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 2024-01-14.

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	11% 4 1 3 3 2 3 3 4 5% 62%	74755
	Night	14% 4 1 3 3 2 3 3 4 5% 58%	75011
Transparency	Day	26% 3 7% 18% 24% 21%	74467
	Night	30% 3 7% 18% 22% 20%	74820
Seeing	Day	14% 9% 26% 33% 14% 5%	13984
	Night	15% 4 14% 34% 28% 6%	23392

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	25% 7% 3 5% 3 3 3 3 4 7% 38%	1519
	11th to 20th	Day	19% 5% 1 4 4 2 3 4 3 6% 49%	1576
	21st to end	Day	21% 7% 2 4 3 2 3 4 5% 7% 42%	1782
February	1st to 10th	Day	22% 6% 2 3 3 2 3 4 4 5% 48%	1620
	11th to 20th	Day	18% 7% 2 4 4 2 3 4 4 6% 46%	1685
	21st to end	Day	22% 8% 2 3 3 2 3 4 4 5% 44%	1480
March	1st to 10th	Day	22% 9% 4 5% 5% 3 4 3 4 7% 35%	1844
	11th to 20th	Day	24% 6% 3 4 4 2 3 4 4 6% 40%	1980
	21st to end	Day	22% 6% 3 3 4 2 3 4 5% 7% 41%	2268
April	1st to 10th	Day	19% 6% 3 3 3 2 3 4 6% 6% 44%	2160
	11th to 20th	Day	16% 4 2 3 3 2 3 4 4 6% 52%	2129
	21st to end	Day	11% 5% 1 3 3 2 3 4 4 7% 58%	2160
May	1st to 10th	Day	8% 4 2 3 3 2 3 3 4 6% 61%	2235
	11th to 20th	Day	6% 3 1 4 3 2 3 4 5% 7% 62%	2477
	21st to end	Day	8% 3 1 3 4 2 3 4 5% 7% 59%	2723
June	1st to 10th	Day	8% 3 1 3 3 2 3 4 5% 8% 61%	2520
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 5% 79%	2520
	21st to end	Day	3 2 2 1 1 2 2 2 3 83%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 2 3 87%	2520
	11th to 20th	Day	1 1 1 1 1 1 1 3 3 87%	2520
	21st to end	Day	1 1 1 2 2 1 2 2 3 3 82%	2590
August	1st to 10th	Day	2 2 1 1 1 1 2 2 2 3 83%	2328
	11th to 20th	Day	1 1 1 1 1 2 2 3 3 85%	2164
	21st to end	Day	1 1 1 1 1 1 1 2 91%	2376
September	1st to 10th	Day	2 1 1 1 2 2 3 87%	2160
	11th to 20th	Day	4 2 1 2 2 1 2 3 3 4 75%	2160
	21st to end	Day	5% 2 2 2 1 2 2 3 4 78%	1946
October	1st to 10th	Day	6% 2 2 2 3 2 3 3 4 5% 69%	1800
	11th to 20th	Day	7% 2 1 3 2 2 3 3 3 6% 68%	1800
	21st to end	Day	9% 3 1 3 3 2 3 4 3 5% 63%	1980
November	1st to 10th	Day	15% 5% 2 4 3 2 4 2 5% 7% 51%	1693
	11th to 20th	Day	16% 3 1 2 4 2 4 5% 5% 6% 51%	1710
	21st to end	Day	13% 5% 2 5% 3 2 3 4 5% 7% 52%	1568
December	1st to 10th	Day	19% 5% 2 3 4 1 3 4 5% 7% 47%	1520
	11th to 20th	Day	31% 7% 3 6% 4 2 2 3 4 5% 31%	1928
	21st to end	Day	17% 7% 3 5% 4 4 5% 5% 5% 8% 37%	2794
January	1st to 10th	Night	24% 7% 2 3 4 2 3 4 5% 7% 40%	2646
	11th to 20th	Night	18% 6% 2 4 4 2 4 4 4 7% 46%	2453
	21st to end	Night	21% 6% 2 4 4 2 3 4 5% 6% 45%	2574
February	1st to 10th	Night	21% 5% 1 3 3 2 3 3 4 5% 50%	2340
	11th to 20th	Night	17% 6% 1 4 4 2 4 4 5% 6% 47%	2161
	21st to end	Night	21% 7% 2 3 3 2 3 4 5% 6% 45%	1776
March	1st to 10th	Night	26% 8% 3 5% 4 2 4 4 3 5% 37%	2111
	11th to 20th	Night	22% 6% 2 4 4 2 4 3 5% 7% 41%	1980
	21st to end	Night	22% 6% 2 4 4 2 4 4 5% 4 45%	2088
April	1st to 10th	Night	17% 6% 3 5% 4 2 4 3 4 6% 47%	1800
	11th to 20th	Night	15% 4 2 3 3 2 3 4 4 7% 52%	1779
	21st to end	Night	11% 3 1 3 3 2 3 4 4 6% 61%	1800
May	1st to 10th	Night	9% 4 1 2 3 2 3 3 4 5% 62%	1718
	11th to 20th	Night	8% 2 1 3 3 2 3 4 6% 7% 61%	1416
	21st to end	Night	8% 3 2 3 4 2 3 3 4 6% 62%	1552
June	1st to 10th	Night	8% 3 1 2 2 2 4 3 4 6% 66%	1440
	11th to 20th	Night	2 2 1 2 2 2 2 2 4 5% 77%	1440
	21st to end	Night	3 2 2 3 1 3 1 2 3 81%	1440
July	1st to 10th	Night	1 1 1 1 1 2 2 1 3 87%	1440
	11th to 20th	Night	1 1 1 2 1 1 1 1 1 2 89%	1440
	21st to end	Night	1 2 1 2 2 2 2 2 2 4 80%	1725
August	1st to 10th	Night	3 2 1 2 2 2 3 2 3 3 78%	1611
	11th to 20th	Night	1 1 1 1 1 1 2 3 3 4 83%	1736
	21st to end	Night	1 1 1 2 1 2 2 2 2 86%	1980
September	1st to 10th	Night	2 1 1 1 1 1 2 1 2 3 87%	1800
	11th to 20th	Night	4 2 1 2 2 1 1 3 3 5% 77%	1800
	21st to end	Night	3 1 1 2 2 1 2 3 4 4 77%	2009
October	1st to 10th	Night	7% 2 1 2 3 2 3 2 3 4 72%	2160
	11th to 20th	Night	8% 2 1 2 2 1 2 2 3 5% 72%	2160
	21st to end	Night	9% 3 1 2 2 1 2 3 4 6% 67%	2376
November	1st to 10th	Night	17% 5% 2 3 2 1 2 3 5% 6% 53%	2383
	11th to 20th	Night	12% 4 1 4 3 1 2 4 5% 7% 55%	2470
	21st to end	Night	14% 4 2 4 4 2 3 3 4 6% 54%	2609
December	1st to 10th	Night	22% 4 1 3 3 2 3 3 4 7% 49%	2660
	11th to 20th	Night	34% 6% 2 4 3 2 3 3 5% 7% 32%	3276
	21st to end	Night	21% 7% 3 6% 6% 3 5% 5% 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	48% 4 8% 21% 14% 5%	1519
	11th to 20th	Day	39% 4 8% 17% 17% 15%	1576
	21st to end	Day	43% 2 8% 15% 21% 10%	1782
February	1st to 10th	Day	41% 2 8% 19% 19% 11%	1620
	11th to 20th	Day	39% 3 9% 18% 24% 7%	1694
	21st to end	Day	43% 3 9% 21% 16% 7%	1480
March	1st to 10th	Day	50% 4 10% 16% 16% 5%	1844
	11th to 20th	Day	46% 3 9% 21% 16% 5%	1980
	21st to end	Day	44% 5% 13% 20% 15% 4	2268
April	1st to 10th	Day	40% 5% 11% 23% 17% 4	2160
	11th to 20th	Day	33% 4 13% 24% 20% 7%	2132
	21st to end	Day	27% 3 13% 28% 25% 4	2160
May	1st to 10th	Day	24% 3 10% 27% 28% 8%	2235
	11th to 20th	Day	22% 4 10% 31% 24% 10%	2480
	21st to end	Day	25% 4 14% 30% 22% 6%	2724
June	1st to 10th	Day	22% 3 11% 25% 27% 13%	2520
	11th to 20th	Day	11% 2 9% 20% 34% 24%	2520
	21st to end	Day	9% 1 3 14% 38% 35%	2520
July	1st to 10th	Day	6% 1 2 10% 26% 55%	2451
	11th to 20th	Day	6% 1 3 12% 25% 53%	2445
	21st to end	Day	10% 2 2 7% 25% 55%	2446
August	1st to 10th	Day	9% 1 3 11% 30% 46%	2312
	11th to 20th	Day	6% 2 1 10% 35% 47%	2164
	21st to end	Day	5% 1 1 11% 36% 46%	2376
September	1st to 10th	Day	6% 1 2 8% 27% 56%	2160
	11th to 20th	Day	14% 1 5% 20% 35% 25%	2160
	21st to end	Day	13% 2 4 18% 34% 29%	1946
October	1st to 10th	Day	19% 3 5% 20% 28% 26%	1800
	11th to 20th	Day	19% 3 6% 18% 27% 27%	1800
	21st to end	Day	25% 3 5% 18% 25% 24%	1980
November	1st to 10th	Day	34% 3 7% 20% 22% 14%	1693
	11th to 20th	Day	34% 4 10% 19% 23% 10%	1710
	21st to end	Day	33% 4 9% 18% 24% 13%	1568
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 11% 21% 14% 7%	2646
	11th to 20th	Night	39% 5% 9% 13% 18% 15%	2453
	21st to end	Night	41% 3 8% 15% 21% 12%	2574
February	1st to 10th	Night	38% 3 6% 19% 20% 14%	2340
	11th to 20th	Night	38% 5% 7% 19% 22% 9%	2163
	21st to end	Night	39% 4 8% 24% 18% 7%	1776
March	1st to 10th	Night	51% 5% 8% 16% 14% 5%	2111
	11th to 20th	Night	42% 3 9% 20% 18% 7%	1980
	21st to end	Night	42% 6% 9% 22% 15% 6%	2088
April	1st to 10th	Night	41% 5% 12% 18% 19% 6%	1800
	11th to 20th	Night	33% 5% 12% 23% 19% 9%	1779
	21st to end	Night	25% 5% 12% 31% 22% 6%	1800
May	1st to 10th	Night	25% 4 10% 21% 29% 11%	1718
	11th to 20th	Night	22% 3 13% 30% 21% 11%	1416
	21st to end	Night	23% 4 14% 29% 23% 6%	1552
June	1st to 10th	Night	21% 5% 11% 25% 26% 12%	1440
	11th to 20th	Night	12% 2 8% 21% 32% 25%	1440
	21st to end	Night	12% 2 3 13% 33% 38%	1440
July	1st to 10th	Night	6% 1 2 11% 28% 51%	1400
	11th to 20th	Night	7% 1 2 12% 21% 58%	1392
	21st to end	Night	11% 2 2 7% 21% 58%	1629
August	1st to 10th	Night	13% 1 2 14% 24% 46%	1602
	11th to 20th	Night	7% 1 2 12% 30% 48%	1736
	21st to end	Night	7% 1 2 10% 27% 52%	1980
September	1st to 10th	Night	7% 1 2 7% 29% 54%	1800
	11th to 20th	Night	12% 2 6% 16% 40% 24%	1800
	21st to end	Night	12% 2 4 19% 32% 32%	2009
October	1st to 10th	Night	19% 3 6% 18% 26% 29%	2160
	11th to 20th	Night	18% 2 4 19% 27% 30%	2160
	21st to end	Night	21% 2 5% 20% 27% 25%	2376
November	1st to 10th	Night	33% 3 9% 20% 18% 17%	2383
	11th to 20th	Night	29% 4 10% 21% 25% 11%	2470
	21st to end	Night	32% 4 7% 19% 22% 16%	2609
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% 7% 25% 27% 12% 3	278
	21st to end	Day	26% 8% 24% 23% 15% 4	374
February	1st to 10th	Day	28% 10% 23% 25% 11% 3	340
	11th to 20th	Day	25% 14% 29% 23% 9% 1	385
	21st to end	Day	24% 16% 29% 23% 8% 1	346
March	1st to 10th	Day	35% 16% 22% 19% 5% 2	420
	11th to 20th	Day	31% 13% 26% 17% 10% 2	420
	21st to end	Day	25% 18% 29% 22% 6%	462
April	1st to 10th	Day	24% 16% 33% 19% 7% 2	420
	11th to 20th	Day	24% 18% 28% 23% 6% 1	412
	21st to end	Day	13% 16% 27% 28% 13% 3	420
May	1st to 10th	Day	14% 18% 29% 26% 11% 3	423
	11th to 20th	Day	13% 12% 29% 27% 13% 5%	448
	21st to end	Day	11% 10% 27% 32% 14% 7%	485
June	1st to 10th	Day	7% 11% 27% 36% 12% 7%	450
	11th to 20th	Day	6% 13% 30% 37% 10% 6%	450
	21st to end	Day	2 6% 27% 43% 14% 8%	450
July	1st to 10th	Day	1 3 26% 50% 13% 7%	446
	11th to 20th	Day	1 1 24% 48% 18% 8%	444
	21st to end	Day	1 2 22% 51% 19% 5%	487
August	1st to 10th	Day	2 4 23% 45% 20% 6%	447
	11th to 20th	Day	1 2 25% 45% 20% 6%	437
	21st to end	Day	1 2 21% 50% 21% 4	495
September	1st to 10th	Day	3 3 26% 44% 19% 5%	450
	11th to 20th	Day	9% 6% 27% 35% 20% 3	450
	21st to end	Day	6% 8% 29% 37% 15% 4	450
October	1st to 10th	Day	5% 12% 28% 31% 17% 6%	450
	11th to 20th	Day	8% 8% 24% 37% 16% 7%	450
	21st to end	Day	13% 9% 23% 31% 19% 5%	495
November	1st to 10th	Day	19% 6% 20% 26% 23% 6%	289
	11th to 20th	Day	15% 4 20% 31% 23% 7%	270
	21st to end	Day	14% 8% 19% 32% 22% 5%	203
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% 40% 15% 2	734
	21st to end	Night	23% 4 18% 29% 23% 4	792
February	1st to 10th	Night	23% 3 18% 34% 19% 2	720
	11th to 20th	Night	21% 5% 17% 34% 19% 4	704
	21st to end	Night	26% 7% 15% 37% 13% 2	592
March	1st to 10th	Night	30% 6% 15% 29% 17% 3	720
	11th to 20th	Night	24% 5% 15% 29% 23% 3	720
	21st to end	Night	24% 6% 18% 32% 18% 2	792
April	1st to 10th	Night	21% 4 17% 36% 20% 2	720
	11th to 20th	Night	18% 6% 17% 33% 22% 5%	711
	21st to end	Night	12% 5% 15% 37% 25% 6%	720
May	1st to 10th	Night	11% 5% 21% 33% 26% 4	638
	11th to 20th	Night	10% 3 13% 34% 35% 5%	354
	21st to end	Night	10% 3 10% 33% 39% 5%	389
June	1st to 10th	Night	8% 3 11% 35% 38% 6%	360
	11th to 20th	Night	2 3 12% 40% 36% 7%	360
	21st to end	Night	3 1 9% 39% 42% 7%	360
July	1st to 10th	Night	1 5% 35% 53% 7%	350
	11th to 20th	Night	1 6% 39% 46% 8%	348
	21st to end	Night	2 1 7% 28% 50% 12%	543
August	1st to 10th	Night	4 1 6% 32% 46% 10%	537
	11th to 20th	Night	1 1 6% 41% 40% 11%	668
	21st to end	Night	1 6% 36% 50% 6%	792
September	1st to 10th	Night	2 1 7% 40% 44% 7%	720
	11th to 20th	Night	4 2 9% 35% 39% 11%	720
	21st to end	Night	4 2 18% 34% 32% 10%	720
October	1st to 10th	Night	8% 3 12% 42% 29% 6%	720
	11th to 20th	Night	9% 4 12% 36% 33% 7%	720
	21st to end	Night	11% 4 14% 33% 28% 10%	792
November	1st to 10th	Night	18% 4 12% 30% 28% 7%	740
	11th to 20th	Night	13% 3 16% 39% 23% 5%	760
	21st to end	Night	15% 6% 18% 36% 20% 5%	760
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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