The Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Ranch Observatory clear sky chart from 2005-11-05 to 2017-09-19.

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
32%
15220
13%
46817
7%
15189
39%
15189
22%
29194

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
16%
897
8%
2682
4%
892
22%
892
14%
1865
January16th to endNight
21%
768
16%
2688
8%
768
33%
768
16%
1701
February1st to 15thNight
23%
699
10%
2375
5%
697
31%
697
14%
1552
February16th to endNight
24%
632
11%
1959
6%
632
32%
632
17%
1314
March1st to 15thNight
20%
720
10%
2106
2%
720
28%
720
19%
1374
March16th to endNight
29%
768
9%
2106
1%
768
36%
768
17%
1458
April1st to 15thNight
28%
635
11%
1715
3%
635
34%
635
20%
1166
April16th to endNight
30%
540
14%
1620
8%
540
36%
540
22%
1080
May1st to 15thNight
40%
478
11%
1518
9%
478
45%
478
25%
926
May16th to endNight
44%
374
8%
1309
8%
374
47%
374
18%
875
June1st to 15thNight
43%
360
4%
1260
2%
360
44%
360
23%
751
June16th to endNight
45%
360
7%
1260
2%
360
49%
360
30%
682
July1st to 15thNight
61%
340
10%
1190
6%
340
65%
340
45%
538
July16th to endNight
62%
376
19%
1332
20%
354
70%
354
44%
577
August1st to 15thNight
67%
444
21%
1516
22%
442
72%
442
43%
730
August16th to endNight
58%
573
29%
1719
21%
573
64%
573
45%
864
September1st to 15thNight
52%
674
20%
1838
12%
674
55%
674
40%
1045
September16th to endNight
47%
671
23%
1856
13%
671
55%
671
39%
1035
October1st to 15thNight
35%
660
16%
1914
8%
660
43%
660
24%
1208
October16th to endNight
31%
704
17%
2288
8%
704
42%
704
23%
1341
November1st to 15thNight
16%
787
15%
2362
4%
787
28%
787
17%
1499
November16th to endNight
18%
900
12%
2624
3%
900
27%
900
17%
1773
December1st to 15thNight
21%
900
11%
2700
5%
900
26%
900
15%
1871
December16th to endNight
15%
960
10%
2880
2%
960
23%
960
14%
1969

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
7%7%35%5%35%5%6%8%42%
48030
Night
5%6%35%5%2446%9%46%
46972
Cloud CoverDay
7%7%35%5%35%5%6%8%42%
48031
Night
5%6%35%5%3446%8%46%
46975
TransparencyDay
37%8%21%21%9%1
47689
Night
34%9%22%21%9%1
46817
SeeingDay
14%18%26%23%12%3
5959
Night
11%20%28%25%11%1
15227

By Month

Percentage Cloud Cover NW (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
25%46%6%4447%12%40%
1265
16th to endDay
27%48%6%47%45%10%37%
1536
February1st to 15thDay
27%47%5%35%5%6%10%39%
1488
16th to endDay
210%5%8%6%35%5%6%9%34%
1512
March1st to 15thDay
1448%6%345%7%10%42%
1854
16th to endDay
5%11%5%7%5%36%6%7%11%30%
2117
April1st to 15thDay
7%12%36%445%6%8%10%30%
2233
16th to endDay
8%13%46%5%345%5%8%33%
2340
May1st to 15thDay
9%10%35%5%35%5%6%7%37%
2430
16th to endDay
21%8%35%6%36%5%6%6%26%
2802
June1st to 15thDay
14%6%35%6%46%5%7%7%33%
2700
16th to endDay
9%4245%35%6%7%8%41%
2700
July1st to 15thDay
5%3145%35%6%7%8%48%
2700
16th to endDay
421332445%8%60%
2743
August1st to 15thDay
9%3143235%6%8%51%
2436
16th to endDay
7%21331445%8%55%
2483
September1st to 15thDay
13%41331345%6%51%
2121
16th to endDay
10%412224348%55%
1849
October1st to 15thDay
11%8%35%6%2446%7%41%
1713
16th to endDay
5%7%25%325%5%8%9%42%
1584
November1st to 15thDay
35%235%1347%11%49%
1485
16th to endDay
111%5%7%5%33449%42%
1335
December1st to 15thDay
26%48%7%45%6%7%9%38%
1260
16th to endDay
18%37%6%25%5%7%10%41%
1344
January1st to 15thNight
26%37%5%35%5%6%11%40%
2682
16th to endNight
27%37%5%3446%10%44%
2688
February1st to 15thNight
39%47%5%25%5%5%10%39%
2364
16th to endNight
110%38%5%45%5%5%9%38%
1959
March1st to 15thNight
5%47%43446%12%46%
2106
16th to endNight
47%5%6%5%25%5%7%10%37%
2103
April1st to 15thNight
6%10%443246%7%9%40%
1715
16th to endNight
8%10%25%5%3445%8%41%
1620
May1st to 15thNight
8%6%345%346%5%7%45%
1517
16th to endNight
19%6%243245%6%9%33%
1309
June1st to 15thNight
13%4135%25%46%6%45%
1260
16th to endNight
8%31442445%8%51%
1260
July1st to 15thNight
5%22213447%64%
1260
16th to endNight
4312313436%67%
1424
August1st to 15thNight
7%21231335%8%59%
1524
16th to endNight
7%22313346%63%
1719
September1st to 15thNight
11%5%12312345%56%
1838
16th to endNight
9%212313449%55%
1856
October1st to 15thNight
9%8%2442345%8%45%
1914
16th to endNight
45%25%4245%7%9%47%
2288
November1st to 15thNight
35%245%345%5%9%49%
2362
16th to endNight
17%46%6%3445%10%43%
2624
December1st to 15thNight
18%5%8%5%35%6%7%10%38%
2700
16th to endNight
7%5%9%8%3446%9%39%
2880

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
25%36%7%35%5%6%12%40%
1265
16th to endDay
27%47%7%46%46%10%37%
1536
February1st to 15thDay
28%47%5%36%5%5%9%39%
1488
16th to endDay
39%6%8%6%35%5%5%10%35%
1512
March1st to 15thDay
15%47%6%35%5%6%9%43%
1854
16th to endDay
5%10%46%6%35%49%10%32%
2118
April1st to 15thDay
7%11%46%5%35%5%7%10%32%
2233
16th to endDay
9%13%35%6%345%6%7%35%
2340
May1st to 15thDay
8%10%35%5%35%5%5%8%37%
2430
16th to endDay
20%8%36%5%35%5%5%7%27%
2802
June1st to 15thDay
14%7%25%5%36%6%6%7%33%
2700
16th to endDay
9%4145%25%5%7%8%43%
2700
July1st to 15thDay
5%314435%5%6%9%49%
2700
16th to endDay
421221345%7%61%
2743
August1st to 15thDay
8%31242446%8%51%
2436
16th to endDay
7%31332446%8%55%
2483
September1st to 15thDay
13%413412446%52%
2121
16th to endDay
10%412324347%54%
1849
October1st to 15thDay
11%9%25%5%2445%7%41%
1713
16th to endDay
5%8%25%325%5%8%10%41%
1584
November1st to 15thDay
46%244235%6%10%49%
1485
16th to endDay
211%47%5%3445%9%42%
1335
December1st to 15thDay
17%5%8%8%35%47%8%38%
1260
16th to endDay
17%5%7%6%246%7%10%40%
1344
January1st to 15thNight
26%5%7%6%346%7%10%39%
2682
16th to endNight
37%37%5%4446%9%44%
2688
February1st to 15thNight
210%48%435%5%6%9%39%
2364
16th to endNight
110%47%6%46%5%5%8%37%
1959
March1st to 15thNight
5%47%5%25%46%11%46%
2106
16th to endNight
5%8%46%6%35%6%6%11%36%
2106
April1st to 15thNight
7%10%344245%6%8%39%
1715
16th to endNight
8%10%345%2435%8%42%
1620
May1st to 15thNight
7%8%24435%5%5%6%45%
1517
16th to endNight
19%6%24425%5%6%8%34%
1309
June1st to 15thNight
12%42342446%6%46%
1260
16th to endNight
7%41443446%7%51%
1260
July1st to 15thNight
5%2212235%8%63%
1260
16th to endNight
4212323346%66%
1424
August1st to 15thNight
7%2222335%8%60%
1524
16th to endNight
7%22412346%63%
1719
September1st to 15thNight
11%5%1322325%5%57%
1838
16th to endNight
9%31222435%8%56%
1856
October1st to 15thNight
9%8%2442346%9%44%
1914
16th to endNight
5%5%25%4246%6%8%47%
2288
November1st to 15thNight
36%25%43445%9%49%
2362
16th to endNight
27%46%6%35%46%9%42%
2624
December1st to 15thNight
28%48%6%35%7%6%9%38%
2700
16th to endNight
18%5%8%8%3446%9%38%
2880

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
38%11%29%15%31
1265
16th to endDay
43%9%21%17%5%2
1536
February1st to 15thDay
39%11%26%14%6%
1488
16th to endDay
44%10%24%14%5%
1512
March1st to 15thDay
36%13%26%20%3
1854
16th to endDay
45%9%26%13%3
2118
April1st to 15thDay
45%10%21%16%5%
2233
16th to endDay
47%7%20%18%5%1
2340
May1st to 15thDay
42%5%23%17%10%
2430
16th to endDay
54%7%17%15%4
2807
June1st to 15thDay
47%9%15%18%8%
2700
16th to endDay
34%6%22%25%9%1
2700
July1st to 15thDay
28%6%16%27%17%3
2541
16th to endDay
21%5%13%27%25%5%
2572
August1st to 15thDay
29%5%17%28%15%3
2419
16th to endDay
25%315%30%20%3
2483
September1st to 15thDay
32%313%27%16%5%
2121
16th to endDay
29%5%19%29%12%3
1849
October1st to 15thDay
41%5%24%20%6%
1713
16th to endDay
34%9%26%22%6%
1584
November1st to 15thDay
31%14%28%20%5%
1485
16th to endDay
39%11%24%15%7%1
1335
December1st to 15thDay
41%12%27%14%4
1260
16th to endDay
36%12%26%16%8%
1344
January1st to 15thNight
38%15%26%12%42
2682
16th to endNight
36%11%21%20%8%2
2688
February1st to 15thNight
40%10%24%17%6%1
2375
16th to endNight
43%8%22%17%7%
1959
March1st to 15thNight
31%14%30%19%2
2106
16th to endNight
40%11%26%17%3
2106
April1st to 15thNight
38%10%22%22%5%
1715
16th to endNight
41%8%18%23%6%
1620
May1st to 15thNight
35%7%22%22%11%
1518
16th to endNight
45%8%21%17%7%
1309
June1st to 15thNight
36%6%20%23%11%
1260
16th to endNight
30%5%24%24%14%1
1260
July1st to 15thNight
19%318%34%17%6%
1190
16th to endNight
18%413%29%26%8%
1332
August1st to 15thNight
21%416%37%15%4
1516
16th to endNight
22%312%33%24%4
1719
September1st to 15thNight
28%314%29%17%5%
1838
16th to endNight
24%5%22%27%15%4
1856
October1st to 15thNight
36%7%24%22%8%1
1914
16th to endNight
31%9%26%24%8%
2288
November1st to 15thNight
33%11%26%21%7%
2362
16th to endNight
37%12%23%17%5%2
2624
December1st to 15thNight
39%14%23%16%41
2700
16th to endNight
41%11%26%15%6%
2880

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
36%36%20%23%10%
60
16th to endDay
6%34%28%18%110%
64
February1st to 15thDay
9%28%33%24%5%
75
16th to endDay
11%28%32%19%8%
171
March1st to 15thDay
10%41%36%10%
270
16th to endDay
16%31%34%15%1
288
April1st to 15thDay
17%31%33%12%5%
269
16th to endDay
23%17%26%18%10%2
270
May1st to 15thDay
18%20%25%20%11%2
327
16th to endDay
26%9%19%19%18%6%
517
June1st to 15thDay
18%17%21%23%15%3
495
16th to endDay
11%15%26%27%14%4
495
July1st to 15thDay
8%7%24%31%22%5%
475
16th to endDay
6%10%32%31%15%3
412
August1st to 15thDay
10%9%25%29%19%6%
315
16th to endDay
11%12%25%33%13%4
332
September1st to 15thDay
18%19%25%21%11%4
315
16th to endDay
13%25%27%23%9%1
268
October1st to 15thDay
22%19%29%16%11%
158
16th to endDay
8%22%28%19%14%6%
96
November1st to 15thDay
15%33%25%22%3
90
16th to endDay
15%23%20%28%6%5%
73
December1st to 15thDay
5%31%13%38%5%6%
60
16th to endDay
10%31%18%21%10%6%
64
January1st to 15thNight
12%31%33%16%5%
897
16th to endNight
11%28%33%19%5%1
768
February1st to 15thNight
8%27%29%22%9%1
704
16th to endNight
9%20%34%26%8%
632
March1st to 15thNight
5%27%37%22%5%1
720
16th to endNight
10%23%25%30%9%1
768
April1st to 15thNight
11%21%30%24%10%1
635
16th to endNight
14%14%32%24%12%1
540
May1st to 15thNight
13%13%25%29%15%1
479
16th to endNight
22%8%17%29%20%2
375
June1st to 15thNight
14%14%23%29%16%1
360
16th to endNight
10%11%26%32%17%1
360
July1st to 15thNight
8%6%18%37%26%2
340
16th to endNight
7%420%42%22%2
376
August1st to 15thNight
7%217%38%30%3
444
16th to endNight
8%8%19%39%21%2
573
September1st to 15thNight
15%6%18%30%19%8%
674
16th to endNight
11%12%24%29%16%4
671
October1st to 15thNight
15%17%23%26%14%2
660
16th to endNight
10%22%30%28%7%1
704
November1st to 15thNight
15%32%30%16%4
787
16th to endNight
9%29%36%19%41
900
December1st to 15thNight
9%26%34%21%7%1
900
16th to endNight
7%29%36%19%6%
960
Page generated at 2017-09-20 02:49:47. Version ac04

Copyright 2017 A. Danko


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