Founder's Hut
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Founder's Hut clear sky chart from 2005-11-06 to 2026-07-01.

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	7% 6% 2 6% 4 4 5% 4 6% 8% 46%	146954
	Night	7% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	142796
Transparency	Day	35% 5% 14% 26% 16% 4	141649
	Night	38% 6% 15% 25% 14% 3	139110
Seeing	Day	13% 11% 25% 29% 17% 4	17825
	Night	12% 10% 23% 39% 14% 1	27224

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	6% 13% 4 9% 5% 5% 6% 6% 8% 11% 27%	4124
	11th to 20th	Day	4 10% 5% 8% 6% 6% 8% 5% 6% 8% 35%	4140
	21st to end	Day	2 7% 3 10% 5% 4 5% 4 8% 13% 39%	5623
February	1st to 10th	Day	9% 13% 4 10% 7% 6% 6% 4 6% 8% 29%	1890
	11th to 20th	Day	6% 15% 5% 11% 6% 5% 6% 4 6% 8% 29%	1845
	21st to end	Day	4 11% 4 10% 7% 6% 6% 4 7% 8% 34%	1557
March	1st to 10th	Day	5% 7% 2 8% 5% 6% 6% 4 7% 9% 40%	4940
	11th to 20th	Day	4 6% 3 7% 5% 6% 7% 5% 8% 8% 40%	5110
	21st to end	Day	11% 14% 4 12% 6% 5% 5% 4 6% 9% 23%	5607
April	1st to 10th	Day	9% 10% 3 7% 6% 5% 6% 5% 7% 9% 35%	2504
	11th to 20th	Day	12% 9% 3 7% 6% 5% 5% 5% 6% 7% 35%	2659
	21st to end	Day	13% 9% 3 7% 5% 4 5% 4 7% 7% 36%	2730
May	1st to 10th	Day	14% 8% 2 5% 3 3 4 3 6% 8% 44%	8016
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	8191
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 8% 46%	9230
June	1st to 10th	Day	12% 5% 2 5% 5% 4 4 5% 7% 9% 42%	3150
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 6% 8% 41%	3150
	21st to end	Day	6% 4 1 4 3 4 4 4 7% 9% 54%	3150
July	1st to 10th	Day	4 3 1 3 3 4 4 4 7% 8% 58%	6780
	11th to 20th	Day	6% 2 1 3 2 2 3 3 5% 6% 68%	6780
	21st to end	Day	3 2 1 3 3 4 4 4 6% 8% 63%	7077
August	1st to 10th	Day	6% 4 1 5% 4 4 5% 4 7% 10% 51%	5786
	11th to 20th	Day	4 2 1 3 2 3 3 3 6% 8% 63%	5798
	21st to end	Day	7% 4 1 4 3 3 3 3 5% 6% 60%	6428
September	1st to 10th	Day	8% 3 1 4 3 3 3 3 5% 8% 58%	2400
	11th to 20th	Day	9% 4 1 5% 4 3 5% 4 6% 8% 50%	2293
	21st to end	Day	11% 4 1 4 2 3 3 2 5% 7% 58%	2174
October	1st to 10th	Day	10% 5% 1 5% 3 3 3 2 4 6% 57%	3270
	11th to 20th	Day	6% 6% 2 4 3 4 4 3 6% 8% 56%	3250
	21st to end	Day	4 6% 2 6% 3 4 5% 4 7% 9% 50%	3356
November	1st to 10th	Day	10% 10% 2 7% 4 4 5% 4 5% 7% 42%	1648
	11th to 20th	Day	4 9% 3 8% 5% 5% 6% 5% 8% 8% 38%	1680
	21st to end	Day	6% 10% 3 8% 7% 5% 5% 4 7% 9% 36%	1680
December	1st to 10th	Day	3 10% 3 8% 7% 8% 7% 6% 7% 10% 32%	2648
	11th to 20th	Day	6% 11% 4 8% 5% 4 4 4 6% 8% 39%	2715
	21st to end	Day	6% 10% 4 10% 5% 6% 5% 4 5% 7% 38%	3575
January	1st to 10th	Night	4 10% 4 10% 6% 5% 6% 5% 7% 9% 32%	8768
	11th to 20th	Night	4 11% 3 9% 6% 5% 6% 4 6% 7% 39%	8820
	21st to end	Night	3 8% 4 10% 6% 5% 6% 4 6% 8% 40%	8641
February	1st to 10th	Night	7% 14% 5% 11% 5% 5% 5% 4 6% 8% 28%	2730
	11th to 20th	Night	6% 12% 5% 10% 6% 5% 5% 5% 7% 8% 31%	2662
	21st to end	Night	5% 12% 5% 10% 7% 6% 5% 4 7% 7% 33%	2249
March	1st to 10th	Night	4 8% 3 7% 6% 6% 6% 6% 8% 8% 37%	5169
	11th to 20th	Night	4 8% 3 8% 5% 5% 5% 4 7% 9% 41%	5110
	21st to end	Night	12% 15% 5% 11% 7% 6% 6% 4 6% 7% 22%	5610
April	1st to 10th	Night	9% 9% 3 8% 6% 5% 5% 4 6% 7% 39%	2116
	11th to 20th	Night	10% 10% 4 7% 5% 4 5% 4 7% 9% 34%	1907
	21st to end	Night	12% 8% 2 7% 4 4 5% 4 7% 8% 40%	1890
May	1st to 10th	Night	14% 7% 1 5% 4 4 4 3 5% 7% 48%	4929
	11th to 20th	Night	11% 9% 3 7% 5% 5% 6% 5% 8% 7% 34%	4701
	21st to end	Night	12% 4 1 5% 4 4 5% 4 6% 9% 47%	4949
June	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	1470
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 4 7% 9% 39%	1470
	21st to end	Night	6% 3 2 6% 4 4 5% 3 6% 7% 53%	1470
July	1st to 10th	Night	5% 2 1 4 3 4 4 4 6% 9% 57%	3180
	11th to 20th	Night	8% 2 1 3 3 2 3 4 6% 8% 62%	3180
	21st to end	Night	5% 3 2 5% 4 4 4 4 7% 9% 50%	3785
August	1st to 10th	Night	9% 4 2 6% 5% 4 5% 5% 7% 10% 44%	4023
	11th to 20th	Night	6% 3 1 4 3 3 4 4 7% 8% 56%	4051
	21st to end	Night	9% 4 2 5% 3 3 3 3 5% 6% 56%	4548
September	1st to 10th	Night	9% 4 1 4 3 3 3 4 4 7% 58%	2000
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 52%	2105
	21st to end	Night	12% 3 1 4 3 3 4 3 6% 8% 54%	2225
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 4 7% 57%	3920
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 6% 7% 53%	3898
	21st to end	Night	6% 5% 2 7% 5% 5% 5% 4 5% 7% 49%	4395
November	1st to 10th	Night	10% 9% 3 7% 5% 4 5% 5% 6% 8% 38%	2870
	11th to 20th	Night	5% 7% 3 6% 5% 6% 6% 6% 6% 8% 41%	2940
	21st to end	Night	5% 10% 3 7% 5% 5% 5% 4 6% 8% 42%	2940
December	1st to 10th	Night	4 8% 3 8% 8% 7% 6% 5% 10% 8% 32%	4756
	11th to 20th	Night	5% 12% 4 10% 7% 5% 5% 4 6% 7% 36%	5696
	21st to end	Night	5% 9% 3 7% 4 5% 5% 4 7% 8% 43%	7623

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	47% 7% 19% 22% 5% 1	4125
	11th to 20th	Day	45% 5% 16% 21% 11% 1	4140
	21st to end	Day	35% 6% 20% 30% 9%	5623
February	1st to 10th	Day	52% 5% 15% 19% 9% 1	1890
	11th to 20th	Day	52% 5% 15% 19% 7% 1	1854
	21st to end	Day	45% 6% 15% 23% 10% 1	1557
March	1st to 10th	Day	36% 7% 13% 28% 14% 1	4940
	11th to 20th	Day	38% 7% 20% 24% 12% 1	5110
	21st to end	Day	56% 8% 16% 18% 3 1	5607
April	1st to 10th	Day	43% 6% 17% 25% 8% 1	2504
	11th to 20th	Day	47% 5% 14% 24% 9% 1	2662
	21st to end	Day	44% 4 14% 27% 10% 1	2730
May	1st to 10th	Day	37% 3 12% 29% 16% 2	8016
	11th to 20th	Day	42% 5% 13% 25% 12% 4	8195
	21st to end	Day	33% 6% 12% 30% 18% 1	9231
June	1st to 10th	Day	36% 6% 18% 27% 12% 1	3150
	11th to 20th	Day	39% 5% 14% 23% 16% 4	3150
	21st to end	Day	26% 5% 14% 34% 19% 3	3150
July	1st to 10th	Day	22% 4 13% 28% 24% 9%	6707
	11th to 20th	Day	18% 3 9% 26% 28% 17%	6699
	21st to end	Day	18% 5% 12% 21% 27% 17%	6875
August	1st to 10th	Day	26% 4 10% 24% 24% 12%	4214
	11th to 20th	Day	24% 4 11% 35% 21% 5%	4197
	21st to end	Day	21% 3 8% 31% 28% 8%	4635
September	1st to 10th	Day	24% 3 9% 30% 23% 11%	2400
	11th to 20th	Day	30% 5% 14% 29% 17% 5%	2293
	21st to end	Day	27% 3 11% 32% 22% 6%	2174
October	1st to 10th	Day	30% 3 8% 34% 20% 5%	3270
	11th to 20th	Day	27% 4 13% 34% 19% 2	3250
	21st to end	Day	30% 3 14% 29% 21% 3	3356
November	1st to 10th	Day	40% 6% 14% 25% 11% 3	1648
	11th to 20th	Day	40% 6% 20% 24% 8% 1	1680
	21st to end	Day	44% 4 21% 21% 9% 1	1680
December	1st to 10th	Day	46% 8% 17% 22% 7% 1	2648
	11th to 20th	Day	42% 5% 24% 23% 4 1	2714
	21st to end	Day	45% 5% 12% 20% 14% 4	3575
January	1st to 10th	Night	45% 6% 18% 22% 7% 1	8769
	11th to 20th	Night	41% 6% 20% 20% 12% 2	8820
	21st to end	Night	40% 7% 17% 27% 7% 1	8641
February	1st to 10th	Night	52% 6% 16% 18% 8% 1	2730
	11th to 20th	Night	49% 6% 16% 21% 8% 1	2663
	21st to end	Night	48% 5% 14% 22% 10% 1	2249
March	1st to 10th	Night	37% 10% 17% 24% 12%	5169
	11th to 20th	Night	37% 6% 20% 29% 8%	5110
	21st to end	Night	58% 7% 16% 14% 4	5610
April	1st to 10th	Night	42% 7% 16% 24% 10% 1	2116
	11th to 20th	Night	45% 6% 14% 24% 10% 1	1907
	21st to end	Night	40% 5% 15% 27% 12% 1	1890
May	1st to 10th	Night	37% 4 10% 28% 20% 2	4927
	11th to 20th	Night	44% 6% 12% 25% 12% 1	4702
	21st to end	Night	32% 6% 12% 32% 17% 1	4950
June	1st to 10th	Night	39% 6% 16% 28% 9% 2	1470
	11th to 20th	Night	40% 4 15% 23% 16% 3	1470
	21st to end	Night	28% 6% 15% 29% 20% 3	1470
July	1st to 10th	Night	24% 4 12% 26% 24% 9%	3145
	11th to 20th	Night	20% 4 10% 28% 28% 10%	3138
	21st to end	Night	26% 5% 12% 17% 26% 14%	3676
August	1st to 10th	Night	31% 5% 11% 22% 20% 11%	2919
	11th to 20th	Night	29% 5% 12% 29% 19% 5%	2928
	21st to end	Night	26% 3 9% 33% 23% 6%	3272
September	1st to 10th	Night	26% 4 8% 25% 24% 13%	2000
	11th to 20th	Night	34% 5% 10% 28% 18% 6%	2105
	21st to end	Night	28% 4 11% 28% 23% 6%	2225
October	1st to 10th	Night	28% 4 8% 33% 23% 4	3920
	11th to 20th	Night	31% 4 12% 31% 21% 2	3898
	21st to end	Night	33% 4 14% 25% 21% 3	4395
November	1st to 10th	Night	42% 6% 15% 24% 10% 3	2870
	11th to 20th	Night	38% 6% 19% 26% 10% 1	2940
	21st to end	Night	39% 5% 20% 24% 10% 2	2940
December	1st to 10th	Night	43% 8% 20% 20% 8% 1	4756
	11th to 20th	Night	47% 6% 22% 20% 6%	5697
	21st to end	Night	38% 4 12% 28% 14% 3	7623

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	11% 15% 18% 28% 20% 9%	220
	11th to 20th	Day	17% 13% 19% 29% 16% 5%	220
	21st to end	Day	9% 14% 25% 31% 19% 1	337
February	1st to 10th	Day	21% 14% 24% 31% 8% 2	330
	11th to 20th	Day	16% 14% 21% 32% 15% 3	330
	21st to end	Day	11% 18% 30% 28% 12% 2	273
March	1st to 10th	Day	15% 11% 27% 30% 14% 2	512
	11th to 20th	Day	14% 14% 29% 26% 14% 3	540
	21st to end	Day	19% 13% 26% 26% 14% 3	594
April	1st to 10th	Day	18% 13% 29% 28% 12% 1	540
	11th to 20th	Day	20% 18% 28% 23% 9% 2	532
	21st to end	Day	20% 11% 27% 23% 16% 2	540
May	1st to 10th	Day	16% 12% 23% 27% 18% 5%	675
	11th to 20th	Day	17% 11% 23% 27% 17% 5%	775
	21st to end	Day	18% 8% 20% 29% 20% 5%	844
June	1st to 10th	Day	15% 13% 23% 24% 20% 6%	780
	11th to 20th	Day	17% 15% 28% 25% 13% 2	780
	21st to end	Day	9% 11% 25% 29% 22% 5%	780
July	1st to 10th	Day	5% 6% 24% 36% 25% 5%	722
	11th to 20th	Day	5% 8% 30% 35% 19% 3	718
	21st to end	Day	4 5% 24% 38% 23% 6%	794
August	1st to 10th	Day	6% 8% 30% 32% 19% 6%	534
	11th to 20th	Day	8% 6% 27% 29% 21% 9%	517
	21st to end	Day	8% 9% 24% 32% 21% 6%	583
September	1st to 10th	Day	9% 7% 27% 32% 20% 5%	530
	11th to 20th	Day	12% 8% 25% 28% 19% 8%	530
	21st to end	Day	16% 11% 24% 20% 21% 8%	504
October	1st to 10th	Day	17% 6% 21% 26% 23% 7%	330
	11th to 20th	Day	16% 12% 23% 30% 15% 5%	330
	21st to end	Day	15% 18% 25% 28% 12% 2	363
November	1st to 10th	Day	22% 11% 25% 26% 14% 2	330
	11th to 20th	Day	11% 17% 23% 31% 16% 2	330
	21st to end	Day	16% 14% 25% 31% 13% 1	330
December	1st to 10th	Day	11% 17% 28% 30% 13% 2	316
	11th to 20th	Day	20% 15% 19% 29% 14% 3	220
	21st to end	Day	19% 7% 19% 32% 17% 5%	242
January	1st to 10th	Night	11% 14% 27% 36% 11% 2	1050
	11th to 20th	Night	12% 15% 28% 33% 10% 2	1050
	21st to end	Night	9% 16% 27% 37% 9% 1	971
February	1st to 10th	Night	14% 12% 23% 38% 12% 1	840
	11th to 20th	Night	12% 12% 26% 39% 10% 1	824
	21st to end	Night	12% 12% 23% 39% 13% 2	692
March	1st to 10th	Night	11% 9% 24% 43% 13% 1	840
	11th to 20th	Night	10% 10% 27% 38% 14% 1	840
	21st to end	Night	16% 11% 26% 35% 12% 1	924
April	1st to 10th	Night	13% 11% 25% 39% 11%	646
	11th to 20th	Night	16% 10% 24% 37% 12% 1	623
	21st to end	Night	17% 10% 23% 36% 12% 1	630
May	1st to 10th	Night	17% 9% 18% 34% 19% 2	630
	11th to 20th	Night	14% 9% 20% 36% 20% 1	621
	21st to end	Night	21% 7% 18% 30% 23% 1	607
June	1st to 10th	Night	14% 8% 21% 35% 20% 2	420
	11th to 20th	Night	15% 11% 25% 37% 12%	420
	21st to end	Night	7% 8% 18% 46% 20% 1	420
July	1st to 10th	Night	8% 2 15% 52% 24%	390
	11th to 20th	Night	6% 5% 21% 53% 15% 1	388
	21st to end	Night	6% 4 22% 48% 19% 1	576
August	1st to 10th	Night	7% 3 22% 47% 19% 1	597
	11th to 20th	Night	8% 3 21% 43% 24% 2	594
	21st to end	Night	11% 5% 17% 43% 22% 1	660
September	1st to 10th	Night	11% 3 15% 47% 23% 1	600
	11th to 20th	Night	13% 5% 18% 40% 22% 2	705
	21st to end	Night	14% 7% 18% 39% 21% 2	800
October	1st to 10th	Night	15% 6% 18% 40% 19% 1	800
	11th to 20th	Night	14% 9% 22% 39% 16% 1	800
	21st to end	Night	13% 13% 24% 38% 12% 1	886
November	1st to 10th	Night	17% 13% 23% 37% 9%	1025
	11th to 20th	Night	11% 15% 28% 34% 11%	1050
	21st to end	Night	12% 12% 25% 42% 8%	1050
December	1st to 10th	Night	10% 14% 25% 38% 11% 2	1050
	11th to 20th	Night	12% 13% 24% 40% 10% 1	1050
	21st to end	Night	14% 12% 26% 35% 12% 1	1155

Page generated at 2026-06-28 00:10:49. Version ac07

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