UNH Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UNH Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	149278
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 39%	142818
Transparency	Day	30% 34% 8% 16% 10% 2	152800
	Night	30% 34% 7% 15% 11% 2	147372
Seeing	Day	23% 15% 32% 20% 7% 2	17537
	Night	22% 12% 30% 26% 9% 1	26324

By Month

Percentage Cloud Cover NE (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	12% 6% 3 8% 5% 6% 5% 4 6% 7% 38%	4661
	11th to 20th	Day	15% 9% 3 6% 3 3 4 3 4 6% 44%	4650
	21st to end	Day	13% 8% 2 5% 4 4 4 3 4 6% 48%	5124
February	1st to 10th	Day	16% 8% 3 6% 5% 4 4 5% 5% 7% 38%	1907
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 7% 42%	2048
	21st to end	Day	15% 11% 3 7% 5% 3 4 3 5% 5% 38%	1730
March	1st to 10th	Day	23% 10% 2 6% 4 4 4 4 5% 5% 33%	4576
	11th to 20th	Day	18% 9% 3 6% 3 4 4 3 5% 7% 38%	4916
	21st to end	Day	27% 8% 2 5% 3 4 4 4 5% 7% 31%	5854
April	1st to 10th	Day	20% 9% 2 7% 5% 4 4 2 4 5% 38%	2520
	11th to 20th	Day	18% 6% 3 6% 5% 4 5% 4 6% 7% 37%	2545
	21st to end	Day	20% 8% 3 7% 6% 4 3 3 4 5% 37%	2730
May	1st to 10th	Day	28% 9% 3 6% 4 4 4 4 5% 6% 27%	7639
	11th to 20th	Day	20% 7% 2 8% 5% 4 4 4 5% 8% 34%	7796
	21st to end	Day	20% 7% 2 7% 5% 5% 5% 4 6% 7% 32%	9060
June	1st to 10th	Day	19% 8% 2 7% 4 5% 4 4 6% 8% 33%	2940
	11th to 20th	Day	16% 5% 2 6% 4 4 4 5% 7% 8% 39%	2940
	21st to end	Day	19% 6% 3 6% 5% 5% 5% 4 6% 7% 34%	2940
July	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 7% 10% 34%	8280
	11th to 20th	Day	8% 6% 2 5% 4 4 5% 4 7% 10% 46%	8280
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 8% 44%	8993
August	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 8% 35%	6092
	11th to 20th	Day	16% 6% 3 6% 4 4 4 4 6% 8% 39%	5953
	21st to end	Day	10% 5% 1 5% 4 4 5% 5% 7% 9% 44%	6127
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 47%	2502
	11th to 20th	Day	14% 4 2 5% 3 3 3 2 5% 5% 54%	2310
	21st to end	Day	16% 6% 2 6% 4 3 4 3 4 7% 45%	2310
October	1st to 10th	Day	19% 7% 2 5% 3 3 4 3 4 5% 44%	2310
	11th to 20th	Day	16% 6% 2 5% 4 4 4 2 5% 7% 46%	2034
	21st to end	Day	21% 6% 2 5% 4 3 3 4 6% 6% 42%	2052
November	1st to 10th	Day	16% 6% 2 5% 4 3 3 2 4 6% 49%	1851
	11th to 20th	Day	18% 6% 2 4 3 2 3 3 5% 5% 49%	1890
	21st to end	Day	21% 7% 2 4 3 4 3 3 4 6% 42%	1890
December	1st to 10th	Day	15% 8% 2 6% 3 3 4 3 4 7% 44%	2714
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 5% 47%	3066
	21st to end	Day	22% 9% 3 5% 4 4 3 3 5% 5% 38%	4048
January	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	8234
	11th to 20th	Night	15% 10% 3 9% 5% 4 5% 4 5% 6% 35%	8235
	21st to end	Night	13% 8% 2 6% 4 4 5% 4 5% 6% 42%	9057
February	1st to 10th	Night	14% 10% 3 7% 4 4 5% 4 6% 6% 39%	2706
	11th to 20th	Night	13% 8% 3 6% 4 4 4 5% 6% 7% 40%	2458
	21st to end	Night	15% 10% 2 5% 5% 4 4 3 5% 6% 41%	2076
March	1st to 10th	Night	24% 9% 2 5% 3 4 4 3 4 5% 36%	5484
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 5% 6% 36%	5261
	21st to end	Night	26% 8% 2 5% 4 3 4 3 6% 6% 32%	5317
April	1st to 10th	Night	20% 9% 3 6% 4 5% 4 3 4 6% 37%	2100
	11th to 20th	Night	19% 9% 2 6% 4 4 3 3 4 6% 39%	2024
	21st to end	Night	21% 7% 3 6% 3 4 3 3 5% 5% 39%	1890
May	1st to 10th	Night	30% 10% 3 6% 4 3 4 3 4 6% 27%	5259
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	5119
	21st to end	Night	21% 9% 3 7% 5% 4 4 4 5% 7% 32%	5114
June	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 5% 7% 36%	1680
	11th to 20th	Night	19% 6% 1 4 4 4 4 3 8% 9% 37%	1680
	21st to end	Night	19% 7% 2 6% 4 4 5% 4 5% 8% 36%	1680
July	1st to 10th	Night	14% 7% 2 7% 4 5% 5% 4 7% 10% 36%	4680
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 6% 10% 43%	4680
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 7% 48%	5084
August	1st to 10th	Night	16% 5% 2 6% 5% 6% 6% 4 6% 8% 36%	4010
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 6% 8% 38%	4092
	21st to end	Night	12% 4 1 6% 4 4 4 4 7% 8% 46%	4994
September	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 8% 48%	2116
	11th to 20th	Night	13% 6% 1 5% 3 3 3 3 5% 7% 52%	2310
	21st to end	Night	18% 5% 2 5% 3 4 3 3 5% 6% 46%	2310
October	1st to 10th	Night	19% 6% 3 6% 4 4 4 3 4 6% 43%	2310
	11th to 20th	Night	16% 5% 2 6% 3 4 4 3 5% 7% 44%	2583
	21st to end	Night	21% 6% 2 6% 4 4 4 3 5% 5% 40%	2965
November	1st to 10th	Night	15% 6% 2 6% 3 3 4 4 4 6% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 47%	2730
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2730
December	1st to 10th	Night	16% 10% 2 6% 4 3 4 3 5% 6% 40%	4685
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 5% 42%	5348
	21st to end	Night	19% 10% 3 6% 4 4 3 3 4 5% 39%	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	35% 23% 9% 20% 13% 1	4662
	11th to 20th	Day	33% 22% 7% 20% 16% 3	4680
	21st to end	Day	31% 23% 6% 18% 19% 3	5124
February	1st to 10th	Day	19% 59% 5% 9% 7% 2	1907
	11th to 20th	Day	19% 58% 3 8% 10% 2	2057
	21st to end	Day	18% 59% 5% 9% 8% 2	1730
March	1st to 10th	Day	41% 29% 5% 9% 9% 8%	4579
	11th to 20th	Day	34% 30% 10% 16% 7% 3	4916
	21st to end	Day	42% 28% 4 11% 11% 5%	5854
April	1st to 10th	Day	23% 58% 3 8% 7% 2	2520
	11th to 20th	Day	19% 60% 4 8% 7% 1	2545
	21st to end	Day	21% 59% 3 8% 9% 1	2730
May	1st to 10th	Day	46% 26% 8% 12% 7% 1	7651
	11th to 20th	Day	40% 25% 13% 15% 7% 1	7800
	21st to end	Day	41% 25% 9% 16% 8%	9061
June	1st to 10th	Day	18% 60% 4 11% 7%	2940
	11th to 20th	Day	15% 59% 5% 10% 9% 1	2940
	21st to end	Day	19% 58% 5% 11% 7% 1	2940
July	1st to 10th	Day	37% 23% 11% 19% 10%	8210
	11th to 20th	Day	25% 24% 11% 25% 14%	8205
	21st to end	Day	31% 21% 10% 23% 14% 1	8795
August	1st to 10th	Day	33% 29% 14% 15% 6% 2	6078
	11th to 20th	Day	31% 29% 13% 21% 6%	5992
	21st to end	Day	24% 30% 13% 20% 13% 1	6127
September	1st to 10th	Day	15% 55% 6% 14% 9% 1	2502
	11th to 20th	Day	17% 54% 3 12% 11% 1	2310
	21st to end	Day	19% 55% 3 14% 9% 1	2310
October	1st to 10th	Day	27% 36% 5% 17% 13% 2	3700
	11th to 20th	Day	19% 35% 5% 21% 18% 2	3224
	21st to end	Day	20% 35% 4 24% 15% 2	3252
November	1st to 10th	Day	19% 53% 6% 12% 8% 1	1851
	11th to 20th	Day	18% 54% 3 9% 14% 2	1890
	21st to end	Day	19% 54% 6% 11% 9% 1	1890
December	1st to 10th	Day	25% 35% 8% 24% 7%	2714
	11th to 20th	Day	27% 31% 9% 12% 14% 6%	3066
	21st to end	Day	37% 27% 5% 15% 12% 5%	4048
January	1st to 10th	Night	32% 21% 9% 24% 12% 2	8234
	11th to 20th	Night	39% 22% 8% 16% 11% 4	8280
	21st to end	Night	35% 23% 5% 17% 16% 5%	9057
February	1st to 10th	Night	19% 59% 5% 9% 7% 2	2706
	11th to 20th	Night	18% 57% 3 10% 9% 2	2460
	21st to end	Night	19% 58% 3 11% 7% 3	2076
March	1st to 10th	Night	40% 29% 6% 8% 11% 7%	5494
	11th to 20th	Night	35% 30% 11% 12% 10% 2	5262
	21st to end	Night	40% 28% 3 8% 16% 5%	5317
April	1st to 10th	Night	22% 59% 3 6% 8% 2	2100
	11th to 20th	Night	20% 59% 3 8% 8% 1	2024
	21st to end	Night	19% 58% 4 7% 10% 1	1890
May	1st to 10th	Night	47% 25% 9% 10% 9%	5265
	11th to 20th	Night	39% 25% 11% 17% 7%	5120
	21st to end	Night	43% 24% 9% 17% 6%	5115
June	1st to 10th	Night	19% 60% 3 12% 6%	1680
	11th to 20th	Night	15% 61% 5% 11% 7% 1	1680
	21st to end	Night	19% 58% 4 12% 6% 1	1680
July	1st to 10th	Night	35% 23% 12% 19% 11%	4640
	11th to 20th	Night	29% 23% 11% 20% 17%	4632
	21st to end	Night	29% 22% 10% 25% 14%	4967
August	1st to 10th	Night	36% 29% 12% 13% 8% 2	4000
	11th to 20th	Night	33% 29% 10% 20% 8%	4119
	21st to end	Night	25% 30% 12% 20% 13%	4994
September	1st to 10th	Night	16% 54% 7% 14% 9% 1	2116
	11th to 20th	Night	18% 54% 3 12% 11% 2	2310
	21st to end	Night	20% 54% 4 13% 8% 1	2310
October	1st to 10th	Night	26% 36% 9% 16% 10% 2	3700
	11th to 20th	Night	14% 36% 6% 19% 22% 3	4119
	21st to end	Night	20% 35% 6% 22% 14% 2	4715
November	1st to 10th	Night	19% 53% 4 14% 8% 2	2667
	11th to 20th	Night	18% 54% 4 11% 13% 2	2730
	21st to end	Night	18% 54% 7% 12% 8% 1	2730
December	1st to 10th	Night	29% 35% 5% 22% 8%	4685
	11th to 20th	Night	33% 31% 7% 10% 17% 2	5348
	21st to end	Night	38% 26% 5% 16% 13% 2	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	24% 26% 31% 16% 2	220
	11th to 20th	Day	29% 23% 35% 10% 3 1	220
	21st to end	Day	23% 25% 35% 13% 4	242
February	1st to 10th	Day	29% 21% 35% 13% 1	409
	11th to 20th	Day	28% 24% 32% 13% 3	536
	21st to end	Day	26% 23% 32% 16% 3	446
March	1st to 10th	Day	32% 21% 32% 13% 2	540
	11th to 20th	Day	27% 26% 32% 11% 4	540
	21st to end	Day	31% 21% 31% 10% 4 2	594
April	1st to 10th	Day	32% 22% 29% 11% 5% 1	540
	11th to 20th	Day	31% 22% 27% 15% 5% 1	532
	21st to end	Day	28% 16% 34% 14% 6% 1	540
May	1st to 10th	Day	33% 19% 31% 13% 4 1	543
	11th to 20th	Day	22% 17% 38% 17% 5% 1	567
	21st to end	Day	22% 14% 36% 22% 6% 1	617
June	1st to 10th	Day	22% 11% 38% 20% 8% 1	570
	11th to 20th	Day	17% 17% 37% 21% 7% 1	570
	21st to end	Day	19% 13% 36% 24% 6% 2	570
July	1st to 10th	Day	17% 6% 36% 27% 10% 2	565
	11th to 20th	Day	16% 8% 32% 32% 10% 2	564
	21st to end	Day	13% 8% 36% 29% 12% 2	620
August	1st to 10th	Day	13% 7% 31% 30% 14% 4	566
	11th to 20th	Day	17% 7% 33% 26% 11% 6%	559
	21st to end	Day	14% 8% 35% 25% 14% 4	627
September	1st to 10th	Day	18% 8% 31% 28% 10% 6%	570
	11th to 20th	Day	19% 8% 31% 24% 12% 8%	570
	21st to end	Day	20% 11% 32% 24% 7% 6%	570
October	1st to 10th	Day	23% 13% 27% 23% 10% 4	570
	11th to 20th	Day	23% 14% 29% 22% 10% 3	486
	21st to end	Day	25% 15% 30% 22% 7% 1	486
November	1st to 10th	Day	21% 18% 30% 21% 9% 2	425
	11th to 20th	Day	28% 20% 28% 18% 5%	430
	21st to end	Day	30% 19% 26% 18% 5% 1	430
December	1st to 10th	Day	22% 23% 33% 16% 5% 1	241
	11th to 20th	Day	32% 21% 32% 11% 3	220
	21st to end	Day	31% 19% 28% 17% 5% 1	242
January	1st to 10th	Night	22% 20% 35% 20% 3	840
	11th to 20th	Night	26% 17% 34% 20% 3	840
	21st to end	Night	21% 20% 37% 18% 4	924
February	1st to 10th	Night	22% 17% 36% 23% 2	840
	11th to 20th	Night	22% 19% 39% 17% 3	823
	21st to end	Night	22% 18% 34% 23% 3	692
March	1st to 10th	Night	28% 18% 34% 18% 2	840
	11th to 20th	Night	24% 18% 33% 20% 5%	840
	21st to end	Night	26% 16% 33% 20% 5%	924
April	1st to 10th	Night	26% 17% 32% 21% 4	840
	11th to 20th	Night	25% 14% 32% 22% 7%	780
	21st to end	Night	24% 13% 33% 25% 5%	630
May	1st to 10th	Night	27% 9% 30% 25% 8% 1	630
	11th to 20th	Night	22% 9% 32% 29% 7% 1	584
	21st to end	Night	23% 7% 27% 32% 9% 1	455
June	1st to 10th	Night	23% 5% 26% 33% 12% 1	420
	11th to 20th	Night	21% 5% 26% 34% 13%	420
	21st to end	Night	21% 4 26% 38% 12%	420
July	1st to 10th	Night	17% 3 20% 45% 14% 1	410
	11th to 20th	Night	13% 4 19% 42% 20% 1	408
	21st to end	Night	14% 3 19% 46% 17% 2	460
August	1st to 10th	Night	15% 3 19% 41% 21% 1	567
	11th to 20th	Night	14% 5% 23% 39% 18% 1	622
	21st to end	Night	16% 3 24% 35% 20% 2	898
September	1st to 10th	Night	15% 4 27% 34% 19% 1	840
	11th to 20th	Night	16% 6% 25% 30% 21% 3	840
	21st to end	Night	21% 4 27% 29% 17% 2	840
October	1st to 10th	Night	22% 10% 25% 27% 15% 1	840
	11th to 20th	Night	19% 10% 29% 28% 13% 1	840
	21st to end	Night	25% 12% 31% 24% 9%	913
November	1st to 10th	Night	20% 12% 32% 28% 7% 1	820
	11th to 20th	Night	22% 16% 32% 22% 6% 1	840
	21st to end	Night	21% 18% 30% 25% 4	840
December	1st to 10th	Night	25% 16% 35% 19% 5%	840
	11th to 20th	Night	24% 20% 30% 22% 3	840
	21st to end	Night	26% 16% 33% 23% 3	924

Page generated at 2026-10-26 04:37:47. Version ac07

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