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 2025-01-22.

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%	104816
	Night	14% 6% 2 5% 4 3 3 3 5% 6% 49%	104648
Transparency	Day	21% 45% 6% 15% 11% 2	108455
	Night	20% 44% 6% 16% 12% 3	109150
Seeing	Day	17% 11% 26% 25% 17% 5%	15715
	Night	17% 10% 27% 29% 15% 2	25625

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	17% 8% 2 6% 3 3 3 2 4 5% 48%	2805
	11th to 20th	Day	13% 7% 2 5% 4 3 3 3 4 4 52%	2940
	21st to end	Day	25% 8% 2 4 2 2 3 2 3 4 45%	3144
February	1st to 10th	Day	18% 8% 2 5% 5% 4 3 3 4 5% 42%	1710
	11th to 20th	Day	15% 8% 2 7% 4 3 4 4 4 6% 43%	1825
	21st to end	Day	17% 9% 2 6% 5% 5% 4 4 5% 5% 38%	1570
March	1st to 10th	Day	23% 8% 3 6% 3 3 3 2 5% 5% 40%	3330
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 3 6% 52%	3480
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 42%	3931
April	1st to 10th	Day	13% 7% 3 6% 4 3 3 4 5% 7% 46%	2280
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	2254
	21st to end	Day	18% 6% 2 8% 5% 5% 4 3 5% 6% 39%	2280
May	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 9% 35%	3765
	11th to 20th	Day	15% 5% 2 7% 5% 5% 5% 4 6% 7% 40%	4123
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	4824
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	2660
	11th to 20th	Day	5% 4 1 4 4 5% 7% 4 7% 10% 49%	2660
	21st to end	Day	5% 4 2 4 4 5% 5% 4 7% 10% 50%	2660
July	1st to 10th	Day	9% 4 2 6% 4 5% 6% 4 6% 9% 45%	4440
	11th to 20th	Day	6% 3 1 4 4 4 4 3 5% 10% 55%	4158
	21st to end	Day	9% 6% 2 8% 6% 6% 7% 5% 7% 10% 34%	4465
August	1st to 10th	Day	4 2 1 4 3 3 3 4 5% 9% 61%	4077
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 8% 47%	3729
	21st to end	Day	6% 3 1 5% 4 4 4 4 5% 9% 55%	4180
September	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 4 5% 8% 48%	2280
	11th to 20th	Day	13% 5% 2 6% 3 4 4 3 5% 7% 47%	2280
	21st to end	Day	10% 5% 2 4 3 3 4 3 5% 7% 55%	2034
October	1st to 10th	Day	9% 3 1 4 4 4 3 3 5% 6% 57%	1900
	11th to 20th	Day	14% 5% 1 5% 3 2 3 2 4 4 57%	1900
	21st to end	Day	19% 5% 2 5% 3 3 3 3 4 6% 48%	2090
November	1st to 10th	Day	13% 5% 2 4 2 3 3 3 4 5% 56%	1877
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 5% 53%	1800
	21st to end	Day	19% 6% 2 6% 4 2 3 3 3 4 48%	1765
December	1st to 10th	Day	12% 7% 2 4 3 3 4 2 4 5% 54%	2610
	11th to 20th	Day	17% 7% 3 6% 3 3 3 3 3 4 47%	3008
	21st to end	Day	22% 13% 3 8% 4 4 4 3 4 5% 32%	3982
January	1st to 10th	Night	16% 7% 2 5% 4 3 4 4 5% 7% 44%	4365
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 4 5% 49%	4250
	21st to end	Night	25% 10% 2 4 3 3 3 2 3 4 42%	4546
February	1st to 10th	Night	18% 9% 3 5% 4 3 3 3 4 5% 42%	2470
	11th to 20th	Night	16% 7% 3 5% 4 4 3 3 4 6% 46%	2238
	21st to end	Night	18% 8% 2 4 4 3 3 3 5% 5% 45%	1884
March	1st to 10th	Night	22% 6% 2 5% 3 3 3 3 4 5% 43%	3551
	11th to 20th	Night	15% 6% 2 5% 3 3 2 3 4 5% 53%	3480
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 5% 6% 43%	3698
April	1st to 10th	Night	10% 5% 2 5% 4 4 3 4 5% 6% 52%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 6% 6% 49%	1875
	21st to end	Night	18% 5% 2 5% 4 4 4 4 6% 7% 42%	1900
May	1st to 10th	Night	18% 5% 2 4 3 3 4 3 5% 8% 45%	3130
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 5% 9% 46%	2775
	21st to end	Night	12% 7% 3 6% 4 4 5% 4 6% 8% 42%	2743
June	1st to 10th	Night	9% 4 2 4 4 4 4 4 6% 9% 51%	1520
	11th to 20th	Night	4 3 1 3 3 2 4 4 6% 10% 60%	1520
	21st to end	Night	4 4 1 3 3 2 4 5% 6% 10% 60%	1520
July	1st to 10th	Night	7% 3 2 4 3 4 5% 4 7% 10% 50%	2530
	11th to 20th	Night	4 2 1 4 3 3 3 4 6% 8% 61%	2807
	21st to end	Night	8% 6% 3 8% 6% 5% 5% 4 6% 8% 41%	3098
August	1st to 10th	Night	3 2 1 4 3 3 3 3 5% 8% 65%	2860
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	3100
	21st to end	Night	4 3 1 4 3 3 4 4 5% 8% 60%	3476
September	1st to 10th	Night	10% 4 2 4 3 3 3 3 5% 7% 55%	1900
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 7% 50%	1900
	21st to end	Night	9% 4 2 4 4 4 3 2 5% 6% 59%	2120
October	1st to 10th	Night	7% 4 2 3 3 2 3 3 5% 6% 64%	2280
	11th to 20th	Night	12% 4 1 4 2 3 3 2 4 6% 58%	2280
	21st to end	Night	18% 5% 2 4 3 3 3 2 3 5% 51%	2508
November	1st to 10th	Night	13% 5% 1 3 3 2 3 3 4 6% 57%	2419
	11th to 20th	Night	13% 4 2 4 3 3 3 3 4 7% 55%	2600
	21st to end	Night	16% 6% 2 4 3 3 3 2 5% 6% 50%	2630
December	1st to 10th	Night	13% 7% 2 4 3 4 3 3 4 6% 53%	4580
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 3 5% 47%	5199
	21st to end	Night	24% 9% 3 7% 4 4 4 3 5% 5% 33%	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	29% 35% 5% 20% 10% 2	2806
	11th to 20th	Day	23% 35% 6% 18% 14% 4	2940
	21st to end	Day	30% 38% 6% 8% 14% 3	3144
February	1st to 10th	Day	15% 65% 5% 9% 5% 1	1710
	11th to 20th	Day	17% 65% 4 7% 6% 1	1835
	21st to end	Day	18% 65% 2 6% 5% 4	1570
March	1st to 10th	Day	33% 41% 4 10% 5% 8%	3330
	11th to 20th	Day	17% 40% 4 20% 14% 5%	3480
	21st to end	Day	23% 41% 7% 16% 13%	3931
April	1st to 10th	Day	15% 65% 4 8% 7% 1	2280
	11th to 20th	Day	17% 66% 4 7% 5% 1	2254
	21st to end	Day	17% 65% 3 8% 7% 1	2280
May	1st to 10th	Day	26% 42% 9% 14% 8% 1	3800
	11th to 20th	Day	28% 42% 9% 16% 6%	4127
	21st to end	Day	27% 42% 5% 15% 11% 1	4837
June	1st to 10th	Day	14% 67% 5% 6% 7% 1	2660
	11th to 20th	Day	12% 66% 6% 9% 6% 1	2660
	21st to end	Day	12% 66% 7% 11% 4	2660
July	1st to 10th	Day	24% 37% 10% 18% 9% 1	4371
	11th to 20th	Day	14% 37% 6% 26% 15% 1	4087
	21st to end	Day	30% 38% 10% 17% 4 1	4325
August	1st to 10th	Day	10% 38% 11% 21% 18% 1	4063
	11th to 20th	Day	25% 38% 8% 17% 11% 1	3765
	21st to end	Day	14% 39% 9% 30% 9% 1	4179
September	1st to 10th	Day	11% 61% 6% 13% 7% 2	2280
	11th to 20th	Day	15% 61% 4 9% 11% 1	2280
	21st to end	Day	13% 60% 4 12% 9%	2034
October	1st to 10th	Day	8% 37% 3 7% 33% 12%	3160
	11th to 20th	Day	9% 36% 3 14% 32% 6%	3140
	21st to end	Day	17% 37% 4 21% 18% 3	3428
November	1st to 10th	Day	18% 56% 3 11% 9% 3	1877
	11th to 20th	Day	15% 57% 2 11% 11% 5%	1800
	21st to end	Day	21% 56% 2 10% 10% 1	1765
December	1st to 10th	Day	21% 35% 4 25% 13% 1	2610
	11th to 20th	Day	27% 31% 4 22% 14% 2	3005
	21st to end	Day	45% 27% 6% 13% 7% 2	3982
January	1st to 10th	Night	27% 36% 5% 20% 11% 1	4365
	11th to 20th	Night	25% 36% 6% 19% 12% 2	4250
	21st to end	Night	32% 39% 7% 12% 10% 2	4546
February	1st to 10th	Night	15% 65% 4 10% 6% 1	2470
	11th to 20th	Night	15% 64% 4 9% 7% 1	2239
	21st to end	Night	14% 64% 4 7% 7% 4	1884
March	1st to 10th	Night	32% 41% 4 10% 8% 6%	3551
	11th to 20th	Night	19% 40% 4 20% 14% 4	3480
	21st to end	Night	22% 41% 6% 21% 10%	3698
April	1st to 10th	Night	13% 65% 4 8% 8% 1	1900
	11th to 20th	Night	14% 66% 4 10% 5% 1	1875
	21st to end	Night	13% 66% 4 8% 8% 1	1900
May	1st to 10th	Night	24% 42% 9% 17% 7% 1	3160
	11th to 20th	Night	22% 42% 11% 16% 8%	2776
	21st to end	Night	26% 41% 6% 16% 9% 1	2751
June	1st to 10th	Night	11% 66% 6% 8% 8% 1	1520
	11th to 20th	Night	6% 65% 6% 13% 9% 1	1520
	21st to end	Night	9% 64% 7% 13% 7% 1	1520
July	1st to 10th	Night	21% 38% 13% 17% 11% 1	2490
	11th to 20th	Night	13% 37% 6% 29% 15% 1	2755
	21st to end	Night	30% 38% 10% 16% 7%	2999
August	1st to 10th	Night	9% 38% 12% 24% 16%	2850
	11th to 20th	Night	19% 38% 14% 18% 10% 1	3130
	21st to end	Night	12% 38% 8% 27% 15% 1	3476
September	1st to 10th	Night	7% 60% 5% 16% 9% 2	1900
	11th to 20th	Night	12% 60% 5% 11% 10% 1	1900
	21st to end	Night	11% 61% 6% 12% 9% 1	2120
October	1st to 10th	Night	6% 36% 3 8% 30% 17%	3800
	11th to 20th	Night	10% 36% 2 10% 26% 16%	3779
	21st to end	Night	16% 36% 4 18% 22% 4	4120
November	1st to 10th	Night	18% 55% 3 9% 11% 3	2419
	11th to 20th	Night	12% 56% 3 10% 12% 6%	2600
	21st to end	Night	16% 57% 4 12% 11% 1	2630
December	1st to 10th	Night	21% 35% 5% 26% 12% 1	4580
	11th to 20th	Night	28% 31% 5% 18% 15% 3	5201
	21st to end	Night	44% 27% 7% 15% 6% 1	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	28% 19% 37% 13% 2	300
	11th to 20th	Day	29% 24% 31% 14% 1	300
	21st to end	Day	23% 24% 37% 13% 3	300
February	1st to 10th	Day	28% 22% 35% 11% 3	270
	11th to 20th	Day	33% 21% 33% 11% 3 1	270
	21st to end	Day	36% 24% 31% 7% 2	225
March	1st to 10th	Day	28% 20% 37% 11% 3	270
	11th to 20th	Day	34% 18% 36% 11%	270
	21st to end	Day	28% 22% 36% 11% 4	363
April	1st to 10th	Day	25% 16% 34% 18% 7% 1	460
	11th to 20th	Day	25% 21% 29% 17% 7%	452
	21st to end	Day	20% 20% 31% 21% 6% 1	460
May	1st to 10th	Day	18% 19% 33% 21% 7% 2	463
	11th to 20th	Day	17% 10% 30% 25% 14% 4	621
	21st to end	Day	11% 8% 32% 30% 14% 5%	734
June	1st to 10th	Day	10% 4 25% 28% 25% 8%	680
	11th to 20th	Day	7% 3 21% 41% 24% 4	680
	21st to end	Day	7% 4 27% 36% 22% 5%	680
July	1st to 10th	Day	8% 2 18% 31% 30% 11%	671
	11th to 20th	Day	6% 2 18% 33% 31% 10%	668
	21st to end	Day	9% 2 12% 32% 34% 11%	739
August	1st to 10th	Day	9% 2 12% 41% 27% 9%	662
	11th to 20th	Day	15% 3 15% 30% 30% 7%	479
	21st to end	Day	10% 1 11% 31% 30% 16%	539
September	1st to 10th	Day	8% 4 13% 35% 32% 8%	490
	11th to 20th	Day	10% 3 16% 29% 30% 11%	490
	21st to end	Day	12% 7% 24% 29% 20% 8%	419
October	1st to 10th	Day	10% 7% 23% 41% 17% 2	300
	11th to 20th	Day	17% 15% 36% 22% 10%	300
	21st to end	Day	28% 14% 31% 22% 5% 1	330
November	1st to 10th	Day	18% 12% 34% 27% 7% 2	300
	11th to 20th	Day	18% 17% 38% 17% 10% 1	300
	21st to end	Day	31% 13% 33% 17% 5% 2	300
December	1st to 10th	Day	21% 26% 29% 16% 6% 1	300
	11th to 20th	Day	28% 20% 31% 15% 6%	300
	21st to end	Day	31% 20% 32% 14% 3	330
January	1st to 10th	Night	23% 18% 33% 22% 4	1000
	11th to 20th	Night	22% 20% 36% 17% 4	1000
	21st to end	Night	22% 15% 38% 22% 3	1051
February	1st to 10th	Night	25% 15% 34% 23% 2	950
	11th to 20th	Night	23% 18% 39% 18% 2	763
	21st to end	Night	26% 20% 36% 17% 2	628
March	1st to 10th	Night	23% 15% 35% 24% 3	656
	11th to 20th	Night	24% 13% 30% 30% 3	570
	21st to end	Night	23% 12% 35% 22% 7%	627
April	1st to 10th	Night	14% 17% 38% 26% 5%	570
	11th to 20th	Night	19% 16% 30% 26% 8% 1	563
	21st to end	Night	19% 12% 36% 26% 6%	570
May	1st to 10th	Night	15% 9% 32% 31% 12%	570
	11th to 20th	Night	18% 6% 31% 28% 15% 2	561
	21st to end	Night	12% 6% 25% 35% 19% 4	617
June	1st to 10th	Night	10% 5% 19% 34% 29% 3	570
	11th to 20th	Night	5% 1 16% 48% 26% 3	570
	21st to end	Night	5% 2 21% 40% 29% 3	570
July	1st to 10th	Night	6% 2 12% 34% 37% 8%	555
	11th to 20th	Night	4 1 9% 38% 41% 7%	552
	21st to end	Night	6% 2 12% 35% 38% 7%	624
August	1st to 10th	Night	5% 1 10% 40% 39% 5%	567
	11th to 20th	Night	7% 1 12% 40% 35% 4	564
	21st to end	Night	7% 1 11% 32% 43% 6%	627
September	1st to 10th	Night	11% 3 12% 34% 35% 5%	570
	11th to 20th	Night	10% 1 13% 32% 37% 7%	570
	21st to end	Night	10% 4 21% 34% 28% 3	724
October	1st to 10th	Night	9% 5% 21% 35% 27% 3	760
	11th to 20th	Night	15% 9% 30% 30% 15% 1	760
	21st to end	Night	21% 11% 28% 30% 10%	836
November	1st to 10th	Night	15% 9% 30% 32% 13% 1	780
	11th to 20th	Night	17% 11% 32% 31% 9%	800
	21st to end	Night	21% 12% 30% 27% 9%	830
December	1st to 10th	Night	22% 13% 32% 27% 5%	1000
	11th to 20th	Night	25% 12% 29% 29% 5%	1000
	21st to end	Night	27% 14% 30% 25% 3	1100

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