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Robert Ferguson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Ferguson Observatory clear sky chart from 2005-11-05 to 2017-02-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
64%
14630
26%
44922
22%
14591
68%
14591
37%
25036

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
56%
718
18%
2390
17%
716
61%
716
26%
1465
January16th to endNight
54%
768
25%
2496
23%
768
59%
768
27%
1529
February1st to 15thNight
52%
699
22%
2288
22%
698
57%
698
23%
1462
February16th to endNight
50%
588
20%
1792
16%
588
57%
588
27%
1127
March1st to 15thNight
49%
660
15%
1828
11%
660
52%
660
24%
1271
March16th to endNight
46%
704
21%
1925
14%
704
52%
704
31%
1244
April1st to 15thNight
48%
659
24%
1649
12%
659
54%
659
33%
1056
April16th to endNight
57%
660
25%
1650
12%
660
61%
660
45%
934
May1st to 15thNight
66%
524
28%
1514
20%
524
71%
524
47%
786
May16th to endNight
70%
357
24%
1384
22%
357
78%
357
39%
634
June1st to 15thNight
71%
330
26%
1320
29%
330
79%
330
40%
586
June16th to endNight
83%
330
32%
1320
41%
330
87%
330
57%
486
July1st to 15thNight
92%
339
34%
1269
42%
339
93%
339
62%
470
July16th to endNight
87%
522
29%
1467
28%
489
88%
489
50%
694
August1st to 15thNight
84%
493
33%
1476
29%
490
85%
490
50%
691
August16th to endNight
87%
691
33%
1741
21%
691
88%
691
59%
904
September1st to 15thNight
88%
660
39%
1650
27%
660
89%
660
69%
802
September16th to endNight
80%
656
38%
1810
34%
656
83%
656
60%
864
October1st to 15thNight
69%
660
30%
1980
29%
660
75%
660
45%
1021
October16th to endNight
69%
704
31%
2112
32%
704
74%
704
40%
1158
November1st to 15thNight
64%
700
27%
2181
23%
700
70%
700
39%
1186
November16th to endNight
59%
720
23%
2472
25%
720
63%
720
30%
1389
December1st to 15thNight
50%
720
19%
2520
17%
720
56%
720
21%
1610
December16th to endNight
48%
768
21%
2688
18%
768
55%
768
23%
1667

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
9%313313336%59%
45248
Night
10%313323346%56%
45108
TransparencyDay
27%49%19%23%14%
44935
Night
29%410%18%22%14%
44923
SeeingDay
11%9%24%32%17%4
5162
Night
11%314%31%31%6%
14639

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
19%6%25%322335%45%
1546
16th to endDay
16%5%25%5%24335%43%
1728
February1st to 15thDay
20%5%25%414345%42%
1576
16th to endDay
20%7%25%423336%40%
1441
March1st to 15thDay
20%6%45%42336%7%33%
1801
16th to endDay
18%4245%2445%5%41%
1947
April1st to 15thDay
17%5%2442445%6%40%
1969
16th to endDay
7%322313348%58%
1980
May1st to 15thDay
6%313213347%63%
2103
16th to endDay
6%32332245%7%58%
2376
June1st to 15thDay
5%423312346%62%
2310
16th to endDay
21112235%77%
2310
July1st to 15thDay
1111122480%
2275
16th to endDay
1111112235%79%
2286
August1st to 15thDay
11212325%77%
2145
16th to endDay
111112135%78%
2108
September1st to 15thDay
11211226%78%
1980
16th to endDay
421111235%73%
1818
October1st to 15thDay
8%212213336%63%
1650
16th to endDay
10%413212336%59%
1760
November1st to 15thDay
10%2144235%47%53%
1675
16th to endDay
16%42443434448%
1488
December1st to 15thDay
20%8%2432432442%
1440
16th to endDay
23%7%23313245%40%
1536
January1st to 15thNight
16%5%24324448%42%
2390
16th to endNight
19%5%23313436%47%
2496
February1st to 15thNight
17%6%24423446%41%
2276
16th to endNight
16%5%14424347%44%
1792
March1st to 15thNight
17%6%34435%436%38%
1828
16th to endNight
16%6%24322345%47%
1925
April1st to 15thNight
12%424423446%50%
1649
16th to endNight
7%213223346%62%
1650
May1st to 15thNight
4212323347%63%
1514
16th to endNight
322323447%63%
1383
June1st to 15thNight
321221325%7%66%
1320
16th to endNight
2121123478%
1320
July1st to 15thNight
1111222481%
1351
16th to endNight
11213349%69%
1575
August1st to 15thNight
12223347%70%
1485
16th to endNight
1121335%8%70%
1741
September1st to 15thNight
1212236%77%
1650
16th to endNight
311211236%73%
1810
October1st to 15thNight
5%313313245%63%
1980
16th to endNight
8%212314347%58%
2112
November1st to 15thNight
9%423312449%54%
2181
16th to endNight
12%5%24323236%52%
2472
December1st to 15thNight
21%6%24323346%40%
2520
16th to endNight
21%424323346%43%
2688

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
42%412%16%15%8%
1546
16th to endDay
44%47%12%19%11%
1728
February1st to 15thDay
44%47%18%19%4
1576
16th to endDay
44%6%10%23%10%4
1441
March1st to 15thDay
48%5%11%17%12%4
1801
16th to endDay
42%6%12%18%16%3
1947
April1st to 15thDay
42%6%12%17%17%2
1969
16th to endDay
24%6%14%25%22%6%
1980
May1st to 15thDay
21%49%29%27%7%
2103
16th to endDay
24%6%15%26%19%6%
2380
June1st to 15thDay
23%5%11%21%23%14%
2310
16th to endDay
11%36%18%39%21%
2310
July1st to 15thDay
11%35%13%28%37%
2129
16th to endDay
11%5%6%11%27%38%
2131
August1st to 15thDay
12%45%13%33%30%
2129
16th to endDay
13%35%15%36%25%
2108
September1st to 15thDay
11%35%13%33%31%
1980
16th to endDay
15%36%22%30%20%
1818
October1st to 15thDay
23%48%18%28%15%
1650
16th to endDay
28%48%23%24%10%
1760
November1st to 15thDay
29%6%13%27%13%8%
1675
16th to endDay
39%5%9%18%19%7%
1488
December1st to 15thDay
45%5%7%15%20%4
1440
16th to endDay
46%411%16%14%6%
1536
January1st to 15thNight
39%5%12%18%15%9%
2390
16th to endNight
39%48%14%18%14%
2496
February1st to 15thNight
42%49%17%17%7%
2288
16th to endNight
39%5%13%22%15%3
1792
March1st to 15thNight
46%5%13%14%15%4
1828
16th to endNight
38%5%10%20%19%5%
1925
April1st to 15thNight
33%6%15%20%18%5%
1649
16th to endNight
24%412%28%21%9%
1650
May1st to 15thNight
21%6%8%19%30%13%
1515
16th to endNight
20%6%14%30%19%8%
1384
June1st to 15thNight
18%7%12%21%23%16%
1320
16th to endNight
12%25%16%38%24%
1320
July1st to 15thNight
9%45%12%28%40%
1269
16th to endNight
16%7%9%11%19%35%
1467
August1st to 15thNight
17%5%7%14%24%30%
1476
16th to endNight
15%5%8%13%29%27%
1741
September1st to 15thNight
11%36%12%32%33%
1650
16th to endNight
16%25%20%32%22%
1810
October1st to 15thNight
22%48%19%25%18%
1980
16th to endNight
25%6%8%22%23%14%
2112
November1st to 15thNight
27%5%14%28%15%8%
2181
16th to endNight
34%39%21%21%9%
2472
December1st to 15thNight
44%49%16%19%6%
2520
16th to endNight
42%410%15%19%7%
2688

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
31%6%23%25%11%1
174
16th to endDay
19%12%23%25%17%3
256
February1st to 15thDay
24%6%24%25%17%1
236
16th to endDay
21%13%23%22%17%1
190
March1st to 15thDay
30%18%26%16%8%
210
16th to endDay
18%15%31%21%10%2
224
April1st to 15thDay
17%21%29%21%7%2
209
16th to endDay
9%23%29%24%10%2
210
May1st to 15thDay
10%15%35%27%10%
228
16th to endDay
9%13%25%29%16%4
266
June1st to 15thDay
6%14%30%36%9%2
255
16th to endDay
3329%41%16%4
255
July1st to 15thDay
2218%50%21%4
245
16th to endDay
116%53%21%7%
271
August1st to 15thDay
420%48%21%4
255
16th to endDay
1125%43%22%5%
268
September1st to 15thDay
1219%45%22%9%
255
16th to endDay
6%6%18%33%25%10%
255
October1st to 15thDay
9%9%24%31%17%7%
255
16th to endDay
16%7%23%29%20%1
272
November1st to 15thDay
11%6%17%24%33%7%
181
16th to endDay
17%10%23%25%22%1
68
December1st to 15thDay
36%15%21%15%11%
60
16th to endDay
20%9%21%25%18%4
64
January1st to 15thNight
18%212%34%25%6%
720
16th to endNight
20%315%29%23%7%
768
February1st to 15thNight
19%5%13%32%25%4
704
16th to endNight
18%418%31%21%5%
588
March1st to 15thNight
18%418%29%23%4
660
16th to endNight
18%423%27%23%2
704
April1st to 15thNight
13%8%22%31%20%3
659
16th to endNight
8%8%21%29%26%6%
660
May1st to 15thNight
5%120%34%33%5%
525
16th to endNight
4318%37%30%5%
358
June1st to 15thNight
4315%40%33%1
330
16th to endNight
39%44%39%2
330
July1st to 15thNight
1429%58%6%
339
16th to endNight
2429%53%9%
522
August1st to 15thNight
116%31%50%8%
493
16th to endNight
15%33%51%7%
691
September1st to 15thNight
16%30%50%10%
660
16th to endNight
419%30%41%12%
656
October1st to 15thNight
7%313%32%33%9%
660
16th to endNight
10%111%30%32%12%
704
November1st to 15thNight
10%216%28%33%8%
700
16th to endNight
14%5%12%31%26%8%
720
December1st to 15thNight
23%413%30%20%7%
720
16th to endNight
23%5%16%33%18%2
768
Page generated at 2017-02-18 13:04:29. Version ac04

Copyright 2017 A. Danko


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