Dawn's Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dawn's Light Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 6% 2 6% 4 4 4 3 5% 7% 46%	131739
	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	131322
Transparency	Day	24% 37% 7% 17% 12% 3	135339
	Night	23% 36% 7% 17% 13% 3	135800
Seeing	Day	17% 11% 26% 25% 16% 5%	17480
	Night	17% 10% 27% 29% 15% 2	27366

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% 8% 2 5% 3 3 3 3 4 5% 48%	5051
	11th to 20th	Day	8% 6% 2 5% 4 3 3 3 4 5% 57%	5310
	21st to end	Day	19% 6% 2 5% 3 3 3 2 4 4 49%	5812
February	1st to 10th	Day	19% 8% 2 5% 5% 3 4 3 4 6% 42%	1890
	11th to 20th	Day	16% 8% 2 7% 4 3 4 4 4 6% 42%	2023
	21st to end	Day	16% 9% 2 6% 4 4 4 4 5% 5% 41%	1730
March	1st to 10th	Day	19% 10% 3 6% 3 3 4 3 5% 5% 40%	4880
	11th to 20th	Day	11% 5% 1 4 4 3 4 3 5% 7% 54%	5090
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 5% 7% 41%	5760
April	1st to 10th	Day	14% 7% 3 5% 3 3 3 4 5% 7% 46%	2520
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 4 5% 7% 41%	2494
	21st to end	Day	18% 6% 2 8% 5% 5% 4 3 5% 6% 38%	2520
May	1st to 10th	Day	14% 6% 2 8% 6% 6% 6% 5% 7% 9% 31%	5525
	11th to 20th	Day	15% 6% 3 8% 5% 5% 5% 4 6% 7% 36%	5993
	21st to end	Day	19% 5% 2 6% 4 4 4 4 6% 7% 38%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 6% 9% 44%	2800
	11th to 20th	Day	5% 4 2 4 4 5% 7% 4 7% 10% 48%	2800
	21st to end	Day	5% 4 2 4 4 5% 5% 4 7% 10% 50%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 4 6% 9% 46%	6360
	11th to 20th	Day	5% 3 1 5% 5% 4 5% 4 6% 11% 51%	5952
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	6422
August	1st to 10th	Day	4 2 1 4 3 3 3 4 5% 9% 61%	4206
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	3849
	21st to end	Day	6% 3 1 5% 4 4 4 4 5% 9% 55%	4312
September	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 4 5% 8% 48%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2400
	21st to end	Day	11% 5% 2 4 3 3 4 3 4 7% 55%	2141
October	1st to 10th	Day	9% 3 1 4 4 4 4 3 5% 6% 57%	2000
	11th to 20th	Day	14% 5% 1 5% 3 3 3 2 4 4 58%	2000
	21st to end	Day	20% 5% 2 5% 3 3 2 3 4 6% 48%	2200
November	1st to 10th	Day	12% 4 2 4 2 3 3 3 4 5% 57%	1973
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 5% 53%	1890
	21st to end	Day	20% 6% 2 6% 3 2 3 3 3 4 47%	1853
December	1st to 10th	Day	12% 7% 2 5% 3 3 4 2 4 5% 54%	2690
	11th to 20th	Day	17% 7% 3 6% 3 3 3 3 3 5% 47%	3088
	21st to end	Day	21% 12% 3 8% 4 3 4 3 4 5% 32%	4070
January	1st to 10th	Night	15% 8% 3 5% 3 3 3 3 5% 6% 45%	7854
	11th to 20th	Night	10% 5% 2 6% 4 3 3 3 4 5% 56%	7680
	21st to end	Night	19% 8% 2 5% 4 4 4 3 4 4 44%	8402
February	1st to 10th	Night	17% 9% 3 5% 4 3 3 3 4 5% 44%	2730
	11th to 20th	Night	17% 7% 3 5% 4 4 3 3 4 6% 45%	2480
	21st to end	Night	17% 7% 2 4 4 3 3 3 5% 6% 47%	2076
March	1st to 10th	Night	18% 8% 2 7% 4 4 4 3 4 5% 42%	5222
	11th to 20th	Night	12% 6% 2 6% 4 3 3 3 5% 6% 52%	5090
	21st to end	Night	12% 5% 2 6% 5% 4 5% 4 5% 6% 45%	5409
April	1st to 10th	Night	11% 5% 2 5% 4 4 3 4 5% 6% 51%	2100
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 6% 7% 49%	2075
	21st to end	Night	17% 5% 2 6% 4 4 4 4 6% 7% 42%	2100
May	1st to 10th	Night	17% 7% 2 6% 3 3 4 4 5% 8% 41%	4590
	11th to 20th	Night	17% 8% 3 6% 4 4 3 3 5% 8% 40%	4032
	21st to end	Night	20% 7% 3 6% 4 3 4 3 5% 7% 38%	3942
June	1st to 10th	Night	9% 4 2 4 4 4 4 4 6% 9% 51%	1600
	11th to 20th	Night	5% 3 1 3 3 3 4 4 6% 10% 58%	1600
	21st to end	Night	4 4 1 3 3 2 4 5% 6% 10% 60%	1600
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 7% 9% 48%	3618
	11th to 20th	Night	4 2 2 5% 3 4 4 4 7% 9% 56%	4020
	21st to end	Night	6% 5% 2 6% 5% 4 4 4 6% 10% 48%	4449
August	1st to 10th	Night	3 2 1 4 3 3 3 3 5% 8% 65%	2951
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 6% 8% 53%	3200
	21st to end	Night	4 3 1 4 3 4 4 4 5% 8% 60%	3586
September	1st to 10th	Night	10% 4 2 4 3 3 3 4 5% 7% 55%	2000
	11th to 20th	Night	10% 4 2 5% 4 4 5% 3 6% 8% 50%	2000
	21st to end	Night	9% 4 1 4 4 3 3 2 4 6% 58%	2232
October	1st to 10th	Night	7% 4 2 3 3 2 3 3 5% 5% 64%	2400
	11th to 20th	Night	12% 4 1 4 2 3 3 2 4 6% 59%	2400
	21st to end	Night	19% 5% 2 4 3 3 3 2 4 5% 51%	2640
November	1st to 10th	Night	13% 4 1 3 3 2 3 3 4 6% 58%	2543
	11th to 20th	Night	13% 4 1 4 3 3 3 3 4 7% 55%	2730
	21st to end	Night	17% 6% 2 4 3 3 3 2 5% 6% 49%	2762
December	1st to 10th	Night	13% 7% 2 4 3 4 3 3 4 6% 52%	4720
	11th to 20th	Night	17% 8% 2 6% 3 3 3 3 3 5% 47%	5339
	21st to end	Night	24% 9% 3 7% 4 4 4 3 5% 5% 33%	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	34% 20% 8% 25% 11% 2	5052
	11th to 20th	Day	22% 21% 4 26% 20% 6%	5310
	21st to end	Day	32% 22% 7% 14% 18% 8%	5812
February	1st to 10th	Day	19% 59% 6% 11% 6% 1	1890
	11th to 20th	Day	21% 58% 4 9% 7% 1	2033
	21st to end	Day	18% 60% 2 8% 7% 5%	1730
March	1st to 10th	Day	36% 28% 4 15% 7% 10%	4880
	11th to 20th	Day	18% 28% 4 21% 18% 10%	5090
	21st to end	Day	29% 29% 7% 22% 13%	5762
April	1st to 10th	Day	18% 59% 5% 8% 8% 1	2520
	11th to 20th	Day	19% 60% 6% 8% 5% 2	2494
	21st to end	Day	20% 59% 4 9% 7% 1	2520
May	1st to 10th	Day	34% 30% 10% 15% 8% 1	5560
	11th to 20th	Day	36% 31% 10% 15% 8% 1	5997
	21st to end	Day	34% 30% 5% 17% 12% 2	6949
June	1st to 10th	Day	15% 64% 5% 7% 8% 1	2800
	11th to 20th	Day	14% 64% 6% 10% 7% 1	2800
	21st to end	Day	13% 63% 8% 12% 5%	2800
July	1st to 10th	Day	27% 28% 9% 19% 16% 1	6291
	11th to 20th	Day	19% 29% 14% 26% 12% 1	5881
	21st to end	Day	26% 28% 12% 25% 8% 1	6240
August	1st to 10th	Day	10% 37% 11% 23% 18% 1	4192
	11th to 20th	Day	24% 37% 9% 18% 11% 1	3885
	21st to end	Day	15% 37% 9% 30% 9% 1	4311
September	1st to 10th	Day	11% 58% 6% 15% 7% 2	2400
	11th to 20th	Day	15% 58% 4 11% 11% 1	2400
	21st to end	Day	15% 58% 4 12% 11% 1	2141
October	1st to 10th	Day	8% 36% 3 8% 33% 12%	3260
	11th to 20th	Day	9% 35% 3 15% 32% 6%	3240
	21st to end	Day	18% 36% 4 21% 18% 3	3538
November	1st to 10th	Day	18% 53% 3 13% 10% 3	1973
	11th to 20th	Day	16% 54% 3 12% 11% 4	1890
	21st to end	Day	22% 53% 2 11% 10% 1	1853
December	1st to 10th	Day	22% 34% 5% 25% 13% 1	2690
	11th to 20th	Day	27% 30% 4 22% 14% 2	3085
	21st to end	Day	45% 27% 6% 14% 7% 2	4070
January	1st to 10th	Night	33% 21% 8% 26% 10% 1	7855
	11th to 20th	Night	24% 21% 4 19% 28% 5%	7680
	21st to end	Night	36% 22% 6% 16% 16% 4	8402
February	1st to 10th	Night	16% 59% 5% 12% 7% 1	2730
	11th to 20th	Night	19% 58% 4 10% 8% 1	2481
	21st to end	Night	14% 59% 4 9% 10% 4	2076
March	1st to 10th	Night	36% 29% 4 12% 12% 8%	5222
	11th to 20th	Night	23% 28% 4 21% 14% 11%	5090
	21st to end	Night	26% 29% 6% 24% 14% 1	5409
April	1st to 10th	Night	16% 59% 6% 10% 9% 1	2100
	11th to 20th	Night	16% 60% 6% 11% 6% 1	2075
	21st to end	Night	16% 60% 5% 10% 8% 1	2100
May	1st to 10th	Night	32% 30% 12% 19% 7%	4620
	11th to 20th	Night	33% 30% 9% 18% 7% 2	4033
	21st to end	Night	37% 30% 5% 15% 10% 3	3950
June	1st to 10th	Night	12% 63% 6% 9% 9% 1	1600
	11th to 20th	Night	9% 62% 6% 12% 9% 1	1600
	21st to end	Night	9% 61% 7% 15% 7% 1	1600
July	1st to 10th	Night	26% 29% 11% 16% 17% 1	3580
	11th to 20th	Night	18% 29% 15% 27% 11%	3971
	21st to end	Night	24% 28% 13% 24% 11%	4320
August	1st to 10th	Night	10% 37% 12% 25% 16%	2941
	11th to 20th	Night	19% 37% 14% 19% 10% 1	3230
	21st to end	Night	13% 37% 8% 27% 15% 1	3586
September	1st to 10th	Night	8% 58% 5% 17% 10% 2	2000
	11th to 20th	Night	13% 57% 5% 14% 10% 1	2000
	21st to end	Night	13% 58% 6% 12% 10% 2	2232
October	1st to 10th	Night	6% 35% 3 8% 31% 17%	3920
	11th to 20th	Night	10% 35% 3 11% 26% 15%	3899
	21st to end	Night	17% 35% 4 18% 22% 4	4252
November	1st to 10th	Night	18% 53% 4 10% 13% 3	2543
	11th to 20th	Night	13% 54% 3 12% 12% 6%	2730
	21st to end	Night	18% 54% 4 13% 11% 1	2762
December	1st to 10th	Night	21% 35% 5% 26% 12% 1	4720
	11th to 20th	Night	28% 30% 5% 18% 16% 3	5341
	21st to end	Night	43% 26% 7% 16% 6% 1	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	29% 17% 38% 14% 2	330
	11th to 20th	Day	29% 24% 33% 13% 1	330
	21st to end	Day	24% 24% 38% 12% 2	363
February	1st to 10th	Day	29% 20% 36% 11% 3	330
	11th to 20th	Day	33% 22% 33% 9% 2 1	330
	21st to end	Day	34% 23% 32% 10% 1	273
March	1st to 10th	Day	29% 21% 36% 12% 3	330
	11th to 20th	Day	31% 19% 39% 10%	330
	21st to end	Day	27% 22% 37% 11% 4	437
April	1st to 10th	Day	26% 15% 33% 19% 6% 1	540
	11th to 20th	Day	24% 22% 31% 16% 6%	532
	21st to end	Day	20% 20% 31% 21% 6% 1	540
May	1st to 10th	Day	17% 20% 34% 21% 7% 1	543
	11th to 20th	Day	17% 11% 31% 24% 14% 3	696
	21st to end	Day	12% 8% 32% 29% 14% 4	789
June	1st to 10th	Day	10% 4 25% 28% 25% 8%	730
	11th to 20th	Day	8% 3 20% 40% 24% 4	730
	21st to end	Day	6% 4 25% 36% 24% 5%	730
July	1st to 10th	Day	8% 2 18% 31% 30% 11%	721
	11th to 20th	Day	6% 2 17% 33% 31% 10%	718
	21st to end	Day	8% 3 12% 32% 35% 11%	794
August	1st to 10th	Day	9% 2 13% 41% 27% 8%	711
	11th to 20th	Day	14% 3 14% 30% 29% 9%	519
	21st to end	Day	10% 1 12% 32% 29% 15%	583
September	1st to 10th	Day	8% 4 13% 36% 31% 8%	530
	11th to 20th	Day	9% 3 15% 28% 32% 12%	530
	21st to end	Day	13% 7% 23% 28% 20% 8%	455
October	1st to 10th	Day	9% 7% 24% 41% 17% 2	330
	11th to 20th	Day	16% 15% 35% 23% 11%	330
	21st to end	Day	29% 14% 31% 20% 4 1	363
November	1st to 10th	Day	18% 13% 34% 26% 7% 2	330
	11th to 20th	Day	18% 17% 38% 17% 9% 1	330
	21st to end	Day	32% 13% 32% 17% 4 2	330
December	1st to 10th	Day	21% 25% 31% 16% 6% 1	330
	11th to 20th	Day	26% 20% 32% 15% 6%	330
	21st to end	Day	30% 20% 34% 13% 3	363
January	1st to 10th	Night	22% 18% 34% 22% 4	1050
	11th to 20th	Night	21% 21% 36% 17% 4	1050
	21st to end	Night	22% 14% 40% 22% 3	1155
February	1st to 10th	Night	24% 15% 35% 24% 2	1050
	11th to 20th	Night	24% 18% 37% 18% 2	845
	21st to end	Night	24% 19% 36% 19% 2	692
March	1st to 10th	Night	23% 16% 35% 24% 3	725
	11th to 20th	Night	24% 14% 31% 28% 3	630
	21st to end	Night	22% 13% 37% 22% 6%	693
April	1st to 10th	Night	15% 17% 37% 27% 5%	630
	11th to 20th	Night	18% 16% 31% 26% 8%	623
	21st to end	Night	19% 12% 35% 26% 7%	630
May	1st to 10th	Night	15% 10% 33% 32% 11%	630
	11th to 20th	Night	18% 6% 31% 29% 14% 2	609
	21st to end	Night	13% 6% 25% 34% 19% 4	650
June	1st to 10th	Night	10% 4 20% 35% 28% 3	600
	11th to 20th	Night	6% 1 17% 47% 26% 3	600
	21st to end	Night	5% 2 20% 40% 31% 4	600
July	1st to 10th	Night	7% 2 12% 33% 39% 8%	585
	11th to 20th	Night	4 1 9% 37% 42% 7%	582
	21st to end	Night	6% 2 12% 35% 38% 7%	657
August	1st to 10th	Night	5% 1 10% 40% 39% 5%	597
	11th to 20th	Night	7% 1 11% 39% 37% 4	594
	21st to end	Night	7% 2 11% 33% 42% 6%	660
September	1st to 10th	Night	11% 3 12% 35% 34% 4	600
	11th to 20th	Night	10% 1 13% 33% 38% 7%	600
	21st to end	Night	10% 4 20% 34% 28% 3	762
October	1st to 10th	Night	9% 5% 20% 36% 27% 3	800
	11th to 20th	Night	15% 9% 29% 30% 17% 1	800
	21st to end	Night	22% 11% 28% 29% 9%	880
November	1st to 10th	Night	15% 9% 30% 32% 12% 1	820
	11th to 20th	Night	17% 10% 33% 31% 9%	840
	21st to end	Night	22% 13% 30% 27% 9%	872
December	1st to 10th	Night	22% 13% 32% 28% 5%	1050
	11th to 20th	Night	25% 12% 29% 29% 5%	1050
	21st to end	Night	27% 14% 31% 25% 4	1155

Page generated at 2026-05-14 00:58:57. Version ac07

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