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 2024-01-17.

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	5% 8% 4 9% 7% 4 5% 4 5% 6% 43%	74778
	Night	2 4 3 7% 7% 3 4 5% 5% 7% 51%	75062
Transparency	Day	39% 5% 11% 21% 18% 6%	74490
	Night	29% 6% 15% 25% 20% 6%	74872
Seeing	Day	16% 24% 28% 19% 10% 2	14366
	Night	8% 18% 29% 29% 14% 1	24568

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	5% 6% 10% 9% 4 4 4 5% 7% 47%	1518
	11th to 20th	Day	4 4 13% 8% 3 5% 5% 3 6% 49%	1626
	21st to end	Day	5% 4 15% 10% 4 3 4 4 7% 44%	1782
February	1st to 10th	Day	4 7% 16% 10% 4 5% 4 5% 4 40%	1620
	11th to 20th	Day	4 6% 11% 13% 4 5% 4 4 6% 43%	1672
	21st to end	Day	4 5% 16% 11% 6% 5% 4 4 5% 41%	1480
March	1st to 10th	Day	1 9% 6% 11% 8% 4 3 4 5% 7% 43%	1854
	11th to 20th	Day	2 10% 6% 13% 8% 3 4 4 4 6% 41%	1980
	21st to end	Day	2 9% 8% 14% 9% 4 5% 4 5% 6% 35%	2258
April	1st to 10th	Day	2 10% 5% 10% 8% 4 4 4 5% 6% 42%	2160
	11th to 20th	Day	2 13% 7% 14% 9% 3 4 4 4 6% 33%	2131
	21st to end	Day	3 15% 8% 11% 8% 4 5% 4 4 5% 33%	2160
May	1st to 10th	Day	5% 15% 7% 8% 6% 4 5% 5% 5% 7% 33%	2247
	11th to 20th	Day	6% 15% 5% 9% 6% 3 5% 5% 5% 8% 34%	2478
	21st to end	Day	7% 10% 5% 9% 7% 4 5% 5% 6% 8% 35%	2724
June	1st to 10th	Day	6% 7% 3 6% 6% 3 5% 6% 6% 9% 45%	2520
	11th to 20th	Day	7% 7% 2 5% 4 3 5% 4 5% 6% 51%	2520
	21st to end	Day	6% 5% 3 6% 6% 4 5% 6% 5% 7% 48%	2520
July	1st to 10th	Day	10% 6% 4 7% 6% 4 6% 4 6% 7% 41%	2520
	11th to 20th	Day	10% 7% 3 7% 7% 3 6% 7% 7% 7% 36%	2520
	21st to end	Day	15% 8% 4 8% 8% 4 6% 6% 5% 7% 28%	2583
August	1st to 10th	Day	16% 9% 4 7% 7% 4 6% 5% 6% 7% 31%	2328
	11th to 20th	Day	11% 8% 3 7% 6% 3 5% 5% 6% 6% 40%	2177
	21st to end	Day	11% 7% 4 8% 7% 4 5% 5% 6% 6% 37%	2376
September	1st to 10th	Day	11% 7% 2 5% 5% 4 4 3 4 5% 50%	2160
	11th to 20th	Day	9% 7% 3 5% 6% 4 4 4 4 5% 49%	2160
	21st to end	Day	6% 9% 3 5% 5% 3 2 3 4 6% 54%	1942
October	1st to 10th	Day	7% 9% 4 6% 6% 3 4 3 4 5% 50%	1800
	11th to 20th	Day	3 8% 3 5% 4 2 3 3 3 6% 61%	1800
	21st to end	Day	3 7% 4 10% 4 2 3 3 4 5% 54%	1980
November	1st to 10th	Day	1 5% 3 7% 4 3 3 4 4 5% 61%	1683
	11th to 20th	Day	1 7% 4 6% 5% 2 3 3 4 8% 57%	1710
	21st to end	Day	1 5% 4 7% 9% 2 2 3 4 6% 56%	1549
December	1st to 10th	Day	4 4 13% 11% 5% 5% 4 4 8% 43%	1520
	11th to 20th	Day	5% 5% 11% 7% 4 4 4 5% 10% 45%	1926
	21st to end	Day	5% 4 12% 12% 4 5% 4 5% 7% 41%	2794
January	1st to 10th	Night	2 3 10% 10% 5% 5% 5% 4 7% 48%	2647
	11th to 20th	Night	3 4 9% 9% 4 5% 5% 4 6% 51%	2488
	21st to end	Night	3 3 10% 11% 6% 6% 4 5% 7% 46%	2574
February	1st to 10th	Night	2 4 14% 11% 4 5% 4 5% 7% 44%	2340
	11th to 20th	Night	3 5% 10% 11% 5% 6% 5% 5% 7% 43%	2173
	21st to end	Night	3 4 12% 9% 7% 6% 5% 4 7% 42%	1776
March	1st to 10th	Night	4 4 8% 9% 4 4 4 5% 7% 50%	2102
	11th to 20th	Night	1 6% 4 10% 7% 3 4 3 5% 7% 52%	1980
	21st to end	Night	2 6% 6% 10% 9% 4 4 4 5% 6% 45%	2097
April	1st to 10th	Night	1 4 4 7% 7% 3 5% 5% 5% 8% 51%	1800
	11th to 20th	Night	1 9% 3 10% 9% 4 5% 4 6% 6% 43%	1777
	21st to end	Night	2 9% 4 8% 6% 3 4 5% 4 8% 47%	1800
May	1st to 10th	Night	5% 9% 3 7% 4 4 4 5% 6% 7% 46%	1709
	11th to 20th	Night	4 9% 3 6% 7% 4 5% 5% 6% 8% 43%	1417
	21st to end	Night	4 8% 3 5% 4 3 4 4 6% 9% 51%	1552
June	1st to 10th	Night	3 3 2 3 5% 2 4 5% 6% 10% 59%	1440
	11th to 20th	Night	4 4 1 3 3 2 4 4 4 7% 62%	1440
	21st to end	Night	3 2 2 3 3 3 4 5% 5% 9% 60%	1440
July	1st to 10th	Night	6% 5% 2 6% 5% 3 5% 7% 6% 10% 46%	1440
	11th to 20th	Night	3 3 2 5% 4 3 5% 4 7% 11% 53%	1440
	21st to end	Night	6% 4 2 6% 6% 4 6% 7% 7% 10% 42%	1731
August	1st to 10th	Night	7% 5% 2 6% 5% 2 4 6% 6% 9% 46%	1611
	11th to 20th	Night	6% 6% 2 4 4 3 4 5% 6% 8% 53%	1722
	21st to end	Night	5% 5% 2 5% 5% 3 4 5% 7% 7% 50%	1980
September	1st to 10th	Night	4 4 2 5% 4 2 4 6% 6% 8% 54%	1800
	11th to 20th	Night	4 4 2 5% 4 2 4 4 5% 6% 60%	1800
	21st to end	Night	3 5% 2 4 4 3 3 4 5% 8% 60%	2013
October	1st to 10th	Night	4 5% 2 4 4 2 5% 5% 5% 7% 56%	2160
	11th to 20th	Night	2 4 2 4 4 1 3 3 4 5% 67%	2160
	21st to end	Night	2 7% 3 7% 4 2 3 4 3 6% 58%	2376
November	1st to 10th	Night	1 4 3 6% 4 2 3 3 4 7% 65%	2377
	11th to 20th	Night	3 3 7% 6% 3 3 4 4 6% 60%	2470
	21st to end	Night	4 4 8% 8% 3 3 4 5% 7% 54%	2626
December	1st to 10th	Night	3 3 9% 10% 4 5% 4 4 6% 53%	2660
	11th to 20th	Night	3 4 8% 9% 5% 5% 5% 6% 7% 46%	3271
	21st to end	Night	4 3 10% 11% 6% 5% 5% 4 7% 46%	4873

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	35% 4 17% 24% 17% 4	1519
	11th to 20th	Day	35% 7% 17% 19% 17% 5%	1626
	21st to end	Day	39% 4 16% 24% 12% 5%	1782
February	1st to 10th	Day	44% 5% 16% 17% 13% 5%	1620
	11th to 20th	Day	39% 3 12% 23% 17% 6%	1681
	21st to end	Day	43% 4 11% 20% 17% 6%	1480
March	1st to 10th	Day	40% 4 10% 24% 20% 2	1854
	11th to 20th	Day	42% 4 14% 23% 14% 4	1980
	21st to end	Day	47% 4 11% 23% 11% 3	2258
April	1st to 10th	Day	41% 5% 13% 20% 16% 6%	2160
	11th to 20th	Day	51% 3 12% 16% 15% 2	2134
	21st to end	Day	50% 4 9% 25% 11% 1	2160
May	1st to 10th	Day	49% 5% 11% 21% 14% 1	2247
	11th to 20th	Day	49% 6% 12% 24% 9% 1	2482
	21st to end	Day	46% 7% 10% 19% 14% 4	2724
June	1st to 10th	Day	34% 7% 12% 23% 19% 6%	2520
	11th to 20th	Day	33% 4 8% 18% 23% 13%	2520
	21st to end	Day	33% 7% 7% 19% 24% 11%	2520
July	1st to 10th	Day	40% 8% 8% 15% 19% 10%	2451
	11th to 20th	Day	42% 9% 12% 17% 12% 8%	2445
	21st to end	Day	52% 8% 14% 14% 10% 2	2439
August	1st to 10th	Day	51% 8% 10% 13% 14% 4	2313
	11th to 20th	Day	43% 6% 8% 18% 18% 7%	2177
	21st to end	Day	45% 7% 8% 20% 16% 5%	2376
September	1st to 10th	Day	37% 5% 6% 15% 26% 11%	2160
	11th to 20th	Day	36% 4 9% 19% 24% 8%	2160
	21st to end	Day	32% 3 6% 27% 27% 6%	1942
October	1st to 10th	Day	36% 5% 7% 21% 25% 7%	1800
	11th to 20th	Day	25% 4 7% 29% 28% 8%	1800
	21st to end	Day	32% 3 11% 26% 21% 7%	1980
November	1st to 10th	Day	23% 5% 12% 24% 31% 5%	1683
	11th to 20th	Day	24% 3 16% 30% 21% 7%	1710
	21st to end	Day	30% 3 10% 28% 17% 11%	1549
December	1st to 10th	Day	37% 6% 16% 22% 15% 4	1520
	11th to 20th	Day	32% 8% 12% 28% 18% 2	1924
	21st to end	Day	35% 6% 12% 16% 22% 8%	2794
January	1st to 10th	Night	30% 6% 18% 26% 16% 4	2647
	11th to 20th	Night	29% 7% 18% 24% 17% 6%	2488
	21st to end	Night	33% 5% 16% 26% 15% 6%	2574
February	1st to 10th	Night	36% 4 16% 23% 16% 4	2340
	11th to 20th	Night	35% 4 14% 23% 19% 4	2174
	21st to end	Night	35% 4 14% 22% 19% 6%	1776
March	1st to 10th	Night	30% 4 13% 27% 22% 4	2102
	11th to 20th	Night	29% 5% 15% 32% 15% 5%	1980
	21st to end	Night	35% 6% 16% 28% 12% 2	2097
April	1st to 10th	Night	29% 6% 17% 24% 20% 5%	1800
	11th to 20th	Night	36% 5% 15% 26% 16% 2	1777
	21st to end	Night	34% 5% 17% 30% 15% 1	1800
May	1st to 10th	Night	34% 6% 15% 30% 13% 1	1709
	11th to 20th	Night	37% 6% 18% 26% 12%	1418
	21st to end	Night	28% 6% 14% 26% 21% 4	1552
June	1st to 10th	Night	20% 7% 16% 30% 21% 6%	1440
	11th to 20th	Night	21% 5% 12% 21% 27% 13%	1440
	21st to end	Night	19% 6% 13% 24% 27% 12%	1440
July	1st to 10th	Night	31% 9% 14% 18% 19% 9%	1400
	11th to 20th	Night	24% 8% 19% 25% 17% 8%	1392
	21st to end	Night	32% 11% 25% 20% 9% 2	1635
August	1st to 10th	Night	34% 8% 20% 19% 14% 5%	1602
	11th to 20th	Night	28% 6% 17% 22% 21% 6%	1722
	21st to end	Night	30% 8% 14% 25% 19% 5%	1980
September	1st to 10th	Night	25% 7% 12% 20% 27% 9%	1800
	11th to 20th	Night	24% 5% 14% 23% 24% 10%	1800
	21st to end	Night	22% 5% 10% 34% 24% 4	2013
October	1st to 10th	Night	25% 6% 12% 26% 23% 9%	2160
	11th to 20th	Night	18% 4 11% 31% 28% 8%	2160
	21st to end	Night	26% 3 12% 29% 22% 7%	2376
November	1st to 10th	Night	20% 5% 13% 28% 27% 7%	2377
	11th to 20th	Night	22% 4 14% 31% 22% 6%	2470
	21st to end	Night	28% 4 10% 28% 21% 9%	2626
December	1st to 10th	Night	29% 6% 19% 24% 17% 6%	2660
	11th to 20th	Night	30% 7% 14% 22% 23% 4	3272
	21st to end	Night	33% 6% 12% 18% 20% 10%	4873

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	21% 27% 24% 19% 8% 2	270
	11th to 20th	Day	22% 30% 27% 17% 3	258
	21st to end	Day	16% 36% 32% 15% 2	264
February	1st to 10th	Day	25% 42% 23% 9% 1 1	240
	11th to 20th	Day	30% 33% 22% 13% 2 1	240
	21st to end	Day	29% 34% 27% 9% 1	198
March	1st to 10th	Day	23% 34% 24% 15% 3	294
	11th to 20th	Day	20% 33% 31% 13% 3	420
	21st to end	Day	19% 42% 30% 7% 2	462
April	1st to 10th	Day	23% 36% 30% 8% 2 1	420
	11th to 20th	Day	29% 34% 25% 10% 2	412
	21st to end	Day	20% 39% 28% 9% 3 1	420
May	1st to 10th	Day	19% 36% 25% 15% 4 1	448
	11th to 20th	Day	16% 20% 33% 20% 11% 1	625
	21st to end	Day	15% 21% 29% 23% 10% 2	679
June	1st to 10th	Day	9% 20% 27% 24% 18% 2	630
	11th to 20th	Day	11% 21% 33% 22% 12% 1	630
	21st to end	Day	7% 20% 30% 24% 17% 2	630
July	1st to 10th	Day	10% 9% 24% 31% 23% 3	621
	11th to 20th	Day	8% 9% 24% 29% 23% 6%	618
	21st to end	Day	14% 11% 26% 27% 17% 5%	525
August	1st to 10th	Day	16% 14% 32% 24% 11% 3	447
	11th to 20th	Day	8% 12% 35% 26% 14% 5%	437
	21st to end	Day	11% 14% 30% 28% 13% 3	495
September	1st to 10th	Day	14% 16% 30% 23% 10% 6%	450
	11th to 20th	Day	12% 17% 37% 23% 10% 2	450
	21st to end	Day	13% 19% 29% 21% 14% 4	299
October	1st to 10th	Day	17% 20% 24% 20% 17% 3	270
	11th to 20th	Day	16% 31% 25% 18% 7% 3	270
	21st to end	Day	21% 29% 26% 15% 7% 2	297
November	1st to 10th	Day	22% 34% 23% 14% 7% 1	270
	11th to 20th	Day	18% 31% 30% 13% 6% 2	270
	21st to end	Day	12% 30% 35% 19% 4	270
December	1st to 10th	Day	22% 31% 25% 13% 8% 1	270
	11th to 20th	Day	19% 35% 28% 13% 5%	270
	21st to end	Day	22% 34% 26% 11% 6%	297
January	1st to 10th	Night	13% 23% 30% 22% 10% 2	950
	11th to 20th	Night	15% 23% 30% 23% 7% 1	810
	21st to end	Night	8% 23% 39% 23% 5% 1	792
February	1st to 10th	Night	10% 30% 36% 19% 5%	720
	11th to 20th	Night	10% 30% 33% 23% 4	704
	21st to end	Night	11% 27% 40% 20% 2	592
March	1st to 10th	Night	10% 24% 34% 25% 6% 1	720
	11th to 20th	Night	5% 18% 39% 29% 9%	720
	21st to end	Night	7% 26% 40% 23% 4	711
April	1st to 10th	Night	9% 25% 37% 24% 4	540
	11th to 20th	Night	9% 25% 32% 26% 8%	533
	21st to end	Night	11% 19% 34% 29% 7% 1	540
May	1st to 10th	Night	9% 12% 40% 28% 10% 1	540
	11th to 20th	Night	8% 10% 30% 36% 15% 1	531
	21st to end	Night	9% 13% 27% 34% 17% 1	584
June	1st to 10th	Night	5% 6% 26% 38% 23% 1	540
	11th to 20th	Night	6% 11% 29% 39% 15% 1	540
	21st to end	Night	3 4 24% 41% 28% 1	540
July	1st to 10th	Night	6% 2 14% 41% 34% 3	525
	11th to 20th	Night	3 2 11% 39% 42% 4	522
	21st to end	Night	6% 3 9% 35% 43% 4	591
August	1st to 10th	Night	8% 1 12% 40% 37% 3	537
	11th to 20th	Night	6% 2 14% 40% 33% 4	534
	21st to end	Night	6% 2 14% 40% 36% 3	594
September	1st to 10th	Night	5% 3 16% 45% 27% 4	540
	11th to 20th	Night	6% 4 19% 44% 25% 3	540
	21st to end	Night	4 10% 23% 35% 23% 4	604
October	1st to 10th	Night	5% 16% 30% 29% 16% 4	720
	11th to 20th	Night	8% 20% 28% 29% 13% 2	720
	21st to end	Night	9% 18% 31% 29% 11% 2	792
November	1st to 10th	Night	7% 22% 32% 28% 10% 1	897
	11th to 20th	Night	11% 25% 33% 24% 6% 2	950
	21st to end	Night	8% 21% 33% 30% 8%	950
December	1st to 10th	Night	10% 28% 35% 21% 6%	950
	11th to 20th	Night	10% 27% 35% 20% 7% 1	950
	21st to end	Night	14% 25% 35% 21% 5% 1	1045

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