This page made possible by the sponsorship of: Frank Willburn.

San Ignacio Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Ignacio Observatory clear sky chart from 2005-11-05 to 2020-06-03.

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
44%
19349
33%
57829
16%
19305
61%
19305
44%
29108

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
33%
993
34%
3005
14%
989
54%
989
44%
1449
January16th to endNight
32%
960
39%
3120
17%
960
59%
960
42%
1428
February1st to 15thNight
30%
881
35%
2686
13%
880
54%
880
41%
1319
February16th to endNight
22%
792
40%
2374
9%
792
53%
792
42%
1162
March1st to 15thNight
35%
900
34%
2621
13%
900
53%
900
42%
1345
March16th to endNight
33%
851
38%
2531
13%
851
56%
851
44%
1231
April1st to 15thNight
28%
674
35%
2248
12%
674
56%
674
39%
1051
April16th to endNight
32%
669
36%
2229
15%
669
63%
669
47%
996
May1st to 15thNight
41%
675
28%
2227
17%
674
60%
674
44%
1069
May16th to endNight
47%
707
33%
2019
18%
705
63%
705
49%
1017
June1st to 15thNight
55%
637
27%
1699
15%
637
64%
637
51%
900
June16th to endNight
61%
630
26%
1680
17%
630
67%
630
52%
895
July1st to 15thNight
73%
600
22%
1616
18%
600
75%
600
42%
940
July16th to endNight
77%
660
22%
1881
21%
627
78%
627
40%
1036
August1st to 15thNight
78%
620
19%
2051
22%
617
78%
617
35%
1149
August16th to endNight
72%
669
23%
2230
26%
669
75%
669
36%
1226
September1st to 15thNight
67%
630
28%
2100
28%
630
71%
630
34%
1171
September16th to endNight
58%
627
36%
2268
27%
627
72%
627
42%
1064
October1st to 15thNight
48%
784
35%
2464
20%
784
62%
784
47%
1222
October16th to endNight
50%
896
41%
2688
17%
896
68%
896
55%
1228
November1st to 15thNight
48%
1044
39%
2804
16%
1044
64%
1044
60%
1334
November16th to endNight
41%
1125
41%
2925
15%
1125
59%
1125
52%
1495
December1st to 15thNight
32%
1125
37%
3003
11%
1125
52%
1125
42%
1579
December16th to endNight
26%
1200
34%
3360
9%
1200
48%
1200
38%
1802

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
431332345%7%62%
58580
Night
4313323347%62%
58017
Cloud CoverDay
431232345%7%62%
58578
Night
4313323447%61%
58017
TransparencyDay
21%37%21%27%18%
58269
Night
22%37%20%27%18%
57831
SeeingDay
8%8%27%30%19%5%
9127
Night
6%9%33%33%14%1
19353

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
3412312346%65%
1934
16th to endDay
3312113246%69%
2160
February1st to 15thDay
3413312346%65%
2163
16th to endDay
2414322436%64%
1980
March1st to 15thDay
4423323346%60%
2325
16th to endDay
2412312346%66%
2749
April1st to 15thDay
3313223347%64%
2690
16th to endDay
4212213336%68%
2666
May1st to 15thDay
6%313214446%61%
2713
16th to endDay
42221335%6%66%
3148
June1st to 15thDay
111232446%8%62%
2970
16th to endDay
211222345%8%65%
2940
July1st to 15thDay
4313435%5%7%9%50%
2920
16th to endDay
4213436%6%7%12%48%
2883
August1st to 15thDay
4213425%6%7%10%48%
2481
16th to endDay
5%313425%5%7%10%50%
2676
September1st to 15thDay
8%3244246%5%8%48%
2520
16th to endDay
6%212212345%67%
2351
October1st to 15thDay
7%21222223467%
2153
16th to endDay
431211122474%
2240
November1st to 15thDay
2211212345%73%
2030
16th to endDay
4313212235%69%
2025
December1st to 15thDay
3314312347%63%
1943
16th to endDay
5%5%2321236%7%60%
1920
January1st to 15thNight
3313313236%66%
3005
16th to endNight
3313313235%67%
3120
February1st to 15thNight
3413312346%64%
2676
16th to endNight
25%12213346%64%
2374
March1st to 15thNight
5%413312346%61%
2621
16th to endNight
3412213346%65%
2531
April1st to 15thNight
5%312323447%63%
2248
16th to endNight
4212222236%69%
2229
May1st to 15thNight
6%212313345%63%
2227
16th to endNight
5%212223346%65%
2018
June1st to 15thNight
22133245%8%9%56%
1699
16th to endNight
221232446%9%57%
1680
July1st to 15thNight
4324436%6%6%10%48%
1697
16th to endNight
3224425%7%8%12%46%
1989
August1st to 15thNight
5%3145%35%7%9%11%42%
2061
16th to endNight
5%2145%36%6%8%11%45%
2230
September1st to 15thNight
7%325%435%5%7%8%48%
2100
16th to endNight
6%313323336%63%
2268
October1st to 15thNight
7%31222233465%
2464
16th to endNight
5%2221223473%
2688
November1st to 15thNight
222212226%73%
2804
16th to endNight
4312211225%69%
2925
December1st to 15thNight
4313212347%64%
3003
16th to endNight
441322435%6%59%
3360

Percentage Cloud Cover SE (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
3422212337%65%
1934
16th to endDay
3312213246%69%
2160
February1st to 15thDay
2413322346%65%
2163
16th to endDay
2414323345%64%
1980
March1st to 15thDay
4424324346%61%
2325
16th to endDay
2422312346%66%
2747
April1st to 15thDay
3313222347%64%
2690
16th to endDay
4212212346%68%
2666
May1st to 15thDay
6%21231345%6%61%
2713
16th to endDay
42221345%6%66%
3148
June1st to 15thDay
211222446%8%62%
2970
16th to endDay
311122345%8%65%
2940
July1st to 15thDay
4313435%5%7%10%49%
2920
16th to endDay
4323435%6%7%11%48%
2883
August1st to 15thDay
4213425%7%8%10%48%
2481
16th to endDay
5%313425%5%7%10%50%
2676
September1st to 15thDay
8%3144245%6%8%48%
2520
16th to endDay
6%22212345%67%
2351
October1st to 15thDay
7%31222223467%
2153
16th to endDay
421211222474%
2240
November1st to 15thDay
221212345%72%
2030
16th to endDay
5%312212235%69%
2025
December1st to 15thDay
3314322347%63%
1943
16th to endDay
5%5%2331225%8%60%
1920
January1st to 15thNight
3313212236%66%
3005
16th to endNight
3312322235%67%
3120
February1st to 15thNight
3413312346%63%
2676
16th to endNight
25%12222347%64%
2374
March1st to 15thNight
5%423312347%61%
2621
16th to endNight
3412213346%65%
2531
April1st to 15thNight
4312323347%62%
2248
16th to endNight
4212312246%68%
2229
May1st to 15thNight
5%312323345%63%
2227
16th to endNight
5%212223446%64%
2018
June1st to 15thNight
21133246%7%10%56%
1699
16th to endNight
22133245%6%9%57%
1680
July1st to 15thNight
5%3145%36%5%6%11%47%
1697
16th to endNight
4223435%7%8%12%45%
1989
August1st to 15thNight
5%2145%26%7%10%11%42%
2061
16th to endNight
5%3145%35%6%7%10%45%
2230
September1st to 15thNight
7%324335%47%8%48%
2100
16th to endNight
7%213213345%62%
2268
October1st to 15thNight
8%31221333465%
2464
16th to endNight
6%21111123472%
2688
November1st to 15thNight
222312236%73%
2804
16th to endNight
4312211235%70%
2925
December1st to 15thNight
4313212347%64%
3003
16th to endNight
441332335%6%59%
3360

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
20%2422%36%13%
1935
16th to endDay
17%1419%37%18%
2160
February1st to 15thDay
19%25%20%32%19%
2164
16th to endDay
22%2115%35%22%
1980
March1st to 15thDay
24%2317%30%20%
2325
16th to endDay
20%2418%32%22%
2749
April1st to 15thDay
20%2319%29%23%
2690
16th to endDay
17%25%18%33%21%
2666
May1st to 15thDay
22%27%19%25%22%
2713
16th to endDay
17%36%22%21%28%
3152
June1st to 15thDay
17%39%19%22%27%
2970
16th to endDay
17%28%20%23%26%
2940
July1st to 15thDay
26%5%14%20%17%15%
2772
16th to endDay
25%417%30%18%3
2728
August1st to 15thDay
25%6%18%26%17%5%
2467
16th to endDay
26%413%27%21%7%
2676
September1st to 15thDay
31%410%25%18%9%
2520
16th to endDay
19%15%22%32%18%
2351
October1st to 15thDay
22%26%16%30%21%
2153
16th to endDay
16%1419%31%27%
2240
November1st to 15thDay
13%1422%34%22%
2030
16th to endDay
19%1216%34%24%
2025
December1st to 15thDay
21%25%29%30%11%
1943
16th to endDay
23%36%23%30%13%
1920
January1st to 15thNight
20%26%24%31%14%
3005
16th to endNight
20%1419%37%16%
3120
February1st to 15thNight
21%3423%28%18%
2686
16th to endNight
20%3217%34%22%
2374
March1st to 15thNight
22%2421%29%19%
2621
16th to endNight
20%2419%29%22%
2531
April1st to 15thNight
21%25%17%30%22%
2248
16th to endNight
17%35%20%30%23%
2229
May1st to 15thNight
22%27%18%25%23%
2228
16th to endNight
20%38%17%20%30%
2020
June1st to 15thNight
20%412%17%17%27%
1699
16th to endNight
21%313%18%17%25%
1680
July1st to 15thNight
28%5%17%19%15%13%
1616
16th to endNight
27%5%21%26%16%2
1881
August1st to 15thNight
29%5%19%24%14%5%
2051
16th to endNight
30%415%24%15%8%
2230
September1st to 15thNight
31%5%15%23%16%8%
2100
16th to endNight
23%27%20%24%21%
2268
October1st to 15thNight
22%25%16%27%25%
2464
16th to endNight
16%1415%33%28%
2688
November1st to 15thNight
14%1321%34%23%
2804
16th to endNight
19%1218%33%24%
2925
December1st to 15thNight
21%25%26%31%12%
3003
16th to endNight
25%26%22%31%11%
3360

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
12%14%35%27%7%2
225
16th to endDay
7%17%37%26%7%3
240
February1st to 15thDay
7%16%34%30%10%
225
16th to endDay
12%21%43%15%31
201
March1st to 15thDay
12%15%41%23%6%2
300
16th to endDay
11%23%37%21%5%1
480
April1st to 15thDay
10%18%41%23%5%
449
16th to endDay
11%13%43%24%7%
438
May1st to 15thDay
6%11%37%27%12%3
490
16th to endDay
7%8%31%36%14%1
757
June1st to 15thDay
2328%33%27%4
656
16th to endDay
4219%35%29%7%
645
July1st to 15thDay
410%33%41%9%
606
16th to endDay
5%5%34%33%21%
455
August1st to 15thDay
426%36%36%13%
419
16th to endDay
5%11%39%32%10%
460
September1st to 15thDay
7%111%43%25%10%
435
16th to endDay
10%322%38%20%4
266
October1st to 15thDay
14%433%29%13%5%
225
16th to endDay
11%330%30%17%6%
240
November1st to 15thDay
7%10%35%26%13%7%
225
16th to endDay
14%17%34%20%11%1
225
December1st to 15thDay
11%11%41%27%8%
225
16th to endDay
12%15%44%23%4
240
January1st to 15thNight
5%15%41%26%9%1
994
16th to endNight
6%17%40%28%6%1
960
February1st to 15thNight
7%14%43%28%5%
884
16th to endNight
7%16%51%22%2
792
March1st to 15thNight
8%13%38%31%7%
900
16th to endNight
7%12%42%29%7%
851
April1st to 15thNight
6%17%44%28%3
674
16th to endNight
7%10%46%28%7%
669
May1st to 15thNight
6%7%40%36%8%
675
16th to endNight
6%5%35%39%11%
707
June1st to 15thNight
3333%43%16%
637
16th to endNight
3226%46%19%
630
July1st to 15thNight
6%9%49%32%2
600
16th to endNight
415%42%43%2
660
August1st to 15thNight
5%439%43%5%
620
16th to endNight
7%8%42%37%3
669
September1st to 15thNight
9%113%45%28%1
630
16th to endNight
10%124%41%21%
627
October1st to 15thNight
9%333%36%16%
784
16th to endNight
8%6%32%32%19%2
896
November1st to 15thNight
48%36%36%13%1
1044
16th to endNight
7%11%35%28%14%1
1125
December1st to 15thNight
7%16%40%28%6%
1125
16th to endNight
7%16%45%24%5%
1200
Page generated at 2020-06-04 01:18:54. Version ac04

Copyright 2020 A. Danko


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