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 2018-03-11.

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%
16010
26%
49141
23%
15971
69%
15971
37%
27270

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%
778
18%
2590
17%
776
60%
776
24%
1622
January16th to endNight
54%
832
24%
2704
22%
832
59%
832
27%
1668
February1st to 15thNight
54%
759
25%
2483
24%
758
60%
758
26%
1528
February16th to endNight
50%
684
23%
2082
18%
684
58%
684
28%
1277
March1st to 15thNight
51%
764
15%
2116
12%
764
54%
764
25%
1457
March16th to endNight
45%
768
21%
2100
14%
768
51%
768
30%
1362
April1st to 15thNight
48%
719
23%
1799
11%
719
53%
719
32%
1160
April16th to endNight
55%
720
26%
1800
12%
720
60%
720
43%
1037
May1st to 15thNight
66%
572
27%
1652
19%
572
70%
572
47%
853
May16th to endNight
71%
390
26%
1513
24%
390
78%
390
39%
694
June1st to 15thNight
71%
360
25%
1440
28%
360
78%
360
40%
635
June16th to endNight
83%
360
34%
1440
44%
360
87%
360
58%
528
July1st to 15thNight
93%
372
32%
1392
40%
372
93%
372
64%
505
July16th to endNight
88%
564
32%
1593
30%
531
89%
531
52%
741
August1st to 15thNight
85%
538
31%
1611
27%
535
85%
535
51%
751
August16th to endNight
88%
754
35%
1900
23%
754
89%
754
59%
981
September1st to 15thNight
87%
720
37%
1800
26%
720
88%
720
66%
891
September16th to endNight
80%
716
41%
1974
36%
716
84%
716
62%
925
October1st to 15thNight
70%
720
30%
2160
29%
720
75%
720
48%
1083
October16th to endNight
71%
768
33%
2304
34%
768
75%
768
42%
1242
November1st to 15thNight
63%
760
25%
2368
22%
760
69%
760
37%
1317
November16th to endNight
59%
780
23%
2678
25%
780
62%
780
29%
1531
December1st to 15thNight
52%
780
20%
2730
19%
780
58%
780
23%
1684
December16th to endNight
49%
832
22%
2912
19%
832
56%
832
23%
1798

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%
49491
Night
10%313323346%56%
49327
TransparencyDay
27%49%19%23%15%
49178
Night
29%49%18%21%14%
49142
SeeingDay
11%9%23%31%18%4
6551
Night
11%313%31%32%6%
16019

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
20%6%25%322335%42%
1676
16th to endDay
16%5%25%5%24336%42%
1872
February1st to 15thDay
18%415%414345%45%
1711
16th to endDay
19%6%25%423346%41%
1678
March1st to 15thDay
19%6%46%43436%7%33%
2079
16th to endDay
18%5%245%3445%5%41%
2124
April1st to 15thDay
17%5%25%42445%6%40%
2149
16th to endDay
8%422313347%57%
2160
May1st to 15thDay
5%313213347%64%
2295
16th to endDay
5%323322447%59%
2599
June1st to 15thDay
5%423312346%63%
2520
16th to endDay
2111223477%
2520
July1st to 15thDay
1111122481%
2482
16th to endDay
11111135%80%
2468
August1st to 15thDay
111212325%76%
2340
16th to endDay
111112135%78%
2301
September1st to 15thDay
111212226%77%
2160
16th to endDay
4211112236%73%
1983
October1st to 15thDay
8%212213235%65%
1800
16th to endDay
9%412213236%61%
1920
November1st to 15thDay
11%314423447%51%
1818
16th to endDay
16%42443434447%
1612
December1st to 15thDay
19%8%24324325%44%
1560
16th to endDay
21%7%24323345%39%
1664
January1st to 15thNight
16%5%25%424448%40%
2590
16th to endNight
18%423323436%46%
2704
February1st to 15thNight
16%5%24423446%45%
2471
16th to endNight
15%5%14424347%47%
2082
March1st to 15thNight
17%6%34425%446%40%
2116
16th to endNight
16%6%24422345%47%
2100
April1st to 15thNight
13%424423346%49%
1799
16th to endNight
8%313223346%60%
1800
May1st to 15thNight
4212323347%64%
1652
16th to endNight
322324447%63%
1512
June1st to 15thNight
4213213247%66%
1440
16th to endNight
21211123478%
1440
July1st to 15thNight
111212482%
1474
16th to endNight
11213348%71%
1701
August1st to 15thNight
12223347%70%
1620
16th to endNight
1121335%8%71%
1900
September1st to 15thNight
111212236%76%
1800
16th to endNight
211211235%74%
1974
October1st to 15thNight
5%313213245%66%
2160
16th to endNight
8%212314347%59%
2304
November1st to 15thNight
10%424312449%53%
2368
16th to endNight
13%6%24323336%51%
2678
December1st to 15thNight
20%5%24323347%42%
2730
16th to endNight
20%5%24323446%43%
2912

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
44%411%16%14%8%
1676
16th to endDay
44%5%8%13%18%10%
1872
February1st to 15thDay
41%47%19%21%5%
1711
16th to endDay
43%6%9%23%12%4
1678
March1st to 15thDay
48%5%11%17%12%4
2079
16th to endDay
42%6%12%18%15%3
2124
April1st to 15thDay
42%6%12%17%17%2
2149
16th to endDay
26%6%14%24%21%6%
2160
May1st to 15thDay
21%49%30%27%7%
2295
16th to endDay
24%6%14%26%20%6%
2603
June1st to 15thDay
23%5%11%23%23%13%
2520
16th to endDay
11%36%17%37%23%
2520
July1st to 15thDay
10%3412%28%39%
2336
16th to endDay
10%46%10%25%41%
2313
August1st to 15thDay
13%45%14%33%28%
2324
16th to endDay
12%35%16%35%26%
2301
September1st to 15thDay
12%36%15%32%30%
2160
16th to endDay
15%37%23%31%18%
1983
October1st to 15thDay
21%48%17%27%20%
1800
16th to endDay
27%49%22%24%12%
1920
November1st to 15thDay
31%6%13%27%13%8%
1818
16th to endDay
40%5%9%19%18%6%
1612
December1st to 15thDay
43%5%7%14%21%7%
1560
16th to endDay
46%410%16%14%6%
1664
January1st to 15thNight
40%5%12%17%14%8%
2590
16th to endNight
39%410%13%18%13%
2704
February1st to 15thNight
39%48%17%18%9%
2483
16th to endNight
37%5%13%23%17%3
2082
March1st to 15thNight
45%5%12%15%15%4
2116
16th to endNight
39%410%20%19%5%
2100
April1st to 15thNight
34%6%14%20%18%5%
1799
16th to endNight
25%411%27%22%8%
1800
May1st to 15thNight
20%6%8%21%30%12%
1653
16th to endNight
20%6%14%29%20%8%
1513
June1st to 15thNight
18%7%11%23%24%15%
1440
16th to endNight
12%25%16%36%25%
1440
July1st to 15thNight
9%3411%28%42%
1392
16th to endNight
15%6%8%11%19%39%
1593
August1st to 15thNight
17%5%8%15%24%28%
1611
16th to endNight
15%5%8%13%29%28%
1900
September1st to 15thNight
13%36%13%30%32%
1800
16th to endNight
14%26%22%31%22%
1974
October1st to 15thNight
21%38%18%24%23%
2160
16th to endNight
24%5%8%21%23%16%
2304
November1st to 15thNight
29%5%13%28%14%8%
2368
16th to endNight
35%39%21%21%8%
2678
December1st to 15thNight
41%48%15%19%10%
2730
16th to endNight
42%410%16%19%7%
2912

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
33%5%22%24%13%
214
16th to endDay
18%11%22%25%17%4
304
February1st to 15thDay
21%6%23%25%19%3
281
16th to endDay
20%14%26%21%15%1
283
March1st to 15thDay
25%14%25%18%13%3
312
16th to endDay
20%15%30%21%9%2
288
April1st to 15thDay
20%19%28%21%7%2
269
16th to endDay
12%23%28%24%9%2
270
May1st to 15thDay
9%15%34%27%11%1
288
16th to endDay
7%11%23%31%19%5%
330
June1st to 15thDay
7%13%29%34%11%3
315
16th to endDay
2226%41%21%5%
315
July1st to 15thDay
1215%47%25%6%
305
16th to endDay
16%50%21%9%
327
August1st to 15thDay
1318%46%25%4
315
16th to endDay
1120%39%28%7%
332
September1st to 15thDay
2220%42%23%8%
315
16th to endDay
5%6%20%33%25%9%
315
October1st to 15thDay
7%10%25%32%17%6%
315
16th to endDay
13%7%20%29%24%5%
336
November1st to 15thDay
14%6%16%26%29%6%
234
16th to endDay
16%8%21%29%22%
102
December1st to 15thDay
25%12%17%16%17%10%
90
16th to endDay
15%10%18%23%28%3
96
January1st to 15thNight
19%213%33%25%5%
780
16th to endNight
19%314%31%23%6%
832
February1st to 15thNight
17%5%13%32%26%4
764
16th to endNight
18%5%18%31%21%4
684
March1st to 15thNight
18%417%29%26%4
764
16th to endNight
18%423%28%22%2
768
April1st to 15thNight
14%7%21%32%20%3
719
16th to endNight
9%8%21%29%25%5%
720
May1st to 15thNight
5%220%33%34%4
573
16th to endNight
4317%37%31%5%
391
June1st to 15thNight
4315%39%34%1
360
16th to endNight
39%43%41%2
360
July1st to 15thNight
1328%59%6%
372
16th to endNight
2429%52%10%
564
August1st to 15thNight
26%29%51%9%
538
16th to endNight
15%32%51%8%
754
September1st to 15thNight
17%31%49%9%
720
16th to endNight
319%31%41%12%
716
October1st to 15thNight
6%313%33%32%9%
720
16th to endNight
10%110%29%35%12%
768
November1st to 15thNight
11%215%30%32%8%
760
16th to endNight
15%5%12%31%27%8%
780
December1st to 15thNight
21%413%29%22%7%
780
16th to endNight
22%5%16%32%20%2
832
Page generated at 2018-03-12 10:28:14. Version ac04

Copyright 2018 A. Danko


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