ASU Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASU Dark Sky Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	125786
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 45%	122295
Transparency	Day	38% 9% 12% 20% 16% 5%	129445
	Night	36% 8% 12% 21% 17% 6%	126705
Seeing	Day	20% 15% 25% 21% 13% 6%	16158
	Night	17% 14% 23% 26% 17% 3	26218

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% 5% 2 5% 3 3 3 3 4 5% 57%	4056
	11th to 20th	Day	11% 6% 2 4 3 2 3 3 5% 5% 57%	4060
	21st to end	Day	17% 5% 2 5% 4 4 3 3 5% 4 49%	4466
February	1st to 10th	Day	12% 8% 2 6% 4 4 4 4 6% 7% 43%	1800
	11th to 20th	Day	17% 8% 3 6% 3 4 4 3 5% 6% 42%	1925
	21st to end	Day	18% 6% 2 4 4 3 5% 3 6% 6% 43%	1660
March	1st to 10th	Day	19% 7% 2 5% 4 4 4 3 5% 6% 42%	4935
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 6% 54%	4980
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 6% 45%	5465
April	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 8% 48%	2200
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 6% 8% 45%	2385
	21st to end	Day	15% 6% 2 5% 5% 4 4 4 5% 6% 44%	2600
May	1st to 10th	Day	13% 6% 2 7% 6% 6% 7% 5% 8% 9% 31%	5862
	11th to 20th	Day	21% 7% 3 7% 5% 5% 5% 4 7% 8% 28%	5861
	21st to end	Day	14% 5% 2 6% 4 5% 5% 5% 7% 9% 38%	6433
June	1st to 10th	Day	14% 5% 2 5% 5% 5% 6% 5% 7% 9% 36%	2600
	11th to 20th	Day	10% 5% 2 7% 6% 6% 5% 6% 8% 11% 35%	2600
	21st to end	Day	9% 5% 2 6% 6% 6% 6% 5% 8% 10% 37%	2600
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 7% 6% 8% 10% 35%	5900
	11th to 20th	Day	8% 6% 3 9% 7% 7% 8% 6% 9% 11% 26%	5900
	21st to end	Day	12% 8% 3 10% 7% 7% 7% 6% 8% 9% 23%	6424
August	1st to 10th	Day	16% 7% 3 7% 6% 6% 6% 6% 8% 10% 25%	4156
	11th to 20th	Day	10% 5% 2 6% 6% 6% 7% 5% 8% 11% 34%	4044
	21st to end	Day	8% 4 2 5% 3 5% 5% 5% 7% 10% 45%	4318
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 4 6% 8% 41%	2095
	11th to 20th	Day	20% 7% 2 6% 4 4 5% 3 5% 8% 36%	2090
	21st to end	Day	22% 6% 2 5% 4 5% 4 4 5% 6% 37%	2090
October	1st to 10th	Day	18% 5% 2 4 3 3 3 2 5% 5% 50%	1990
	11th to 20th	Day	15% 5% 1 4 3 3 3 2 4 6% 54%	1900
	21st to end	Day	16% 4 2 5% 3 2 2 2 4 5% 55%	2090
November	1st to 10th	Day	12% 4 2 5% 4 2 3 3 4 6% 55%	1932
	11th to 20th	Day	13% 5% 1 4 3 3 5% 3 4 7% 54%	1800
	21st to end	Day	15% 4 2 5% 3 3 4 3 5% 7% 51%	1800
December	1st to 10th	Day	11% 6% 2 6% 4 4 3 3 4 5% 51%	2930
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 5% 7% 46%	3373
	21st to end	Day	21% 6% 2 4 2 3 4 3 5% 7% 41%	4466
January	1st to 10th	Night	12% 6% 2 3 3 3 3 3 4 5% 56%	5889
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 5% 54%	5900
	21st to end	Night	16% 5% 2 5% 3 3 3 3 4 5% 51%	6490
February	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 7% 7% 44%	2600
	11th to 20th	Night	19% 6% 2 5% 3 4 3 4 5% 6% 44%	2358
	21st to end	Night	13% 7% 3 6% 4 5% 5% 4 6% 6% 43%	1970
March	1st to 10th	Night	20% 7% 2 5% 4 4 3 3 4 5% 43%	4918
	11th to 20th	Night	11% 3 1 4 3 3 4 4 5% 7% 55%	4980
	21st to end	Night	16% 5% 2 5% 3 4 3 4 6% 7% 45%	5467
April	1st to 10th	Night	13% 6% 1 4 4 3 4 3 6% 8% 48%	2200
	11th to 20th	Night	15% 5% 1 4 3 4 3 3 4 7% 50%	1980
	21st to end	Night	14% 6% 2 5% 3 3 4 4 5% 6% 48%	1800
May	1st to 10th	Night	15% 7% 2 6% 5% 4 4 4 6% 11% 36%	4033
	11th to 20th	Night	22% 6% 2 6% 4 4 4 3 6% 8% 35%	4032
	21st to end	Night	18% 4 2 5% 4 3 4 3 6% 9% 40%	4421
June	1st to 10th	Night	11% 4 2 5% 5% 5% 6% 5% 7% 11% 40%	1800
	11th to 20th	Night	12% 5% 1 5% 4 4 4 5% 8% 10% 43%	1800
	21st to end	Night	7% 4 2 5% 5% 4 5% 5% 7% 13% 44%	1800
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 9% 10% 38%	4058
	11th to 20th	Night	8% 6% 2 7% 5% 6% 6% 6% 8% 11% 34%	4057
	21st to end	Night	11% 7% 2 9% 6% 6% 7% 5% 8% 10% 26%	4427
August	1st to 10th	Night	16% 8% 3 7% 4 4 5% 5% 7% 10% 31%	2861
	11th to 20th	Night	11% 6% 2 7% 5% 6% 5% 4 7% 9% 39%	2785
	21st to end	Night	8% 5% 2 5% 3 4 4 3 7% 9% 51%	3306
September	1st to 10th	Night	15% 5% 2 5% 4 4 5% 4 6% 6% 44%	2080
	11th to 20th	Night	20% 6% 3 6% 3 3 4 3 4 8% 39%	2090
	21st to end	Night	22% 5% 2 6% 4 4 4 3 3 6% 42%	2090
October	1st to 10th	Night	18% 6% 1 4 3 2 3 3 4 6% 49%	2190
	11th to 20th	Night	16% 5% 2 3 3 3 4 2 4 5% 54%	2280
	21st to end	Night	15% 4 1 3 2 3 3 2 4 4 58%	2508
November	1st to 10th	Night	15% 6% 1 4 2 3 3 3 4 5% 53%	2363
	11th to 20th	Night	14% 4 1 4 3 3 3 3 5% 6% 55%	2600
	21st to end	Night	13% 4 2 4 4 3 4 4 5% 7% 52%	2600
December	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 4 5% 52%	4250
	11th to 20th	Night	17% 11% 2 6% 4 4 4 3 4 5% 41%	4822
	21st to end	Night	21% 6% 2 4 3 3 4 3 5% 7% 42%	6490

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	29% 4 7% 19% 33% 7%	4057
	11th to 20th	Day	29% 5% 7% 16% 34% 9%	4060
	21st to end	Day	37% 5% 7% 21% 24% 6%	4466
February	1st to 10th	Day	39% 7% 11% 20% 18% 5%	1800
	11th to 20th	Day	42% 6% 9% 18% 19% 5%	1935
	21st to end	Day	41% 7% 12% 18% 17% 6%	1660
March	1st to 10th	Day	42% 5% 7% 17% 16% 12%	4935
	11th to 20th	Day	31% 6% 7% 20% 22% 15%	4980
	21st to end	Day	39% 5% 9% 26% 16% 5%	5465
April	1st to 10th	Day	33% 8% 12% 23% 19% 5%	2200
	11th to 20th	Day	36% 8% 13% 20% 19% 6%	2385
	21st to end	Day	40% 10% 12% 23% 14% 3	2600
May	1st to 10th	Day	45% 12% 14% 18% 10% 1	5897
	11th to 20th	Day	50% 11% 12% 19% 6% 1	5865
	21st to end	Day	38% 11% 14% 19% 15% 3	6447
June	1st to 10th	Day	41% 12% 16% 21% 9% 1	2600
	11th to 20th	Day	39% 13% 15% 18% 12% 2	2600
	21st to end	Day	37% 13% 16% 19% 13% 1	2600
July	1st to 10th	Day	38% 16% 16% 19% 9% 2	5835
	11th to 20th	Day	45% 16% 23% 13% 4	5831
	21st to end	Day	51% 14% 15% 15% 5% 1	6240
August	1st to 10th	Day	48% 13% 17% 17% 4	4143
	11th to 20th	Day	37% 14% 23% 16% 7% 3	4083
	21st to end	Day	30% 11% 18% 32% 8% 2	4318
September	1st to 10th	Day	39% 10% 13% 19% 15% 4	2095
	11th to 20th	Day	44% 9% 11% 19% 15% 2	2090
	21st to end	Day	47% 7% 12% 18% 14% 3	2090
October	1st to 10th	Day	29% 5% 9% 19% 26% 12%	3302
	11th to 20th	Day	22% 4 6% 23% 24% 21%	3140
	21st to end	Day	27% 4 10% 24% 18% 17%	3428
November	1st to 10th	Day	31% 5% 9% 22% 26% 7%	1932
	11th to 20th	Day	30% 5% 9% 22% 27% 7%	1800
	21st to end	Day	34% 5% 9% 22% 26% 4	1800
December	1st to 10th	Day	35% 6% 6% 19% 28% 6%	2930
	11th to 20th	Day	38% 6% 11% 18% 21% 7%	3370
	21st to end	Day	40% 6% 12% 24% 14% 3	4466
January	1st to 10th	Night	28% 6% 8% 18% 28% 12%	5889
	11th to 20th	Night	30% 7% 6% 17% 29% 10%	5900
	21st to end	Night	36% 5% 7% 18% 28% 7%	6490
February	1st to 10th	Night	37% 7% 12% 22% 18% 4	2600
	11th to 20th	Night	42% 6% 9% 19% 17% 7%	2359
	21st to end	Night	39% 7% 13% 17% 18% 6%	1970
March	1st to 10th	Night	41% 6% 6% 19% 18% 9%	4918
	11th to 20th	Night	27% 6% 7% 21% 22% 16%	4980
	21st to end	Night	36% 6% 11% 25% 15% 6%	5467
April	1st to 10th	Night	34% 8% 12% 22% 19% 5%	2200
	11th to 20th	Night	34% 7% 11% 23% 18% 7%	1980
	21st to end	Night	35% 8% 15% 24% 15% 2	1800
May	1st to 10th	Night	41% 12% 15% 22% 10% 1	4060
	11th to 20th	Night	47% 9% 15% 20% 7%	4033
	21st to end	Night	38% 10% 15% 24% 11% 2	4429
June	1st to 10th	Night	37% 12% 18% 22% 10% 1	1800
	11th to 20th	Night	34% 12% 20% 23% 11% 1	1800
	21st to end	Night	30% 13% 19% 23% 11% 3	1800
July	1st to 10th	Night	37% 13% 17% 23% 9% 1	4015
	11th to 20th	Night	39% 15% 24% 17% 3 1	4006
	21st to end	Night	47% 14% 18% 14% 6%	4297
August	1st to 10th	Night	46% 11% 21% 16% 5%	2850
	11th to 20th	Night	39% 11% 22% 18% 9% 1	2812
	21st to end	Night	29% 8% 18% 35% 8% 2	3306
September	1st to 10th	Night	38% 10% 15% 18% 16% 4	2080
	11th to 20th	Night	43% 9% 12% 19% 17% 1	2090
	21st to end	Night	45% 6% 12% 18% 16% 2	2090
October	1st to 10th	Night	30% 5% 10% 20% 22% 13%	3658
	11th to 20th	Night	21% 4 7% 23% 26% 18%	3779
	21st to end	Night	27% 4 7% 23% 24% 15%	4120
November	1st to 10th	Night	34% 5% 10% 20% 23% 8%	2363
	11th to 20th	Night	30% 5% 9% 20% 26% 10%	2600
	21st to end	Night	32% 6% 8% 23% 26% 5%	2600
December	1st to 10th	Night	34% 6% 9% 20% 27% 4	4250
	11th to 20th	Night	45% 5% 8% 18% 17% 7%	4824
	21st to end	Night	38% 7% 15% 22% 14% 3	6490

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	30% 33% 27% 9% 1	300
	11th to 20th	Day	34% 26% 30% 10%	300
	21st to end	Day	28% 35% 27% 8% 2	330
February	1st to 10th	Day	26% 29% 31% 12% 1	300
	11th to 20th	Day	42% 29% 24% 4	300
	21st to end	Day	35% 29% 29% 7%	259
March	1st to 10th	Day	25% 26% 29% 15% 5%	500
	11th to 20th	Day	28% 27% 28% 14% 3 1	500
	21st to end	Day	30% 23% 28% 13% 5%	550
April	1st to 10th	Day	23% 26% 29% 16% 5% 1	500
	11th to 20th	Day	21% 24% 25% 18% 10% 2	492
	21st to end	Day	20% 19% 30% 18% 9% 4	500
May	1st to 10th	Day	18% 21% 32% 17% 8% 4	503
	11th to 20th	Day	20% 15% 26% 22% 11% 6%	527
	21st to end	Day	21% 8% 24% 23% 15% 9%	573
June	1st to 10th	Day	12% 11% 25% 28% 16% 7%	530
	11th to 20th	Day	10% 7% 22% 31% 21% 9%	530
	21st to end	Day	9% 7% 26% 32% 18% 8%	530
July	1st to 10th	Day	10% 3 16% 32% 27% 12%	525
	11th to 20th	Day	7% 3 16% 30% 29% 14%	524
	21st to end	Day	10% 3 18% 29% 27% 12%	576
August	1st to 10th	Day	17% 5% 24% 23% 17% 14%	502
	11th to 20th	Day	11% 3 20% 28% 24% 14%	479
	21st to end	Day	13% 3 14% 31% 24% 16%	539
September	1st to 10th	Day	13% 4 17% 29% 25% 12%	490
	11th to 20th	Day	21% 4 14% 27% 24% 10%	490
	21st to end	Day	24% 10% 19% 26% 16% 5%	490
October	1st to 10th	Day	23% 8% 22% 24% 18% 6%	490
	11th to 20th	Day	16% 18% 26% 24% 14% 2	490
	21st to end	Day	19% 14% 30% 24% 9% 3	539
November	1st to 10th	Day	19% 16% 28% 23% 11% 3	470
	11th to 20th	Day	20% 25% 32% 18% 4 1	300
	21st to end	Day	20% 25% 32% 18% 5% 1	300
December	1st to 10th	Day	23% 32% 33% 10% 2	300
	11th to 20th	Day	22% 23% 35% 16% 4	300
	21st to end	Day	32% 19% 31% 12% 4 2	330
January	1st to 10th	Night	19% 29% 29% 19% 4	800
	11th to 20th	Night	21% 24% 30% 22% 4	800
	21st to end	Night	18% 26% 32% 21% 3	880
February	1st to 10th	Night	17% 21% 32% 27% 4	800
	11th to 20th	Night	25% 30% 29% 16% 1	783
	21st to end	Night	20% 24% 34% 19% 3	660
March	1st to 10th	Night	18% 24% 31% 23% 4	800
	11th to 20th	Night	22% 18% 33% 20% 7% 1	800
	21st to end	Night	20% 19% 31% 23% 7%	880
April	1st to 10th	Night	18% 22% 28% 25% 7%	800
	11th to 20th	Night	19% 18% 27% 23% 12% 1	780
	21st to end	Night	17% 13% 26% 32% 11%	600
May	1st to 10th	Night	17% 11% 28% 29% 15% 1	600
	11th to 20th	Night	18% 9% 20% 32% 18% 3	591
	21st to end	Night	19% 5% 16% 24% 30% 5%	650
June	1st to 10th	Night	12% 3 19% 34% 28% 5%	600
	11th to 20th	Night	12% 2 19% 30% 31% 6%	600
	21st to end	Night	8% 3 14% 37% 35% 3	600
July	1st to 10th	Night	10% 3 9% 30% 42% 6%	585
	11th to 20th	Night	10% 2 10% 28% 43% 8%	582
	21st to end	Night	7% 2 11% 32% 40% 8%	657
August	1st to 10th	Night	16% 2 12% 28% 36% 6%	579
	11th to 20th	Night	15% 3 10% 28% 36% 8%	564
	21st to end	Night	13% 1 8% 29% 41% 7%	731
September	1st to 10th	Night	15% 3 13% 26% 36% 8%	760
	11th to 20th	Night	20% 4 13% 25% 33% 6%	760
	21st to end	Night	24% 6% 17% 27% 25% 2	760
October	1st to 10th	Night	19% 9% 19% 24% 24% 5%	760
	11th to 20th	Night	19% 11% 25% 28% 14% 3	760
	21st to end	Night	18% 14% 20% 32% 13% 3	836
November	1st to 10th	Night	18% 12% 22% 30% 14% 3	780
	11th to 20th	Night	16% 16% 25% 32% 10% 1	800
	21st to end	Night	14% 21% 25% 28% 10% 1	800
December	1st to 10th	Night	20% 21% 29% 24% 6%	800
	11th to 20th	Night	18% 20% 29% 26% 6% 1	800
	21st to end	Night	26% 19% 24% 27% 5%	880

Page generated at 2025-08-02 01:32:26. Version ac07

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