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 2024-11-04.

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	15% 6% 2 6% 5% 4 5% 4 6% 8% 41%	101216
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	97761
Transparency	Day	38% 9% 13% 20% 15% 5%	104919
	Night	36% 8% 13% 21% 16% 6%	102197
Seeing	Day	19% 15% 25% 22% 13% 6%	15173
	Night	17% 14% 22% 27% 17% 3	25232

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	16% 4 1 4 3 3 3 3 4 6% 54%	2836
	11th to 20th	Day	14% 6% 2 5% 4 3 4 4 6% 5% 48%	2840
	21st to end	Day	22% 7% 2 5% 3 3 3 3 5% 4 44%	3124
February	1st to 10th	Day	12% 8% 2 6% 4 4 4 4 6% 6% 43%	1710
	11th to 20th	Day	16% 8% 3 6% 3 4 4 3 5% 5% 42%	1826
	21st to end	Day	19% 6% 2 4 4 4 5% 3 5% 6% 41%	1579
March	1st to 10th	Day	23% 7% 2 4 3 4 4 3 5% 6% 40%	3435
	11th to 20th	Day	12% 5% 2 5% 4 4 4 2 4 6% 54%	3480
	21st to end	Day	20% 7% 2 5% 4 4 4 3 5% 5% 41%	3815
April	1st to 10th	Day	11% 5% 2 4 4 4 4 3 6% 7% 49%	2090
	11th to 20th	Day	17% 5% 2 5% 4 4 4 3 6% 8% 44%	2264
	21st to end	Day	15% 6% 2 5% 5% 4 4 4 5% 6% 44%	2470
May	1st to 10th	Day	16% 6% 2 7% 5% 5% 6% 4 7% 8% 34%	4082
	11th to 20th	Day	19% 7% 2 6% 5% 5% 5% 4 6% 8% 32%	4081
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 9% 42%	4476
June	1st to 10th	Day	14% 5% 2 6% 6% 5% 6% 4 7% 9% 36%	2470
	11th to 20th	Day	10% 5% 2 6% 6% 6% 5% 6% 8% 11% 35%	2470
	21st to end	Day	9% 5% 2 6% 6% 6% 6% 5% 8% 10% 36%	2470
July	1st to 10th	Day	8% 4 2 6% 6% 6% 7% 7% 9% 10% 34%	4120
	11th to 20th	Day	9% 6% 3 7% 7% 7% 8% 6% 8% 10% 30%	4120
	21st to end	Day	14% 9% 3 11% 8% 7% 7% 6% 7% 8% 20%	4466
August	1st to 10th	Day	16% 7% 3 7% 6% 6% 6% 6% 8% 10% 25%	4104
	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 2 4 6% 56%	1864
	11th to 20th	Day	13% 5% 1 4 3 2 4 3 4 6% 54%	1710
	21st to end	Day	15% 4 2 5% 3 3 4 2 5% 6% 51%	1710
December	1st to 10th	Day	16% 8% 2 8% 4 4 4 4 5% 6% 39%	1710
	11th to 20th	Day	14% 4 2 4 3 2 3 3 6% 7% 53%	2153
	21st to end	Day	20% 6% 2 4 3 4 4 4 6% 6% 41%	3124
January	1st to 10th	Night	15% 7% 2 3 3 3 4 3 5% 6% 49%	4109
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 5% 49%	4120
	21st to end	Night	22% 7% 2 4 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2470
	11th to 20th	Night	18% 6% 2 5% 3 4 3 4 5% 6% 44%	2237
	21st to end	Night	14% 7% 3 6% 4 4 5% 4 6% 6% 41%	1875
March	1st to 10th	Night	24% 9% 2 6% 4 4 4 3 4 5% 36%	3418
	11th to 20th	Night	10% 4 1 5% 3 4 5% 4 5% 7% 54%	3480
	21st to end	Night	19% 5% 1 4 3 3 3 4 6% 7% 45%	3817
April	1st to 10th	Night	12% 6% 1 4 4 3 4 3 6% 8% 48%	2090
	11th to 20th	Night	16% 5% 1 4 4 4 3 3 4 7% 49%	1880
	21st to end	Night	14% 6% 2 4 3 3 4 4 5% 6% 50%	1710
May	1st to 10th	Night	16% 7% 2 5% 5% 4 4 4 5% 10% 39%	2813
	11th to 20th	Night	21% 6% 2 5% 3 3 4 3 6% 7% 40%	2812
	21st to end	Night	11% 4 2 5% 4 3 5% 4 7% 11% 44%	3079
June	1st to 10th	Night	12% 4 2 5% 4 5% 6% 5% 7% 11% 40%	1710
	11th to 20th	Night	12% 5% 1 5% 4 4 4 5% 8% 10% 44%	1710
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 5% 7% 13% 43%	1710
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 6% 9% 10% 38%	2840
	11th to 20th	Night	8% 5% 2 5% 5% 5% 5% 6% 7% 11% 42%	2840
	21st to end	Night	12% 8% 3 10% 7% 7% 7% 5% 7% 9% 26%	3087
August	1st to 10th	Night	16% 8% 3 7% 4 4 5% 5% 7% 10% 31%	2825
	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% 54%	2284
	11th to 20th	Night	15% 4 1 3 2 3 3 3 4 6% 55%	2470
	21st to end	Night	13% 5% 2 4 4 3 4 3 5% 7% 51%	2470
December	1st to 10th	Night	17% 7% 2 6% 4 3 4 4 4 5% 44%	2470
	11th to 20th	Night	14% 6% 2 5% 4 4 4 3 4 6% 49%	3042
	21st to end	Night	23% 7% 2 5% 3 4 4 3 6% 7% 37%	4532

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	32% 5% 8% 23% 26% 7%	2837
	11th to 20th	Day	35% 8% 9% 16% 28% 5%	2840
	21st to end	Day	42% 5% 6% 20% 21% 5%	3124
February	1st to 10th	Day	39% 7% 11% 20% 19% 5%	1710
	11th to 20th	Day	42% 6% 9% 18% 19% 5%	1836
	21st to end	Day	42% 7% 12% 17% 16% 6%	1579
March	1st to 10th	Day	43% 6% 9% 20% 14% 8%	3435
	11th to 20th	Day	32% 7% 8% 23% 20% 10%	3480
	21st to end	Day	42% 6% 10% 23% 15% 4	3815
April	1st to 10th	Day	32% 8% 12% 23% 20% 5%	2090
	11th to 20th	Day	37% 8% 12% 19% 19% 6%	2264
	21st to end	Day	39% 10% 11% 23% 14% 3	2470
May	1st to 10th	Day	44% 13% 13% 19% 10% 2	4120
	11th to 20th	Day	46% 11% 12% 23% 7% 1	4085
	21st to end	Day	33% 12% 16% 20% 15% 2	4489
June	1st to 10th	Day	42% 12% 16% 20% 9% 1	2470
	11th to 20th	Day	39% 13% 15% 19% 12% 2	2470
	21st to end	Day	38% 13% 15% 18% 13% 1	2470
July	1st to 10th	Day	38% 18% 17% 17% 7% 2	4055
	11th to 20th	Day	43% 14% 22% 16% 5% 1	4051
	21st to end	Day	57% 13% 12% 13% 4 1	4324
August	1st to 10th	Day	48% 14% 17% 17% 4	4091
	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	30% 5% 9% 22% 27% 7%	1864
	11th to 20th	Day	30% 5% 9% 22% 26% 7%	1710
	21st to end	Day	35% 5% 8% 21% 26% 5%	1710
December	1st to 10th	Day	45% 8% 7% 19% 15% 5%	1710
	11th to 20th	Day	31% 6% 10% 20% 24% 10%	2150
	21st to end	Day	40% 7% 10% 23% 17% 3	3124
January	1st to 10th	Night	33% 7% 11% 18% 24% 7%	4109
	11th to 20th	Night	35% 8% 8% 15% 26% 8%	4120
	21st to end	Night	43% 5% 8% 17% 20% 7%	4532
February	1st to 10th	Night	37% 7% 12% 22% 19% 4	2470
	11th to 20th	Night	41% 6% 9% 20% 18% 7%	2238
	21st to end	Night	40% 7% 13% 17% 17% 6%	1875
March	1st to 10th	Night	48% 8% 8% 16% 15% 5%	3418
	11th to 20th	Night	29% 8% 8% 23% 19% 13%	3480
	21st to end	Night	37% 7% 11% 24% 14% 7%	3817
April	1st to 10th	Night	33% 8% 12% 23% 19% 5%	2090
	11th to 20th	Night	34% 7% 11% 22% 18% 7%	1880
	21st to end	Night	34% 8% 15% 25% 16% 3	1710
May	1st to 10th	Night	41% 11% 15% 24% 8% 1	2840
	11th to 20th	Night	43% 9% 15% 23% 9% 1	2813
	21st to end	Night	30% 12% 18% 26% 12% 2	3087
June	1st to 10th	Night	36% 12% 18% 22% 10% 1	1710
	11th to 20th	Night	33% 12% 20% 23% 11% 1	1710
	21st to end	Night	31% 13% 19% 22% 11% 4	1710
July	1st to 10th	Night	37% 14% 18% 21% 8% 1	2795
	11th to 20th	Night	34% 13% 24% 23% 5% 1	2786
	21st to end	Night	51% 13% 16% 15% 5%	2988
August	1st to 10th	Night	46% 11% 22% 16% 5%	2814
	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	33% 5% 10% 20% 23% 8%	2284
	11th to 20th	Night	30% 5% 9% 20% 26% 10%	2470
	21st to end	Night	32% 6% 8% 24% 25% 5%	2470
December	1st to 10th	Night	42% 7% 9% 19% 18% 5%	2470
	11th to 20th	Night	35% 6% 9% 19% 24% 7%	3044
	21st to end	Night	42% 8% 13% 20% 15% 2	4532

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	27% 34% 29% 9% 1	270
	11th to 20th	Day	32% 26% 32% 10%	270
	21st to end	Day	29% 34% 27% 8% 2	297
February	1st to 10th	Day	27% 27% 31% 13% 1	270
	11th to 20th	Day	39% 30% 26% 4	270
	21st to end	Day	35% 30% 27% 7%	234
March	1st to 10th	Day	23% 26% 30% 15% 6%	460
	11th to 20th	Day	27% 28% 27% 14% 3 1	460
	21st to end	Day	29% 23% 28% 14% 6%	506
April	1st to 10th	Day	22% 26% 29% 17% 5% 1	460
	11th to 20th	Day	22% 23% 25% 18% 11% 2	452
	21st to end	Day	20% 20% 29% 17% 10% 4	460
May	1st to 10th	Day	18% 20% 32% 17% 8% 5%	463
	11th to 20th	Day	19% 14% 26% 23% 12% 7%	487
	21st to end	Day	19% 7% 23% 24% 16% 10%	529
June	1st to 10th	Day	13% 9% 24% 28% 17% 7%	490
	11th to 20th	Day	11% 7% 22% 31% 20% 9%	490
	21st to end	Day	10% 7% 28% 31% 16% 8%	490
July	1st to 10th	Day	10% 4 16% 31% 27% 13%	485
	11th to 20th	Day	7% 3 16% 31% 29% 14%	484
	21st to end	Day	11% 3 19% 30% 26% 12%	532
August	1st to 10th	Day	15% 5% 24% 24% 17% 15%	486
	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% 27% 23% 11% 3	444
	11th to 20th	Day	20% 25% 31% 17% 4 1	270
	21st to end	Day	19% 24% 33% 19% 6% 1	270
December	1st to 10th	Day	23% 29% 35% 11% 2	270
	11th to 20th	Day	21% 23% 35% 16% 5%	270
	21st to end	Day	32% 21% 30% 11% 4 2	297
January	1st to 10th	Night	18% 29% 30% 20% 4	760
	11th to 20th	Night	21% 24% 29% 23% 4	760
	21st to end	Night	19% 26% 31% 22% 3	836
February	1st to 10th	Night	16% 21% 31% 28% 4	760
	11th to 20th	Night	24% 30% 29% 16% 1	743
	21st to end	Night	20% 24% 33% 20% 3	628
March	1st to 10th	Night	17% 23% 32% 24% 4	760
	11th to 20th	Night	22% 18% 33% 20% 7% 1	760
	21st to end	Night	19% 19% 30% 24% 7%	836
April	1st to 10th	Night	17% 23% 28% 25% 8%	760
	11th to 20th	Night	20% 18% 26% 24% 12% 1	741
	21st to end	Night	17% 13% 26% 32% 12%	570
May	1st to 10th	Night	18% 11% 27% 29% 15% 1	570
	11th to 20th	Night	17% 9% 20% 33% 18% 3	561
	21st to end	Night	17% 5% 16% 25% 32% 5%	617
June	1st to 10th	Night	12% 3 18% 35% 29% 5%	570
	11th to 20th	Night	12% 2 19% 29% 32% 7%	570
	21st to end	Night	8% 3 14% 36% 35% 3	570
July	1st to 10th	Night	10% 3 9% 30% 43% 6%	555
	11th to 20th	Night	10% 1 10% 28% 43% 8%	552
	21st to end	Night	7% 2 11% 32% 40% 8%	624
August	1st to 10th	Night	16% 2 12% 28% 36% 6%	567
	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% 23% 30% 14% 3	755
	11th to 20th	Night	17% 16% 24% 32% 10% 1	760
	21st to end	Night	14% 21% 24% 29% 10% 1	760
December	1st to 10th	Night	20% 21% 27% 25% 6%	760
	11th to 20th	Night	18% 20% 29% 26% 7% 1	760
	21st to end	Night	26% 19% 24% 27% 4	836

Page generated at 2024-11-01 22:47:29. Version ac07

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