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 2005-11-06 to 2024-01-12.

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
30%
23474
15%
74729
7%
23432
39%
23432
22%
45676

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 10thNight
17%
950
11%
2836
3%
946
25%
946
15%
1874
January11th to 20thNight
18%
814
15%
2628
6%
814
29%
814
16%
1703
January21st to endNight
19%
792
18%
2772
6%
792
35%
792
18%
1670
February1st to 10thNight
21%
720
12%
2469
5%
720
30%
720
13%
1636
February11th to 20thNight
18%
702
11%
2273
4%
700
28%
700
14%
1500
February21st to endNight
25%
592
15%
1785
7%
592
34%
592
16%
1212
March1st to 10thNight
23%
720
12%
2160
4%
720
31%
720
18%
1454
March11th to 20thNight
27%
720
17%
1990
6%
720
37%
720
22%
1280
March21st to endNight
31%
792
11%
2170
2%
792
37%
792
20%
1456
April1st to 10thNight
30%
684
14%
1764
4%
684
36%
684
23%
1165
April11th to 20thNight
26%
533
16%
1601
7%
533
34%
533
20%
1066
April21st to endNight
31%
540
13%
1620
7%
540
37%
540
22%
1076
May1st to 10thNight
35%
539
10%
1619
7%
539
40%
539
22%
1058
May11th to 20thNight
48%
354
14%
1269
16%
354
52%
354
24%
750
May21st to endNight
40%
388
8%
1358
7%
388
44%
388
18%
903
June1st to 10thNight
38%
360
7%
1260
3%
360
42%
360
21%
772
June11th to 20thNight
33%
360
6%
1260
1%
360
38%
360
20%
779
June21st to endNight
49%
360
8%
1260
3%
360
53%
360
36%
627
July1st to 10thNight
62%
350
12%
1225
8%
350
66%
350
46%
553
July11th to 20thNight
53%
348
17%
1218
15%
348
62%
348
43%
573
July21st to endNight
62%
394
24%
1446
26%
372
75%
372
49%
580
August1st to 10thNight
62%
393
23%
1459
25%
391
70%
391
41%
674
August11th to 20thNight
63%
534
25%
1602
19%
534
68%
534
49%
793
August21st to endNight
52%
594
29%
1782
18%
594
60%
594
41%
939
September1st to 10thNight
48%
650
21%
1765
12%
650
52%
650
39%
1015
September11th to 20thNight
50%
720
22%
1980
13%
720
55%
720
38%
1150
September21st to endNight
39%
720
23%
1980
11%
720
47%
720
36%
1138
October1st to 10thNight
40%
720
19%
2047
11%
720
48%
720
29%
1246
October11th to 20thNight
34%
720
20%
2254
9%
720
46%
720
26%
1285
October21st to endNight
22%
792
14%
2574
5%
792
33%
792
17%
1626
November1st to 10thNight
18%
782
14%
2447
5%
782
29%
782
16%
1611
November11th to 20thNight
14%
950
17%
2660
3%
950
27%
950
18%
1690
November21st to endNight
17%
950
13%
2825
3%
950
28%
950
17%
1839
December1st to 10thNight
20%
950
15%
2850
5%
950
30%
950
19%
1868
December11th to 20thNight
18%
949
10%
3472
3%
949
25%
949
13%
2369
December21st to endNight
18%
1038
8%
5049
4%
1026
26%
1026
11%
2746

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%36%5%35%5%6%9%43%
74614
Night
5%7%36%5%35%5%6%9%47%
75101
TransparencyDay
36%8%20%22%11%3
74304
Night
34%9%21%22%11%3
74929
SeeingDay
16%19%28%22%12%3
14148
Night
12%20%30%26%10%2
23485

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 10thDay
26%47%8%5%5%5%9%12%38%
1329
11th to 20thDay
16%46%5%48%7%7%10%42%
1375
21st to endDay
39%48%6%46%45%12%38%
1584
February1st to 10thDay
27%410%7%5%7%6%7%10%34%
1490
11th to 20thDay
29%6%9%6%46%6%7%9%35%
1575
21st to endDay
28%5%10%8%45%5%7%9%37%
1467
March1st to 10thDay
16%5%9%7%446%7%9%43%
1800
11th to 20thDay
38%47%5%345%7%12%43%
1969
21st to endDay
410%5%8%6%36%6%8%10%34%
2185
April1st to 10thDay
411%5%8%6%5%6%6%8%9%33%
2195
11th to 20thDay
6%12%48%7%445%6%9%34%
2310
21st to endDay
9%13%47%435%6%6%8%35%
2340
May1st to 10thDay
9%12%46%6%46%6%7%8%34%
2340
11th to 20thDay
13%10%36%6%46%5%6%8%35%
2622
21st to endDay
22%9%36%5%35%5%6%7%29%
2917
June1st to 10thDay
13%8%36%7%46%5%7%8%35%
2700
11th to 20thDay
15%6%36%6%46%7%7%8%32%
2700
21st to endDay
7%3245%35%6%7%9%49%
2700
July1st to 10thDay
5%4145%35%6%7%8%50%
2700
11th to 20thDay
5%314425%6%7%7%56%
2700
21st to endDay
421322345%8%64%
2783
August1st to 10thDay
8%4243345%6%8%55%
2472
11th to 20thDay
7%3143245%5%9%56%
2299
21st to endDay
9%3144245%6%9%53%
2574
September1st to 10thDay
12%5%1442445%7%53%
2192
11th to 20thDay
9%5%243245%7%7%52%
1980
21st to endDay
10%7%2332345%8%53%
1980
October1st to 10thDay
11%9%35%5%2446%8%45%
1912
11th to 20thDay
5%6%36%6%35%5%7%8%47%
1702
21st to endDay
49%46%5%345%7%10%43%
1782
November1st to 10thDay
5%8%36%7%345%7%10%43%
1612
11th to 20thDay
19%35%4235%6%12%49%
1520
21st to endDay
29%5%8%6%35%5%5%9%45%
1354
December1st to 10thDay
17%5%7%6%45%6%7%9%43%
1330
11th to 20thDay
28%37%8%35%6%7%10%40%
1682
21st to endDay
27%36%6%45%5%6%10%47%
2442
January1st to 10thNight
25%48%6%45%6%7%10%44%
2836
11th to 20thNight
27%37%7%45%6%7%11%41%
2628
21st to endNight
26%36%5%45%5%8%11%45%
2772
February1st to 10thNight
29%5%10%7%45%6%5%10%37%
2469
11th to 20thNight
110%5%8%6%46%5%6%10%39%
2272
21st to endNight
18%410%7%5%6%6%6%9%38%
1785
March1st to 10thNight
16%5%10%7%45%46%11%43%
2160
11th to 20thNight
36%36%5%245%7%11%47%
1990
21st to endNight
47%5%6%5%35%6%7%10%42%
2170
April1st to 10thNight
5%9%5%6%5%345%6%9%43%
1764
11th to 20thNight
6%12%47%5%345%6%8%40%
1601
21st to endNight
7%10%36%6%35%5%6%8%41%
1620
May1st to 10thNight
9%9%36%7%45%5%5%7%40%
1619
11th to 20thNight
9%7%25%435%7%6%8%44%
1269
21st to endNight
21%8%343345%7%8%35%
1358
June1st to 10thNight
12%5%35%6%3447%8%43%
1260
11th to 20thNight
16%6%25%345%5%6%9%41%
1260
21st to endNight
5%3235%2447%8%57%
1260
July1st to 10thNight
6%312223447%65%
1260
11th to 20thNight
431232445%6%65%
1260
21st to endNight
5%212213446%70%
1531
August1st to 10thNight
7%31342445%7%62%
1467
11th to 20thNight
7%21331346%9%61%
1602
21st to endNight
8%413423346%62%
1782
September1st to 10thNight
11%5%2243446%7%54%
1765
11th to 20thNight
9%6%23433448%54%
1980
21st to endNight
9%5%233235%5%8%55%
1980
October1st to 10thNight
9%9%24433448%51%
2047
11th to 20thNight
46%35%4346%6%8%53%
2254
21st to endNight
47%47%6%345%7%10%44%
2574
November1st to 10thNight
49%47%5%35%5%6%8%43%
2447
11th to 20thNight
16%25%5%335%6%10%51%
2660
21st to endNight
17%47%7%3446%11%45%
2825
December1st to 10thNight
16%48%6%35%6%7%10%45%
2850
11th to 20thNight
26%49%7%46%6%7%10%38%
3510
21st to endNight
17%47%7%35%46%9%47%
5214

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 10thDay
39%11%27%15%6%2
1329
11th to 20thDay
36%11%26%16%7%3
1375
21st to endDay
41%9%21%21%7%1
1584
February1st to 10thDay
44%11%24%13%6%2
1490
11th to 20thDay
42%10%21%20%6%1
1584
21st to endDay
43%10%19%18%8%2
1467
March1st to 10thDay
36%13%23%23%5%
1800
11th to 20thDay
36%10%25%22%7%1
1970
21st to endDay
43%9%27%15%5%1
2185
April1st to 10thDay
43%9%22%22%4
2195
11th to 20thDay
47%8%18%20%7%1
2310
21st to endDay
45%8%20%20%7%1
2340
May1st to 10thDay
45%7%21%20%8%
2340
11th to 20thDay
45%7%19%21%9%
2626
21st to endDay
52%8%18%17%5%
2918
June1st to 10thDay
45%9%17%21%8%
2700
11th to 20thDay
47%8%17%19%9%1
2700
21st to endDay
28%6%23%28%13%2
2700
July1st to 10thDay
26%7%15%29%18%4
2626
11th to 20thDay
24%7%13%25%24%7%
2619
21st to endDay
17%5%11%28%31%8%
2628
August1st to 10thDay
26%6%14%25%21%8%
2455
11th to 20thDay
25%5%15%32%19%4
2299
21st to endDay
27%5%14%29%21%5%
2574
September1st to 10thDay
31%413%28%16%8%
2192
11th to 20thDay
29%6%16%29%15%4
1980
21st to endDay
31%6%20%30%11%3
1980
October1st to 10thDay
38%5%20%26%8%2
1912
11th to 20thDay
33%7%24%25%9%2
1702
21st to endDay
37%9%20%24%9%1
1782
November1st to 10thDay
39%11%22%20%7%1
1612
11th to 20thDay
30%13%27%22%7%1
1520
21st to endDay
38%10%24%17%9%2
1354
December1st to 10thDay
38%13%22%17%6%3
1330
11th to 20thDay
37%12%29%16%5%1
1684
21st to endDay
33%11%20%18%14%4
2442
January1st to 10thNight
37%12%26%16%7%2
2836
11th to 20thNight
37%12%22%20%6%3
2628
21st to endNight
33%11%22%24%10%1
2772
February1st to 10thNight
43%9%20%18%7%3
2469
11th to 20thNight
41%11%23%18%8%
2273
21st to endNight
44%8%19%18%9%2
1785
March1st to 10thNight
37%14%23%20%6%
2160
11th to 20thNight
32%12%25%22%8%1
1990
21st to endNight
37%11%27%19%5%1
2170
April1st to 10thNight
37%8%22%26%7%
1764
11th to 20thNight
41%9%17%25%8%
1601
21st to endNight
41%8%19%23%8%1
1620
May1st to 10thNight
41%7%21%23%9%
1619
11th to 20thNight
35%8%22%23%11%
1269
21st to endNight
46%7%21%20%7%
1358
June1st to 10thNight
39%7%21%23%9%
1260
11th to 20thNight
41%6%19%21%12%1
1260
21st to endNight
23%7%23%28%16%3
1260
July1st to 10thNight
18%5%18%33%20%6%
1225
11th to 20thNight
20%5%13%28%24%10%
1218
21st to endNight
15%411%28%30%11%
1446
August1st to 10thNight
22%5%14%31%19%9%
1459
11th to 20thNight
19%5%16%34%21%4
1602
21st to endNight
25%411%30%25%5%
1782
September1st to 10thNight
28%5%15%28%17%6%
1765
11th to 20thNight
29%5%16%30%15%5%
1980
21st to endNight
28%5%23%27%15%3
1980
October1st to 10thNight
33%6%20%26%13%2
2047
11th to 20thNight
29%8%22%27%11%3
2254
21st to endNight
36%11%22%21%9%1
2574
November1st to 10thNight
40%10%22%19%7%2
2447
11th to 20thNight
30%12%25%24%8%1
2660
21st to endNight
35%12%23%19%8%3
2825
December1st to 10thNight
35%13%23%19%7%3
2850
11th to 20thNight
40%14%24%17%41
3507
21st to endNight
34%9%22%17%12%6%
5214

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 10thDay
14%24%30%18%8%5%
180
11th to 20thDay
11%23%30%26%7%2
162
21st to endDay
14%32%32%14%35%
176
February1st to 10thDay
21%19%35%20%5%
182
11th to 20thDay
13%27%33%19%8%
240
21st to endDay
15%26%33%18%6%1
333
March1st to 10thDay
12%28%35%17%6%2
420
11th to 20thDay
13%25%30%19%10%3
420
21st to endDay
15%26%33%18%6%2
462
April1st to 10thDay
17%25%35%15%6%
420
11th to 20thDay
18%26%29%16%8%2
412
21st to endDay
22%18%27%21%8%3
420
May1st to 10thDay
19%18%24%21%14%4
423
11th to 20thDay
24%13%22%20%17%5%
593
21st to endDay
27%10%17%23%18%6%
679
June1st to 10thDay
19%18%24%23%14%3
630
11th to 20thDay
22%21%23%20%11%3
630
21st to endDay
10%12%26%29%18%6%
630
July1st to 10thDay
9%10%26%29%21%5%
621
11th to 20thDay
8%10%29%33%16%3
618
21st to endDay
6%10%28%33%18%5%
529
August1st to 10thDay
11%10%26%29%18%6%
447
11th to 20thDay
7%8%31%31%18%5%
437
21st to endDay
12%14%28%29%13%4
495
September1st to 10thDay
16%19%27%21%13%5%
450
11th to 20thDay
12%15%31%23%15%3
450
21st to endDay
21%20%28%20%9%2
450
October1st to 10thDay
18%17%28%20%14%3
382
11th to 20thDay
15%21%29%21%10%3
270
21st to endDay
20%25%32%13%8%2
297
November1st to 10thDay
21%27%30%16%5%
270
11th to 20thDay
19%34%29%15%3
270
21st to endDay
17%38%19%13%9%4
192
December1st to 10thDay
13%32%31%17%6%3
180
11th to 20thDay
22%23%27%22%41
180
21st to endDay
11%26%28%21%11%4
198
January1st to 10thNight
13%30%34%18%5%1
950
11th to 20thNight
11%29%34%19%6%1
814
21st to endNight
11%29%36%17%5%2
792
February1st to 10thNight
11%23%34%23%7%2
720
11th to 20thNight
11%26%33%22%7%
704
21st to endNight
9%23%32%26%9%1
592
March1st to 10thNight
7%22%36%26%8%2
720
11th to 20thNight
8%26%30%26%8%2
720
21st to endNight
10%20%28%28%11%2
792
April1st to 10thNight
10%19%32%26%11%1
684
11th to 20thNight
14%18%33%24%9%1
533
21st to endNight
13%16%32%27%11%1
540
May1st to 10thNight
17%14%25%28%15%1
539
11th to 20thNight
12%8%24%36%19%2
354
21st to endNight
26%8%19%27%18%2
389
June1st to 10thNight
15%14%24%30%16%1
360
11th to 20thNight
18%16%27%29%9%1
360
21st to endNight
7%8%29%36%20%1
360
July1st to 10thNight
8%8%17%40%25%2
350
11th to 20thNight
7%5%30%40%16%1
348
21st to endNight
7%422%42%22%2
394
August1st to 10thNight
8%222%41%26%2
393
11th to 20thNight
6%422%40%24%3
534
21st to endNight
10%9%23%42%14%2
594
September1st to 10thNight
15%7%24%32%17%5%
650
11th to 20thNight
13%8%23%34%17%5%
720
21st to endNight
14%13%30%27%13%3
720
October1st to 10thNight
16%15%22%31%14%2
720
11th to 20thNight
11%20%29%28%10%1
720
21st to endNight
14%25%32%21%5%2
792
November1st to 10thNight
16%25%34%20%4
782
11th to 20thNight
14%32%35%16%21
950
21st to endNight
12%30%33%19%5%1
950
December1st to 10thNight
10%24%37%20%7%1
950
11th to 20thNight
9%28%37%20%5%
950
21st to endNight
11%26%35%22%6%1
1045
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