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 2025-02-25.

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% 7% 2 6% 4 4 5% 4 6% 8% 45%	112309
	Night	7% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	115122
Transparency	Day	35% 5% 14% 26% 15% 5%	112037
	Night	38% 6% 15% 24% 13% 3	114985
Seeing	Day	13% 11% 25% 29% 18% 4	15917
	Night	12% 10% 23% 39% 15% 1	25527

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	8% 16% 5% 11% 6% 5% 6% 6% 7% 8% 22%	3179
	11th to 20th	Day	6% 12% 3 9% 6% 7% 8% 4 5% 7% 32%	3180
	21st to end	Day	2 7% 3 8% 5% 4 5% 4 8% 15% 38%	4331
February	1st to 10th	Day	9% 13% 4 10% 7% 6% 6% 5% 6% 7% 27%	1800
	11th to 20th	Day	6% 15% 5% 11% 6% 6% 6% 4 6% 7% 28%	1755
	21st to end	Day	4 12% 5% 10% 7% 6% 6% 4 6% 8% 32%	1449
March	1st to 10th	Day	4 7% 3 9% 5% 6% 6% 5% 8% 10% 37%	3363
	11th to 20th	Day	4 6% 3 7% 4 5% 6% 4 7% 8% 45%	3490
	21st to end	Day	11% 17% 5% 12% 6% 4 5% 4 6% 8% 22%	3825
April	1st to 10th	Day	9% 10% 3 7% 6% 5% 6% 5% 7% 9% 33%	2266
	11th to 20th	Day	13% 10% 4 7% 6% 6% 4 5% 5% 8% 33%	2403
	21st to end	Day	13% 9% 3 7% 5% 4 5% 4 7% 7% 35%	2470
May	1st to 10th	Day	19% 11% 3 6% 4 4 4 3 6% 7% 33%	4320
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	4388
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 8% 33%	4938
June	1st to 10th	Day	12% 5% 2 5% 5% 4 4 5% 7% 9% 42%	2850
	11th to 20th	Day	13% 5% 2 5% 5% 4 5% 5% 6% 8% 41%	2850
	21st to end	Day	5% 4 1 4 3 4 4 4 7% 9% 55%	2850
July	1st to 10th	Day	4 3 1 3 3 4 4 4 7% 8% 59%	4740
	11th to 20th	Day	2 1 1 2 2 2 3 3 5% 6% 73%	4740
	21st to end	Day	2 1 1 3 2 3 3 3 5% 6% 70%	4924
August	1st to 10th	Day	6% 4 1 5% 4 3 5% 4 6% 8% 55%	4099
	11th to 20th	Day	6% 3 2 4 3 3 4 4 6% 8% 57%	4028
	21st to end	Day	6% 3 1 3 2 3 3 3 5% 7% 63%	4494
September	1st to 10th	Day	8% 3 1 4 3 3 3 3 5% 8% 58%	2280
	11th to 20th	Day	9% 4 2 5% 4 3 5% 4 6% 8% 50%	2179
	21st to end	Day	11% 4 1 4 2 4 3 2 5% 7% 57%	2066
October	1st to 10th	Day	11% 5% 1 5% 3 3 3 2 4 6% 56%	3170
	11th to 20th	Day	5% 5% 1 4 3 4 4 3 6% 8% 57%	3150
	21st to end	Day	4 6% 2 6% 4 4 5% 4 7% 9% 49%	3252
November	1st to 10th	Day	10% 10% 2 7% 5% 3 5% 4 5% 7% 42%	1568
	11th to 20th	Day	4 9% 3 8% 5% 5% 7% 5% 8% 8% 38%	1600
	21st to end	Day	6% 10% 3 8% 7% 5% 5% 4 7% 8% 37%	1600
December	1st to 10th	Day	3 10% 3 8% 7% 8% 7% 6% 7% 10% 32%	2569
	11th to 20th	Day	5% 11% 4 8% 5% 4 4 4 6% 8% 40%	2645
	21st to end	Day	6% 10% 4 10% 5% 6% 5% 4 5% 7% 38%	3498
January	1st to 10th	Night	5% 12% 4 11% 6% 5% 5% 5% 8% 9% 29%	6766
	11th to 20th	Night	5% 13% 4 10% 7% 5% 6% 4 6% 6% 34%	6780
	21st to end	Night	3 8% 3 8% 5% 5% 6% 4 6% 9% 43%	6685
February	1st to 10th	Night	7% 14% 5% 12% 6% 6% 5% 5% 6% 8% 28%	2600
	11th to 20th	Night	6% 13% 5% 10% 6% 5% 5% 5% 7% 8% 30%	2532
	21st to end	Night	5% 13% 5% 10% 7% 6% 5% 4 7% 7% 32%	2099
March	1st to 10th	Night	5% 9% 4 9% 7% 7% 6% 5% 6% 8% 35%	3507
	11th to 20th	Night	4 6% 2 7% 5% 4 5% 4 7% 8% 48%	3490
	21st to end	Night	13% 16% 5% 12% 7% 6% 5% 5% 5% 6% 20%	3828
April	1st to 10th	Night	9% 10% 3 8% 6% 5% 6% 4 6% 7% 37%	1914
	11th to 20th	Night	11% 10% 4 7% 5% 4 5% 4 7% 9% 34%	1723
	21st to end	Night	13% 7% 2 6% 3 4 5% 4 7% 8% 39%	1710
May	1st to 10th	Night	20% 10% 2 6% 4 4 4 4 5% 6% 36%	2648
	11th to 20th	Night	8% 7% 2 7% 4 5% 5% 5% 8% 7% 41%	2517
	21st to end	Night	14% 6% 2 7% 5% 4 5% 4 6% 9% 38%	2642
June	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	1330
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 7% 9% 40%	1330
	21st to end	Night	5% 3 2 5% 4 4 5% 3 6% 8% 54%	1330
July	1st to 10th	Night	5% 2 1 3 3 3 4 4 6% 9% 59%	2220
	11th to 20th	Night	3 1 1 3 3 2 2 3 6% 8% 68%	2220
	21st to end	Night	3 2 1 5% 3 4 4 4 7% 8% 59%	2632
August	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	2837
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 8% 8% 50%	2811
	21st to end	Night	8% 3 1 4 3 3 4 3 6% 7% 57%	3172
September	1st to 10th	Night	10% 4 1 4 3 3 4 4 4 7% 58%	1900
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 5% 7% 51%	2000
	21st to end	Night	12% 3 1 4 3 3 4 3 6% 8% 52%	2114
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 4 7% 57%	3800
	11th to 20th	Night	7% 5% 2 5% 4 3 4 3 6% 7% 54%	3778
	21st to end	Night	6% 6% 2 7% 5% 5% 5% 4 5% 7% 49%	4258
November	1st to 10th	Night	11% 10% 3 7% 5% 4 5% 5% 6% 8% 38%	2730
	11th to 20th	Night	6% 7% 3 6% 5% 6% 6% 6% 6% 8% 41%	2800
	21st to end	Night	5% 10% 3 7% 5% 5% 5% 3 7% 8% 42%	2800
December	1st to 10th	Night	4 7% 3 8% 8% 7% 6% 5% 10% 8% 33%	4615
	11th to 20th	Night	5% 12% 4 10% 6% 5% 5% 4 6% 7% 36%	5546
	21st to end	Night	5% 9% 3 7% 4 5% 5% 4 7% 8% 43%	7458

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	56% 6% 16% 18% 3	3179
	11th to 20th	Day	50% 4 15% 18% 12% 1	3180
	21st to end	Day	34% 7% 18% 34% 7%	4331
February	1st to 10th	Day	53% 5% 14% 19% 8% 1	1800
	11th to 20th	Day	53% 6% 16% 18% 7% 1	1764
	21st to end	Day	47% 7% 15% 21% 9% 1	1449
March	1st to 10th	Day	36% 10% 16% 30% 9%	3363
	11th to 20th	Day	35% 8% 25% 26% 7%	3490
	21st to end	Day	58% 8% 16% 14% 3 1	3825
April	1st to 10th	Day	45% 7% 17% 23% 8%	2266
	11th to 20th	Day	48% 5% 14% 23% 9% 1	2406
	21st to end	Day	45% 4 15% 27% 9% 1	2470
May	1st to 10th	Day	50% 4 13% 23% 9% 1	4320
	11th to 20th	Day	35% 6% 15% 29% 14% 2	4392
	21st to end	Day	42% 8% 14% 25% 9% 1	4938
June	1st to 10th	Day	36% 6% 19% 27% 11% 1	2850
	11th to 20th	Day	39% 5% 14% 23% 16% 4	2850
	21st to end	Day	24% 5% 14% 34% 19% 3	2850
July	1st to 10th	Day	20% 5% 12% 31% 24% 9%	4666
	11th to 20th	Day	12% 3 6% 24% 32% 23%	4659
	21st to end	Day	14% 3 6% 20% 34% 24%	4769
August	1st to 10th	Day	26% 4 9% 24% 25% 12%	4084
	11th to 20th	Day	24% 4 11% 35% 21% 5%	4067
	21st to end	Day	21% 3 9% 32% 28% 8%	4493
September	1st to 10th	Day	25% 4 9% 29% 23% 11%	2280
	11th to 20th	Day	30% 5% 14% 28% 17% 5%	2179
	21st to end	Day	28% 3 11% 32% 21% 6%	2066
October	1st to 10th	Day	30% 3 8% 34% 20% 5%	3170
	11th to 20th	Day	26% 4 13% 35% 19% 3	3150
	21st to end	Day	30% 3 14% 29% 20% 3	3252
November	1st to 10th	Day	40% 7% 14% 25% 11% 3	1568
	11th to 20th	Day	40% 7% 20% 24% 8% 1	1600
	21st to end	Day	43% 4 21% 21% 9% 1	1600
December	1st to 10th	Day	45% 8% 17% 22% 7% 1	2569
	11th to 20th	Day	42% 5% 25% 24% 4 1	2644
	21st to end	Day	45% 5% 12% 20% 14% 4	3498
January	1st to 10th	Night	49% 7% 18% 21% 5%	6766
	11th to 20th	Night	47% 6% 18% 16% 13% 1	6780
	21st to end	Night	37% 8% 17% 29% 7% 2	6685
February	1st to 10th	Night	53% 6% 15% 17% 7% 1	2600
	11th to 20th	Night	49% 6% 16% 20% 8% 1	2533
	21st to end	Night	49% 5% 14% 21% 10% 1	2099
March	1st to 10th	Night	40% 12% 14% 23% 11%	3507
	11th to 20th	Night	31% 6% 25% 30% 7%	3490
	21st to end	Night	62% 8% 14% 13% 3	3828
April	1st to 10th	Night	43% 7% 16% 24% 10% 1	1914
	11th to 20th	Night	44% 6% 14% 24% 10% 1	1723
	21st to end	Night	40% 5% 16% 27% 12% 1	1710
May	1st to 10th	Night	47% 6% 10% 26% 10% 1	2648
	11th to 20th	Night	36% 7% 14% 30% 12% 1	2518
	21st to end	Night	40% 8% 14% 25% 11% 2	2642
June	1st to 10th	Night	39% 7% 17% 28% 8% 1	1330
	11th to 20th	Night	39% 3 15% 24% 15% 3	1330
	21st to end	Night	26% 6% 15% 29% 21% 3	1330
July	1st to 10th	Night	21% 4 12% 30% 24% 8%	2185
	11th to 20th	Night	14% 3 6% 28% 34% 15%	2178
	21st to end	Night	19% 4 7% 19% 32% 20%	2550
August	1st to 10th	Night	31% 5% 10% 22% 20% 11%	2829
	11th to 20th	Night	29% 5% 11% 29% 20% 5%	2838
	21st to end	Night	26% 4 9% 33% 23% 6%	3172
September	1st to 10th	Night	27% 4 8% 24% 24% 14%	1900
	11th to 20th	Night	34% 5% 10% 26% 19% 6%	2000
	21st to end	Night	29% 4 12% 28% 21% 5%	2114
October	1st to 10th	Night	28% 4 8% 33% 23% 4	3800
	11th to 20th	Night	30% 3 12% 32% 21% 2	3778
	21st to end	Night	33% 4 14% 24% 21% 3	4258
November	1st to 10th	Night	42% 6% 15% 23% 11% 3	2730
	11th to 20th	Night	38% 6% 18% 26% 11% 1	2800
	21st to end	Night	39% 5% 20% 24% 10% 2	2800
December	1st to 10th	Night	43% 8% 20% 20% 8% 2	4615
	11th to 20th	Night	46% 6% 22% 20% 5%	5547
	21st to end	Night	38% 4 12% 29% 14% 3	7458

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	13% 15% 17% 27% 21% 10%	200
	11th to 20th	Day	16% 11% 19% 31% 18% 6%	200
	21st to end	Day	10% 14% 25% 30% 20% 2	306
February	1st to 10th	Day	21% 13% 24% 32% 8% 2	300
	11th to 20th	Day	17% 14% 21% 30% 14% 3	300
	21st to end	Day	11% 16% 31% 28% 12% 3	237
March	1st to 10th	Day	14% 11% 27% 32% 14% 1	434
	11th to 20th	Day	11% 14% 28% 28% 16% 3	460
	21st to end	Day	17% 12% 26% 26% 16% 3	506
April	1st to 10th	Day	18% 12% 30% 29% 11% 1	460
	11th to 20th	Day	22% 17% 28% 23% 9% 2	452
	21st to end	Day	21% 11% 27% 22% 17% 2	460
May	1st to 10th	Day	17% 12% 23% 26% 17% 5%	582
	11th to 20th	Day	16% 10% 23% 28% 19% 5%	675
	21st to end	Day	19% 9% 19% 28% 19% 6%	734
June	1st to 10th	Day	14% 13% 23% 24% 21% 6%	680
	11th to 20th	Day	17% 16% 27% 24% 14% 2	680
	21st to end	Day	7% 11% 25% 29% 23% 5%	680
July	1st to 10th	Day	5% 7% 23% 35% 25% 5%	671
	11th to 20th	Day	4 8% 31% 35% 19% 3	668
	21st to end	Day	4 5% 24% 37% 24% 6%	739
August	1st to 10th	Day	6% 8% 30% 31% 19% 6%	494
	11th to 20th	Day	9% 6% 26% 29% 21% 10%	477
	21st to end	Day	8% 10% 23% 31% 21% 6%	539
September	1st to 10th	Day	10% 7% 28% 32% 19% 4	490
	11th to 20th	Day	13% 9% 25% 28% 18% 7%	490
	21st to end	Day	17% 11% 25% 21% 18% 8%	466
October	1st to 10th	Day	18% 6% 21% 26% 22% 7%	300
	11th to 20th	Day	14% 10% 23% 31% 16% 5%	300
	21st to end	Day	15% 17% 25% 29% 12% 2	330
November	1st to 10th	Day	22% 10% 24% 26% 15% 3	300
	11th to 20th	Day	10% 17% 24% 32% 15% 2	300
	21st to end	Day	16% 13% 26% 31% 14% 1	300
December	1st to 10th	Day	11% 15% 28% 30% 14% 2	287
	11th to 20th	Day	19% 14% 20% 29% 16% 4	200
	21st to end	Day	20% 8% 19% 30% 18% 5%	220
January	1st to 10th	Night	11% 13% 26% 36% 12% 2	1000
	11th to 20th	Night	12% 15% 27% 34% 11% 2	1000
	21st to end	Night	9% 16% 27% 37% 10% 1	925
February	1st to 10th	Night	14% 12% 23% 38% 12% 1	800
	11th to 20th	Night	13% 12% 26% 38% 10% 1	784
	21st to end	Night	11% 12% 21% 40% 14% 2	646
March	1st to 10th	Night	11% 9% 24% 42% 13% 1	760
	11th to 20th	Night	10% 9% 26% 39% 15% 1	760
	21st to end	Night	16% 10% 25% 35% 13% 1	836
April	1st to 10th	Night	14% 11% 25% 39% 11%	584
	11th to 20th	Night	16% 9% 24% 37% 13% 1	563
	21st to end	Night	17% 10% 24% 35% 13% 1	570
May	1st to 10th	Night	18% 10% 18% 32% 20% 2	570
	11th to 20th	Night	13% 9% 19% 36% 22% 1	561
	21st to end	Night	22% 7% 17% 28% 23% 2	549
June	1st to 10th	Night	14% 8% 21% 34% 21% 2	380
	11th to 20th	Night	15% 11% 24% 36% 12% 1	380
	21st to end	Night	6% 8% 18% 46% 22% 1	380
July	1st to 10th	Night	8% 2 16% 49% 25%	370
	11th to 20th	Night	5% 4 22% 53% 15% 1	368
	21st to end	Night	5% 4 21% 48% 20% 1	547
August	1st to 10th	Night	7% 3 22% 47% 20% 1	567
	11th to 20th	Night	8% 3 20% 42% 25% 2	564
	21st to end	Night	11% 6% 17% 43% 22% 1	627
September	1st to 10th	Night	11% 4 15% 46% 23% 1	570
	11th to 20th	Night	13% 5% 19% 40% 21% 2	670
	21st to end	Night	14% 7% 18% 38% 21% 2	760
October	1st to 10th	Night	15% 6% 18% 39% 19% 1	760
	11th to 20th	Night	14% 8% 21% 40% 16% 1	760
	21st to end	Night	13% 13% 23% 37% 12% 1	841
November	1st to 10th	Night	17% 13% 22% 38% 9% 1	975
	11th to 20th	Night	11% 15% 28% 35% 11%	1000
	21st to end	Night	12% 12% 25% 43% 8%	1000
December	1st to 10th	Night	10% 14% 25% 37% 12% 2	1000
	11th to 20th	Night	12% 13% 25% 40% 11% 1	1000
	21st to end	Night	15% 12% 25% 35% 13% 1	1100

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