Allen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Allen Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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% 5% 9% 5% 4 4 3 5% 6% 34%	149540
	Night	13% 12% 5% 11% 6% 5% 4 3 4 6% 30%	142846
Transparency	Day	51% 4 10% 22% 13% 1	153040
	Night	55% 4 10% 20% 10% 1	147423
Seeing	Day	22% 18% 32% 19% 7% 2	18658
	Night	21% 14% 30% 26% 9% 1	28443

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	13% 25% 12% 24% 8% 4 2 1 2 2 6%	4691
	11th to 20th	Day	9% 17% 12% 23% 14% 10% 5% 2 2 2 4	4710
	21st to end	Day	11% 16% 9% 17% 12% 8% 7% 3 2 3 13%	5156
February	1st to 10th	Day	10% 17% 7% 16% 14% 6% 4 2 3 5% 15%	1858
	11th to 20th	Day	11% 18% 8% 15% 11% 6% 4 3 4 4 17%	2048
	21st to end	Day	12% 19% 8% 15% 7% 4 4 3 5% 4 20%	1730
March	1st to 10th	Day	12% 11% 5% 11% 6% 4 5% 3 5% 6% 31%	4580
	11th to 20th	Day	13% 9% 7% 11% 5% 4 4 3 5% 8% 33%	4895
	21st to end	Day	21% 12% 7% 11% 5% 4 3 3 4 5% 26%	5881
April	1st to 10th	Day	18% 16% 4 8% 5% 4 4 3 4 6% 29%	2520
	11th to 20th	Day	20% 12% 3 6% 4 4 4 3 5% 5% 33%	2550
	21st to end	Day	16% 11% 4 8% 4 4 4 4 5% 6% 35%	2730
May	1st to 10th	Day	13% 8% 3 6% 4 3 4 3 5% 8% 43%	7616
	11th to 20th	Day	13% 10% 3 7% 4 4 5% 4 6% 6% 40%	7882
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 7% 45%	9047
June	1st to 10th	Day	12% 6% 2 5% 5% 4 4 4 5% 8% 44%	2940
	11th to 20th	Day	13% 7% 2 6% 5% 3 4 4 6% 8% 42%	2940
	21st to end	Day	14% 6% 2 6% 5% 4 4 3 6% 8% 43%	2940
July	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 9% 41%	8280
	11th to 20th	Day	8% 4 1 4 3 4 4 3 6% 9% 53%	8280
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	8990
August	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	6230
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 7% 46%	5951
	21st to end	Day	10% 6% 3 7% 4 5% 5% 4 6% 9% 40%	6149
September	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 3 6% 7% 41%	2515
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 47%	2310
	21st to end	Day	13% 7% 3 7% 5% 4 5% 3 6% 7% 41%	2310
October	1st to 10th	Day	15% 7% 3 7% 4 4 3 3 4 6% 44%	2310
	11th to 20th	Day	22% 13% 3 9% 6% 5% 5% 3 4 6% 25%	2006
	21st to end	Day	22% 12% 4 10% 5% 4 4 4 5% 6% 25%	2052
November	1st to 10th	Day	15% 14% 3 7% 5% 5% 4 3 5% 6% 31%	1852
	11th to 20th	Day	15% 22% 7% 12% 5% 4 5% 3 5% 5% 18%	1890
	21st to end	Day	20% 22% 8% 12% 6% 4 3 3 3 5% 16%	1853
December	1st to 10th	Day	11% 27% 16% 15% 5% 3 5% 2 3 3 10%	2690
	11th to 20th	Day	13% 23% 14% 15% 10% 4 3 2 3 4 9%	3088
	21st to end	Day	22% 23% 8% 13% 6% 5% 3 2 3 4 12%	4070
January	1st to 10th	Night	12% 21% 11% 23% 7% 4 3 2 2 3 11%	8232
	11th to 20th	Night	9% 19% 9% 23% 11% 12% 4 2 2 3 6%	8280
	21st to end	Night	10% 17% 9% 20% 12% 8% 7% 2 3 3 10%	9058
February	1st to 10th	Night	11% 17% 6% 15% 11% 6% 5% 3 4 4 17%	2758
	11th to 20th	Night	12% 15% 6% 13% 10% 5% 5% 3 5% 5% 19%	2458
	21st to end	Night	13% 17% 5% 12% 6% 5% 4 4 5% 6% 22%	2076
March	1st to 10th	Night	16% 9% 4 11% 5% 4 4 3 4 6% 35%	5493
	11th to 20th	Night	12% 10% 4 8% 5% 4 4 3 5% 8% 38%	5284
	21st to end	Night	17% 14% 4 9% 5% 4 4 3 5% 5% 29%	5321
April	1st to 10th	Night	17% 12% 3 8% 4 4 4 3 5% 7% 33%	2100
	11th to 20th	Night	21% 10% 3 6% 4 4 4 4 5% 5% 35%	2019
	21st to end	Night	17% 9% 3 7% 5% 3 4 3 5% 6% 38%	1890
May	1st to 10th	Night	12% 7% 3 6% 5% 5% 5% 4 6% 8% 41%	5237
	11th to 20th	Night	12% 8% 2 6% 4 4 5% 4 6% 8% 42%	4976
	21st to end	Night	16% 6% 2 5% 4 3 3 3 5% 6% 46%	5139
June	1st to 10th	Night	11% 8% 2 5% 4 3 5% 4 7% 8% 43%	1680
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	1680
	21st to end	Night	10% 6% 3 7% 3 4 4 4 7% 8% 44%	1680
July	1st to 10th	Night	13% 7% 2 6% 4 4 5% 4 6% 8% 40%	4708
	11th to 20th	Night	6% 4 1 4 3 4 4 4 6% 8% 57%	4707
	21st to end	Night	9% 6% 2 6% 3 3 4 4 5% 7% 50%	5117
August	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 6% 8% 42%	3928
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 5% 8% 50%	4095
	21st to end	Night	9% 6% 3 6% 5% 4 5% 4 6% 8% 45%	4974
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 4 6% 51%	2100
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 54%	2294
	21st to end	Night	14% 6% 3 7% 4 4 4 3 6% 7% 42%	2310
October	1st to 10th	Night	17% 8% 2 4 4 4 4 3 4 6% 45%	2310
	11th to 20th	Night	21% 11% 4 6% 5% 4 4 3 5% 5% 32%	2610
	21st to end	Night	23% 11% 3 8% 3 4 4 4 4 6% 30%	2965
November	1st to 10th	Night	14% 13% 4 8% 5% 5% 4 3 4 6% 34%	2666
	11th to 20th	Night	15% 18% 5% 10% 6% 5% 6% 4 5% 5% 21%	2730
	21st to end	Night	18% 21% 6% 10% 6% 5% 4 3 3 5% 19%	2762
December	1st to 10th	Night	12% 20% 9% 17% 7% 5% 5% 3 4 5% 13%	4720
	11th to 20th	Night	15% 25% 8% 15% 10% 5% 3 2 3 4 10%	5339
	21st to end	Night	23% 21% 7% 11% 5% 4 3 3 4 5% 14%	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	87% 3 4 6% 1	4692
	11th to 20th	Day	90% 3 2 3 1	4710
	21st to end	Day	80% 3 6% 9% 2	5156
February	1st to 10th	Day	73% 5% 7% 13% 3	1858
	11th to 20th	Day	70% 5% 9% 11% 5%	2058
	21st to end	Day	67% 5% 10% 13% 5%	1730
March	1st to 10th	Day	54% 4 13% 18% 9% 1	4580
	11th to 20th	Day	50% 4 17% 19% 9% 1	4895
	21st to end	Day	62% 4 9% 16% 9% 1	5883
April	1st to 10th	Day	57% 4 10% 20% 10%	2520
	11th to 20th	Day	52% 4 9% 20% 13% 2	2550
	21st to end	Day	50% 4 12% 19% 13% 2	2730
May	1st to 10th	Day	40% 4 8% 28% 17% 3	7651
	11th to 20th	Day	45% 4 10% 22% 19% 1	7886
	21st to end	Day	39% 4 8% 23% 22% 4	9061
June	1st to 10th	Day	37% 5% 11% 27% 18% 2	2940
	11th to 20th	Day	39% 5% 13% 23% 19% 1	2940
	21st to end	Day	39% 5% 13% 25% 16% 1	2940
July	1st to 10th	Day	41% 2 13% 27% 16% 1	8211
	11th to 20th	Day	29% 2 11% 32% 24% 1	8205
	21st to end	Day	34% 3 13% 29% 19% 1	8794
August	1st to 10th	Day	40% 4 12% 29% 15% 1	6215
	11th to 20th	Day	38% 3 15% 29% 14% 1	5990
	21st to end	Day	41% 3 14% 29% 12% 1	6148
September	1st to 10th	Day	42% 5% 12% 26% 13% 2	2515
	11th to 20th	Day	36% 5% 13% 31% 14% 1	2310
	21st to end	Day	43% 5% 11% 27% 13% 1	2310
October	1st to 10th	Day	30% 3 8% 39% 18% 2	3700
	11th to 20th	Day	50% 4 11% 24% 12%	3170
	21st to end	Day	56% 3 14% 19% 8%	3252
November	1st to 10th	Day	52% 6% 12% 20% 9%	1852
	11th to 20th	Day	67% 5% 11% 12% 5%	1890
	21st to end	Day	72% 4 11% 10% 3	1853
December	1st to 10th	Day	81% 3 8% 7% 1	2690
	11th to 20th	Day	81% 3 7% 7% 1	3085
	21st to end	Day	78% 3 5% 10% 4	4070
January	1st to 10th	Night	81% 3 6% 7% 2	8233
	11th to 20th	Night	87% 3 5% 4 1	8280
	21st to end	Night	81% 3 6% 8% 2	9058
February	1st to 10th	Night	71% 5% 7% 11% 5% 1	2758
	11th to 20th	Night	66% 5% 7% 15% 6%	2459
	21st to end	Night	62% 5% 12% 15% 5%	2076
March	1st to 10th	Night	51% 4 15% 22% 8%	5493
	11th to 20th	Night	45% 5% 18% 22% 9% 1	5284
	21st to end	Night	57% 4 10% 20% 9% 1	5321
April	1st to 10th	Night	51% 5% 10% 20% 13% 1	2100
	11th to 20th	Night	50% 5% 7% 21% 14% 2	2019
	21st to end	Night	47% 5% 10% 21% 13% 3	1890
May	1st to 10th	Night	41% 4 11% 26% 15% 2	5264
	11th to 20th	Night	40% 4 10% 28% 16% 1	4977
	21st to end	Night	40% 3 8% 24% 19% 7%	5149
June	1st to 10th	Night	37% 4 11% 29% 17% 2	1680
	11th to 20th	Night	37% 6% 13% 25% 18% 2	1680
	21st to end	Night	36% 6% 14% 27% 15% 3	1680
July	1st to 10th	Night	43% 3 13% 23% 14% 4	4670
	11th to 20th	Night	26% 2 11% 37% 22% 2	4662
	21st to end	Night	34% 3 12% 27% 22% 1	5002
August	1st to 10th	Night	41% 3 12% 26% 17% 1	3919
	11th to 20th	Night	34% 3 17% 32% 13%	4122
	21st to end	Night	38% 3 12% 29% 16% 2	4974
September	1st to 10th	Night	35% 5% 11% 30% 18% 2	2100
	11th to 20th	Night	31% 4 12% 35% 16% 2	2294
	21st to end	Night	41% 5% 12% 27% 14% 1	2310
October	1st to 10th	Night	35% 3 8% 36% 16% 1	3700
	11th to 20th	Night	43% 3 13% 26% 14%	4185
	21st to end	Night	47% 4 12% 27% 10%	4715
November	1st to 10th	Night	51% 6% 11% 21% 10% 1	2666
	11th to 20th	Night	63% 7% 12% 13% 5% 1	2730
	21st to end	Night	68% 5% 13% 12% 2	2762
December	1st to 10th	Night	74% 3 13% 9% 1	4720
	11th to 20th	Night	80% 4 6% 8% 1	5341
	21st to end	Night	74% 4 8% 10% 4	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	24% 25% 33% 12% 5%	330
	11th to 20th	Day	26% 26% 38% 7% 2	330
	21st to end	Day	28% 24% 34% 13% 1	363
February	1st to 10th	Day	24% 36% 25% 10% 5% 1	330
	11th to 20th	Day	25% 38% 29% 9%	330
	21st to end	Day	32% 34% 29% 4 1	273
March	1st to 10th	Day	30% 31% 32% 7% 1	330
	11th to 20th	Day	32% 18% 29% 14% 6% 1	449
	21st to end	Day	33% 20% 31% 12% 4 1	594
April	1st to 10th	Day	32% 21% 32% 12% 2 1	540
	11th to 20th	Day	35% 22% 24% 11% 6% 1	532
	21st to end	Day	27% 20% 31% 16% 6% 1	540
May	1st to 10th	Day	25% 22% 33% 13% 5% 2	543
	11th to 20th	Day	21% 19% 35% 17% 7% 2	654
	21st to end	Day	19% 16% 35% 21% 7% 2	844
June	1st to 10th	Day	16% 16% 38% 22% 6% 2	780
	11th to 20th	Day	19% 18% 34% 20% 6% 2	780
	21st to end	Day	19% 14% 37% 22% 7% 2	780
July	1st to 10th	Day	10% 11% 34% 30% 11% 4	771
	11th to 20th	Day	11% 12% 33% 30% 9% 4	768
	21st to end	Day	11% 10% 38% 26% 11% 4	849
August	1st to 10th	Day	12% 11% 28% 27% 16% 6%	666
	11th to 20th	Day	15% 5% 26% 29% 16% 9%	559
	21st to end	Day	17% 9% 28% 26% 16% 5%	627
September	1st to 10th	Day	17% 10% 31% 22% 15% 5%	570
	11th to 20th	Day	12% 9% 26% 29% 17% 7%	570
	21st to end	Day	18% 14% 23% 26% 15% 5%	570
October	1st to 10th	Day	21% 16% 29% 23% 10% 1	570
	11th to 20th	Day	29% 17% 36% 16% 3	416
	21st to end	Day	34% 18% 32% 13% 3 1	387
November	1st to 10th	Day	29% 22% 32% 12% 4 1	330
	11th to 20th	Day	31% 28% 31% 8% 2	330
	21st to end	Day	33% 25% 34% 8% 1	330
December	1st to 10th	Day	34% 28% 24% 10% 3	330
	11th to 20th	Day	32% 32% 26% 9% 2	330
	21st to end	Day	35% 28% 26% 8% 1 2	363
January	1st to 10th	Night	24% 22% 31% 18% 4	1050
	11th to 20th	Night	27% 20% 31% 16% 5%	1050
	21st to end	Night	22% 16% 35% 22% 5%	1155
February	1st to 10th	Night	20% 19% 34% 23% 4	939
	11th to 20th	Night	22% 20% 34% 20% 4	823
	21st to end	Night	26% 20% 34% 17% 2	692
March	1st to 10th	Night	20% 20% 33% 20% 7% 1	840
	11th to 20th	Night	21% 18% 36% 19% 6%	840
	21st to end	Night	24% 15% 30% 25% 6% 1	812
April	1st to 10th	Night	25% 16% 32% 22% 4 1	630
	11th to 20th	Night	29% 14% 31% 20% 5% 2	623
	21st to end	Night	22% 13% 35% 22% 8%	630
May	1st to 10th	Night	16% 14% 36% 27% 5% 1	630
	11th to 20th	Night	18% 12% 33% 27% 9%	621
	21st to end	Night	16% 7% 32% 32% 12% 1	683
June	1st to 10th	Night	12% 9% 33% 33% 12% 1	630
	11th to 20th	Night	14% 8% 34% 30% 12% 1	630
	21st to end	Night	13% 8% 30% 35% 13%	630
July	1st to 10th	Night	10% 4 23% 41% 20% 2	615
	11th to 20th	Night	9% 5% 25% 39% 20% 1	612
	21st to end	Night	10% 3 25% 40% 20% 2	690
August	1st to 10th	Night	11% 4 22% 43% 20% 2	627
	11th to 20th	Night	12% 2 19% 42% 21% 4	624
	21st to end	Night	14% 2 23% 41% 18% 2	693
September	1st to 10th	Night	13% 8% 23% 34% 18% 4	630
	11th to 20th	Night	13% 7% 22% 35% 20% 3	824
	21st to end	Night	16% 10% 27% 27% 19% 2	840
October	1st to 10th	Night	21% 10% 27% 30% 12% 1	840
	11th to 20th	Night	28% 13% 28% 24% 7%	840
	21st to end	Night	31% 11% 28% 23% 6%	913
November	1st to 10th	Night	24% 17% 31% 21% 6% 1	820
	11th to 20th	Night	26% 20% 30% 19% 4	840
	21st to end	Night	30% 17% 28% 21% 4	872
December	1st to 10th	Night	27% 22% 30% 18% 3	1050
	11th to 20th	Night	29% 21% 30% 16% 3 1	1050
	21st to end	Night	32% 18% 30% 19% 1	1155

Page generated at 2026-10-26 04:37:52. Version ac07

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