Astro Hut
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro Hut clear sky chart from 2005-11-06 to 2025-05-26.

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	13% 12% 4 9% 5% 4 4 3 5% 7% 33%	113772
	Night	15% 14% 5% 11% 5% 4 4 3 4 5% 29%	115686
Transparency	Day	50% 5% 11% 21% 12% 1	117345
	Night	56% 5% 10% 18% 11% 1	120243
Seeing	Day	20% 17% 31% 20% 9% 3	16495
	Night	22% 14% 27% 25% 11% 1	26456

By Month

Percentage Cloud Cover SE (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	12% 23% 12% 23% 6% 4 3 2 3 3 10%	3617
	11th to 20th	Day	11% 18% 8% 24% 14% 11% 4 2 2 2 5%	3620
	21st to end	Day	17% 19% 8% 18% 8% 4 6% 2 3 3 12%	3982
February	1st to 10th	Day	13% 18% 6% 16% 12% 6% 4 3 3 3 16%	1861
	11th to 20th	Day	14% 15% 6% 18% 9% 5% 3 3 4 3 20%	1948
	21st to end	Day	16% 15% 6% 14% 6% 5% 4 4 4 6% 21%	1650
March	1st to 10th	Day	14% 12% 5% 11% 5% 4 4 3 5% 6% 31%	4480
	11th to 20th	Day	10% 10% 5% 9% 5% 4 4 3 6% 8% 36%	4791
	21st to end	Day	18% 15% 7% 10% 5% 4 4 3 4 6% 25%	5737
April	1st to 10th	Day	18% 13% 3 7% 5% 5% 5% 4 5% 6% 29%	2400
	11th to 20th	Day	16% 12% 2 6% 4 3 5% 4 4 6% 38%	2399
	21st to end	Day	17% 9% 3 7% 4 4 4 4 6% 7% 35%	2600
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 38%	4212
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 9% 42%	4239
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 46%	4895
June	1st to 10th	Day	10% 6% 2 6% 3 5% 4 4 6% 8% 47%	2660
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 8% 48%	2660
	21st to end	Day	10% 7% 2 6% 4 4 3 4 6% 7% 45%	2660
July	1st to 10th	Day	12% 7% 3 6% 4 4 5% 3 6% 10% 40%	4440
	11th to 20th	Day	7% 4 1 4 4 4 4 4 6% 9% 53%	4440
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 6% 9% 48%	4813
August	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 6% 9% 46%	4172
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 6% 7% 50%	4017
	21st to end	Day	7% 6% 3 6% 4 4 4 4 6% 8% 49%	4180
September	1st to 10th	Day	10% 7% 2 5% 4 4 5% 4 5% 8% 45%	2269
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 6% 8% 49%	2090
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 3 6% 8% 40%	2090
October	1st to 10th	Day	16% 7% 2 6% 5% 4 4 2 3 5% 44%	2090
	11th to 20th	Day	23% 11% 3 8% 5% 4 5% 3 6% 7% 25%	1853
	21st to end	Day	26% 12% 3 8% 6% 5% 5% 4 5% 5% 22%	1881
November	1st to 10th	Day	14% 15% 3 6% 5% 4 4 4 4 5% 35%	1762
	11th to 20th	Day	16% 25% 4 10% 5% 4 5% 3 3 5% 20%	1800
	21st to end	Day	22% 21% 7% 10% 4 4 3 3 3 5% 19%	1800
December	1st to 10th	Day	12% 28% 12% 15% 4 4 2 2 3 3 14%	2674
	11th to 20th	Day	16% 27% 13% 14% 7% 4 3 3 3 3 9%	3008
	21st to end	Day	23% 24% 9% 13% 5% 3 3 2 3 4 10%	3982
January	1st to 10th	Night	12% 24% 12% 23% 6% 4 2 2 3 3 9%	6348
	11th to 20th	Night	13% 21% 12% 22% 10% 9% 3 2 2 2 6%	6360
	21st to end	Night	18% 20% 9% 17% 5% 6% 5% 2 3 3 13%	6996
February	1st to 10th	Night	13% 19% 6% 17% 9% 4 4 2 4 3 18%	2534
	11th to 20th	Night	14% 17% 8% 15% 8% 5% 3 2 4 3 20%	2338
	21st to end	Night	18% 15% 5% 13% 7% 4 3 2 4 5% 24%	1980
March	1st to 10th	Night	15% 11% 4 10% 4 4 4 3 5% 6% 34%	5373
	11th to 20th	Night	10% 9% 3 8% 5% 4 4 3 5% 7% 42%	5169
	21st to end	Night	18% 13% 5% 9% 5% 4 4 3 4 5% 29%	5220
April	1st to 10th	Night	15% 11% 3 7% 4 5% 4 5% 5% 7% 34%	2000
	11th to 20th	Night	19% 10% 3 6% 4 3 4 4 5% 6% 37%	1948
	21st to end	Night	16% 9% 2 7% 4 3 5% 3 6% 8% 38%	1800
May	1st to 10th	Night	14% 9% 3 9% 5% 4 4 3 6% 7% 34%	2903
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 4 7% 41%	2872
	21st to end	Night	15% 7% 3 6% 4 4 4 4 5% 7% 42%	2788
June	1st to 10th	Night	10% 6% 1 4 5% 3 5% 4 7% 9% 46%	1520
	11th to 20th	Night	9% 6% 2 5% 6% 5% 4 4 6% 8% 44%	1520
	21st to end	Night	10% 5% 2 6% 5% 5% 4 4 6% 9% 44%	1520
July	1st to 10th	Night	9% 6% 2 8% 5% 5% 5% 4 7% 9% 41%	2530
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 8% 54%	2530
	21st to end	Night	6% 5% 2 7% 4 5% 5% 5% 6% 10% 45%	2750
August	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	2759
	11th to 20th	Night	10% 6% 2 5% 3 4 4 3 5% 7% 50%	2809
	21st to end	Night	8% 6% 3 8% 4 5% 5% 4 5% 7% 46%	3476
September	1st to 10th	Night	10% 5% 2 5% 5% 4 4 3 4 7% 50%	1900
	11th to 20th	Night	11% 4 1 4 3 4 4 4 5% 7% 52%	2081
	21st to end	Night	13% 7% 3 9% 5% 4 3 3 5% 7% 41%	2090
October	1st to 10th	Night	17% 6% 2 6% 4 4 3 3 5% 6% 45%	2090
	11th to 20th	Night	22% 12% 3 7% 4 4 4 2 4 5% 34%	2323
	21st to end	Night	26% 12% 4 6% 4 3 3 2 4 5% 30%	2717
November	1st to 10th	Night	17% 13% 4 8% 4 4 3 3 4 5% 35%	2536
	11th to 20th	Night	18% 20% 6% 10% 4 4 4 3 4 5% 22%	2600
	21st to end	Night	18% 22% 6% 10% 5% 3 3 2 4 5% 21%	2600
December	1st to 10th	Night	14% 26% 9% 16% 5% 4 4 2 4 3 12%	4511
	11th to 20th	Night	17% 27% 10% 15% 6% 3 3 2 3 4 9%	5199
	21st to end	Night	24% 23% 7% 12% 5% 4 4 3 4 4 10%	6996

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	80% 5% 6% 8% 1	3618
	11th to 20th	Day	87% 3 6% 3 1	3620
	21st to end	Day	78% 4 8% 8% 2	3982
February	1st to 10th	Day	74% 5% 7% 10% 3	1861
	11th to 20th	Day	69% 4 9% 12% 6% 1	1958
	21st to end	Day	64% 5% 11% 13% 5% 1	1650
March	1st to 10th	Day	53% 5% 14% 19% 9% 1	4480
	11th to 20th	Day	45% 5% 16% 25% 8% 1	4791
	21st to end	Day	61% 5% 9% 17% 8%	5739
April	1st to 10th	Day	54% 5% 10% 19% 11%	2400
	11th to 20th	Day	46% 5% 9% 23% 13% 3	2399
	21st to end	Day	47% 5% 13% 20% 13% 2	2600
May	1st to 10th	Day	43% 7% 11% 24% 14% 1	4250
	11th to 20th	Day	39% 7% 12% 25% 15% 1	4243
	21st to end	Day	34% 6% 12% 25% 23% 1	4908
June	1st to 10th	Day	35% 5% 11% 27% 19% 2	2660
	11th to 20th	Day	32% 7% 12% 27% 20% 2	2660
	21st to end	Day	36% 7% 14% 23% 18% 3	2660
July	1st to 10th	Day	40% 6% 14% 22% 15% 3	4371
	11th to 20th	Day	27% 6% 14% 33% 18% 2	4365
	21st to end	Day	32% 8% 16% 21% 21% 2	4662
August	1st to 10th	Day	34% 7% 12% 31% 15% 1	4157
	11th to 20th	Day	32% 7% 13% 32% 15% 1	4056
	21st to end	Day	32% 6% 15% 28% 18% 2	4179
September	1st to 10th	Day	36% 6% 13% 26% 15% 4	2269
	11th to 20th	Day	33% 4 12% 32% 17% 2	2090
	21st to end	Day	41% 6% 11% 25% 16% 1	2090
October	1st to 10th	Day	31% 3 8% 40% 18% 1	3480
	11th to 20th	Day	43% 5% 11% 24% 17%	3043
	21st to end	Day	52% 5% 12% 22% 8% 1	3081
November	1st to 10th	Day	51% 6% 13% 19% 12% 1	1762
	11th to 20th	Day	68% 4 8% 15% 5% 1	1800
	21st to end	Day	70% 5% 9% 12% 4	1800
December	1st to 10th	Day	76% 5% 10% 7% 2	2674
	11th to 20th	Day	80% 6% 7% 6% 1	3005
	21st to end	Day	79% 4 6% 9% 2	3982
January	1st to 10th	Night	81% 4 6% 7% 2	6348
	11th to 20th	Night	88% 2 5% 3 2	6360
	21st to end	Night	79% 3 10% 6% 2	6996
February	1st to 10th	Night	71% 4 8% 10% 6%	2534
	11th to 20th	Night	70% 3 7% 14% 6%	2339
	21st to end	Night	63% 4 10% 15% 6% 1	1980
March	1st to 10th	Night	51% 5% 13% 23% 7% 1	5373
	11th to 20th	Night	42% 5% 13% 29% 10% 2	5169
	21st to end	Night	57% 5% 9% 17% 12% 1	5220
April	1st to 10th	Night	49% 7% 11% 19% 14% 1	2000
	11th to 20th	Night	49% 5% 8% 18% 18% 3	1948
	21st to end	Night	45% 7% 11% 19% 15% 3	1800
May	1st to 10th	Night	47% 6% 11% 22% 13% 1	2930
	11th to 20th	Night	44% 5% 9% 23% 17% 3	2873
	21st to end	Night	41% 6% 9% 19% 23% 2	2796
June	1st to 10th	Night	34% 6% 12% 29% 17% 3	1520
	11th to 20th	Night	36% 6% 12% 24% 19% 3	1520
	21st to end	Night	37% 7% 15% 22% 16% 3	1520
July	1st to 10th	Night	39% 6% 13% 23% 12% 7%	2490
	11th to 20th	Night	27% 7% 12% 25% 26% 3	2482
	21st to end	Night	33% 8% 13% 17% 26% 2	2662
August	1st to 10th	Night	33% 8% 13% 28% 18% 1	2750
	11th to 20th	Night	33% 6% 9% 31% 19% 1	2836
	21st to end	Night	36% 6% 9% 27% 20% 2	3476
September	1st to 10th	Night	34% 5% 12% 26% 18% 4	1900
	11th to 20th	Night	32% 5% 8% 31% 21% 2	2081
	21st to end	Night	42% 6% 10% 24% 16% 1	2090
October	1st to 10th	Night	31% 4 8% 34% 22% 1	3480
	11th to 20th	Night	42% 4 10% 23% 20%	3859
	21st to end	Night	47% 4 9% 26% 15%	4467
November	1st to 10th	Night	53% 4 9% 20% 13% 1	2536
	11th to 20th	Night	65% 4 10% 14% 6% 1	2600
	21st to end	Night	66% 4 12% 15% 3	2600
December	1st to 10th	Night	77% 5% 10% 7% 2	4511
	11th to 20th	Night	80% 4 8% 6% 1	5201
	21st to end	Night	76% 6% 9% 7% 2	6996

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	23% 26% 32% 15% 4 1	300
	11th to 20th	Day	26% 22% 39% 10% 3	300
	21st to end	Day	30% 24% 33% 12% 1	330
February	1st to 10th	Day	29% 28% 26% 13% 3 1	300
	11th to 20th	Day	27% 33% 30% 9% 1	300
	21st to end	Day	30% 36% 26% 7% 1	249
March	1st to 10th	Day	23% 33% 34% 9% 2	300
	11th to 20th	Day	26% 24% 30% 15% 4	415
	21st to end	Day	32% 20% 29% 14% 4 1	550
April	1st to 10th	Day	30% 23% 30% 13% 4	500
	11th to 20th	Day	29% 25% 27% 13% 5% 2	492
	21st to end	Day	27% 20% 30% 15% 8% 1	500
May	1st to 10th	Day	23% 21% 33% 16% 5% 2	503
	11th to 20th	Day	20% 15% 32% 21% 8% 4	542
	21st to end	Day	14% 14% 35% 23% 11% 4	760
June	1st to 10th	Day	13% 13% 40% 23% 9% 3	680
	11th to 20th	Day	14% 12% 35% 28% 9% 2	680
	21st to end	Day	15% 12% 36% 24% 11% 2	680
July	1st to 10th	Day	9% 9% 30% 31% 15% 7%	671
	11th to 20th	Day	10% 9% 33% 31% 11% 6%	668
	21st to end	Day	8% 6% 33% 33% 14% 6%	739
August	1st to 10th	Day	11% 6% 26% 26% 21% 10%	529
	11th to 20th	Day	11% 7% 26% 30% 18% 7%	479
	21st to end	Day	13% 5% 27% 30% 18% 6%	539
September	1st to 10th	Day	15% 9% 26% 27% 17% 6%	490
	11th to 20th	Day	9% 7% 24% 28% 25% 7%	490
	21st to end	Day	18% 15% 23% 23% 15% 5%	490
October	1st to 10th	Day	19% 16% 25% 25% 13% 2	490
	11th to 20th	Day	29% 21% 28% 18% 4	369
	21st to end	Day	36% 20% 29% 12% 2 1	330
November	1st to 10th	Day	27% 21% 31% 16% 4 1	300
	11th to 20th	Day	32% 26% 29% 11% 3	300
	21st to end	Day	35% 22% 36% 7% 1	300
December	1st to 10th	Day	30% 30% 24% 13% 3	300
	11th to 20th	Day	26% 32% 30% 10% 1 1	300
	21st to end	Day	29% 27% 32% 8% 2 2	330
January	1st to 10th	Night	26% 22% 30% 17% 4	1000
	11th to 20th	Night	29% 19% 32% 16% 3 1	1000
	21st to end	Night	26% 18% 31% 22% 3	1100
February	1st to 10th	Night	24% 19% 33% 19% 4	847
	11th to 20th	Night	25% 21% 31% 20% 4	783
	21st to end	Night	27% 22% 32% 16% 2	660
March	1st to 10th	Night	23% 19% 28% 23% 7% 1	800
	11th to 20th	Night	20% 19% 33% 22% 6% 1	800
	21st to end	Night	24% 15% 29% 25% 6% 1	775
April	1st to 10th	Night	21% 17% 31% 24% 6% 1	600
	11th to 20th	Night	27% 12% 30% 23% 6% 1	593
	21st to end	Night	20% 13% 32% 26% 8% 1	600
May	1st to 10th	Night	21% 11% 33% 26% 8% 1	600
	11th to 20th	Night	20% 9% 28% 31% 12%	591
	21st to end	Night	15% 5% 28% 33% 17% 1	634
June	1st to 10th	Night	12% 6% 27% 37% 17% 1	570
	11th to 20th	Night	12% 5% 32% 34% 16% 1	570
	21st to end	Night	11% 7% 24% 35% 21% 2	570
July	1st to 10th	Night	9% 3 19% 38% 29% 3	555
	11th to 20th	Night	10% 4 21% 35% 26% 3	552
	21st to end	Night	8% 3 18% 42% 26% 2	624
August	1st to 10th	Night	8% 3 17% 42% 27% 4	567
	11th to 20th	Night	12% 4 15% 39% 26% 5%	564
	21st to end	Night	10% 4 20% 36% 26% 4	627
September	1st to 10th	Night	11% 6% 21% 36% 22% 4	570
	11th to 20th	Night	13% 5% 19% 34% 26% 3	751
	21st to end	Night	17% 10% 24% 26% 18% 5%	760
October	1st to 10th	Night	20% 9% 23% 29% 18% 2	760
	11th to 20th	Night	29% 15% 26% 23% 6% 1	760
	21st to end	Night	35% 14% 26% 19% 5%	836
November	1st to 10th	Night	26% 16% 29% 20% 9% 1	780
	11th to 20th	Night	31% 21% 28% 16% 4 1	800
	21st to end	Night	33% 15% 31% 18% 3	800
December	1st to 10th	Night	31% 22% 28% 15% 4	957
	11th to 20th	Night	31% 20% 31% 15% 3	1000
	21st to end	Night	37% 18% 30% 14% 2	1100

Page generated at 2025-05-23 09:21:03. Version ac07

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