Rachel's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rachel's Observatory clear sky chart from 2005-11-05 to 2018-10-08.

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
23%
16023
12%
50944
5%
15991
30%
15991
13%
37561

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
12%
811
8%
2854
2%
807
20%
807
6%
2268
January16th to endNight
17%
832
12%
2912
4%
832
28%
832
10%
2150
February1st to 15thNight
16%
764
9%
2508
3%
761
24%
761
8%
1943
February16th to endNight
14%
683
10%
2105
3%
683
24%
683
9%
1658
March1st to 15thNight
17%
780
13%
2259
6%
780
25%
780
11%
1746
March16th to endNight
20%
832
15%
2288
5%
832
28%
832
16%
1670
April1st to 15thNight
17%
688
15%
1875
4%
688
24%
688
13%
1408
April16th to endNight
20%
578
15%
1734
5%
578
29%
578
14%
1230
May1st to 15thNight
20%
585
10%
1746
5%
585
27%
585
13%
1310
May16th to endNight
21%
428
10%
1443
4%
427
30%
427
12%
1045
June1st to 15thNight
30%
390
9%
1365
5%
390
34%
390
15%
942
June16th to endNight
29%
390
7%
1365
4%
390
33%
390
13%
974
July1st to 15thNight
32%
370
9%
1295
7%
370
39%
370
18%
858
July16th to endNight
39%
408
9%
1495
10%
386
47%
386
16%
995
August1st to 15thNight
40%
536
10%
1677
7%
534
44%
534
17%
1184
August16th to endNight
40%
621
16%
1863
8%
621
46%
621
23%
1222
September1st to 15thNight
33%
721
14%
2021
7%
721
39%
721
22%
1360
September16th to endNight
36%
777
18%
2145
10%
777
42%
777
26%
1401
October1st to 15thNight
29%
752
17%
2192
8%
752
37%
752
21%
1498
October16th to endNight
21%
768
11%
2454
5%
768
27%
768
12%
1840
November1st to 15thNight
22%
779
14%
2553
8%
779
29%
779
12%
1860
November16th to endNight
20%
825
10%
2750
6%
825
27%
825
9%
2132
December1st to 15thNight
13%
825
7%
2925
2%
825
20%
825
7%
2340
December16th to endNight
14%
880
9%
3120
3%
880
22%
880
5%
2527

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
20%9%37%6%34446%27%
52201
Night
18%9%36%6%3445%6%29%
51104
TransparencyDay
56%6%11%16%7%
51879
Night
54%6%12%17%8%
50944
SeeingDay
25%21%26%17%7%1
7959
Night
21%24%25%20%7%
16023

By Month

Percentage Cloud Cover NE (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
17%12%5%9%6%5%5%5%5%7%18%
1413
16th to endDay
13%11%5%10%7%45%6%7%6%22%
1664
February1st to 15thDay
9%15%6%8%7%6%7%6%6%7%18%
1681
16th to endDay
17%13%6%9%8%45%445%21%
1653
March1st to 15thDay
20%13%48%6%35%445%23%
2020
16th to endDay
17%12%5%8%7%3333430%
2288
April1st to 15thDay
22%12%47%5%2335%6%26%
2403
16th to endDay
25%9%36%42344432%
2501
May1st to 15thDay
25%8%35%5%33345%30%
2533
16th to endDay
26%7%37%5%34345%28%
3034
June1st to 15thDay
23%8%26%6%46%45%6%25%
2925
16th to endDay
24%8%37%6%35%45%6%23%
2925
July1st to 15thDay
18%6%35%5%36%5%6%8%29%
2925
16th to endDay
18%6%36%6%35%5%5%8%29%
2927
August1st to 15thDay
20%6%36%7%45%6%5%7%26%
2511
16th to endDay
14%6%36%5%35%5%6%7%34%
2691
September1st to 15thDay
20%6%25%423336%39%
2265
16th to endDay
20%5%35%423345%42%
2145
October1st to 15thDay
21%9%25%5%33435%33%
1938
16th to endDay
27%12%36%5%23345%24%
1767
November1st to 15thDay
22%12%46%6%345%4425%
1634
16th to endDay
24%14%38%6%24335%22%
1537
December1st to 15thDay
23%13%48%6%34345%20%
1365
16th to endDay
20%15%6%7%8%44445%18%
1456
January1st to 15thNight
17%14%5%9%8%45%435%21%
2854
16th to endNight
13%10%5%7%7%46%5%7%7%23%
2912
February1st to 15thNight
10%12%5%8%7%5%8%7%6%7%20%
2507
16th to endNight
14%17%48%7%345%3424%
2105
March1st to 15thNight
17%10%47%6%46%5%45%27%
2247
16th to endNight
13%11%5%7%6%34445%33%
2288
April1st to 15thNight
21%11%35%5%2443431%
1875
16th to endNight
25%8%24423445%34%
1734
May1st to 15thNight
22%9%46%5%35%345%29%
1746
16th to endNight
23%7%36%5%235%5%6%31%
1441
June1st to 15thNight
22%6%25%42435%6%34%
1365
16th to endNight
20%6%46%6%24446%32%
1365
July1st to 15thNight
19%5%24415%46%7%38%
1365
16th to endNight
16%5%25%446%46%8%34%
1590
August1st to 15thNight
15%6%25%7%45%5%5%6%35%
1687
16th to endNight
12%5%25%5%3346%8%41%
1863
September1st to 15thNight
17%5%24433447%41%
2021
16th to endNight
18%424423346%46%
2145
October1st to 15thNight
19%5%245%23446%40%
2192
16th to endNight
25%9%36%424345%28%
2454
November1st to 15thNight
23%9%36%5%23445%30%
2553
16th to endNight
21%13%48%5%24445%25%
2750
December1st to 15thNight
24%12%5%7%7%3444421%
2925
16th to endNight
20%14%48%8%45%445%19%
3120

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
63%8%11%11%5%
1414
16th to endDay
58%6%10%15%8%
1664
February1st to 15thDay
62%7%10%12%5%
1684
16th to endDay
65%48%14%6%
1653
March1st to 15thDay
61%48%13%9%1
2031
16th to endDay
57%47%17%11%1
2288
April1st to 15thDay
59%47%16%9%2
2403
16th to endDay
55%5%6%17%12%2
2501
May1st to 15thDay
56%5%9%16%11%1
2533
16th to endDay
57%5%9%18%8%
3038
June1st to 15thDay
57%7%10%13%8%1
2925
16th to endDay
60%7%10%14%6%
2925
July1st to 15thDay
49%9%16%17%7%
2766
16th to endDay
53%8%13%17%6%
2759
August1st to 15thDay
54%10%15%15%3
2497
16th to endDay
46%8%17%20%6%
2691
September1st to 15thDay
47%5%12%23%10%
2265
16th to endDay
44%5%13%24%11%
2145
October1st to 15thDay
52%5%12%21%7%
1938
16th to endDay
61%5%11%15%5%
1767
November1st to 15thDay
60%6%10%14%6%
1634
16th to endDay
65%6%11%13%3
1537
December1st to 15thDay
66%5%11%12%4
1365
16th to endDay
66%6%9%10%5%
1456
January1st to 15thNight
64%6%11%12%4
2854
16th to endNight
54%8%10%18%7%
2912
February1st to 15thNight
58%8%11%15%5%
2508
16th to endNight
62%5%7%13%9%
2105
March1st to 15thNight
58%5%8%16%9%
2259
16th to endNight
52%5%10%18%11%1
2288
April1st to 15thNight
56%48%13%14%2
1875
16th to endNight
51%48%17%16%1
1734
May1st to 15thNight
56%5%9%16%10%1
1746
16th to endNight
52%7%12%16%10%
1443
June1st to 15thNight
50%6%11%21%9%1
1365
16th to endNight
52%7%13%16%9%
1365
July1st to 15thNight
44%7%17%22%7%
1295
16th to endNight
45%8%16%19%9%
1495
August1st to 15thNight
47%9%16%22%4
1677
16th to endNight
39%8%17%26%7%
1863
September1st to 15thNight
42%7%12%25%11%1
2021
16th to endNight
39%5%16%25%11%1
2145
October1st to 15thNight
44%5%13%26%8%
2192
16th to endNight
56%6%13%18%4
2454
November1st to 15thNight
55%5%14%15%8%
2553
16th to endNight
61%6%11%15%4
2750
December1st to 15thNight
65%6%10%13%3
2925
16th to endNight
66%6%8%12%5%
3120

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
32%17%33%16%
90
16th to endDay
22%26%33%12%5%
96
February1st to 15thDay
18%25%30%17%8%
252
16th to endDay
26%29%22%15%5%
278
March1st to 15thDay
31%24%23%16%3
330
16th to endDay
22%23%27%15%9%2
352
April1st to 15thDay
28%29%25%10%5%1
330
16th to endDay
28%21%27%15%6%1
319
May1st to 15thDay
31%23%26%14%31
354
16th to endDay
28%24%23%16%5%
599
June1st to 15thDay
28%22%22%17%6%1
570
16th to endDay
25%22%27%15%7%1
570
July1st to 15thDay
19%18%26%20%13%1
550
16th to endDay
18%16%33%20%9%1
438
August1st to 15thDay
20%10%32%21%12%2
358
16th to endDay
20%16%24%23%12%3
396
September1st to 15thDay
20%15%31%20%9%2
375
16th to endDay
21%16%30%17%9%4
374
October1st to 15thDay
23%19%27%16%9%3
312
16th to endDay
33%20%23%14%6%1
288
November1st to 15thDay
30%24%20%17%6%
280
16th to endDay
32%20%24%14%6%1
262
December1st to 15thDay
31%18%28%10%8%2
90
16th to endDay
34%20%34%10%
96
January1st to 15thNight
21%32%28%15%2
811
16th to endNight
16%29%30%18%4
832
February1st to 15thNight
14%32%30%16%5%
764
16th to endNight
24%29%26%13%41
683
March1st to 15thNight
22%27%27%18%3
780
16th to endNight
18%28%28%17%6%
832
April1st to 15thNight
23%29%26%14%5%
688
16th to endNight
27%24%23%20%4
578
May1st to 15thNight
23%23%27%19%5%
585
16th to endNight
24%26%22%20%6%
428
June1st to 15thNight
21%19%22%28%7%
390
16th to endNight
20%20%22%25%10%
390
July1st to 15thNight
19%15%27%25%11%
370
16th to endNight
16%18%20%31%12%
408
August1st to 15thNight
17%12%21%32%15%
536
16th to endNight
12%14%27%30%15%
621
September1st to 15thNight
20%15%28%24%10%
721
16th to endNight
19%15%25%27%10%
777
October1st to 15thNight
20%18%27%21%10%
752
16th to endNight
29%22%23%17%6%
768
November1st to 15thNight
24%25%22%18%7%
779
16th to endNight
26%28%19%17%6%
825
December1st to 15thNight
29%26%23%16%3
825
16th to endNight
27%30%20%16%4
880
Page generated at 2018-10-09 11:39:14. Version ac04

Copyright 2018 A. Danko


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