Ashcroft
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashcroft clear sky chart from 2005-11-06 to 2025-06-10.

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
42%
24355
9%
118362
7%
24087
45%
24087
12%
79547

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
24%
866
4%
6688
2%
836
28%
836
3%
5217
January11th to 20thNight
26%
862
9%
6508
2%
837
29%
837
4%
4343
January21st to endNight
30%
856
9%
6733
4%
836
35%
836
4%
5021
February1st to 10thNight
31%
800
9%
2706
6%
800
36%
800
11%
2120
February11th to 20thNight
41%
780
11%
2534
8%
780
47%
780
17%
1790
February21st to endNight
37%
660
10%
2080
8%
660
41%
660
15%
1561
March1st to 10thNight
38%
788
12%
4870
9%
764
42%
764
10%
3035
March11th to 20thNight
39%
777
10%
4581
7%
760
46%
760
13%
2571
March21st to endNight
40%
854
8%
4766
6%
836
42%
836
9%
3407
April1st to 10thNight
44%
800
10%
2000
4%
800
46%
800
22%
1449
April11th to 20thNight
41%
742
12%
1930
6%
742
45%
742
22%
1367
April21st to endNight
44%
430
14%
1630
14%
430
49%
430
19%
1051
May1st to 10thNight
43%
388
9%
3109
12%
383
48%
383
8%
1964
May11th to 20thNight
53%
380
6%
3271
17%
374
56%
374
8%
2132
May21st to endNight
50%
420
2%
3443
5%
410
53%
410
8%
2229
June1st to 10thNight
43%
399
3%
1261
0%
399
44%
399
20%
864
June11th to 20thNight
46%
380
1%
1140
0%
380
46%
380
20%
813
June21st to endNight
55%
380
2%
1165
0%
380
55%
380
29%
722
July1st to 10thNight
58%
362
2%
1906
0%
360
59%
360
21%
971
July11th to 20thNight
60%
359
6%
1798
7%
359
61%
359
26%
791
July21st to endNight
72%
408
9%
2379
20%
383
76%
383
22%
1146
August1st to 10thNight
72%
368
20%
2253
25%
366
74%
366
25%
1005
August11th to 20thNight
70%
431
15%
2474
25%
431
74%
431
21%
1341
August21st to endNight
61%
715
13%
3163
12%
709
64%
709
22%
1822
September1st to 10thNight
63%
760
21%
1900
12%
760
65%
760
39%
1162
September11th to 20thNight
61%
760
20%
1900
11%
760
64%
760
33%
1238
September21st to endNight
48%
760
16%
2154
11%
760
52%
760
27%
1389
October1st to 10thNight
52%
748
16%
3626
12%
740
55%
740
16%
2168
October11th to 20thNight
44%
748
11%
3591
10%
740
49%
740
14%
2355
October21st to endNight
36%
822
11%
4278
9%
813
42%
813
12%
2729
November1st to 10thNight
32%
780
9%
2730
6%
780
37%
780
12%
2054
November11th to 20thNight
34%
800
9%
2800
6%
800
39%
800
12%
2093
November21st to endNight
32%
820
7%
2820
4%
820
36%
820
10%
2218
December1st to 10thNight
35%
994
9%
4819
6%
989
38%
989
9%
3262
December11th to 20thNight
26%
1079
2%
5839
2%
1069
28%
1069
5%
4692
December21st to endNight
25%
1079
8%
7517
4%
1041
30%
1041
4%
5455

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
5%8%38%6%6%6%5%7%8%36%
122789
Night
5%10%49%6%6%6%5%7%8%33%
124121
TransparencyDay
43%10%17%20%9%2
122540
Night
45%11%16%19%8%1
124013
SeeingDay
11%14%30%27%14%3
16336
Night
10%8%29%33%18%2
24708

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
6%14%6%16%8%8%7%6%6%7%15%
3179
11th to 20thDay
7%8%5%9%6%6%6%6%11%12%25%
3177
21st to endDay
415%6%11%7%7%6%5%8%9%23%
4004
February1st to 10thDay
6%14%5%10%6%7%7%6%7%8%24%
1690
11th to 20thDay
411%410%6%6%6%5%8%9%31%
1755
21st to endDay
212%411%9%10%7%6%8%7%23%
1549
March1st to 10thDay
18%49%6%7%8%6%9%10%32%
4518
11th to 20thDay
15%38%7%7%8%7%9%10%35%
4944
21st to endDay
310%410%8%7%7%6%8%10%26%
5784
April1st to 10thDay
410%5%10%8%7%8%6%8%9%26%
2400
11th to 20thDay
5%10%5%9%8%7%6%5%8%8%28%
2419
21st to endDay
6%11%410%7%6%5%5%7%9%29%
2770
May1st to 10thDay
6%9%38%5%5%5%46%8%42%
6320
11th to 20thDay
7%9%311%7%8%7%6%8%10%25%
6277
21st to endDay
10%10%38%6%5%6%5%7%8%31%
7031
June1st to 10thDay
11%9%39%7%6%6%47%7%31%
3118
11th to 20thDay
14%9%49%9%7%6%5%7%7%23%
3040
21st to endDay
7%6%39%7%7%6%5%7%8%37%
3010
July1st to 10thDay
431444445%7%59%
4740
11th to 20thDay
331333335%8%64%
4740
21st to endDay
5%41444446%8%55%
4855
August1st to 10thDay
3314345%47%9%57%
4401
11th to 20thDay
2326%5%5%7%5%9%11%47%
4214
21st to endDay
7%5%27%45%5%5%7%9%44%
4349
September1st to 10thDay
6%5%26%45%436%8%51%
2280
11th to 20thDay
9%7%38%45%5%5%6%7%42%
2280
21st to endDay
9%7%38%5%6%5%46%8%39%
2024
October1st to 10thDay
5%7%38%5%5%5%46%7%44%
3180
11th to 20thDay
9%7%37%46%6%5%8%9%37%
3143
21st to endDay
6%9%37%6%6%6%5%9%9%35%
3088
November1st to 10thDay
7%12%5%10%7%6%6%5%6%7%28%
1568
11th to 20thDay
410%49%7%6%6%5%7%9%35%
1600
21st to endDay
5%12%5%11%8%6%6%5%8%9%24%
1556
December1st to 10thDay
411%5%10%7%6%10%9%10%8%21%
2241
11th to 20thDay
6%16%7%13%8%7%8%6%6%5%17%
2262
21st to endDay
311%5%13%9%9%9%6%8%7%19%
3283
January1st to 10thNight
6%16%7%15%7%7%8%6%8%7%13%
6766
11th to 20thNight
7%11%412%7%5%5%49%9%28%
6720
21st to endNight
412%6%11%7%7%8%5%8%8%24%
6952
February1st to 10thNight
8%12%5%12%9%7%6%5%6%7%23%
2706
11th to 20thNight
5%12%38%7%6%7%5%7%8%33%
2532
21st to endNight
314%5%11%7%7%7%5%7%8%26%
2080
March1st to 10thNight
110%49%5%6%7%5%8%10%33%
5376
11th to 20thNight
16%26%5%5%6%6%9%10%44%
5053
21st to endNight
6%10%310%7%6%7%5%8%9%30%
5191
April1st to 10thNight
39%310%7%6%7%5%7%8%35%
2000
11th to 20thNight
28%48%7%6%6%6%8%9%36%
1930
21st to endNight
5%9%49%6%46%5%7%10%36%
1630
May1st to 10thNight
5%9%410%6%5%5%35%6%42%
3640
11th to 20thNight
7%9%48%6%6%5%5%7%9%34%
3616
21st to endNight
7%8%38%6%6%6%47%10%35%
3839
June1st to 10thNight
12%6%38%6%5%5%5%8%8%35%
1261
11th to 20thNight
9%8%38%7%6%6%6%6%9%31%
1140
21st to endNight
5%6%37%5%6%6%5%7%7%44%
1165
July1st to 10thNight
5%5%25%44446%7%55%
2220
11th to 20thNight
3323335%36%7%63%
2220
21st to endNight
4414445%47%8%53%
2695
August1st to 10thNight
3216%34447%8%57%
2529
11th to 20thNight
4415%5%5%6%5%8%11%45%
2615
21st to endNight
7%5%27%45%5%5%7%7%45%
3323
September1st to 10thNight
4426%445%35%7%55%
1900
11th to 20thNight
8%6%25%5%45%46%8%47%
1900
21st to endNight
7%7%36%445%46%8%45%
2154
October1st to 10thNight
5%7%27%5%6%6%5%7%8%43%
3800
11th to 20thNight
5%7%38%5%5%6%5%8%10%37%
3793
21st to endNight
6%8%39%7%6%6%7%9%7%31%
4457
November1st to 10thNight
9%12%410%7%6%5%5%7%8%27%
2730
11th to 20thNight
410%410%7%6%6%6%8%8%30%
2800
21st to endNight
6%15%5%11%7%7%6%5%7%9%22%
2820
December1st to 10thNight
411%49%7%6%10%7%9%7%27%
4923
11th to 20thNight
7%15%6%15%8%7%7%6%6%6%16%
5930
21st to endNight
5%13%5%12%8%7%8%7%7%7%21%
7715

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
60%16%11%11%2
3179
11th to 20thDay
46%13%18%16%34
3198
21st to endDay
56%11%14%14%5%
4004
February1st to 10thDay
56%12%13%10%8%1
1690
11th to 20thDay
47%11%16%17%8%
1764
21st to endDay
57%10%14%14%5%1
1549
March1st to 10thDay
43%10%20%18%8%
4518
11th to 20thDay
41%9%20%23%6%
4944
21st to endDay
50%11%17%14%7%
5784
April1st to 10thDay
52%12%17%14%4
2400
11th to 20thDay
51%10%13%16%9%1
2419
21st to endDay
51%8%16%18%8%
2770
May1st to 10thDay
37%7%13%28%14%1
6320
11th to 20thDay
49%10%18%18%5%
6280
21st to endDay
46%10%18%21%32
7032
June1st to 10thDay
51%8%13%15%9%3
3118
11th to 20thDay
58%8%15%13%6%
3040
21st to endDay
42%7%19%18%11%2
3010
July1st to 10thDay
24%5%11%23%24%13%
4666
11th to 20thDay
19%5%12%29%28%7%
4659
21st to endDay
25%5%19%26%18%7%
4701
August1st to 10thDay
22%7%13%35%21%2
4384
11th to 20thDay
27%12%25%24%11%1
4256
21st to endDay
34%9%24%27%5%1
4348
September1st to 10thDay
31%8%17%28%13%3
2280
11th to 20thDay
41%8%14%23%12%2
2280
21st to endDay
44%9%20%17%8%1
2024
October1st to 10thDay
39%8%19%25%7%1
3180
11th to 20thDay
44%10%16%25%5%
3143
21st to endDay
42%11%22%22%3
3088
November1st to 10thDay
55%10%13%17%41
1568
11th to 20thDay
46%15%15%17%6%
1600
21st to endDay
51%16%17%12%41
1556
December1st to 10thDay
52%13%21%12%1
2241
11th to 20thDay
62%13%11%9%5%
2264
21st to endDay
56%18%13%11%2
3283
January1st to 10thNight
59%15%13%10%3
6766
11th to 20thNight
51%10%16%15%44
6761
21st to endNight
55%13%14%12%5%
6952
February1st to 10thNight
57%12%11%13%6%1
2706
11th to 20thNight
48%9%17%18%8%1
2534
21st to endNight
53%10%17%14%5%
2080
March1st to 10thNight
39%11%21%21%7%
5376
11th to 20thNight
33%10%21%23%12%
5054
21st to endNight
48%11%16%16%10%
5191
April1st to 10thNight
44%11%20%19%6%
2000
11th to 20thNight
42%12%16%19%11%
1930
21st to endNight
41%11%17%20%11%
1630
May1st to 10thNight
40%9%11%26%13%2
3637
11th to 20thNight
41%13%15%25%6%
3616
21st to endNight
40%13%18%23%6%
3839
June1st to 10thNight
45%8%14%19%11%3
1261
11th to 20thNight
47%9%16%17%10%
1140
21st to endNight
35%8%21%21%11%4
1165
July1st to 10thNight
28%7%11%16%27%11%
2185
11th to 20thNight
18%8%12%25%33%4
2178
21st to endNight
27%7%14%25%15%12%
2610
August1st to 10thNight
22%8%12%30%26%2
2521
11th to 20thNight
30%10%20%25%13%2
2639
21st to endNight
34%8%24%24%8%1
3323
September1st to 10thNight
28%8%16%30%15%4
1900
11th to 20thNight
36%7%17%24%14%3
1900
21st to endNight
36%10%22%19%11%2
2154
October1st to 10thNight
39%8%19%26%8%
3800
11th to 20thNight
44%9%16%24%7%
3793
21st to endNight
45%9%18%25%4
4457
November1st to 10thNight
54%11%16%14%5%1
2730
11th to 20thNight
49%16%15%14%6%1
2800
21st to endNight
55%15%15%10%4
2820
December1st to 10thNight
48%12%19%18%4
4923
11th to 20thNight
61%13%13%9%3
5927
21st to endNight
53%14%14%15%3
7715

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
7%14%36%29%10%6%
200
11th to 20thDay
7%13%31%34%15%3
200
21st to endDay
15%9%36%27%13%1
220
February1st to 10thDay
9%15%44%21%8%3
290
11th to 20thDay
10%11%37%27%13%2
395
21st to endDay
9%16%39%24%10%2
364
March1st to 10thDay
10%19%34%24%13%1
500
11th to 20thDay
8%20%35%20%14%3
500
21st to endDay
13%23%31%22%9%2
550
April1st to 10thDay
14%22%30%25%8%1
500
11th to 20thDay
16%21%29%20%11%3
492
21st to endDay
12%21%29%24%11%3
500
May1st to 10thDay
12%20%31%25%10%2
503
11th to 20thDay
13%11%29%28%14%4
528
21st to endDay
14%15%27%29%12%3
623
June1st to 10thDay
14%18%30%27%10%1
856
11th to 20thDay
17%13%27%27%14%2
870
21st to endDay
10%10%28%30%19%2
840
July1st to 10thDay
8%11%31%31%18%2
671
11th to 20thDay
6%13%27%30%19%5%
668
21st to endDay
5%7%24%33%26%4
570
August1st to 10thDay
6%7%22%30%28%7%
487
11th to 20thDay
5%10%21%34%22%8%
477
21st to endDay
8%12%27%32%16%6%
539
September1st to 10thDay
8%14%28%29%16%4
490
11th to 20thDay
14%12%26%26%17%5%
490
21st to endDay
15%18%32%23%10%2
490
October1st to 10thDay
14%14%31%24%13%4
490
11th to 20thDay
14%20%28%23%14%1
484
21st to endDay
16%12%33%26%12%1
329
November1st to 10thDay
15%18%36%18%13%1
200
11th to 20thDay
12%10%39%24%13%4
200
21st to endDay
10%11%35%29%9%7%
200
December1st to 10thDay
9%7%30%28%22%6%
200
11th to 20thDay
10%10%32%29%14%6%
200
21st to endDay
10%9%34%34%10%3
220
January1st to 10thNight
15%9%35%29%10%2
900
11th to 20thNight
13%7%34%34%11%1
896
21st to endNight
10%6%36%32%14%2
880
February1st to 10thNight
14%7%33%32%13%2
800
11th to 20thNight
10%6%31%37%15%1
784
21st to endNight
7%9%36%29%17%2
660
March1st to 10thNight
10%9%32%31%17%1
800
11th to 20thNight
5%12%35%31%16%2
800
21st to endNight
8%9%31%32%17%2
880
April1st to 10thNight
9%10%26%33%20%2
800
11th to 20thNight
5%9%35%34%16%1
742
21st to endNight
7%9%33%32%18%1
430
May1st to 10thNight
8%8%30%34%19%2
400
11th to 20thNight
9%5%23%39%22%2
394
21st to endNight
15%423%34%22%2
433
June1st to 10thNight
13%8%25%36%18%1
399
11th to 20thNight
13%6%23%38%18%2
380
21st to endNight
6%6%25%36%26%2
380
July1st to 10thNight
7%422%41%24%2
370
11th to 20thNight
7%5%18%38%29%3
368
21st to endNight
6%313%42%34%3
416
August1st to 10thNight
5%213%34%40%6%
378
11th to 20thNight
5%415%35%36%6%
442
21st to endNight
8%5%19%37%30%2
726
September1st to 10thNight
7%5%19%40%26%4
760
11th to 20thNight
11%5%16%29%31%7%
760
21st to endNight
13%9%24%31%19%4
760
October1st to 10thNight
10%9%23%34%22%2
760
11th to 20thNight
13%9%27%31%17%3
760
21st to endNight
14%9%32%32%12%2
836
November1st to 10thNight
16%9%33%32%10%1
780
11th to 20thNight
12%9%33%31%13%2
800
21st to endNight
14%11%30%32%10%3
820
December1st to 10thNight
10%6%30%37%15%2
1010
11th to 20thNight
14%8%35%32%10%1
1100
21st to endNight
13%9%36%32%10%1
1104
Page generated at 2025-06-07 07:28:15. Version ac07

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