Climax Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Climax Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	4 8% 4 11% 6% 5% 5% 3 5% 6% 43%	140504
	Night	1 5% 3 10% 6% 5% 4 4 5% 7% 50%	139193
Transparency	Day	38% 6% 10% 21% 19% 7%	135430
	Night	30% 5% 13% 25% 20% 6%	135274
Seeing	Day	17% 25% 29% 19% 9% 2	17416
	Night	9% 17% 30% 30% 13% 1	27607

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	1 4 5% 19% 11% 7% 3 2 3 5% 39%	4720
	11th to 20th	Day	3 3 14% 10% 8% 6% 3 2 5% 45%	5197
	21st to end	Day	4 4 18% 6% 5% 6% 2 3 4 48%	5844
February	1st to 10th	Day	5% 6% 19% 8% 5% 4 4 4 5% 40%	1890
	11th to 20th	Day	5% 6% 17% 9% 6% 4 3 4 6% 41%	1959
	21st to end	Day	5% 5% 18% 7% 7% 4 3 4 4 43%	1730
March	1st to 10th	Day	5% 6% 20% 6% 3 3 2 4 4 47%	4741
	11th to 20th	Day	1 6% 8% 20% 8% 4 4 3 5% 6% 35%	5110
	21st to end	Day	1 11% 9% 17% 7% 5% 4 2 5% 5% 34%	5821
April	1st to 10th	Day	2 10% 5% 12% 6% 4 4 3 5% 6% 43%	2520
	11th to 20th	Day	1 14% 8% 14% 7% 4 4 3 4 6% 35%	2444
	21st to end	Day	2 17% 6% 13% 7% 5% 5% 3 4 5% 33%	2400
May	1st to 10th	Day	2 16% 8% 13% 6% 6% 5% 4 5% 5% 28%	5687
	11th to 20th	Day	3 15% 5% 9% 5% 5% 5% 4 6% 6% 37%	6298
	21st to end	Day	4 9% 3 7% 4 5% 5% 4 7% 8% 45%	6925
June	1st to 10th	Day	6% 8% 2 6% 5% 4 5% 4 7% 8% 45%	2800
	11th to 20th	Day	7% 8% 2 5% 4 4 5% 3 5% 6% 51%	2800
	21st to end	Day	6% 6% 3 7% 5% 5% 5% 4 6% 7% 47%	2800
July	1st to 10th	Day	6% 5% 2 6% 4 5% 6% 4 6% 6% 50%	6339
	11th to 20th	Day	6% 6% 2 8% 7% 7% 7% 7% 9% 8% 33%	6340
	21st to end	Day	11% 7% 3 9% 6% 6% 6% 4 5% 6% 36%	6495
August	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 8% 10% 34%	5779
	11th to 20th	Day	11% 8% 3 9% 6% 5% 5% 4 6% 6% 37%	5495
	21st to end	Day	11% 8% 4 10% 6% 6% 5% 4 6% 6% 32%	5984
September	1st to 10th	Day	10% 7% 2 6% 5% 5% 4 3 4 5% 49%	2400
	11th to 20th	Day	9% 8% 3 6% 4 5% 4 3 4 5% 49%	2400
	21st to end	Day	6% 9% 3 6% 4 4 2 2 4 6% 53%	2156
October	1st to 10th	Day	4 7% 2 4 4 3 3 2 3 5% 64%	3270
	11th to 20th	Day	3 11% 2 7% 5% 5% 4 3 6% 8% 46%	3250
	21st to end	Day	2 9% 3 9% 2 2 3 2 4 5% 60%	3548
November	1st to 10th	Day	6% 3 8% 3 4 3 3 4 5% 60%	1863
	11th to 20th	Day	8% 3 7% 4 3 3 3 4 8% 58%	1890
	21st to end	Day	1 6% 4 10% 8% 3 2 2 5% 6% 55%	1711
December	1st to 10th	Day	3 2 10% 8% 6% 4 2 3 6% 55%	2700
	11th to 20th	Day	4 4 12% 5% 4 3 3 4 7% 55%	3106
	21st to end	Day	6% 6% 14% 8% 5% 4 4 5% 7% 42%	4092
January	1st to 10th	Night	3 4 19% 11% 6% 4 3 4 6% 42%	8201
	11th to 20th	Night	1 3 14% 8% 7% 7% 2 3 4 49%	7850
	21st to end	Night	2 3 15% 8% 6% 5% 3 4 5% 49%	8370
February	1st to 10th	Night	3 4 16% 9% 6% 4 3 5% 7% 44%	2730
	11th to 20th	Night	4 4 14% 9% 7% 5% 4 5% 6% 42%	2546
	21st to end	Night	5% 4 12% 7% 8% 5% 4 5% 6% 45%	2076
March	1st to 10th	Night	3 7% 19% 7% 4 3 3 5% 6% 44%	5342
	11th to 20th	Night	5% 5% 15% 8% 3 3 3 4 6% 47%	5110
	21st to end	Night	1 9% 8% 17% 6% 5% 4 4 5% 6% 37%	5385
April	1st to 10th	Night	5% 4 9% 6% 5% 5% 3 5% 7% 50%	2100
	11th to 20th	Night	1 10% 3 11% 7% 5% 5% 3 6% 6% 44%	2037
	21st to end	Night	2 10% 4 9% 5% 4 4 4 5% 8% 47%	2000
May	1st to 10th	Night	2 10% 5% 10% 4 5% 5% 5% 7% 7% 41%	4289
	11th to 20th	Night	2 10% 8% 8% 5% 5% 5% 4 6% 7% 40%	3617
	21st to end	Night	2 7% 2 5% 3 3 4 3 6% 7% 57%	3972
June	1st to 10th	Night	3 4 2 4 4 2 4 4 6% 9% 59%	1600
	11th to 20th	Night	4 4 1 4 3 3 4 3 4 7% 63%	1600
	21st to end	Night	3 3 2 4 3 4 3 4 5% 9% 60%	1600
July	1st to 10th	Night	4 5% 2 4 4 4 4 5% 5% 7% 57%	3640
	11th to 20th	Night	2 2 1 4 4 5% 6% 6% 10% 12% 48%	3640
	21st to end	Night	3 3 1 5% 4 5% 6% 5% 8% 9% 49%	4374
August	1st to 10th	Night	4 4 2 5% 3 3 5% 5% 8% 11% 49%	4033
	11th to 20th	Night	3 3 1 4 4 4 4 5% 7% 9% 55%	4352
	21st to end	Night	5% 5% 3 7% 5% 6% 6% 5% 8% 8% 42%	4993
September	1st to 10th	Night	4 5% 2 6% 3 3 5% 4 6% 7% 56%	2000
	11th to 20th	Night	4 4 2 6% 3 3 4 3 5% 6% 60%	2000
	21st to end	Night	3 5% 2 5% 3 3 4 3 4 7% 60%	2239
October	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 73%	3920
	11th to 20th	Night	1 5% 2 5% 4 3 4 3 5% 7% 59%	3898
	21st to end	Night	1 6% 2 6% 3 3 3 2 3 6% 65%	4252
November	1st to 10th	Night	4 3 7% 3 3 3 3 4 6% 64%	2635
	11th to 20th	Night	4 3 8% 5% 4 3 3 4 6% 60%	2730
	21st to end	Night	5% 3 10% 7% 4 3 3 5% 7% 53%	2903
December	1st to 10th	Night	2 2 8% 6% 6% 3 2 4 5% 62%	4710
	11th to 20th	Night	3 3 10% 7% 5% 4 3 5% 6% 55%	5321
	21st to end	Night	6% 6% 16% 7% 6% 4 3 5% 6% 40%	7128

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% 5% 9% 25% 13% 5%	4723
	11th to 20th	Day	40% 4 12% 21% 16% 7%	5197
	21st to end	Day	39% 4 8% 20% 19% 10%	5844
February	1st to 10th	Day	43% 4 14% 19% 14% 6%	1890
	11th to 20th	Day	41% 3 11% 22% 17% 6%	1968
	21st to end	Day	41% 3 10% 19% 19% 8%	1730
March	1st to 10th	Day	38% 4 6% 19% 21% 12%	4741
	11th to 20th	Day	45% 7% 9% 24% 13% 2	5110
	21st to end	Day	47% 4 12% 25% 10% 1	5821
April	1st to 10th	Day	40% 5% 12% 20% 17% 6%	2520
	11th to 20th	Day	49% 4 11% 18% 16% 3	2447
	21st to end	Day	50% 5% 9% 25% 12% 1	2400
May	1st to 10th	Day	50% 7% 8% 18% 15% 3	5687
	11th to 20th	Day	41% 7% 8% 23% 18% 2	6302
	21st to end	Day	31% 8% 8% 24% 25% 3	6926
June	1st to 10th	Day	34% 6% 12% 24% 18% 5%	2800
	11th to 20th	Day	32% 5% 8% 18% 24% 13%	2800
	21st to end	Day	33% 7% 7% 18% 24% 11%	2800
July	1st to 10th	Day	28% 8% 8% 12% 22% 23%	6271
	11th to 20th	Day	38% 11% 13% 19% 14% 6%	6265
	21st to end	Day	44% 8% 11% 11% 15% 11%	6308
August	1st to 10th	Day	47% 10% 13% 12% 13% 4	4208
	11th to 20th	Day	45% 7% 9% 13% 19% 7%	3977
	21st to end	Day	43% 8% 9% 15% 19% 6%	4311
September	1st to 10th	Day	37% 6% 7% 16% 26% 10%	2400
	11th to 20th	Day	36% 5% 9% 18% 25% 8%	2400
	21st to end	Day	32% 3 6% 26% 27% 6%	2156
October	1st to 10th	Day	23% 5% 6% 24% 35% 9%	3270
	11th to 20th	Day	32% 7% 9% 28% 19% 5%	3250
	21st to end	Day	27% 3 8% 30% 28% 4	3548
November	1st to 10th	Day	24% 5% 12% 24% 30% 5%	1863
	11th to 20th	Day	23% 3 15% 30% 22% 7%	1890
	21st to end	Day	31% 3 10% 28% 18% 10%	1711
December	1st to 10th	Day	29% 4 11% 33% 17% 6%	2700
	11th to 20th	Day	27% 5% 16% 30% 20% 2	3104
	21st to end	Day	35% 6% 13% 17% 23% 6%	4092
January	1st to 10th	Night	39% 4 11% 26% 14% 5%	8202
	11th to 20th	Night	36% 4 11% 23% 18% 8%	7850
	21st to end	Night	33% 4 11% 19% 22% 11%	8370
February	1st to 10th	Night	35% 5% 15% 23% 18% 5%	2730
	11th to 20th	Night	37% 4 14% 22% 18% 5%	2547
	21st to end	Night	34% 3 12% 21% 22% 8%	2076
March	1st to 10th	Night	37% 4 8% 18% 23% 9%	5342
	11th to 20th	Night	36% 5% 10% 22% 24% 3	5110
	21st to end	Night	43% 6% 16% 25% 9% 1	5385
April	1st to 10th	Night	29% 5% 16% 25% 20% 4	2100
	11th to 20th	Night	35% 5% 14% 27% 17% 2	2037
	21st to end	Night	33% 5% 17% 31% 15% 1	2000
May	1st to 10th	Night	35% 9% 11% 26% 17% 2	4286
	11th to 20th	Night	38% 6% 12% 29% 16%	3618
	21st to end	Night	22% 6% 12% 33% 25% 2	3972
June	1st to 10th	Night	21% 7% 16% 31% 20% 6%	1600
	11th to 20th	Night	20% 5% 12% 21% 29% 13%	1600
	21st to end	Night	20% 6% 12% 23% 28% 12%	1600
July	1st to 10th	Night	24% 6% 9% 17% 23% 20%	3600
	11th to 20th	Night	22% 8% 21% 28% 15% 5%	3592
	21st to end	Night	25% 10% 20% 23% 12% 10%	4249
August	1st to 10th	Night	32% 9% 22% 19% 14% 3	2928
	11th to 20th	Night	26% 8% 22% 17% 18% 8%	3146
	21st to end	Night	31% 9% 15% 19% 22% 4	3597
September	1st to 10th	Night	24% 7% 12% 23% 26% 8%	2000
	11th to 20th	Night	24% 5% 13% 24% 24% 9%	2000
	21st to end	Night	22% 5% 10% 33% 25% 5%	2239
October	1st to 10th	Night	15% 3 8% 31% 35% 8%	3920
	11th to 20th	Night	21% 6% 13% 36% 20% 5%	3898
	21st to end	Night	20% 3 9% 32% 31% 6%	4252
November	1st to 10th	Night	20% 5% 13% 28% 27% 6%	2635
	11th to 20th	Night	22% 4 14% 31% 23% 6%	2730
	21st to end	Night	28% 4 11% 28% 20% 8%	2903
December	1st to 10th	Night	23% 4 12% 34% 21% 5%	4710
	11th to 20th	Night	25% 5% 13% 28% 25% 3	5322
	21st to end	Night	40% 5% 12% 18% 19% 7%	7128

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	22% 29% 24% 17% 7% 2	330
	11th to 20th	Day	23% 30% 27% 15% 4	330
	21st to end	Day	17% 34% 29% 14% 5%	363
February	1st to 10th	Day	28% 38% 22% 9% 2 1	330
	11th to 20th	Day	30% 35% 24% 10% 1 1	330
	21st to end	Day	34% 30% 24% 12%	273
March	1st to 10th	Day	22% 33% 25% 16% 3	393
	11th to 20th	Day	23% 31% 29% 12% 4	540
	21st to end	Day	19% 39% 31% 9% 2	594
April	1st to 10th	Day	22% 36% 29% 10% 2 1	540
	11th to 20th	Day	28% 34% 26% 10% 2	517
	21st to end	Day	19% 38% 30% 10% 2 1	500
May	1st to 10th	Day	21% 36% 24% 15% 4 1	532
	11th to 20th	Day	15% 20% 34% 19% 10% 1	725
	21st to end	Day	15% 21% 30% 22% 9% 2	789
June	1st to 10th	Day	9% 20% 29% 23% 17% 2	730
	11th to 20th	Day	11% 22% 33% 22% 12% 1	730
	21st to end	Day	8% 21% 29% 24% 16% 2	730
July	1st to 10th	Day	10% 11% 25% 31% 20% 3	721
	11th to 20th	Day	7% 9% 26% 30% 23% 5%	718
	21st to end	Day	13% 12% 27% 28% 16% 4	617
August	1st to 10th	Day	15% 16% 32% 24% 11% 3	527
	11th to 20th	Day	8% 15% 33% 26% 14% 5%	517
	21st to end	Day	10% 15% 32% 27% 13% 3	583
September	1st to 10th	Day	13% 16% 29% 25% 11% 6%	530
	11th to 20th	Day	12% 18% 36% 22% 9% 2	530
	21st to end	Day	12% 20% 30% 21% 14% 4	361
October	1st to 10th	Day	16% 17% 28% 20% 16% 3	330
	11th to 20th	Day	20% 29% 26% 16% 8% 2	330
	21st to end	Day	22% 27% 27% 16% 7% 1	363
November	1st to 10th	Day	21% 33% 24% 14% 7% 1	330
	11th to 20th	Day	18% 30% 31% 14% 6% 2	330
	21st to end	Day	15% 29% 33% 18% 4	330
December	1st to 10th	Day	25% 30% 23% 14% 8% 1	330
	11th to 20th	Day	21% 35% 25% 13% 5%	330
	21st to end	Day	25% 32% 26% 11% 6%	363
January	1st to 10th	Night	13% 24% 30% 22% 10% 2	1050
	11th to 20th	Night	15% 22% 31% 24% 7% 1	910
	21st to end	Night	7% 23% 40% 23% 6% 1	924
February	1st to 10th	Night	11% 29% 35% 20% 5%	840
	11th to 20th	Night	12% 31% 33% 21% 3	824
	21st to end	Night	12% 26% 40% 19% 2	692
March	1st to 10th	Night	10% 23% 34% 26% 6%	840
	11th to 20th	Night	7% 18% 39% 27% 8%	840
	21st to end	Night	7% 25% 40% 25% 4	828
April	1st to 10th	Night	9% 23% 38% 24% 4	630
	11th to 20th	Night	8% 24% 34% 26% 7%	611
	21st to end	Night	10% 18% 36% 29% 7% 1	600
May	1st to 10th	Night	10% 13% 39% 28% 10% 1	600
	11th to 20th	Night	8% 10% 31% 35% 15% 1	591
	21st to end	Night	8% 13% 28% 34% 16% 1	650
June	1st to 10th	Night	6% 7% 25% 40% 21% 1	600
	11th to 20th	Night	5% 11% 28% 41% 14% 1	600
	21st to end	Night	3 4 25% 42% 26% 1	600
July	1st to 10th	Night	6% 2 15% 42% 32% 3	585
	11th to 20th	Night	3 2 10% 42% 40% 3	582
	21st to end	Night	6% 3 10% 38% 40% 3	657
August	1st to 10th	Night	8% 1 13% 42% 34% 3	597
	11th to 20th	Night	6% 2 15% 42% 32% 4	594
	21st to end	Night	7% 1 14% 42% 34% 2	660
September	1st to 10th	Night	5% 3 15% 47% 27% 4	600
	11th to 20th	Night	5% 4 21% 44% 23% 3	600
	21st to end	Night	4 10% 22% 38% 23% 4	672
October	1st to 10th	Night	5% 15% 29% 32% 16% 4	800
	11th to 20th	Night	8% 20% 29% 29% 13% 2	800
	21st to end	Night	8% 18% 32% 29% 10% 2	880
November	1st to 10th	Night	7% 22% 33% 28% 9% 1	995
	11th to 20th	Night	10% 24% 35% 24% 5% 1	1050
	21st to end	Night	8% 21% 34% 29% 8%	1050
December	1st to 10th	Night	10% 27% 34% 22% 7%	1050
	11th to 20th	Night	11% 26% 35% 20% 7% 1	1050
	21st to end	Night	15% 24% 35% 21% 5% 1	1155

Page generated at 2026-04-14 00:44:31. Version ac07

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