Therefore, to find the
highest SVP, look for the highest temp and vice versa
for the lowest SVP (look for the lowest temp).
Vapor pressure tells us the amount of water present in
the atmos. VP is related to dew point temp. The higher
the dew point temp, the more water that is in the
atmos. So to find the city with the highest VP, look
for the station with the highest dew point temp and
vice versa for the lowest vapor pressure.
Relative Humidity (RH) is a function of how close the
temp is to the dew point temp (i.e., size of the dew
point depression). The closer these are together, the
higher the RH. The further they are a part, the lower
the relative humidity. The relative, humidity tells us
how close we are to saturation. When RH=100%, the
VP=SVP and dew point temp = actual temp. To
find the highest RH look for the city with the smallest
difference between the temp and the dew point temp
(smallest dew point depression) and for the lowest
RH, look for the city with the largest difference
between temp and dew point temp (largest dew point
depression).
11. In order to get fog to form at
City A, how much does the
temperature have to fall?
Needs to fall 3F so that the temp equals the dew point
12. For City B, at what temperature
would the air become saturated?
25F
13. Which city is furthest from being
saturated?
C (dew point depression is largest here at 37)
14. What is the heat index for City A? 79F
*** To find this answer we assume that 1 of dew point depression is equal to about
3% relative humidity. Therefore the dew point depression is 3 and that is equal to ~10% relative
humidity. So we subtract that 10% from 100% and see that when the temp is 72 and the dew
point is 72 that the RH is 90%. Using the chart on page 87 we can see that the heat index temp is
around 79F.
To calculate the exact relative humidity, you have to use a sounding chart that has mixing ratio
lines on it. Therefore you can find the mixing ratio of the air at temperature of the air (which is
called the saturation vapor pressure) and the mixing ratio of the air at the dew point (or vapor
pressure) and then divide VP/SVP and multiply by 100 to get the true relative humidity. DON’T
WORRY ABOUT UNDERSTANDING THIS LAST PART; I just wanted to let you know how
it is truly calculated.