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 2026-01-17.

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% 65%	136091
	Night	13% 5% 1 3 2 2 3 2 4 5% 60%	133030
Transparency	Day	24% 2 6% 18% 25% 24%	131026
	Night	29% 3 7% 17% 22% 22%	129079
Seeing	Day	14% 9% 26% 32% 14% 5%	16603
	Night	15% 4 15% 35% 26% 5%	25982

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	22% 7% 1 5% 3 3 3 2 4 6% 44%	3719
	11th to 20th	Day	17% 7% 2 5% 3 3 2 2 2 3 54%	3985
	21st to end	Day	21% 7% 2 4 2 3 2 2 4 6% 47%	4510
February	1st to 10th	Day	24% 7% 2 4 2 2 3 3 4 4 45%	1800
	11th to 20th	Day	19% 8% 2 5% 3 3 4 3 4 6% 43%	1878
	21st to end	Day	21% 8% 2 4 3 3 3 3 4 6% 44%	1650
March	1st to 10th	Day	20% 15% 3 6% 4 3 3 3 4 5% 33%	4640
	11th to 20th	Day	19% 9% 2 4 3 2 3 2 3 5% 47%	5000
	21st to end	Day	23% 10% 3 5% 3 4 4 3 5% 6% 34%	5705
April	1st to 10th	Day	18% 7% 2 4 3 3 4 3 6% 6% 45%	2400
	11th to 20th	Day	14% 5% 2 4 3 3 3 4 4 6% 53%	2369
	21st to end	Day	11% 5% 1 4 3 3 3 3 5% 7% 58%	2400
May	1st to 10th	Day	6% 3 1 2 2 2 2 2 3 5% 73%	5645
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 5% 75%	6277
	21st to end	Day	3 1 1 2 2 1 2 2 3 5% 79%	6902
June	1st to 10th	Day	7% 3 1 3 2 2 3 3 5% 7% 63%	2800
	11th to 20th	Day	2 2 1 2 1 1 2 1 3 5% 79%	2800
	21st to end	Day	2 2 2 1 1 1 1 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6320
	21st to end	Day	1 1 1 2 1 2 2 2 3 4 83%	6498
August	1st to 10th	Day	1 1 1 2 1 1 2 1 2 3 84%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 2 91%	5456
	21st to end	Day	2 1 1 2 1 1 2 1 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 86%	2400
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 4 75%	2400
	21st to end	Day	5% 2 1 2 1 1 2 1 3 4 78%	2161
October	1st to 10th	Day	4 2 1 2 2 2 3 2 4 5% 73%	3280
	11th to 20th	Day	7% 2 1 3 1 2 2 2 4 6% 70%	3261
	21st to end	Day	6% 3 1 5% 3 3 3 3 5% 7% 61%	3558
November	1st to 10th	Day	14% 5% 1 4 3 3 3 2 6% 7% 53%	1875
	11th to 20th	Day	17% 4 1 3 4 3 5% 4 4 7% 47%	1890
	21st to end	Day	14% 5% 2 4 2 3 3 3 4 6% 53%	1732
December	1st to 10th	Day	11% 3 1 4 3 2 3 3 5% 7% 58%	2700
	11th to 20th	Day	30% 7% 2 5% 4 3 3 3 5% 6% 32%	3108
	21st to end	Day	29% 7% 2 5% 4 4 4 3 5% 7% 30%	4092
January	1st to 10th	Night	16% 8% 2 5% 3 3 4 3 5% 7% 42%	6446
	11th to 20th	Night	17% 5% 1 4 3 3 3 2 3 4 54%	6117
	21st to end	Night	19% 7% 2 6% 4 3 3 2 4 4 46%	6468
February	1st to 10th	Night	23% 6% 1 3 3 2 3 3 4 5% 48%	2600
	11th to 20th	Night	18% 7% 1 5% 4 3 4 3 5% 5% 45%	2408
	21st to end	Night	19% 8% 2 4 3 3 3 2 4 6% 46%	1980
March	1st to 10th	Night	24% 14% 3 5% 3 3 3 2 3 6% 34%	5249
	11th to 20th	Night	23% 10% 2 3 2 2 2 2 3 4 46%	5000
	21st to end	Night	25% 10% 2 5% 4 3 4 3 5% 5% 34%	5270
April	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 48%	2000
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 7% 53%	1979
	21st to end	Night	11% 4 1 3 3 2 3 3 4 6% 61%	2000
May	1st to 10th	Night	5% 4 1 3 2 2 3 2 4 5% 69%	4338
	11th to 20th	Night	4 2 1 2 3 2 3 2 4 4 73%	3616
	21st to end	Night	3 1 1 2 2 1 2 1 3 4 80%	3972
June	1st to 10th	Night	7% 3 1 2 2 3 3 3 4 6% 66%	1600
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 5% 79%	1600
	21st to end	Night	2 2 2 2 2 2 1 3 2 82%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	3640
	11th to 20th	Night	1 2 1 1 1 1 1 1 1 2 87%	3640
	21st to end	Night	1 2 1 2 2 2 3 2 4 4 78%	4368
August	1st to 10th	Night	2 2 1 1 1 2 2 2 3 3 81%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 91%	4408
	21st to end	Night	3 2 1 2 1 1 2 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 1 1 2 1 2 4 86%	2000
	11th to 20th	Night	4 3 1 3 2 1 1 3 3 5% 75%	2000
	21st to end	Night	3 2 1 2 2 2 2 2 4 4 78%	2233
October	1st to 10th	Night	4 2 1 2 2 2 2 1 2 3 77%	3920
	11th to 20th	Night	5% 2 1 2 2 2 2 2 3 4 77%	3897
	21st to end	Night	10% 3 1 3 2 3 3 3 5% 6% 62%	4253
November	1st to 10th	Night	16% 5% 2 3 2 2 2 3 5% 7% 53%	2641
	11th to 20th	Night	13% 5% 1 5% 3 2 3 3 6% 7% 51%	2730
	21st to end	Night	14% 5% 2 4 3 3 3 2 4 6% 54%	2885
December	1st to 10th	Night	13% 3 1 2 2 2 3 2 4 6% 61%	4700
	11th to 20th	Night	29% 8% 2 5% 3 3 4 3 7% 8% 28%	5316
	21st to end	Night	30% 7% 3 7% 4 4 4 3 4 5% 30%	7106

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	43% 3 11% 20% 15% 8%	3719
	11th to 20th	Day	38% 2 5% 14% 24% 17%	3985
	21st to end	Day	42% 2 6% 14% 19% 17%	4510
February	1st to 10th	Day	43% 3 8% 18% 19% 10%	1800
	11th to 20th	Day	43% 3 9% 17% 22% 6%	1887
	21st to end	Day	42% 3 9% 21% 16% 8%	1650
March	1st to 10th	Day	53% 4 7% 16% 16% 4	4640
	11th to 20th	Day	42% 3 11% 26% 11% 7%	5000
	21st to end	Day	50% 4 13% 21% 10% 1	5705
April	1st to 10th	Day	39% 4 11% 25% 17% 3	2400
	11th to 20th	Day	32% 4 14% 24% 19% 7%	2372
	21st to end	Day	27% 3 13% 29% 24% 4	2400
May	1st to 10th	Day	17% 2 7% 26% 32% 16%	5645
	11th to 20th	Day	14% 2 7% 28% 39% 9%	6280
	21st to end	Day	11% 2 9% 28% 43% 8%	6904
June	1st to 10th	Day	21% 2 11% 25% 29% 12%	2800
	11th to 20th	Day	10% 2 8% 19% 36% 24%	2800
	21st to end	Day	9% 1 2 13% 38% 36%	2800
July	1st to 10th	Day	3 1 1 7% 19% 69%	6251
	11th to 20th	Day	5% 1 1 5% 11% 76%	6245
	21st to end	Day	8% 2 2 15% 27% 46%	6310
August	1st to 10th	Day	11% 2 2 7% 24% 55%	4206
	11th to 20th	Day	5% 1 1 7% 41% 46%	3949
	21st to end	Day	6% 1 1 9% 41% 41%	4311
September	1st to 10th	Day	6% 1 2 10% 27% 54%	2400
	11th to 20th	Day	15% 1 5% 20% 35% 23%	2400
	21st to end	Day	13% 2 4 17% 34% 30%	2161
October	1st to 10th	Day	15% 2 3 13% 26% 41%	3280
	11th to 20th	Day	18% 2 4 19% 38% 20%	3261
	21st to end	Day	24% 3 6% 20% 32% 16%	3558
November	1st to 10th	Day	32% 3 8% 21% 23% 14%	1875
	11th to 20th	Day	36% 4 10% 19% 21% 9%	1890
	21st to end	Day	32% 4 8% 18% 23% 14%	1732
December	1st to 10th	Day	27% 4 6% 23% 27% 14%	2700
	11th to 20th	Day	53% 3 7% 16% 14% 7%	3108
	21st to end	Day	54% 3 12% 17% 10% 3	4092
January	1st to 10th	Night	41% 4 14% 19% 14% 9%	6446
	11th to 20th	Night	36% 3 8% 12% 24% 17%	6117
	21st to end	Night	43% 3 7% 10% 17% 21%	6468
February	1st to 10th	Night	41% 3 6% 18% 19% 13%	2600
	11th to 20th	Night	41% 4 7% 20% 20% 8%	2410
	21st to end	Night	38% 4 8% 23% 18% 8%	1980
March	1st to 10th	Night	54% 3 6% 18% 12% 7%	5249
	11th to 20th	Night	43% 2 9% 22% 16% 8%	5000
	21st to end	Night	52% 4 11% 20% 11% 3	5270
April	1st to 10th	Night	39% 4 11% 20% 20% 5%	2000
	11th to 20th	Night	33% 4 12% 23% 19% 9%	1979
	21st to end	Night	25% 5% 12% 31% 23% 5%	2000
May	1st to 10th	Night	18% 3 11% 23% 26% 19%	4335
	11th to 20th	Night	16% 2 7% 27% 36% 12%	3616
	21st to end	Night	11% 2 8% 30% 40% 9%	3972
June	1st to 10th	Night	21% 5% 11% 26% 27% 11%	1600
	11th to 20th	Night	11% 2 7% 20% 33% 27%	1600
	21st to end	Night	11% 2 3 12% 34% 39%	1600
July	1st to 10th	Night	3 1 1 6% 23% 66%	3600
	11th to 20th	Night	7% 1 1 5% 10% 75%	3592
	21st to end	Night	10% 2 2 13% 24% 49%	4244
August	1st to 10th	Night	14% 2 1 8% 23% 52%	2929
	11th to 20th	Night	4 1 1 9% 36% 49%	3183
	21st to end	Night	9% 1 2 12% 33% 45%	3608
September	1st to 10th	Night	7% 1 2 8% 30% 52%	2000
	11th to 20th	Night	13% 2 6% 17% 39% 23%	2000
	21st to end	Night	12% 2 4 18% 32% 33%	2233
October	1st to 10th	Night	15% 2 3 11% 20% 48%	3920
	11th to 20th	Night	14% 2 5% 25% 34% 21%	3897
	21st to end	Night	24% 3 4 18% 32% 19%	4253
November	1st to 10th	Night	31% 3 10% 21% 19% 17%	2641
	11th to 20th	Night	31% 4 11% 20% 23% 10%	2730
	21st to end	Night	32% 4 7% 19% 22% 16%	2885
December	1st to 10th	Night	25% 3 6% 23% 23% 20%	4700
	11th to 20th	Night	53% 4 6% 13% 17% 7%	5316
	21st to end	Night	58% 3 10% 15% 10% 4	7106

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	40% 5% 26% 18% 9% 2	220
	11th to 20th	Day	23% 7% 25% 27% 14% 4	340
	21st to end	Day	26% 8% 25% 23% 14% 4	440
February	1st to 10th	Day	32% 10% 23% 23% 10% 3	400
	11th to 20th	Day	27% 14% 29% 22% 8% 1	458
	21st to end	Day	23% 14% 29% 25% 7% 1	414
March	1st to 10th	Day	35% 17% 22% 18% 5% 2	500
	11th to 20th	Day	31% 14% 26% 17% 9% 3	500
	21st to end	Day	26% 18% 28% 21% 6% 1	550
April	1st to 10th	Day	22% 15% 34% 20% 8% 2	500
	11th to 20th	Day	21% 16% 28% 26% 8% 2	492
	21st to end	Day	12% 15% 28% 28% 13% 3	500
May	1st to 10th	Day	13% 19% 30% 25% 10% 3	503
	11th to 20th	Day	13% 12% 29% 27% 14% 5%	528
	21st to end	Day	9% 9% 29% 31% 14% 7%	573
June	1st to 10th	Day	6% 10% 27% 36% 14% 8%	530
	11th to 20th	Day	5% 12% 30% 38% 9% 6%	530
	21st to end	Day	2 6% 28% 41% 15% 8%	530
July	1st to 10th	Day	1 4 27% 47% 14% 7%	526
	11th to 20th	Day	1 1 23% 49% 18% 8%	524
	21st to end	Day	1 2 22% 50% 19% 6%	575
August	1st to 10th	Day	2 3 24% 44% 21% 6%	527
	11th to 20th	Day	1 3 27% 43% 21% 6%	517
	21st to end	Day	1 3 21% 49% 22% 5%	583
September	1st to 10th	Day	2 3 28% 45% 18% 5%	530
	11th to 20th	Day	9% 6% 28% 33% 20% 3	530
	21st to end	Day	6% 8% 27% 39% 15% 4	530
October	1st to 10th	Day	6% 12% 27% 32% 16% 6%	530
	11th to 20th	Day	8% 9% 24% 36% 16% 6%	530
	21st to end	Day	13% 9% 23% 31% 18% 5%	583
November	1st to 10th	Day	16% 7% 21% 27% 22% 7%	351
	11th to 20th	Day	18% 6% 21% 30% 18% 6%	330
	21st to end	Day	17% 8% 18% 31% 21% 6%	247
December	1st to 10th	Day	23% 6% 20% 25% 16% 10%	220
	11th to 20th	Day	25% 5% 25% 20% 17% 8%	220
	21st to end	Day	30% 8% 22% 24% 15% 2	242
January	1st to 10th	Night	28% 4 19% 27% 18% 4	840
	11th to 20th	Night	21% 4 19% 39% 15% 2	824
	21st to end	Night	23% 4 19% 30% 21% 3	880
February	1st to 10th	Night	26% 4 19% 32% 17% 2	800
	11th to 20th	Night	22% 5% 19% 33% 18% 3	784
	21st to end	Night	24% 7% 18% 37% 12% 2	660
March	1st to 10th	Night	31% 7% 16% 29% 16% 3	800
	11th to 20th	Night	26% 6% 16% 29% 22% 2	800
	21st to end	Night	25% 6% 18% 32% 17% 2	880
April	1st to 10th	Night	20% 5% 18% 37% 18% 2	800
	11th to 20th	Night	17% 6% 17% 35% 21% 4	791
	21st to end	Night	12% 5% 16% 37% 25% 5%	800
May	1st to 10th	Night	10% 5% 23% 34% 24% 4	708
	11th to 20th	Night	9% 3 15% 34% 34% 5%	394
	21st to end	Night	9% 3 12% 36% 37% 5%	433
June	1st to 10th	Night	7% 3 10% 37% 37% 6%	400
	11th to 20th	Night	2 3 13% 43% 33% 6%	400
	21st to end	Night	3 1 9% 41% 41% 6%	400
July	1st to 10th	Night	1 7% 38% 48% 7%	390
	11th to 20th	Night	1 6% 41% 44% 7%	388
	21st to end	Night	1 1 7% 29% 51% 11%	605
August	1st to 10th	Night	4 1 6% 37% 43% 9%	597
	11th to 20th	Night	1 1 6% 43% 39% 10%	744
	21st to end	Night	1 1 7% 37% 48% 6%	880
September	1st to 10th	Night	2 1 8% 43% 40% 6%	800
	11th to 20th	Night	5% 2 10% 36% 37% 10%	800
	21st to end	Night	4 2 18% 37% 31% 9%	800
October	1st to 10th	Night	7% 4 13% 44% 27% 5%	800
	11th to 20th	Night	9% 4 13% 37% 31% 6%	800
	21st to end	Night	11% 4 15% 35% 25% 9%	880
November	1st to 10th	Night	17% 4 14% 30% 28% 6%	820
	11th to 20th	Night	15% 5% 18% 38% 21% 4	840
	21st to end	Night	16% 6% 18% 37% 19% 5%	840
December	1st to 10th	Night	23% 5% 17% 32% 19% 5%	840
	11th to 20th	Night	31% 4 14% 32% 16% 3	840
	21st to end	Night	30% 6% 19% 31% 14% 2	924

Page generated at 2026-01-13 22:14:29. Version ac07

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