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 2026-09-29.

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% 11% 4 9% 5% 4 4 3 5% 7% 35%	148997
	Night	14% 13% 5% 11% 5% 4 4 3 4 6% 30%	142720
Transparency	Day	48% 6% 11% 22% 12% 1	152523
	Night	55% 5% 9% 18% 12% 1	147258
Seeing	Day	20% 17% 31% 21% 9% 3	18450
	Night	22% 14% 27% 25% 10% 1	28205

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	14% 23% 12% 23% 6% 3 3 2 3 3 8%	4691
	11th to 20th	Day	9% 17% 9% 26% 15% 11% 3 1 2 2 6%	4710
	21st to end	Day	14% 17% 6% 18% 13% 5% 7% 2 3 3 12%	5156
February	1st to 10th	Day	13% 17% 6% 16% 12% 6% 4 3 4 3 17%	1955
	11th to 20th	Day	13% 15% 7% 18% 9% 5% 3 3 4 3 21%	2048
	21st to end	Day	15% 14% 6% 14% 6% 5% 4 4 4 6% 21%	1730
March	1st to 10th	Day	14% 12% 5% 11% 5% 4 4 3 5% 6% 31%	4580
	11th to 20th	Day	10% 10% 5% 9% 5% 4 4 3 6% 8% 35%	4897
	21st to end	Day	18% 15% 7% 10% 5% 4 4 3 4 6% 25%	5864
April	1st to 10th	Day	18% 13% 3 7% 5% 5% 5% 4 5% 7% 29%	2520
	11th to 20th	Day	16% 12% 3 7% 4 3 5% 4 4 6% 38%	2520
	21st to end	Day	16% 9% 3 7% 4 4 4 4 6% 7% 35%	2730
May	1st to 10th	Day	15% 8% 2 7% 4 4 4 3 6% 8% 40%	7616
	11th to 20th	Day	12% 8% 2 7% 5% 4 4 4 6% 8% 40%	7681
	21st to end	Day	14% 6% 2 5% 3 4 4 3 6% 8% 46%	9047
June	1st to 10th	Day	9% 6% 2 6% 3 5% 4 4 6% 8% 47%	2940
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 9% 47%	2940
	21st to end	Day	10% 7% 2 6% 4 4 4 4 6% 7% 46%	2940
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 43%	8280
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 10% 52%	8280
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 9% 49%	8990
August	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 9% 42%	6112
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 4 5% 7% 42%	5909
	21st to end	Day	7% 6% 3 6% 4 4 4 4 6% 9% 48%	6080
September	1st to 10th	Day	10% 7% 2 5% 4 4 5% 4 5% 8% 45%	2507
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 8% 48%	2310
	21st to end	Day	12% 7% 3 7% 5% 5% 4 3 6% 8% 41%	2288
October	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 2 3 5% 45%	2200
	11th to 20th	Day	22% 11% 3 8% 5% 4 5% 3 6% 7% 26%	1950
	21st to end	Day	25% 11% 3 8% 6% 5% 5% 4 5% 5% 24%	1980
November	1st to 10th	Day	14% 15% 3 6% 5% 4 4 4 5% 6% 35%	1852
	11th to 20th	Day	16% 24% 4 9% 6% 4 5% 3 3 5% 21%	1890
	21st to end	Day	22% 21% 7% 10% 5% 3 3 2 3 4 19%	1890
December	1st to 10th	Day	13% 28% 12% 15% 4 4 3 2 3 3 14%	2756
	11th to 20th	Day	16% 27% 12% 14% 7% 3 3 3 3 3 9%	3088
	21st to end	Day	23% 24% 9% 13% 5% 3 3 2 3 4 11%	4070
January	1st to 10th	Night	12% 24% 12% 23% 6% 4 2 2 2 3 10%	8232
	11th to 20th	Night	11% 20% 12% 24% 9% 9% 3 2 2 2 6%	8280
	21st to end	Night	14% 18% 9% 19% 10% 7% 5% 2 3 3 11%	9058
February	1st to 10th	Night	13% 19% 6% 17% 9% 4 4 2 4 4 19%	2660
	11th to 20th	Night	14% 16% 8% 15% 8% 5% 3 2 4 4 21%	2458
	21st to end	Night	17% 15% 5% 13% 7% 4 4 2 4 5% 25%	2076
March	1st to 10th	Night	15% 11% 4 10% 4 4 4 3 5% 6% 34%	5493
	11th to 20th	Night	11% 9% 3 8% 5% 4 4 3 4 7% 42%	5283
	21st to end	Night	18% 13% 5% 9% 5% 4 4 3 4 5% 29%	5336
April	1st to 10th	Night	15% 11% 3 7% 4 5% 4 5% 5% 7% 33%	2100
	11th to 20th	Night	19% 10% 3 6% 4 3 4 4 5% 6% 36%	2047
	21st to end	Night	15% 8% 2 7% 4 3 5% 3 6% 8% 38%	1890
May	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 5% 7% 37%	5237
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 5% 8% 44%	5179
	21st to end	Night	14% 7% 3 7% 4 3 4 3 5% 7% 43%	5139
June	1st to 10th	Night	10% 5% 1 5% 5% 4 5% 4 7% 9% 45%	1680
	11th to 20th	Night	10% 6% 3 5% 6% 5% 4 4 6% 8% 43%	1680
	21st to end	Night	10% 5% 2 6% 5% 5% 4 4 6% 9% 44%	1680
July	1st to 10th	Night	10% 6% 2 6% 4 4 5% 3 6% 8% 45%	4708
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 9% 53%	4707
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 9% 46%	5117
August	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 9% 44%	4041
	11th to 20th	Night	12% 8% 3 7% 4 5% 4 3 5% 6% 42%	4134
	21st to end	Night	7% 6% 2 7% 4 4 5% 4 6% 8% 46%	5046
September	1st to 10th	Night	10% 5% 2 5% 5% 5% 4 3 5% 7% 49%	2100
	11th to 20th	Night	11% 4 2 5% 3 4 4 4 5% 6% 52%	2301
	21st to end	Night	12% 6% 3 9% 5% 4 4 3 5% 7% 42%	2289
October	1st to 10th	Night	16% 6% 2 6% 4 4 3 3 5% 6% 46%	2200
	11th to 20th	Night	21% 11% 3 8% 4 4 4 2 4 5% 34%	2445
	21st to end	Night	25% 12% 4 6% 4 3 3 2 4 5% 31%	2860
November	1st to 10th	Night	17% 13% 5% 8% 4 4 4 3 4 5% 34%	2666
	11th to 20th	Night	17% 20% 6% 10% 4 4 4 3 4 5% 23%	2730
	21st to end	Night	18% 22% 6% 9% 5% 3 3 2 4 5% 21%	2730
December	1st to 10th	Night	14% 27% 10% 16% 5% 4 3 2 4 3 13%	4649
	11th to 20th	Night	17% 27% 10% 15% 6% 3 3 2 3 4 9%	5339
	21st to end	Night	23% 23% 7% 12% 5% 4 4 3 4 4 11%	7150

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	83% 5% 5% 7% 1	4692
	11th to 20th	Day	87% 3 6% 3 1	4710
	21st to end	Day	79% 4 7% 8% 2	5156
February	1st to 10th	Day	73% 5% 8% 10% 4	1955
	11th to 20th	Day	68% 4 9% 12% 6% 1	2058
	21st to end	Day	65% 5% 11% 13% 5% 1	1730
March	1st to 10th	Day	53% 5% 14% 19% 8% 1	4580
	11th to 20th	Day	46% 5% 16% 24% 8% 1	4897
	21st to end	Day	61% 5% 9% 16% 8%	5866
April	1st to 10th	Day	55% 5% 10% 19% 11%	2520
	11th to 20th	Day	47% 5% 9% 23% 13% 3	2520
	21st to end	Day	46% 5% 13% 21% 13% 2	2730
May	1st to 10th	Day	42% 6% 10% 26% 15% 1	7651
	11th to 20th	Day	41% 7% 12% 24% 15% 1	7685
	21st to end	Day	36% 5% 10% 24% 20% 4	9061
June	1st to 10th	Day	35% 5% 11% 27% 20% 2	2940
	11th to 20th	Day	33% 7% 13% 26% 20% 2	2940
	21st to end	Day	36% 7% 14% 23% 18% 2	2940
July	1st to 10th	Day	35% 7% 14% 26% 16% 2	8211
	11th to 20th	Day	27% 6% 12% 33% 21% 1	8205
	21st to end	Day	31% 6% 13% 26% 22% 1	8794
August	1st to 10th	Day	39% 7% 11% 28% 15% 1	6097
	11th to 20th	Day	39% 8% 13% 28% 11% 1	5948
	21st to end	Day	32% 6% 16% 31% 14% 1	6079
September	1st to 10th	Day	36% 6% 13% 27% 14% 3	2507
	11th to 20th	Day	34% 4 12% 31% 17% 2	2310
	21st to end	Day	40% 6% 11% 26% 16% 1	2288
October	1st to 10th	Day	31% 3 8% 40% 18% 1	3590
	11th to 20th	Day	43% 5% 10% 24% 17%	3140
	21st to end	Day	52% 5% 12% 22% 9% 1	3180
November	1st to 10th	Day	50% 6% 13% 19% 12% 1	1852
	11th to 20th	Day	67% 4 8% 15% 5% 1	1890
	21st to end	Day	70% 5% 9% 12% 3	1890
December	1st to 10th	Day	76% 5% 10% 7% 2	2756
	11th to 20th	Day	80% 6% 7% 6% 1	3085
	21st to end	Day	79% 4 6% 9% 2	4070
January	1st to 10th	Night	80% 3 6% 7% 3	8233
	11th to 20th	Night	88% 2 5% 3 2	8280
	21st to end	Night	81% 3 8% 6% 2	9058
February	1st to 10th	Night	70% 4 8% 11% 6%	2660
	11th to 20th	Night	68% 3 7% 15% 6%	2459
	21st to end	Night	63% 5% 10% 16% 6% 1	2076
March	1st to 10th	Night	51% 5% 13% 24% 7% 1	5493
	11th to 20th	Night	43% 5% 12% 28% 10% 2	5283
	21st to end	Night	57% 5% 9% 17% 11% 1	5336
April	1st to 10th	Night	49% 6% 11% 19% 13% 1	2100
	11th to 20th	Night	49% 5% 8% 18% 18% 3	2047
	21st to end	Night	44% 7% 12% 20% 14% 3	1890
May	1st to 10th	Night	46% 5% 10% 22% 14% 3	5264
	11th to 20th	Night	39% 6% 11% 22% 20% 2	5180
	21st to end	Night	40% 5% 8% 20% 22% 4	5149
June	1st to 10th	Night	34% 6% 11% 28% 18% 3	1680
	11th to 20th	Night	38% 7% 12% 22% 19% 3	1680
	21st to end	Night	35% 7% 16% 23% 16% 2	1680
July	1st to 10th	Night	36% 6% 12% 28% 15% 4	4670
	11th to 20th	Night	27% 6% 11% 27% 26% 3	4662
	21st to end	Night	34% 7% 11% 23% 23% 2	5002
August	1st to 10th	Night	36% 6% 12% 26% 19% 1	4032
	11th to 20th	Night	41% 7% 11% 27% 14% 1	4161
	21st to end	Night	34% 6% 10% 31% 17% 1	5046
September	1st to 10th	Night	34% 6% 12% 27% 17% 4	2100
	11th to 20th	Night	32% 5% 8% 30% 22% 2	2301
	21st to end	Night	42% 6% 10% 24% 16% 2	2289
October	1st to 10th	Night	30% 4 8% 34% 23% 1	3590
	11th to 20th	Night	42% 4 10% 23% 20% 1	3981
	21st to end	Night	46% 4 9% 26% 15%	4610
November	1st to 10th	Night	53% 5% 9% 20% 13% 1	2666
	11th to 20th	Night	64% 4 10% 15% 6% 1	2730
	21st to end	Night	67% 4 12% 15% 3	2730
December	1st to 10th	Night	77% 5% 9% 7% 2	4649
	11th to 20th	Night	81% 4 7% 6% 1	5341
	21st to end	Night	76% 5% 9% 7% 2	7150

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	25% 26% 31% 14% 4 1	330
	11th to 20th	Day	27% 23% 38% 9% 3	330
	21st to end	Day	31% 23% 33% 12% 1	363
February	1st to 10th	Day	27% 28% 28% 13% 3 1	330
	11th to 20th	Day	27% 31% 31% 11% 1	330
	21st to end	Day	29% 35% 28% 6% 1	273
March	1st to 10th	Day	23% 32% 34% 9% 2	330
	11th to 20th	Day	27% 25% 29% 14% 4	451
	21st to end	Day	32% 20% 30% 13% 4 1	594
April	1st to 10th	Day	30% 23% 30% 13% 4	540
	11th to 20th	Day	29% 25% 27% 13% 5% 2	532
	21st to end	Day	26% 20% 30% 16% 8% 1	540
May	1st to 10th	Day	22% 22% 33% 15% 5% 2	543
	11th to 20th	Day	20% 17% 32% 19% 8% 3	583
	21st to end	Day	14% 14% 35% 23% 10% 3	844
June	1st to 10th	Day	12% 12% 40% 24% 9% 3	780
	11th to 20th	Day	15% 13% 35% 27% 9% 2	780
	21st to end	Day	14% 12% 36% 25% 11% 2	780
July	1st to 10th	Day	9% 8% 29% 32% 15% 6%	771
	11th to 20th	Day	10% 10% 32% 31% 11% 6%	768
	21st to end	Day	8% 8% 33% 32% 14% 5%	849
August	1st to 10th	Day	11% 6% 25% 27% 21% 9%	613
	11th to 20th	Day	11% 7% 26% 31% 17% 7%	559
	21st to end	Day	11% 7% 28% 29% 18% 7%	627
September	1st to 10th	Day	15% 9% 25% 28% 17% 6%	570
	11th to 20th	Day	11% 7% 24% 27% 25% 8%	570
	21st to end	Day	17% 14% 25% 23% 15% 6%	562
October	1st to 10th	Day	18% 15% 25% 25% 15% 2	530
	11th to 20th	Day	28% 19% 29% 19% 4	402
	21st to end	Day	35% 20% 30% 13% 1 1	363
November	1st to 10th	Day	27% 21% 32% 17% 3 1	330
	11th to 20th	Day	33% 25% 28% 12% 3	330
	21st to end	Day	35% 22% 36% 6% 1	330
December	1st to 10th	Day	29% 30% 24% 13% 4	330
	11th to 20th	Day	28% 31% 30% 10% 1 1	330
	21st to end	Day	31% 28% 31% 8% 2 1	363
January	1st to 10th	Night	26% 22% 30% 17% 4	1050
	11th to 20th	Night	29% 20% 32% 16% 3	1050
	21st to end	Night	26% 18% 32% 22% 3	1155
February	1st to 10th	Night	24% 19% 34% 20% 4	889
	11th to 20th	Night	24% 22% 31% 20% 4	823
	21st to end	Night	27% 22% 33% 16% 2	692
March	1st to 10th	Night	23% 20% 29% 22% 6% 1	840
	11th to 20th	Night	21% 18% 33% 22% 5% 1	840
	21st to end	Night	24% 16% 29% 25% 6% 1	814
April	1st to 10th	Night	22% 17% 31% 23% 6% 1	630
	11th to 20th	Night	27% 13% 29% 23% 7% 1	623
	21st to end	Night	20% 13% 32% 27% 8% 1	630
May	1st to 10th	Night	20% 11% 34% 26% 8% 1	630
	11th to 20th	Night	19% 9% 29% 31% 11%	621
	21st to end	Night	15% 5% 28% 34% 17% 1	683
June	1st to 10th	Night	12% 6% 26% 39% 16% 1	630
	11th to 20th	Night	13% 5% 33% 34% 15% 1	630
	21st to end	Night	11% 7% 24% 36% 20% 2	630
July	1st to 10th	Night	10% 3 18% 39% 28% 3	615
	11th to 20th	Night	9% 4 20% 37% 26% 3	612
	21st to end	Night	9% 3 19% 42% 26% 2	690
August	1st to 10th	Night	7% 4 16% 43% 26% 4	627
	11th to 20th	Night	13% 4 15% 40% 24% 4	624
	21st to end	Night	10% 4 20% 37% 26% 3	693
September	1st to 10th	Night	10% 7% 21% 35% 22% 4	630
	11th to 20th	Night	12% 6% 19% 33% 26% 4	831
	21st to end	Night	16% 10% 24% 27% 18% 5%	833
October	1st to 10th	Night	19% 8% 23% 30% 18% 2	800
	11th to 20th	Night	28% 15% 26% 24% 6% 1	800
	21st to end	Night	34% 14% 26% 20% 5%	880
November	1st to 10th	Night	25% 16% 30% 19% 8%	820
	11th to 20th	Night	30% 21% 28% 17% 4 1	840
	21st to end	Night	34% 16% 30% 17% 3	840
December	1st to 10th	Night	31% 22% 28% 15% 3	1005
	11th to 20th	Night	31% 20% 31% 14% 3	1050
	21st to end	Night	37% 18% 30% 13% 2	1155

Page generated at 2026-09-26 04:18:44. Version ac07

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