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 2026-07-16.

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% 3 7% 5% 5% 5% 4 6% 7% 40%	137841
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	134854
Transparency	Day	41% 7% 12% 20% 17% 4	141603
	Night	39% 7% 11% 20% 18% 5%	139163
Seeing	Day	17% 12% 24% 23% 17% 8%	16969
	Night	16% 10% 23% 28% 19% 3	26972

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	19% 10% 3 6% 3 4 3 3 3 4 41%	5258
	11th to 20th	Day	12% 7% 4 5% 3 3 3 2 3 4 54%	5280
	21st to end	Day	21% 9% 3 7% 5% 4 3 3 4 5% 35%	6097
February	1st to 10th	Day	17% 10% 2 6% 4 3 4 4 5% 7% 37%	2100
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 3 3 6% 36%	2048
	21st to end	Day	21% 8% 3 6% 4 4 3 3 4 6% 38%	1730
March	1st to 10th	Day	19% 8% 3 6% 5% 5% 5% 4 6% 6% 33%	4616
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 5% 6% 50%	5090
	21st to end	Day	16% 6% 2 7% 4 4 4 3 4 6% 44%	5586
April	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 6% 8% 42%	2460
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 3 4 6% 48%	2494
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 3 6% 8% 43%	2656
May	1st to 10th	Day	17% 7% 2 7% 4 4 5% 4 7% 8% 33%	7616
	11th to 20th	Day	14% 7% 3 8% 5% 6% 5% 4 6% 8% 33%	7626
	21st to end	Day	18% 9% 3 9% 5% 5% 5% 4 5% 6% 30%	8391
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 9% 37%	2730
	11th to 20th	Day	5% 4 2 5% 5% 6% 7% 6% 8% 11% 41%	2730
	21st to end	Day	6% 5% 2 6% 6% 6% 5% 5% 8% 12% 39%	2730
July	1st to 10th	Day	6% 5% 2 8% 6% 6% 7% 6% 9% 11% 35%	6030
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 5% 9% 12% 34%	5965
	21st to end	Day	11% 8% 3 9% 7% 7% 7% 6% 9% 11% 23%	6424
August	1st to 10th	Day	6% 3 2 6% 5% 6% 7% 5% 8% 12% 41%	4234
	11th to 20th	Day	7% 4 2 6% 4 6% 5% 5% 7% 11% 42%	3849
	21st to end	Day	8% 4 2 5% 4 4 6% 3 6% 8% 50%	4312
September	1st to 10th	Day	9% 5% 3 7% 5% 6% 5% 5% 7% 9% 40%	2256
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 3 5% 8% 43%	2200
	21st to end	Day	13% 4 2 6% 4 5% 4 4 5% 7% 47%	2200
October	1st to 10th	Day	11% 5% 3 4 4 4 4 2 5% 5% 53%	2200
	11th to 20th	Day	10% 4 2 5% 3 4 3 4 3 5% 57%	2122
	21st to end	Day	14% 5% 2 5% 3 3 3 3 5% 5% 53%	2131
November	1st to 10th	Day	12% 6% 2 5% 3 4 3 3 5% 5% 54%	1852
	11th to 20th	Day	13% 5% 2 5% 4 3 3 3 5% 6% 51%	1890
	21st to end	Day	15% 6% 2 5% 4 4 4 3 4 6% 45%	1890
December	1st to 10th	Day	17% 8% 2 5% 3 4 4 3 4 5% 45%	3020
	11th to 20th	Day	15% 11% 4 9% 4 4 4 3 4 6% 36%	3463
	21st to end	Day	17% 9% 2 6% 4 4 4 3 5% 7% 37%	4565
January	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 4 5% 42%	7635
	11th to 20th	Night	16% 7% 2 6% 3 3 3 2 3 4 51%	7680
	21st to end	Night	20% 9% 2 5% 3 4 3 3 4 5% 41%	8070
February	1st to 10th	Night	15% 7% 3 7% 4 3 4 3 5% 6% 43%	2520
	11th to 20th	Night	21% 7% 3 7% 5% 4 4 3 4 4 38%	2458
	21st to end	Night	21% 8% 2 5% 4 4 4 3 5% 5% 38%	2076
March	1st to 10th	Night	21% 8% 3 7% 5% 4 4 4 5% 7% 32%	5455
	11th to 20th	Night	12% 4 2 4 4 4 4 3 6% 7% 50%	5090
	21st to end	Night	14% 6% 2 7% 4 4 4 4 4 6% 43%	5588
April	1st to 10th	Night	13% 4 2 4 3 4 4 4 5% 9% 49%	2177
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 5% 7% 52%	2075
	21st to end	Night	10% 5% 2 7% 4 4 4 3 5% 7% 48%	1963
May	1st to 10th	Night	14% 7% 3 8% 4 5% 5% 4 7% 8% 34%	5237
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	5234
	21st to end	Night	16% 9% 3 9% 5% 5% 5% 4 6% 7% 31%	5761
June	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 8% 11% 39%	1890
	11th to 20th	Night	5% 4 1 5% 5% 4 4 5% 9% 11% 47%	1890
	21st to end	Night	5% 4 2 5% 4 4 5% 5% 8% 13% 44%	1890
July	1st to 10th	Night	8% 5% 3 8% 6% 5% 6% 5% 8% 11% 36%	4148
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 11% 38%	4102
	21st to end	Night	10% 8% 3 9% 7% 6% 6% 5% 7% 11% 29%	4427
August	1st to 10th	Night	6% 3 2 6% 5% 5% 5% 6% 8% 11% 44%	2915
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 48%	3195
	21st to end	Night	7% 4 1 5% 4 3 5% 4 7% 8% 52%	3586
September	1st to 10th	Night	9% 6% 2 7% 6% 6% 5% 5% 7% 8% 40%	2121
	11th to 20th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	2200
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 2 5% 6% 57%	2200
	11th to 20th	Night	11% 5% 2 6% 4 3 3 2 4 5% 57%	2275
	21st to end	Night	13% 5% 2 5% 4 3 3 3 4 4 55%	2705
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 6% 53%	2666
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 4 6% 53%	2730
	21st to end	Night	16% 5% 2 6% 4 3 4 4 5% 5% 46%	2730
December	1st to 10th	Night	18% 8% 2 6% 3 3 3 3 4 5% 45%	4380
	11th to 20th	Night	18% 10% 3 7% 4 3 4 2 4 5% 41%	4952
	21st to end	Night	20% 7% 2 6% 4 5% 5% 4 5% 6% 37%	6633

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	47% 5% 8% 22% 17% 1	5259
	11th to 20th	Day	35% 3 6% 17% 33% 6%	5280
	21st to end	Day	51% 3 7% 18% 16% 4	6097
February	1st to 10th	Day	46% 5% 8% 20% 18% 3	2100
	11th to 20th	Day	51% 4 6% 19% 16% 4	2058
	21st to end	Day	49% 3 7% 15% 20% 5%	1730
March	1st to 10th	Day	48% 6% 10% 11% 18% 7%	4616
	11th to 20th	Day	34% 5% 7% 17% 24% 13%	5090
	21st to end	Day	42% 4 9% 26% 17% 2	5586
April	1st to 10th	Day	38% 8% 12% 22% 17% 4	2460
	11th to 20th	Day	37% 4 7% 21% 24% 6%	2494
	21st to end	Day	38% 7% 11% 25% 16% 3	2656
May	1st to 10th	Day	46% 8% 12% 21% 12% 1	7651
	11th to 20th	Day	46% 9% 15% 20% 9%	7630
	21st to end	Day	53% 8% 8% 17% 13% 2	8405
June	1st to 10th	Day	40% 9% 16% 21% 12% 2	2730
	11th to 20th	Day	33% 12% 18% 24% 12% 2	2730
	21st to end	Day	35% 11% 18% 20% 12% 3	2730
July	1st to 10th	Day	37% 13% 19% 22% 9% 1	5965
	11th to 20th	Day	38% 13% 26% 18% 4 1	5896
	21st to end	Day	50% 13% 18% 15% 4	6240
August	1st to 10th	Day	31% 11% 17% 32% 9%	4221
	11th to 20th	Day	33% 9% 18% 24% 14% 2	3885
	21st to end	Day	32% 8% 12% 28% 18% 2	4311
September	1st to 10th	Day	39% 10% 15% 16% 16% 5%	2256
	11th to 20th	Day	39% 8% 13% 21% 18% 2	2200
	21st to end	Day	35% 8% 13% 17% 20% 6%	2200
October	1st to 10th	Day	26% 5% 8% 14% 35% 12%	3590
	11th to 20th	Day	20% 4 6% 19% 31% 20%	3440
	21st to end	Day	27% 4 7% 25% 27% 10%	3420
November	1st to 10th	Day	33% 4 8% 18% 29% 8%	1852
	11th to 20th	Day	35% 4 7% 22% 25% 7%	1890
	21st to end	Day	40% 5% 8% 19% 24% 4	1890
December	1st to 10th	Day	44% 3 5% 19% 25% 4	3020
	11th to 20th	Day	51% 4 8% 18% 16% 3	3460
	21st to end	Day	45% 5% 10% 27% 11% 1	4565
January	1st to 10th	Night	44% 5% 11% 22% 16% 2	7636
	11th to 20th	Night	37% 4 7% 19% 28% 5%	7680
	21st to end	Night	46% 4 6% 19% 19% 7%	8070
February	1st to 10th	Night	42% 5% 9% 20% 20% 4	2520
	11th to 20th	Night	51% 5% 6% 16% 16% 5%	2459
	21st to end	Night	48% 5% 8% 16% 18% 6%	2076
March	1st to 10th	Night	50% 6% 8% 14% 18% 4	5455
	11th to 20th	Night	32% 5% 8% 18% 24% 13%	5090
	21st to end	Night	41% 5% 8% 23% 19% 4	5588
April	1st to 10th	Night	32% 6% 13% 21% 21% 5%	2177
	11th to 20th	Night	33% 4 7% 25% 22% 8%	2075
	21st to end	Night	35% 6% 13% 25% 19% 2	1963
May	1st to 10th	Night	45% 9% 14% 18% 12% 2	5264
	11th to 20th	Night	45% 8% 16% 19% 12%	5235
	21st to end	Night	49% 10% 12% 18% 11% 1	5771
June	1st to 10th	Night	36% 11% 17% 20% 14% 2	1890
	11th to 20th	Night	28% 9% 22% 25% 14% 2	1890
	21st to end	Night	30% 11% 21% 20% 14% 5%	1890
July	1st to 10th	Night	40% 12% 19% 22% 7% 1	4105
	11th to 20th	Night	36% 14% 25% 18% 5% 1	4051
	21st to end	Night	47% 13% 18% 17% 5% 1	4297
August	1st to 10th	Night	29% 11% 17% 32% 11% 1	2904
	11th to 20th	Night	30% 9% 19% 27% 14% 1	3225
	21st to end	Night	28% 9% 14% 23% 24% 2	3586
September	1st to 10th	Night	38% 10% 16% 17% 15% 5%	2121
	11th to 20th	Night	37% 10% 12% 21% 18% 2	2200
	21st to end	Night	35% 7% 13% 19% 19% 7%	2200
October	1st to 10th	Night	23% 5% 10% 16% 31% 14%	3590
	11th to 20th	Night	20% 3 6% 18% 35% 18%	3696
	21st to end	Night	23% 4 4 27% 29% 13%	4366
November	1st to 10th	Night	32% 5% 8% 21% 24% 9%	2666
	11th to 20th	Night	33% 4 7% 20% 27% 9%	2730
	21st to end	Night	39% 6% 7% 21% 23% 5%	2730
December	1st to 10th	Night	43% 4 6% 18% 26% 2	4380
	11th to 20th	Night	46% 6% 7% 17% 21% 3	4954
	21st to end	Night	45% 6% 9% 26% 12% 1	6633

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% 25% 29% 13% 2	330
	11th to 20th	Day	30% 25% 32% 10% 3	330
	21st to end	Day	26% 27% 33% 12% 2	363
February	1st to 10th	Day	28% 22% 37% 11% 2	330
	11th to 20th	Day	39% 24% 28% 9% 1	330
	21st to end	Day	37% 22% 30% 8% 2	273
March	1st to 10th	Day	29% 22% 29% 16% 3	340
	11th to 20th	Day	24% 22% 32% 16% 6%	540
	21st to end	Day	19% 21% 37% 16% 6%	594
April	1st to 10th	Day	21% 19% 30% 17% 11% 2	540
	11th to 20th	Day	16% 17% 30% 24% 11% 1	532
	21st to end	Day	17% 12% 31% 25% 11% 3	540
May	1st to 10th	Day	17% 16% 27% 24% 12% 4	543
	11th to 20th	Day	13% 8% 28% 29% 16% 7%	567
	21st to end	Day	14% 8% 23% 30% 16% 10%	617
June	1st to 10th	Day	12% 6% 19% 27% 23% 12%	570
	11th to 20th	Day	8% 4 16% 29% 26% 16%	570
	21st to end	Day	8% 4 18% 27% 25% 19%	570
July	1st to 10th	Day	9% 3 13% 25% 28% 22%	565
	11th to 20th	Day	10% 2 10% 24% 32% 22%	544
	21st to end	Day	11% 3 9% 24% 32% 21%	576
August	1st to 10th	Day	11% 3 15% 25% 28% 18%	526
	11th to 20th	Day	11% 4 15% 30% 25% 16%	519
	21st to end	Day	16% 3 11% 22% 30% 18%	583
September	1st to 10th	Day	9% 3 15% 29% 32% 12%	530
	11th to 20th	Day	15% 5% 11% 27% 31% 10%	530
	21st to end	Day	14% 6% 17% 30% 28% 5%	530
October	1st to 10th	Day	12% 7% 23% 29% 21% 8%	530
	11th to 20th	Day	12% 11% 29% 28% 19% 2	530
	21st to end	Day	18% 11% 27% 27% 15% 3	514
November	1st to 10th	Day	23% 11% 31% 22% 11% 2	330
	11th to 20th	Day	17% 16% 36% 24% 6% 1	330
	21st to end	Day	16% 24% 37% 16% 5% 2	330
December	1st to 10th	Day	29% 22% 31% 14% 3 1	330
	11th to 20th	Day	22% 21% 34% 19% 4 1	330
	21st to end	Day	27% 21% 31% 17% 4	363
January	1st to 10th	Night	25% 21% 29% 20% 4	840
	11th to 20th	Night	23% 18% 31% 22% 6%	840
	21st to end	Night	21% 19% 32% 25% 2	924
February	1st to 10th	Night	20% 15% 34% 27% 3	840
	11th to 20th	Night	27% 22% 30% 20% 1	823
	21st to end	Night	27% 20% 31% 19% 2	692
March	1st to 10th	Night	20% 16% 34% 26% 4	840
	11th to 20th	Night	19% 17% 31% 24% 8% 1	840
	21st to end	Night	18% 14% 32% 28% 8%	924
April	1st to 10th	Night	14% 15% 32% 30% 10% 1	707
	11th to 20th	Night	15% 11% 33% 29% 12%	623
	21st to end	Night	12% 10% 28% 34% 14% 1	630
May	1st to 10th	Night	12% 8% 28% 33% 18% 1	630
	11th to 20th	Night	12% 7% 23% 33% 21% 4	621
	21st to end	Night	11% 5% 14% 35% 30% 5%	683
June	1st to 10th	Night	7% 4 17% 37% 32% 3	630
	11th to 20th	Night	6% 3 12% 35% 35% 9%	630
	21st to end	Night	6% 2 9% 29% 44% 10%	630
July	1st to 10th	Night	11% 2 8% 25% 43% 11%	615
	11th to 20th	Night	9% 2 7% 25% 45% 12%	597
	21st to end	Night	10% 2 9% 25% 45% 10%	657
August	1st to 10th	Night	9% 1 8% 28% 44% 9%	597
	11th to 20th	Night	10% 3 10% 31% 37% 9%	594
	21st to end	Night	13% 2 10% 28% 39% 8%	660
September	1st to 10th	Night	10% 3 12% 30% 38% 7%	721
	11th to 20th	Night	13% 3 12% 31% 38% 4	800
	21st to end	Night	14% 3 17% 33% 33% 2	800
October	1st to 10th	Night	11% 6% 21% 31% 26% 6%	800
	11th to 20th	Night	13% 7% 29% 31% 19% 1	800
	21st to end	Night	15% 9% 22% 36% 15% 2	880
November	1st to 10th	Night	14% 9% 24% 34% 18% 1	820
	11th to 20th	Night	14% 15% 30% 31% 10% 1	840
	21st to end	Night	19% 15% 29% 27% 10%	840
December	1st to 10th	Night	25% 18% 27% 23% 7%	840
	11th to 20th	Night	22% 15% 29% 27% 6%	840
	21st to end	Night	25% 14% 29% 26% 6%	924

Page generated at 2026-07-13 03:14:52. Version ac07

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