Dalton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dalton clear sky chart from 2005-11-06 to 2025-03-12.

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% 7% 2 6% 5% 5% 5% 4 6% 7% 41%	109213
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	109825
Transparency	Day	40% 7% 11% 20% 18% 4	113019
	Night	38% 7% 11% 21% 19% 5%	114157
Seeing	Day	17% 12% 24% 23% 17% 8%	15139
	Night	16% 10% 23% 28% 19% 3	25267

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% 11% 3 5% 3 3 3 3 4 4 44%	4056
	11th to 20th	Day	14% 9% 5% 5% 4 3 3 2 3 4 48%	4060
	21st to end	Day	21% 9% 2 6% 4 4 3 3 5% 6% 37%	4714
February	1st to 10th	Day	17% 10% 3 7% 4 3 4 4 4 7% 37%	2000
	11th to 20th	Day	22% 8% 3 8% 4 4 4 3 3 6% 35%	1948
	21st to end	Day	22% 8% 3 6% 4 3 3 3 4 6% 39%	1650
March	1st to 10th	Day	24% 9% 3 6% 4 4 4 3 6% 6% 31%	3243
	11th to 20th	Day	14% 6% 2 6% 4 3 3 3 4 5% 49%	3491
	21st to end	Day	18% 6% 2 8% 5% 4 5% 3 5% 7% 39%	3815
April	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 5% 6% 8% 43%	2226
	11th to 20th	Day	13% 6% 3 6% 6% 4 4 3 4 6% 45%	2254
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 3 6% 7% 44%	2403
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 6% 8% 40%	4082
	11th to 20th	Day	17% 7% 3 7% 5% 5% 4 4 6% 6% 36%	4081
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4476
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 6% 5% 8% 9% 37%	2470
	11th to 20th	Day	5% 4 2 5% 5% 5% 7% 6% 8% 10% 43%	2470
	21st to end	Day	7% 5% 2 6% 6% 6% 5% 5% 8% 11% 38%	2470
July	1st to 10th	Day	7% 5% 3 8% 7% 7% 7% 7% 9% 11% 29%	4120
	11th to 20th	Day	9% 6% 2 6% 5% 6% 7% 5% 8% 10% 36%	4120
	21st to end	Day	14% 10% 4 10% 7% 7% 7% 6% 8% 8% 19%	4466
August	1st to 10th	Day	5% 3 2 6% 5% 6% 7% 6% 9% 12% 41%	4104
	11th to 20th	Day	7% 4 2 6% 4 6% 5% 5% 7% 11% 42%	3729
	21st to end	Day	8% 4 2 5% 4 4 6% 3 6% 8% 50%	4180
September	1st to 10th	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 9% 40%	2143
	11th to 20th	Day	15% 6% 3 6% 4 4 5% 3 5% 7% 42%	2090
	21st to end	Day	14% 4 2 6% 4 4 3 3 5% 7% 48%	2090
October	1st to 10th	Day	11% 4 3 4 4 4 4 2 5% 5% 54%	2090
	11th to 20th	Day	11% 4 2 5% 3 4 3 4 3 5% 55%	2016
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 6% 53%	2025
November	1st to 10th	Day	12% 6% 2 4 3 3 3 2 4 5% 54%	1762
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 5% 5% 50%	1800
	21st to end	Day	15% 6% 2 5% 4 4 4 3 4 6% 46%	1800
December	1st to 10th	Day	17% 8% 2 5% 3 4 4 3 4 5% 45%	2930
	11th to 20th	Day	16% 11% 4 9% 4 4 4 3 4 6% 35%	3373
	21st to end	Day	18% 9% 3 6% 4 4 4 3 5% 7% 37%	4466
January	1st to 10th	Night	15% 9% 2 7% 4 4 4 2 4 5% 44%	5889
	11th to 20th	Night	17% 7% 3 6% 3 3 3 2 3 4 48%	5900
	21st to end	Night	19% 9% 2 5% 3 4 4 3 4 5% 43%	6241
February	1st to 10th	Night	16% 7% 3 6% 4 3 4 3 5% 6% 43%	2400
	11th to 20th	Night	21% 7% 3 7% 5% 4 4 3 4 4 38%	2338
	21st to end	Night	21% 8% 2 5% 4 4 4 3 5% 5% 38%	1980
March	1st to 10th	Night	22% 9% 3 8% 5% 4 4 3 4 6% 32%	3829
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 6% 7% 49%	3498
	21st to end	Night	16% 6% 2 8% 5% 4 4 4 4 6% 41%	3817
April	1st to 10th	Night	13% 4 2 4 3 4 4 4 5% 8% 49%	1970
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 49%	1875
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	1777
May	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 6% 8% 40%	2813
	11th to 20th	Night	16% 6% 2 6% 5% 4 5% 4 6% 7% 39%	2812
	21st to end	Night	8% 4 2 6% 4 5% 6% 4 8% 10% 44%	3079
June	1st to 10th	Night	8% 5% 2 7% 6% 6% 5% 5% 7% 11% 38%	1710
	11th to 20th	Night	5% 3 1 4 5% 4 4 5% 9% 11% 49%	1710
	21st to end	Night	6% 5% 2 5% 4 5% 5% 5% 8% 13% 44%	1710
July	1st to 10th	Night	8% 5% 3 7% 6% 5% 7% 5% 8% 11% 34%	2840
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 5% 8% 11% 42%	2840
	21st to end	Night	13% 10% 4 9% 8% 6% 6% 5% 7% 9% 25%	3087
August	1st to 10th	Night	5% 3 2 6% 5% 5% 5% 6% 8% 11% 45%	2825
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 49%	3095
	21st to end	Night	7% 4 1 5% 4 3 5% 4 7% 8% 53%	3476
September	1st to 10th	Night	9% 6% 2 7% 6% 6% 5% 5% 7% 8% 41%	2015
	11th to 20th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	2090
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 7% 49%	2090
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 2 5% 5% 58%	2090
	11th to 20th	Night	11% 5% 2 6% 4 3 3 2 4 4 56%	2161
	21st to end	Night	12% 5% 2 5% 4 3 3 3 4 4 55%	2570
November	1st to 10th	Night	12% 5% 2 5% 4 2 3 3 4 6% 53%	2536
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 52%	2600
	21st to end	Night	16% 5% 2 5% 4 3 4 4 5% 5% 47%	2600
December	1st to 10th	Night	18% 8% 2 6% 3 3 3 3 4 5% 45%	4250
	11th to 20th	Night	18% 10% 3 7% 4 3 4 2 4 5% 40%	4822
	21st to end	Night	20% 7% 2 6% 4 5% 5% 4 4 6% 37%	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	43% 5% 8% 21% 22% 2	4057
	11th to 20th	Day	40% 4 7% 19% 29% 2	4060
	21st to end	Day	48% 4 7% 19% 19% 3	4714
February	1st to 10th	Day	47% 5% 8% 19% 18% 3	2000
	11th to 20th	Day	51% 4 6% 18% 16% 4	1958
	21st to end	Day	49% 3 7% 15% 20% 6%	1650
March	1st to 10th	Day	51% 7% 10% 10% 14% 9%	3243
	11th to 20th	Day	37% 6% 7% 20% 22% 7%	3491
	21st to end	Day	45% 5% 10% 21% 16% 3	3815
April	1st to 10th	Day	38% 8% 11% 21% 17% 5%	2226
	11th to 20th	Day	40% 5% 7% 18% 24% 7%	2254
	21st to end	Day	37% 7% 11% 26% 16% 3	2403
May	1st to 10th	Day	40% 9% 13% 21% 15% 2	4120
	11th to 20th	Day	45% 9% 13% 22% 11% 1	4085
	21st to end	Day	36% 9% 12% 23% 19% 2	4489
June	1st to 10th	Day	41% 10% 15% 21% 12% 1	2470
	11th to 20th	Day	31% 11% 18% 25% 13% 2	2470
	21st to end	Day	37% 11% 16% 19% 13% 3	2470
July	1st to 10th	Day	41% 14% 19% 19% 7% 1	4055
	11th to 20th	Day	39% 11% 21% 20% 6% 2	4051
	21st to end	Day	58% 12% 13% 11% 5%	4324
August	1st to 10th	Day	30% 11% 17% 32% 9%	4091
	11th to 20th	Day	33% 9% 17% 24% 15% 2	3765
	21st to end	Day	31% 8% 12% 28% 18% 2	4179
September	1st to 10th	Day	39% 11% 15% 15% 15% 5%	2143
	11th to 20th	Day	41% 8% 12% 19% 18% 2	2090
	21st to end	Day	35% 7% 13% 17% 21% 7%	2090
October	1st to 10th	Day	25% 5% 8% 14% 36% 12%	3480
	11th to 20th	Day	21% 4 6% 19% 30% 19%	3334
	21st to end	Day	27% 4 7% 25% 28% 10%	3314
November	1st to 10th	Day	33% 4 7% 18% 29% 8%	1762
	11th to 20th	Day	36% 4 7% 21% 25% 8%	1800
	21st to end	Day	40% 5% 8% 18% 25% 5%	1800
December	1st to 10th	Day	43% 3 5% 19% 25% 5%	2930
	11th to 20th	Day	52% 4 7% 18% 15% 3	3370
	21st to end	Day	45% 5% 10% 27% 11% 1	4466
January	1st to 10th	Night	43% 5% 8% 22% 20% 2	5889
	11th to 20th	Night	39% 4 8% 19% 24% 5%	5900
	21st to end	Night	44% 4 5% 20% 22% 5%	6241
February	1st to 10th	Night	42% 5% 9% 19% 20% 4	2400
	11th to 20th	Night	50% 5% 7% 16% 17% 6%	2339
	21st to end	Night	47% 5% 8% 16% 18% 6%	1980
March	1st to 10th	Night	51% 7% 10% 12% 15% 5%	3829
	11th to 20th	Night	32% 6% 9% 20% 22% 10%	3498
	21st to end	Night	43% 6% 8% 21% 19% 3	3817
April	1st to 10th	Night	33% 6% 12% 21% 23% 5%	1970
	11th to 20th	Night	35% 4 7% 24% 21% 9%	1875
	21st to end	Night	34% 6% 12% 26% 19% 3	1777
May	1st to 10th	Night	39% 11% 15% 19% 15% 2	2840
	11th to 20th	Night	41% 9% 16% 22% 11%	2813
	21st to end	Night	29% 11% 18% 23% 17% 1	3087
June	1st to 10th	Night	37% 12% 17% 20% 13% 2	1710
	11th to 20th	Night	25% 9% 23% 26% 15% 2	1710
	21st to end	Night	30% 11% 21% 19% 14% 5%	1710
July	1st to 10th	Night	42% 13% 18% 20% 7% 1	2795
	11th to 20th	Night	34% 11% 22% 23% 8% 2	2786
	21st to end	Night	53% 12% 13% 14% 6% 1	2988
August	1st to 10th	Night	28% 11% 17% 32% 11% 1	2814
	11th to 20th	Night	30% 9% 19% 28% 14% 1	3125
	21st to end	Night	27% 9% 14% 23% 24% 2	3476
September	1st to 10th	Night	37% 10% 16% 17% 14% 6%	2015
	11th to 20th	Night	39% 10% 12% 19% 18% 2	2090
	21st to end	Night	34% 7% 12% 20% 20% 7%	2090
October	1st to 10th	Night	23% 5% 10% 16% 31% 15%	3480
	11th to 20th	Night	20% 3 6% 17% 35% 19%	3582
	21st to end	Night	23% 4 4 27% 29% 13%	4231
November	1st to 10th	Night	32% 5% 9% 21% 24% 10%	2536
	11th to 20th	Night	34% 4 7% 19% 27% 9%	2600
	21st to end	Night	38% 6% 7% 21% 23% 5%	2600
December	1st to 10th	Night	43% 4 6% 19% 26% 2	4250
	11th to 20th	Night	47% 6% 7% 18% 20% 3	4824
	21st to end	Night	46% 6% 9% 26% 12% 1	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	31% 26% 29% 13% 1	300
	11th to 20th	Day	32% 23% 31% 10% 4	300
	21st to end	Day	25% 26% 34% 12% 2	330
February	1st to 10th	Day	30% 22% 34% 12% 2	300
	11th to 20th	Day	41% 23% 27% 9%	300
	21st to end	Day	38% 22% 31% 7% 2	249
March	1st to 10th	Day	30% 23% 29% 15% 3	310
	11th to 20th	Day	25% 20% 31% 16% 6%	464
	21st to end	Day	21% 21% 35% 16% 7%	506
April	1st to 10th	Day	21% 20% 29% 17% 12% 2	460
	11th to 20th	Day	17% 18% 30% 23% 11% 1	452
	21st to end	Day	18% 12% 32% 25% 11% 3	460
May	1st to 10th	Day	15% 16% 27% 24% 13% 5%	463
	11th to 20th	Day	13% 7% 28% 28% 17% 7%	487
	21st to end	Day	12% 6% 21% 32% 18% 11%	529
June	1st to 10th	Day	13% 6% 20% 28% 22% 12%	490
	11th to 20th	Day	7% 4 15% 29% 27% 18%	490
	21st to end	Day	9% 4 20% 28% 22% 17%	490
July	1st to 10th	Day	10% 4 13% 25% 28% 21%	485
	11th to 20th	Day	10% 2 10% 26% 31% 20%	484
	21st to end	Day	12% 3 9% 24% 32% 20%	532
August	1st to 10th	Day	10% 2 16% 26% 28% 18%	486
	11th to 20th	Day	10% 3 15% 32% 26% 14%	479
	21st to end	Day	16% 3 10% 23% 30% 19%	539
September	1st to 10th	Day	9% 3 14% 28% 32% 13%	490
	11th to 20th	Day	16% 6% 12% 27% 30% 10%	490
	21st to end	Day	14% 6% 17% 29% 29% 5%	490
October	1st to 10th	Day	12% 7% 23% 29% 21% 8%	490
	11th to 20th	Day	12% 12% 29% 27% 18% 2	490
	21st to end	Day	17% 11% 25% 28% 16% 3	474
November	1st to 10th	Day	23% 12% 31% 22% 11% 2	300
	11th to 20th	Day	19% 17% 35% 23% 5% 1	300
	21st to end	Day	15% 24% 37% 18% 6% 2	300
December	1st to 10th	Day	30% 20% 32% 14% 3 1	300
	11th to 20th	Day	22% 22% 33% 18% 4 1	300
	21st to end	Day	29% 21% 30% 16% 4	330
January	1st to 10th	Night	25% 22% 29% 20% 4	800
	11th to 20th	Night	23% 17% 31% 22% 6%	800
	21st to end	Night	20% 19% 32% 26% 2	880
February	1st to 10th	Night	20% 15% 35% 27% 4	800
	11th to 20th	Night	27% 21% 31% 20% 2	783
	21st to end	Night	27% 20% 31% 20% 2	660
March	1st to 10th	Night	20% 16% 35% 25% 4	800
	11th to 20th	Night	19% 18% 30% 24% 9% 1	767
	21st to end	Night	19% 15% 30% 28% 8%	836
April	1st to 10th	Night	14% 15% 32% 30% 10% 1	640
	11th to 20th	Night	16% 12% 33% 28% 12%	563
	21st to end	Night	12% 10% 29% 35% 13% 1	570
May	1st to 10th	Night	11% 8% 28% 32% 19% 1	570
	11th to 20th	Night	12% 7% 22% 32% 23% 4	561
	21st to end	Night	9% 4 13% 37% 31% 5%	617
June	1st to 10th	Night	7% 4 18% 36% 32% 4	570
	11th to 20th	Night	4 3 12% 35% 37% 10%	570
	21st to end	Night	7% 2 9% 29% 43% 9%	570
July	1st to 10th	Night	12% 2 9% 25% 41% 11%	555
	11th to 20th	Night	9% 1 7% 26% 44% 12%	552
	21st to end	Night	10% 2 9% 26% 44% 9%	624
August	1st to 10th	Night	7% 1 7% 29% 46% 10%	567
	11th to 20th	Night	10% 3 10% 32% 37% 8%	564
	21st to end	Night	13% 2 9% 27% 41% 9%	627
September	1st to 10th	Night	10% 2 12% 30% 39% 7%	685
	11th to 20th	Night	13% 3 12% 31% 36% 3	760
	21st to end	Night	14% 3 17% 32% 33% 2	760
October	1st to 10th	Night	11% 5% 22% 30% 26% 6%	760
	11th to 20th	Night	13% 8% 29% 30% 19% 1	760
	21st to end	Night	15% 9% 22% 36% 16% 2	836
November	1st to 10th	Night	14% 9% 24% 33% 19% 2	780
	11th to 20th	Night	14% 14% 30% 30% 11% 1	800
	21st to end	Night	19% 14% 29% 27% 10% 1	800
December	1st to 10th	Night	25% 18% 27% 23% 7%	800
	11th to 20th	Night	22% 15% 30% 26% 7%	800
	21st to end	Night	26% 14% 29% 25% 6%	880

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