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 2025-07-06.

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	14% 11% 5% 9% 6% 4 4 3 5% 6% 32%	119738
	Night	14% 13% 5% 11% 6% 5% 4 3 4 6% 29%	119251
Transparency	Day	52% 4 11% 20% 12% 1	123283
	Night	56% 4 10% 19% 10% 1	123847
Seeing	Day	22% 18% 31% 18% 8% 2	16795
	Night	21% 14% 30% 26% 9% 1	26691

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	10% 24% 13% 26% 6% 5% 3 2 2 2 8%	3617
	11th to 20th	Day	11% 17% 9% 22% 17% 8% 7% 2 2 2 4	3620
	21st to end	Day	14% 20% 11% 18% 8% 4 6% 3 2 3 12%	3982
February	1st to 10th	Day	10% 17% 7% 15% 14% 6% 4 2 3 4 15%	1770
	11th to 20th	Day	12% 18% 8% 15% 11% 6% 4 3 4 3 16%	1948
	21st to end	Day	12% 19% 8% 15% 7% 4 4 3 5% 4 20%	1650
March	1st to 10th	Day	12% 12% 5% 11% 6% 4 5% 3 5% 6% 31%	4480
	11th to 20th	Day	12% 9% 7% 11% 4 4 4 3 5% 8% 33%	4789
	21st to end	Day	21% 12% 7% 11% 5% 4 3 3 4 4 26%	5754
April	1st to 10th	Day	17% 16% 4 8% 5% 4 4 3 5% 5% 29%	2400
	11th to 20th	Day	20% 13% 3 6% 4 4 4 4 5% 5% 33%	2427
	21st to end	Day	16% 11% 4 8% 4 4 4 4 5% 6% 34%	2600
May	1st to 10th	Day	16% 8% 3 6% 4 3 4 3 5% 8% 41%	5862
	11th to 20th	Day	13% 8% 2 7% 4 4 5% 4 6% 6% 41%	6061
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 41%	6935
June	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 8% 44%	2800
	11th to 20th	Day	13% 7% 2 5% 5% 3 4 4 6% 8% 42%	2800
	21st to end	Day	14% 6% 2 6% 5% 4 4 3 6% 8% 43%	2800
July	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 9% 38%	4511
	11th to 20th	Day	7% 4 1 4 4 3 4 4 6% 9% 54%	4440
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 6% 8% 46%	4813
August	1st to 10th	Day	11% 5% 2 6% 4 4 6% 5% 7% 9% 41%	4245
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 6% 49%	4044
	21st to end	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 43%	4222
September	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 3 5% 7% 43%	2275
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 7% 47%	2090
	21st to end	Day	13% 7% 3 8% 5% 4 5% 3 6% 7% 39%	2090
October	1st to 10th	Day	16% 8% 3 7% 4 4 3 3 4 5% 42%	2090
	11th to 20th	Day	23% 13% 3 9% 6% 5% 5% 3 5% 6% 22%	1815
	21st to end	Day	24% 13% 4 10% 5% 3 4 3 5% 5% 23%	1881
November	1st to 10th	Day	16% 14% 3 7% 6% 5% 4 3 5% 6% 31%	1762
	11th to 20th	Day	15% 21% 7% 12% 5% 4 5% 4 5% 5% 18%	1800
	21st to end	Day	20% 21% 8% 12% 5% 4 3 3 3 4 16%	1765
December	1st to 10th	Day	11% 26% 16% 15% 5% 3 5% 2 3 3 11%	2610
	11th to 20th	Day	14% 23% 14% 15% 10% 4 3 2 3 4 9%	3008
	21st to end	Day	22% 23% 8% 13% 6% 5% 3 2 3 4 12%	3982
January	1st to 10th	Night	11% 20% 11% 24% 7% 5% 3 2 2 3 11%	6348
	11th to 20th	Night	11% 17% 8% 24% 11% 12% 4 2 2 3 6%	6360
	21st to end	Night	12% 21% 10% 19% 6% 6% 5% 2 3 3 11%	6996
February	1st to 10th	Night	11% 17% 6% 15% 11% 7% 5% 3 4 4 17%	2628
	11th to 20th	Night	12% 16% 7% 13% 11% 6% 5% 3 5% 5% 18%	2338
	21st to end	Night	13% 17% 5% 13% 6% 6% 4 4 5% 5% 22%	1980
March	1st to 10th	Night	16% 9% 4 11% 5% 4 4 3 4 6% 35%	5373
	11th to 20th	Night	12% 10% 4 8% 4 4 4 3 5% 8% 38%	5170
	21st to end	Night	18% 14% 4 10% 5% 4 4 3 5% 5% 29%	5205
April	1st to 10th	Night	17% 11% 3 8% 5% 4 4 3 5% 7% 33%	2000
	11th to 20th	Night	20% 10% 3 6% 4 4 4 4 5% 6% 35%	1922
	21st to end	Night	18% 9% 3 7% 5% 3 4 3 5% 5% 38%	1800
May	1st to 10th	Night	15% 8% 3 6% 4 5% 4 4 5% 8% 38%	4033
	11th to 20th	Night	12% 8% 3 7% 5% 4 5% 4 5% 8% 39%	3826
	21st to end	Night	20% 6% 2 5% 3 3 3 3 4 6% 45%	3942
June	1st to 10th	Night	11% 8% 2 5% 4 3 5% 4 7% 8% 43%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 6% 8% 44%	1600
	21st to end	Night	10% 6% 3 7% 4 4 4 4 7% 9% 44%	1600
July	1st to 10th	Night	13% 8% 2 5% 3 5% 5% 4 6% 8% 41%	2570
	11th to 20th	Night	6% 4 1 4 4 4 4 3 5% 6% 58%	2530
	21st to end	Night	9% 5% 2 6% 3 4 4 4 6% 7% 50%	2750
August	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 7% 8% 44%	2688
	11th to 20th	Night	13% 5% 2 4 3 4 3 3 5% 6% 52%	2785
	21st to end	Night	9% 7% 2 5% 4 5% 4 4 5% 7% 48%	3431
September	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 4 6% 52%	1900
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 54%	2076
	21st to end	Night	14% 6% 3 7% 4 4 5% 3 5% 7% 42%	2090
October	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 4 6% 44%	2090
	11th to 20th	Night	22% 11% 4 6% 4 4 4 3 5% 5% 32%	2362
	21st to end	Night	24% 12% 3 8% 3 4 4 3 4 6% 27%	2717
November	1st to 10th	Night	14% 13% 4 8% 5% 5% 4 3 4 6% 34%	2536
	11th to 20th	Night	15% 19% 5% 10% 6% 5% 6% 4 5% 5% 20%	2600
	21st to end	Night	18% 20% 6% 10% 6% 5% 4 3 3 5% 19%	2630
December	1st to 10th	Night	12% 20% 9% 17% 7% 5% 5% 3 4 5% 14%	4580
	11th to 20th	Night	15% 25% 8% 14% 10% 4 3 2 3 4 10%	5199
	21st to end	Night	23% 22% 7% 11% 5% 4 3 3 4 5% 14%	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	85% 3 4 7% 1	3618
	11th to 20th	Day	89% 3 3 3 1	3620
	21st to end	Day	80% 4 7% 8% 1	3982
February	1st to 10th	Day	72% 5% 7% 13% 3	1770
	11th to 20th	Day	72% 6% 8% 10% 5%	1958
	21st to end	Day	66% 5% 10% 13% 5%	1650
March	1st to 10th	Day	54% 4 14% 18% 9% 1	4480
	11th to 20th	Day	50% 4 17% 19% 9% 1	4789
	21st to end	Day	62% 4 9% 16% 9%	5756
April	1st to 10th	Day	57% 4 10% 20% 10%	2400
	11th to 20th	Day	52% 4 9% 20% 13% 2	2427
	21st to end	Day	50% 4 12% 18% 14% 2	2600
May	1st to 10th	Day	42% 4 10% 25% 16% 4	5897
	11th to 20th	Day	42% 5% 11% 22% 18% 2	6065
	21st to end	Day	41% 5% 9% 24% 19% 2	6949
June	1st to 10th	Day	37% 5% 11% 28% 17% 2	2800
	11th to 20th	Day	39% 5% 14% 23% 19% 1	2800
	21st to end	Day	39% 5% 14% 24% 17% 1	2800
July	1st to 10th	Day	44% 3 13% 24% 14% 2	4442
	11th to 20th	Day	27% 3 15% 33% 20% 2	4365
	21st to end	Day	34% 5% 13% 26% 21% 2	4662
August	1st to 10th	Day	39% 5% 12% 29% 14% 1	4230
	11th to 20th	Day	36% 4 13% 29% 17% 1	4083
	21st to end	Day	38% 3 15% 27% 16% 1	4221
September	1st to 10th	Day	41% 5% 12% 26% 14% 2	2275
	11th to 20th	Day	35% 5% 13% 31% 14% 1	2090
	21st to end	Day	44% 6% 12% 27% 12%	2090
October	1st to 10th	Day	31% 3 8% 38% 19% 2	3480
	11th to 20th	Day	50% 4 11% 23% 12%	2979
	21st to end	Day	57% 3 15% 18% 7%	3081
November	1st to 10th	Day	53% 6% 11% 20% 9%	1762
	11th to 20th	Day	68% 5% 11% 11% 5%	1800
	21st to end	Day	72% 5% 11% 9% 3	1765
December	1st to 10th	Day	80% 3 8% 7% 1	2610
	11th to 20th	Day	81% 3 8% 7% 1	3005
	21st to end	Day	78% 3 5% 10% 4	3982
January	1st to 10th	Night	80% 4 6% 9% 2	6348
	11th to 20th	Night	86% 3 5% 4 2	6360
	21st to end	Night	80% 4 7% 8% 1	6996
February	1st to 10th	Night	70% 5% 7% 11% 5% 1	2628
	11th to 20th	Night	68% 6% 8% 14% 5%	2339
	21st to end	Night	62% 5% 12% 15% 5%	1980
March	1st to 10th	Night	51% 4 15% 22% 8%	5373
	11th to 20th	Night	45% 5% 18% 22% 9% 1	5170
	21st to end	Night	57% 4 9% 20% 9% 1	5205
April	1st to 10th	Night	50% 5% 10% 21% 13% 1	2000
	11th to 20th	Night	50% 5% 7% 21% 14% 2	1922
	21st to end	Night	48% 5% 10% 20% 14% 3	1800
May	1st to 10th	Night	43% 5% 12% 19% 17% 3	4060
	11th to 20th	Night	43% 6% 9% 25% 15% 2	3827
	21st to end	Night	42% 3 9% 24% 16% 5%	3950
June	1st to 10th	Night	37% 5% 12% 29% 15% 2	1600
	11th to 20th	Night	37% 6% 13% 25% 18% 2	1600
	21st to end	Night	36% 6% 14% 26% 15% 3	1600
July	1st to 10th	Night	42% 4 11% 25% 12% 6%	2530
	11th to 20th	Night	26% 4 14% 36% 17% 3	2482
	21st to end	Night	33% 6% 14% 22% 24% 2	2662
August	1st to 10th	Night	36% 4 13% 29% 17% 1	2679
	11th to 20th	Night	33% 4 10% 35% 17% 1	2812
	21st to end	Night	36% 4 12% 25% 21% 2	3431
September	1st to 10th	Night	34% 5% 11% 29% 19% 2	1900
	11th to 20th	Night	30% 4 13% 36% 15% 2	2076
	21st to end	Night	42% 5% 13% 28% 12% 1	2090
October	1st to 10th	Night	35% 4 8% 36% 16% 1	3480
	11th to 20th	Night	42% 4 13% 27% 14%	3937
	21st to end	Night	48% 4 12% 27% 9%	4467
November	1st to 10th	Night	51% 6% 12% 20% 10% 1	2536
	11th to 20th	Night	64% 7% 11% 11% 5% 1	2600
	21st to end	Night	68% 5% 13% 12% 2	2630
December	1st to 10th	Night	74% 3 13% 8% 1	4580
	11th to 20th	Night	80% 4 6% 9% 1	5201
	21st to end	Night	74% 4 9% 9% 4	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% 25% 34% 13% 6%	300
	11th to 20th	Day	27% 24% 39% 8% 2	300
	21st to end	Day	28% 24% 33% 13% 2	330
February	1st to 10th	Day	24% 36% 24% 10% 5% 1	300
	11th to 20th	Day	25% 37% 29% 9%	300
	21st to end	Day	33% 34% 28% 4 1	249
March	1st to 10th	Day	30% 31% 31% 7% 1	300
	11th to 20th	Day	30% 18% 30% 15% 6% 1	413
	21st to end	Day	33% 19% 31% 12% 4 1	550
April	1st to 10th	Day	31% 21% 33% 12% 2 1	500
	11th to 20th	Day	35% 23% 24% 11% 6% 1	492
	21st to end	Day	27% 21% 31% 15% 6% 1	500
May	1st to 10th	Day	25% 21% 33% 13% 6% 2	503
	11th to 20th	Day	20% 17% 36% 18% 7% 2	610
	21st to end	Day	19% 16% 34% 21% 7% 2	789
June	1st to 10th	Day	16% 17% 38% 21% 6% 2	730
	11th to 20th	Day	18% 17% 35% 22% 6% 2	730
	21st to end	Day	19% 14% 36% 22% 7% 2	730
July	1st to 10th	Day	9% 11% 34% 30% 11% 4	697
	11th to 20th	Day	11% 12% 33% 29% 10% 5%	668
	21st to end	Day	10% 9% 37% 26% 12% 5%	739
August	1st to 10th	Day	11% 11% 29% 26% 17% 6%	576
	11th to 20th	Day	15% 5% 25% 28% 16% 10%	479
	21st to end	Day	17% 9% 26% 27% 17% 4	539
September	1st to 10th	Day	17% 10% 31% 22% 16% 5%	490
	11th to 20th	Day	11% 10% 27% 31% 17% 5%	490
	21st to end	Day	18% 16% 23% 25% 14% 4	490
October	1st to 10th	Day	22% 15% 28% 22% 11% 2	490
	11th to 20th	Day	31% 18% 34% 15% 2	351
	21st to end	Day	38% 19% 29% 10% 3	330
November	1st to 10th	Day	29% 23% 31% 12% 4 1	300
	11th to 20th	Day	29% 28% 32% 8% 2	300
	21st to end	Day	32% 25% 34% 8% 1	300
December	1st to 10th	Day	33% 29% 23% 11% 3	300
	11th to 20th	Day	29% 32% 27% 10% 2	300
	21st to end	Day	35% 28% 25% 8% 2 2	330
January	1st to 10th	Night	23% 23% 31% 19% 4 1	1000
	11th to 20th	Night	27% 20% 31% 17% 5%	1000
	21st to end	Night	22% 16% 34% 23% 5%	1100
February	1st to 10th	Night	20% 19% 34% 23% 4	895
	11th to 20th	Night	22% 21% 33% 20% 4	783
	21st to end	Night	25% 20% 35% 17% 3	660
March	1st to 10th	Night	21% 20% 32% 20% 7% 1	800
	11th to 20th	Night	19% 19% 37% 19% 6%	800
	21st to end	Night	25% 14% 30% 25% 6% 1	773
April	1st to 10th	Night	24% 16% 33% 23% 5% 1	600
	11th to 20th	Night	28% 13% 32% 20% 5% 1	593
	21st to end	Night	22% 13% 34% 22% 8%	600
May	1st to 10th	Night	17% 14% 36% 27% 6% 1	600
	11th to 20th	Night	18% 12% 33% 28% 10%	591
	21st to end	Night	17% 7% 31% 32% 12% 1	650
June	1st to 10th	Night	12% 10% 33% 33% 12% 1	600
	11th to 20th	Night	14% 8% 33% 32% 13% 1	600
	21st to end	Night	13% 8% 31% 35% 14%	600
July	1st to 10th	Night	9% 4 23% 41% 21% 2	570
	11th to 20th	Night	9% 5% 26% 38% 20% 1	552
	21st to end	Night	10% 3 24% 40% 21% 2	624
August	1st to 10th	Night	9% 4 21% 44% 20% 2	567
	11th to 20th	Night	11% 3 19% 40% 22% 4	564
	21st to end	Night	15% 2 22% 42% 18% 2	627
September	1st to 10th	Night	12% 8% 23% 35% 19% 4	570
	11th to 20th	Night	13% 8% 23% 35% 19% 3	746
	21st to end	Night	17% 10% 26% 27% 17% 2	760
October	1st to 10th	Night	22% 10% 26% 29% 13% 1	760
	11th to 20th	Night	28% 14% 25% 25% 7% 1	760
	21st to end	Night	33% 12% 28% 22% 5%	836
November	1st to 10th	Night	24% 17% 31% 21% 7% 1	780
	11th to 20th	Night	27% 20% 31% 18% 4	800
	21st to end	Night	30% 17% 28% 21% 4	830
December	1st to 10th	Night	28% 22% 30% 17% 3	1000
	11th to 20th	Night	29% 21% 30% 16% 3 1	1000
	21st to end	Night	31% 17% 31% 19% 1	1100

Page generated at 2025-07-03 01:30:16. Version ac07

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