Script for presentation
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- Word – spray_wx03
SDSU home page
SDSU_web
Ready page
ETA meteogram
Introduction
SDSU web page – “South Dakota Climate has top billing – weather icon W2
Al Bender, State Climatologist W3
The topics page W1
The wind humidity –
Direct effects on the evaporation of the droplets containing pesticide
The vertical movements that cause mixing under normal conditions
The visual indication of different stability conditions
What a inversion looks like when we measure the vertical profile of temperature
and humidity W4
Using a meteogram forecast to determine the weather and winds W1 – W5
Other tools from the SDSU web page W2
Weather impacts on
Spray Drift
 Wind velocity and
direction
 Relative humidity and
temperature
 Atmospheric stability
and inversions
Weather impacts on Spray Drift
Wind velocity and direction
Relative humidity and temperature
Atmospheric stability and inversions
The rate of evaporation is important because it determines how far a droplet can travel
before the liquid changes to a vapor. The general rule is that the greater the difference
between the air temperature and the dew point (called the dew point depression) the faster
liquid will evaporate.
[Click on the figure for the source of this information.]
Fig.12. Nozzle selection is a critical force in reducing drift.
Small droplets evaporate faster but they fall much slower. In conditions when the dew
point depression is less than 3oF (temp-dew point) the small droplets may remain in the
air stream for long enough to move off-site.
The dew point depression typically becomes greater as the air warms during the day. At
sunrise when the lowest temperature often occurs the temperature and dew point are
nearly equal and the evaporation rate would be low. This is also a time when the wind
may be light because of a thermal inversion.
(Use some hourly observations to illustrate the point.)
http://weather.noaa.gov/weather/metar.shtml#reports
Take this opportunity to provide information on the sources of current and past weather
observations. Here is a good place to go to the SDSU homepage and click on the weather
icon for weather forecasts and observations. http://www.sdstate.org/Index.cfm
The concerns with inversions
The temperature normally decreases as the we mover vertically above the land surface..
When the reverse is true, i.e., the temperature does not decrease with height, and may
actually increase with height, a thermal inversion exists. When an inversion exists, the
warmer layer puts a cap on the air near the surface.
When there is an inversion, air does not rise and the spray droplets are not mixed with the
air above the field being sprayed. Secondly the dew point depression is typically low
when an inversion exists. Thirdly, the wind are light so it seems like a good time to spray.
There may even be problems after the pesticide has vaporized under inversions because
the vapor may move miles off-site and condense when the temperature cools. This is
most likely to occur in the afternoon or other time when temperature decreases after
application.
There are ways to see an inversion by looking at smoke rising from a surface source as
illustrated below. [Click on the figure for the source of this information.]
The use of observed and forecast information that reveal the location and extent of
inversions and the low level winds associated with them is one way to avoid situation that
lead to potential spray drift problems.
There are some very good tools available to help us evaluate what the conditions above
the land surface are like even when we cannot see evidence of inversions or very stable
conditions as shown by the smoke stack picture. The temperature and humidity above a
surface location is called a sounding.
The plot of the sounding shown below was for a cold December morning at Huron
Regional Airport. The red line is the temperature with height, 700 is about 10,000 feet
above sea level. The temperature at the surface is –20oC or –4oF while the temperature at
700mb is -12oC. The temperature increases with height so it is an inversion.
[Click on the figure for the source of this information.]
Sounding from the RUC analysis
data is for nearest grid point, 8.8 nm / 314 deg from HON
FAA 604 format for SHARP <- ?
(DD = 'A' means analysis)
Pressure_Alt DD Dir Spd ---Temp--- DewPt
(ft)
(mb)
(kts) (F)
(C)
(C)
1404
1463
1584
1768
2014
2388
2769
2844
3523
980.0
978.0
973.0
966.0
956.0
942.0
927.0
925.0
900.0
A 37
A 35
A 36
A 40
A 42
A 41
A 38
A 27
A 295
4
5
6
6
7
8
5
5
6
-3.6
-1.8
-0.2
1.6
2.8
2.8
3.7
3.9
3.6
-19.8
-18.8
-17.9
-16.9
-16.2
-16.2
-15.7
-15.6
-15.8
-24.1
-23.6
-23.2
-23.1
-23.1
-26.1
-26.7
-26.9
-29.3
4816
4937
5626
6443
7493
8569
9803
10288
13274
16040
17985
19297
21722
23116
23986
26079
27844
29370
30039
31804
32145
32368
32565
32752
32972
33159
33556
34268
34793
35390
36040
36807
37893
39540
41387
43717
43881
48162
52473
55643
854.0
850.0
826.0
800.0
767.0
735.0
700.0
686.0
608.0
543.0
500.0
473.0
425.0
400.0
384.0
350.0
322.0
300.0
290.0
267.0
262.0
259.0
257.0
255.0
252.0
250.0
245.0
237.0
231.0
224.0
217.0
210.0
199.0
184.0
168.0
151.0
150.0
122.0
100.0
86.0
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
330
331
337
340
339
337
332
330
312
301
297
295
295
297
298
296
290
289
289
287
287
288
288
289
290
291
294
294
292
292
295
297
300
303
303
303
303
298
314
325
8
9
16
26
27
27
26
26
31
41
47
51
52
52
52
56
60
62
63
68
70
71
73
74
76
76
77
73
72
70
69
68
67
64
60
55
55
52
42
34
5.0
5.7
8.4
10.4
11.1
12.2
10.4
9.3
-0.4
-10.7
-18.2
-23.6
-33.7
-39.3
-43.1
-51.5
-58.4
-64.5
-67.5
-74.4
-73.8
-72.6
-71.1
-69.7
-68.4
-68.1
-67.9
-68.1
-67.4
-67.0
-67.0
-66.5
-65.4
-65.2
-65.4
-66.1
-65.9
-64.3
-60.2
-57.5
-15.0
-14.6
-13.1
-12.0
-11.6
-11.0
-12.0
-12.6
-18.0
-23.7
-27.9
-30.9
-36.5
-39.6
-41.7
-46.4
-50.2
-53.6
-55.3
-59.1
-58.8
-58.1
-57.3
-56.5
-55.8
-55.6
-55.5
-55.6
-55.2
-55.0
-55.0
-54.7
-54.1
-54.0
-54.1
-54.5
-54.4
-53.5
-51.2
-49.7
-34.4
-34.1
-33.1
-19.2
-25.6
-31.3
-31.9
-32.3
-28.3
-27.5
-31.6
-34.5
-38.6
-41.3
-43.1
-48.8
-52.3
-56.1
-57.9
-61.8
-61.9
-61.4
-60.7
-60.1
-59.4
-59.4
-59.7
-60.5
-60.7
-60.9
-61.4
-62.0
-63.3
-66.6
-70.4
-74.7
-74.7
-76.9
-77.0
-77.3
Here are some checks of the your competency in reading the data. At what level is the
atmosphere the warmest? What is the dew point at that level? How does the dew point
depression at that level compare to the surface? What is the pressure at the surface?
Look at the changes that have taken place in 6 hours. Look at the winds and the
temperatures. Notice the changes in the inversion. What has happened?
These changes can be determined from the Eta model outputs for forecast times
throughout the day for up to 36 hours in the future from the Eta model. The surface
conditions are frequently used as the basis for whether spray operations can proceed, but
the sounding or conditions above the ground are the key to understanding the risks of
spray drift.
A careful consideration of the projected changes will improve the basis for decision
making about spray operations. Using a meteogram clearly displays the changes in wind
speed and direction that can be expected. The example shown below is produced from the
Eta 40km model which is the most advanced 48-60 hour model that is used for planning
the operations for the next day.
The meteogram also provides a bridge between the information on regional or national
maps which are typically 12 to 24 hours apart and what you really need to know for hour
by hour operations. The interactive forecast maps permit the selection of a critical time
period to display the forecast of critical parameters such as the air temperature at 2 meters
(6.5 ft) above the ground (AGL) and the relative humidity at 2 m. AGL. Next the
meteogram for selected locations can be checked for the changes over time.
[Click on the figure for the source of this information and interactive forecast maps.]
List of links:
The ready forecast site: http://www.arl.noaa.gov/ready/cmet.html