James Clerk Maxwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James Clerk Maxwell Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	10% 4 1 3 2 2 2 2 3 5% 64%	105347
	Night	13% 5% 1 4 3 2 3 2 4 5% 59%	102863
Transparency	Day	25% 2 6% 18% 24% 25%	105097
	Night	29% 3 7% 17% 22% 22%	102702
Seeing	Day	14% 9% 26% 32% 14% 5%	15190
	Night	15% 4 14% 35% 27% 5%	24484

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 10% 2 5% 3 3 3 2 3 6% 39%	2539
	11th to 20th	Day	24% 10% 2 8% 4 4 3 2 3 5% 34%	2736
	21st to end	Day	27% 9% 3 6% 3 3 3 3 4 6% 33%	3146
February	1st to 10th	Day	22% 7% 2 4 3 2 3 3 4 5% 46%	1710
	11th to 20th	Day	19% 8% 2 4 3 3 4 3 4 6% 44%	1781
	21st to end	Day	21% 8% 2 4 3 3 3 3 4 6% 44%	1570
March	1st to 10th	Day	23% 17% 3 6% 4 3 3 2 4 5% 29%	3221
	11th to 20th	Day	17% 5% 2 3 2 2 2 2 3 5% 56%	3490
	21st to end	Day	21% 11% 3 5% 4 4 4 4 6% 7% 31%	3979
April	1st to 10th	Day	18% 7% 2 4 3 3 4 3 6% 6% 45%	2280
	11th to 20th	Day	15% 5% 2 4 3 3 3 4 4 6% 52%	2249
	21st to end	Day	10% 5% 1 3 3 3 3 3 5% 7% 58%	2280
May	1st to 10th	Day	8% 3 1 2 2 2 2 2 3 5% 69%	3940
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 78%	4377
	21st to end	Day	5% 2 1 2 2 2 2 2 4 5% 73%	4813
June	1st to 10th	Day	8% 3 1 3 3 2 3 3 5% 7% 62%	2660
	11th to 20th	Day	3 2 1 2 1 2 2 1 3 5% 79%	2660
	21st to end	Day	2 2 2 1 1 1 2 2 3 83%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	4420
	11th to 20th	Day	2 1 2 1 1 1 1 2 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4525
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	4091
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	3790
	21st to end	Day	2 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	2 1 1 1 1 1 2 3 88%	2280
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 5% 75%	2280
	21st to end	Day	4 2 2 1 1 2 1 3 4 79%	2053
October	1st to 10th	Day	3 2 1 3 2 2 3 2 4 5% 73%	3180
	11th to 20th	Day	7% 2 1 2 1 2 2 2 4 6% 70%	3161
	21st to end	Day	6% 3 1 5% 3 3 3 3 5% 7% 61%	3448
November	1st to 10th	Day	14% 6% 2 4 3 3 3 2 5% 7% 52%	1784
	11th to 20th	Day	16% 4 1 3 4 3 5% 4 4 7% 49%	1800
	21st to end	Day	15% 6% 2 5% 3 3 3 3 4 6% 51%	1602
December	1st to 10th	Day	19% 5% 1 4 3 2 3 3 5% 7% 46%	1520
	11th to 20th	Day	31% 8% 3 6% 4 3 2 2 4 5% 31%	1928
	21st to end	Day	17% 8% 3 6% 4 5% 4 4 6% 8% 37%	2794
January	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 5% 6% 40%	4406
	11th to 20th	Night	25% 7% 2 6% 4 4 4 3 4 5% 36%	4227
	21st to end	Night	27% 9% 2 7% 4 4 4 3 4 5% 31%	4521
February	1st to 10th	Night	21% 6% 1 3 3 2 3 3 4 5% 48%	2470
	11th to 20th	Night	18% 7% 1 4 4 3 4 3 5% 5% 45%	2285
	21st to end	Night	20% 8% 2 4 3 2 3 2 5% 6% 46%	1884
March	1st to 10th	Night	28% 16% 3 5% 3 3 2 3 3 4 30%	3648
	11th to 20th	Night	19% 5% 1 3 2 2 2 2 4 4 55%	3490
	21st to end	Night	25% 10% 2 6% 4 3 4 3 5% 5% 33%	3674
April	1st to 10th	Night	16% 7% 2 5% 3 3 3 2 4 5% 48%	1900
	11th to 20th	Night	15% 5% 1 4 3 3 3 3 4 7% 52%	1879
	21st to end	Night	11% 3 1 3 3 2 3 3 4 6% 61%	1900
May	1st to 10th	Night	7% 5% 1 3 2 2 3 2 4 5% 65%	3028
	11th to 20th	Night	5% 1 1 2 2 1 2 2 4 4 76%	2516
	21st to end	Night	5% 2 1 2 2 2 2 2 3 4 75%	2762
June	1st to 10th	Night	7% 3 1 2 2 3 3 3 4 6% 66%	1520
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 5% 78%	1520
	21st to end	Night	2 2 2 2 2 2 1 3 2 81%	1520
July	1st to 10th	Night	1 1 1 1 1 1 93%	2540
	11th to 20th	Night	2 2 1 2 2 1 2 1 2 3 82%	2540
	21st to end	Night	1 1 1 1 1 2 1 3 3 85%	3033
August	1st to 10th	Night	3 2 1 2 2 2 3 2 4 5% 74%	2848
	11th to 20th	Night	1 1 1 1 1 1 2 2 89%	3056
	21st to end	Night	2 1 1 1 1 1 1 2 2 87%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 87%	1900
	11th to 20th	Night	4 2 1 3 2 1 1 3 3 5% 76%	1900
	21st to end	Night	3 1 1 2 2 2 2 2 4 4 78%	2121
October	1st to 10th	Night	4 2 1 2 2 2 2 1 2 3 77%	3800
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 4 77%	3777
	21st to end	Night	10% 3 1 3 3 3 3 3 5% 6% 62%	4121
November	1st to 10th	Night	16% 5% 2 3 2 2 2 3 5% 7% 53%	2512
	11th to 20th	Night	12% 5% 1 5% 3 2 3 3 6% 7% 53%	2600
	21st to end	Night	15% 5% 2 5% 3 3 3 2 4 5% 53%	2669
December	1st to 10th	Night	22% 4 1 3 2 3 3 2 4 7% 49%	2660
	11th to 20th	Night	33% 7% 2 4 3 2 3 3 6% 6% 31%	3276
	21st to end	Night	21% 7% 3 7% 5% 4 5% 4 5% 6% 34%	4862

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	50% 3 6% 24% 15% 3	2539
	11th to 20th	Day	55% 3 7% 14% 11% 9%	2736
	21st to end	Day	54% 3 8% 18% 12% 6%	3146
February	1st to 10th	Day	43% 3 8% 18% 18% 10%	1710
	11th to 20th	Day	41% 3 9% 17% 22% 7%	1790
	21st to end	Day	42% 3 10% 21% 16% 7%	1570
March	1st to 10th	Day	59% 4 8% 14% 11% 4	3221
	11th to 20th	Day	33% 2 9% 30% 16% 10%	3490
	21st to end	Day	51% 5% 13% 20% 10% 2	3979
April	1st to 10th	Day	40% 5% 11% 24% 17% 4	2280
	11th to 20th	Day	33% 4 14% 24% 19% 6%	2252
	21st to end	Day	26% 3 13% 29% 25% 4	2280
May	1st to 10th	Day	20% 2 9% 30% 23% 16%	3940
	11th to 20th	Day	13% 2 7% 24% 42% 12%	4380
	21st to end	Day	15% 3 12% 36% 29% 6%	4814
June	1st to 10th	Day	22% 2 11% 25% 27% 12%	2660
	11th to 20th	Day	11% 2 8% 20% 36% 23%	2660
	21st to end	Day	9% 1 3 13% 37% 37%	2660
July	1st to 10th	Day	3 1 1 5% 15% 75%	4351
	11th to 20th	Day	7% 2 2 7% 16% 66%	4345
	21st to end	Day	6% 1 1 4 26% 62%	4382
August	1st to 10th	Day	12% 2 2 7% 22% 55%	4076
	11th to 20th	Day	5% 1 1 6% 41% 46%	3827
	21st to end	Day	6% 1 1 9% 41% 41%	4179
September	1st to 10th	Day	6% 1 2 8% 27% 57%	2280
	11th to 20th	Day	15% 1 5% 20% 35% 23%	2280
	21st to end	Day	13% 2 4 17% 33% 31%	2053
October	1st to 10th	Day	15% 2 3 12% 25% 42%	3180
	11th to 20th	Day	17% 2 4 19% 37% 21%	3161
	21st to end	Day	24% 3 6% 20% 31% 17%	3448
November	1st to 10th	Day	33% 3 7% 21% 23% 14%	1784
	11th to 20th	Day	35% 4 10% 20% 22% 9%	1800
	21st to end	Day	34% 4 8% 17% 23% 13%	1602
December	1st to 10th	Day	38% 4 7% 21% 19% 11%	1520
	11th to 20th	Day	57% 2 6% 14% 14% 6%	1928
	21st to end	Day	46% 4 14% 20% 13% 4	2794
January	1st to 10th	Night	44% 4 13% 22% 12% 4	4406
	11th to 20th	Night	51% 5% 11% 11% 13% 9%	4227
	21st to end	Night	56% 3 9% 13% 12% 7%	4521
February	1st to 10th	Night	39% 3 6% 19% 19% 13%	2470
	11th to 20th	Night	40% 4 7% 20% 20% 9%	2287
	21st to end	Night	39% 4 8% 24% 18% 7%	1884
March	1st to 10th	Night	60% 3 8% 14% 10% 5%	3648
	11th to 20th	Night	34% 2 7% 26% 20% 11%	3490
	21st to end	Night	52% 5% 10% 20% 9% 4	3674
April	1st to 10th	Night	40% 5% 12% 20% 19% 5%	1900
	11th to 20th	Night	33% 4 12% 24% 18% 8%	1879
	21st to end	Night	25% 5% 12% 31% 22% 5%	1900
May	1st to 10th	Night	22% 3 13% 20% 25% 17%	3028
	11th to 20th	Night	13% 2 8% 23% 39% 16%	2516
	21st to end	Night	14% 2 12% 37% 28% 7%	2762
June	1st to 10th	Night	21% 5% 11% 25% 26% 12%	1520
	11th to 20th	Night	11% 2 7% 21% 33% 25%	1520
	21st to end	Night	12% 2 3 12% 33% 39%	1520
July	1st to 10th	Night	3 1 1 6% 16% 73%	2500
	11th to 20th	Night	11% 2 1 7% 15% 65%	2492
	21st to end	Night	7% 1 1 4 22% 64%	2938
August	1st to 10th	Night	14% 2 1 8% 22% 52%	2839
	11th to 20th	Night	4 1 1 9% 36% 49%	3085
	21st to end	Night	8% 1 2 12% 32% 45%	3498
September	1st to 10th	Night	7% 1 2 7% 29% 54%	1900
	11th to 20th	Night	13% 2 6% 16% 40% 23%	1900
	21st to end	Night	11% 2 4 18% 31% 33%	2121
October	1st to 10th	Night	15% 2 4 11% 20% 49%	3800
	11th to 20th	Night	13% 2 5% 24% 34% 21%	3777
	21st to end	Night	24% 2 4 18% 32% 19%	4121
November	1st to 10th	Night	32% 3 9% 21% 18% 17%	2512
	11th to 20th	Night	29% 4 11% 21% 24% 10%	2600
	21st to end	Night	34% 4 7% 19% 21% 16%	2669
December	1st to 10th	Night	37% 3 8% 19% 18% 15%	2660
	11th to 20th	Night	54% 3 6% 14% 17% 7%	3276
	21st to end	Night	51% 4 11% 17% 12% 5%	4862

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	42% 5% 24% 17% 9% 2	180
	11th to 20th	Day	26% 8% 24% 27% 12% 3	298
	21st to end	Day	28% 8% 24% 23% 14% 4	407
February	1st to 10th	Day	30% 10% 24% 24% 10% 2	370
	11th to 20th	Day	27% 14% 29% 22% 8% 1	421
	21st to end	Day	24% 15% 30% 23% 8% 1	382
March	1st to 10th	Day	37% 18% 21% 18% 5% 2	460
	11th to 20th	Day	31% 13% 26% 17% 10% 3	460
	21st to end	Day	25% 19% 29% 21% 6%	506
April	1st to 10th	Day	22% 16% 33% 19% 8% 2	460
	11th to 20th	Day	23% 17% 27% 25% 8% 1	452
	21st to end	Day	12% 15% 28% 28% 14% 3	460
May	1st to 10th	Day	13% 19% 29% 25% 10% 3	463
	11th to 20th	Day	12% 11% 28% 28% 14% 6%	488
	21st to end	Day	10% 9% 28% 31% 14% 7%	529
June	1st to 10th	Day	7% 11% 27% 36% 13% 7%	490
	11th to 20th	Day	6% 12% 30% 38% 10% 6%	490
	21st to end	Day	2 6% 28% 42% 14% 8%	490
July	1st to 10th	Day	1 3 26% 49% 14% 7%	486
	11th to 20th	Day	1 1 23% 48% 18% 7%	484
	21st to end	Day	1 2 23% 50% 19% 5%	531
August	1st to 10th	Day	2 3 23% 45% 20% 6%	487
	11th to 20th	Day	1 3 27% 43% 20% 6%	477
	21st to end	Day	1 3 21% 49% 22% 5%	539
September	1st to 10th	Day	2 3 28% 45% 18% 5%	490
	11th to 20th	Day	10% 7% 27% 34% 19% 3	490
	21st to end	Day	6% 8% 28% 39% 16% 4	490
October	1st to 10th	Day	5% 11% 28% 33% 17% 6%	490
	11th to 20th	Day	8% 8% 24% 37% 16% 7%	490
	21st to end	Day	12% 10% 24% 31% 18% 5%	539
November	1st to 10th	Day	17% 8% 22% 26% 22% 6%	320
	11th to 20th	Day	16% 6% 20% 31% 20% 6%	300
	21st to end	Day	18% 8% 18% 30% 21% 5%	213
December	1st to 10th	Day	28% 6% 20% 23% 14% 9%	180
	11th to 20th	Day	23% 6% 28% 20% 15% 8%	180
	21st to end	Day	25% 8% 24% 24% 16% 3	198
January	1st to 10th	Night	28% 4 17% 28% 20% 4	760
	11th to 20th	Night	23% 4 17% 39% 14% 2	760
	21st to end	Night	24% 4 18% 29% 22% 3	836
February	1st to 10th	Night	24% 4 19% 33% 18% 2	760
	11th to 20th	Night	22% 5% 17% 34% 19% 3	744
	21st to end	Night	25% 7% 17% 36% 13% 2	628
March	1st to 10th	Night	31% 7% 15% 28% 17% 3	760
	11th to 20th	Night	24% 6% 16% 29% 23% 3	760
	21st to end	Night	24% 6% 18% 32% 17% 2	836
April	1st to 10th	Night	20% 5% 17% 37% 19% 2	760
	11th to 20th	Night	18% 6% 16% 34% 22% 4	751
	21st to end	Night	12% 5% 16% 36% 25% 6%	760
May	1st to 10th	Night	11% 5% 23% 33% 25% 4	673
	11th to 20th	Night	9% 3 13% 34% 36% 5%	374
	21st to end	Night	9% 3 11% 34% 38% 5%	411
June	1st to 10th	Night	8% 3 11% 36% 37% 6%	380
	11th to 20th	Night	2 3 13% 41% 34% 6%	380
	21st to end	Night	3 1 9% 40% 41% 6%	380
July	1st to 10th	Night	1 5% 36% 50% 7%	370
	11th to 20th	Night	1 7% 40% 44% 8%	368
	21st to end	Night	2 1 7% 29% 51% 11%	574
August	1st to 10th	Night	4 1 6% 35% 44% 10%	567
	11th to 20th	Night	1 1 7% 43% 38% 10%	706
	21st to end	Night	1 1 7% 36% 49% 6%	836
September	1st to 10th	Night	1 1 7% 42% 42% 6%	760
	11th to 20th	Night	4 2 10% 36% 38% 11%	760
	21st to end	Night	4 2 18% 36% 31% 9%	760
October	1st to 10th	Night	7% 4 11% 44% 28% 6%	760
	11th to 20th	Night	9% 4 13% 36% 32% 6%	760
	21st to end	Night	11% 4 15% 34% 26% 10%	836
November	1st to 10th	Night	18% 4 14% 29% 28% 7%	780
	11th to 20th	Night	14% 4 18% 38% 22% 5%	800
	21st to end	Night	17% 6% 17% 35% 20% 5%	778
December	1st to 10th	Night	25% 5% 16% 30% 19% 5%	760
	11th to 20th	Night	31% 4 14% 30% 17% 3	760
	21st to end	Night	27% 5% 18% 33% 15% 2	836

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