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 2025-06-10.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	119208
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	119027
Transparency	Day	28% 41% 7% 14% 9% 2	122773
	Night	29% 39% 6% 13% 10% 2	123610
Seeing	Day	23% 15% 32% 20% 7% 2	15708
	Night	22% 12% 30% 26% 9% 1	24631

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	14% 6% 3 7% 5% 5% 5% 4 6% 7% 38%	3597
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 4 6% 46%	3570
	21st to end	Day	16% 8% 2 5% 4 4 4 3 4 5% 46%	3960
February	1st to 10th	Day	17% 8% 3 6% 5% 4 4 5% 5% 7% 37%	1816
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 7% 42%	1948
	21st to end	Day	15% 11% 3 7% 5% 3 4 3 5% 5% 38%	1650
March	1st to 10th	Day	23% 10% 2 6% 4 4 4 4 5% 5% 33%	4476
	11th to 20th	Day	18% 9% 3 6% 3 3 4 3 5% 7% 38%	4810
	21st to end	Day	27% 8% 2 5% 3 3 4 4 5% 6% 31%	5728
April	1st to 10th	Day	20% 9% 2 7% 5% 4 4 2 4 5% 38%	2400
	11th to 20th	Day	18% 6% 3 6% 5% 3 5% 4 6% 7% 37%	2423
	21st to end	Day	20% 7% 3 7% 6% 4 3 3 4 6% 37%	2600
May	1st to 10th	Day	31% 9% 3 7% 4 4 4 4 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 5% 8% 31%	5990
	21st to end	Day	20% 6% 2 7% 4 5% 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	20% 8% 2 7% 4 5% 4 4 6% 7% 33%	2786
	11th to 20th	Day	17% 5% 2 5% 4 4 4 4 7% 8% 39%	2660
	21st to end	Day	19% 7% 3 7% 5% 5% 5% 4 6% 7% 33%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 4 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 7% 10% 39%	4440
	21st to end	Day	17% 8% 3 7% 5% 4 4 4 5% 8% 36%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 4 7% 8% 35%	4152
	11th to 20th	Day	17% 7% 3 7% 4 4 5% 4 6% 7% 34%	4045
	21st to end	Day	8% 5% 1 5% 4 4 5% 5% 7% 10% 46%	4213
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 47%	2264
	11th to 20th	Day	14% 5% 2 5% 3 3 3 2 4 5% 53%	2090
	21st to end	Day	15% 6% 2 6% 4 3 4 3 4 7% 46%	2090
October	1st to 10th	Day	21% 8% 2 6% 4 3 4 3 4 5% 41%	2090
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 5% 7% 46%	1840
	21st to end	Day	22% 6% 2 5% 4 3 3 3 5% 6% 40%	1881
November	1st to 10th	Day	16% 5% 2 5% 4 3 3 2 4 6% 49%	1761
	11th to 20th	Day	19% 6% 2 4 3 2 3 3 5% 5% 49%	1800
	21st to end	Day	21% 7% 2 4 3 4 3 3 4 6% 42%	1800
December	1st to 10th	Day	15% 8% 2 6% 3 3 4 3 4 7% 44%	2633
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 5% 46%	2986
	21st to end	Day	22% 9% 3 5% 4 3 3 3 5% 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 44%	6349
	11th to 20th	Night	13% 9% 3 9% 5% 4 5% 4 5% 6% 37%	6315
	21st to end	Night	15% 9% 3 6% 4 4 5% 4 5% 5% 39%	6996
February	1st to 10th	Night	15% 10% 3 7% 4 4 5% 4 5% 5% 38%	2577
	11th to 20th	Night	14% 8% 3 6% 5% 4 4 5% 5% 7% 40%	2338
	21st to end	Night	15% 10% 2 6% 5% 4 4 2 5% 6% 41%	1980
March	1st to 10th	Night	24% 9% 2 5% 3 4 4 3 4 5% 35%	5364
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 5% 6% 37%	5147
	21st to end	Night	26% 8% 2 5% 4 3 4 3 6% 6% 32%	5202
April	1st to 10th	Night	20% 9% 2 6% 4 5% 4 3 4 6% 37%	2000
	11th to 20th	Night	19% 9% 2 6% 4 4 4 4 4 6% 39%	1926
	21st to end	Night	21% 7% 3 6% 3 4 3 3 5% 5% 39%	1800
May	1st to 10th	Night	33% 10% 2 6% 4 4 4 3 5% 6% 23%	4054
	11th to 20th	Night	16% 9% 3 9% 5% 6% 6% 4 7% 7% 29%	3945
	21st to end	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 32%	3926
June	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 7% 35%	1598
	11th to 20th	Night	20% 7% 1 4 5% 4 4 4 8% 8% 37%	1520
	21st to end	Night	19% 7% 3 6% 4 4 5% 4 5% 8% 35%	1520
July	1st to 10th	Night	12% 6% 2 7% 5% 7% 6% 5% 8% 9% 34%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2520
	21st to end	Night	15% 6% 2 6% 5% 5% 4 4 6% 7% 40%	2739
August	1st to 10th	Night	21% 6% 2 6% 4 6% 5% 4 5% 7% 34%	2745
	11th to 20th	Night	14% 8% 3 7% 5% 5% 6% 4 7% 8% 33%	2782
	21st to end	Night	11% 4 1 5% 4 4 4 4 6% 8% 51%	3439
September	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 8% 49%	1914
	11th to 20th	Night	14% 6% 1 4 3 2 3 3 5% 7% 52%	2090
	21st to end	Night	16% 5% 2 5% 3 4 3 3 5% 7% 47%	2090
October	1st to 10th	Night	20% 6% 3 6% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 6% 3 4 5% 3 5% 7% 44%	2337
	21st to end	Night	22% 6% 3 6% 4 4 4 3 5% 5% 39%	2717
November	1st to 10th	Night	15% 6% 2 6% 3 3 4 4 4 6% 47%	2537
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 47%	2600
	21st to end	Night	16% 8% 2 5% 4 4 4 3 4 7% 44%	2600
December	1st to 10th	Night	16% 10% 2 6% 4 3 4 3 5% 6% 40%	4546
	11th to 20th	Night	23% 8% 2 5% 3 3 4 3 4 5% 41%	5208
	21st to end	Night	19% 10% 3 6% 4 4 3 3 4 5% 39%	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	32% 28% 6% 20% 13% 1	3598
	11th to 20th	Day	28% 28% 7% 20% 14% 3	3600
	21st to end	Day	31% 29% 4 14% 17% 4	3960
February	1st to 10th	Day	18% 62% 5% 8% 6% 2	1816
	11th to 20th	Day	18% 61% 3 7% 9% 2	1957
	21st to end	Day	16% 61% 4 8% 8% 2	1650
March	1st to 10th	Day	41% 29% 5% 8% 9% 8%	4479
	11th to 20th	Day	34% 30% 10% 16% 7% 3	4810
	21st to end	Day	42% 28% 3 10% 11% 5%	5728
April	1st to 10th	Day	21% 61% 2 7% 6% 2	2400
	11th to 20th	Day	17% 63% 4 8% 7% 2	2423
	21st to end	Day	20% 62% 3 7% 8% 1	2600
May	1st to 10th	Day	47% 32% 8% 9% 4	5897
	11th to 20th	Day	40% 31% 11% 13% 5% 1	5994
	21st to end	Day	36% 31% 10% 14% 9%	6949
June	1st to 10th	Day	17% 63% 3 10% 6%	2786
	11th to 20th	Day	13% 65% 4 9% 8% 1	2660
	21st to end	Day	17% 64% 3 9% 6%	2660
July	1st to 10th	Day	28% 38% 7% 16% 10% 1	4370
	11th to 20th	Day	23% 38% 11% 18% 9%	4365
	21st to end	Day	32% 37% 8% 14% 9% 1	4661
August	1st to 10th	Day	30% 39% 9% 16% 5%	4138
	11th to 20th	Day	32% 39% 10% 15% 4	4084
	21st to end	Day	18% 40% 10% 19% 12% 1	4213
September	1st to 10th	Day	13% 61% 6% 12% 8% 1	2264
	11th to 20th	Day	17% 59% 3 11% 10% 1	2090
	21st to end	Day	16% 60% 3 13% 8% 1	2090
October	1st to 10th	Day	28% 38% 5% 16% 12% 1	3480
	11th to 20th	Day	17% 37% 5% 21% 18% 2	3030
	21st to end	Day	19% 37% 4 23% 15% 2	3081
November	1st to 10th	Day	18% 55% 6% 12% 7% 1	1761
	11th to 20th	Day	17% 56% 2 8% 14% 2	1800
	21st to end	Day	18% 57% 6% 10% 9% 1	1800
December	1st to 10th	Day	25% 36% 8% 24% 7%	2633
	11th to 20th	Day	27% 32% 9% 12% 14% 6%	2986
	21st to end	Day	37% 27% 5% 15% 12% 5%	3960
January	1st to 10th	Night	28% 27% 8% 26% 10% 2	6349
	11th to 20th	Night	34% 28% 9% 17% 10% 3	6360
	21st to end	Night	35% 29% 5% 13% 13% 5%	6996
February	1st to 10th	Night	19% 62% 5% 7% 6% 2	2577
	11th to 20th	Night	17% 60% 3 8% 9% 2	2340
	21st to end	Night	18% 61% 3 9% 6% 3	1980
March	1st to 10th	Night	40% 29% 6% 8% 10% 7%	5374
	11th to 20th	Night	35% 30% 11% 12% 10% 2	5148
	21st to end	Night	40% 29% 3 8% 16% 5%	5202
April	1st to 10th	Night	21% 62% 3 6% 8% 2	2000
	11th to 20th	Night	18% 62% 3 8% 7% 1	1926
	21st to end	Night	18% 61% 4 7% 9% 1	1800
May	1st to 10th	Night	48% 31% 8% 7% 5%	4060
	11th to 20th	Night	39% 32% 12% 12% 6%	3946
	21st to end	Night	41% 31% 9% 13% 6% 1	3927
June	1st to 10th	Night	19% 62% 3 9% 6%	1598
	11th to 20th	Night	14% 66% 4 10% 6% 1	1520
	21st to end	Night	16% 63% 3 10% 6%	1520
July	1st to 10th	Night	27% 39% 9% 15% 10%	2480
	11th to 20th	Night	25% 39% 10% 15% 11%	2472
	21st to end	Night	30% 36% 7% 16% 10%	2651
August	1st to 10th	Night	36% 39% 7% 11% 7% 1	2735
	11th to 20th	Night	31% 40% 10% 12% 7%	2809
	21st to end	Night	17% 39% 10% 18% 16%	3439
September	1st to 10th	Night	13% 60% 6% 12% 8% 1	1914
	11th to 20th	Night	17% 59% 3 11% 10% 1	2090
	21st to end	Night	16% 59% 3 13% 7% 1	2090
October	1st to 10th	Night	27% 38% 9% 16% 8% 1	3480
	11th to 20th	Night	13% 38% 6% 18% 22% 3	3873
	21st to end	Night	20% 37% 6% 22% 13% 2	4467
November	1st to 10th	Night	18% 56% 4 13% 7% 2	2537
	11th to 20th	Night	17% 56% 3 10% 12% 2	2600
	21st to end	Night	17% 56% 7% 12% 8% 1	2600
December	1st to 10th	Night	29% 36% 5% 22% 8%	4546
	11th to 20th	Night	33% 32% 7% 10% 17% 2	5208
	21st to end	Night	37% 27% 5% 16% 13% 2	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	25% 26% 31% 16% 3	200
	11th to 20th	Day	28% 23% 35% 12% 3 1	200
	21st to end	Day	20% 25% 36% 14% 4	220
February	1st to 10th	Day	29% 22% 34% 13% 1	378
	11th to 20th	Day	30% 24% 31% 13% 3	496
	21st to end	Day	26% 23% 32% 16% 3	414
March	1st to 10th	Day	30% 21% 32% 14% 2	500
	11th to 20th	Day	26% 25% 32% 12% 4	500
	21st to end	Day	30% 21% 32% 11% 5% 2	550
April	1st to 10th	Day	31% 23% 28% 11% 5% 1	500
	11th to 20th	Day	31% 22% 26% 15% 5% 1	492
	21st to end	Day	28% 16% 35% 14% 6% 2	500
May	1st to 10th	Day	32% 18% 31% 14% 4 1	503
	11th to 20th	Day	21% 17% 38% 17% 6% 1	527
	21st to end	Day	21% 13% 35% 23% 6% 1	573
June	1st to 10th	Day	23% 12% 37% 19% 8% 1	526
	11th to 20th	Day	16% 16% 37% 23% 7% 1	490
	21st to end	Day	20% 12% 36% 24% 6% 2	490
July	1st to 10th	Day	17% 7% 36% 27% 11% 3	485
	11th to 20th	Day	16% 6% 31% 34% 11% 2	484
	21st to end	Day	13% 7% 36% 30% 12% 2	532
August	1st to 10th	Day	15% 7% 29% 31% 14% 4	486
	11th to 20th	Day	18% 7% 32% 27% 11% 6%	479
	21st to end	Day	14% 7% 33% 26% 15% 5%	539
September	1st to 10th	Day	18% 8% 29% 28% 11% 7%	490
	11th to 20th	Day	20% 7% 31% 21% 13% 8%	490
	21st to end	Day	19% 10% 32% 26% 8% 6%	490
October	1st to 10th	Day	25% 11% 27% 22% 11% 4	490
	11th to 20th	Day	20% 14% 29% 24% 10% 3	420
	21st to end	Day	26% 14% 30% 23% 6%	429
November	1st to 10th	Day	19% 17% 30% 22% 10% 2	395
	11th to 20th	Day	29% 20% 27% 19% 5% 1	400
	21st to end	Day	30% 19% 27% 19% 5% 2	400
December	1st to 10th	Day	20% 23% 34% 16% 5% 1	220
	11th to 20th	Day	31% 21% 33% 13% 3	200
	21st to end	Day	28% 20% 29% 18% 5% 1	220
January	1st to 10th	Night	21% 20% 35% 21% 3	800
	11th to 20th	Night	25% 17% 35% 20% 3	800
	21st to end	Night	20% 20% 38% 19% 4	880
February	1st to 10th	Night	23% 17% 36% 22% 2	800
	11th to 20th	Night	23% 20% 38% 16% 3	783
	21st to end	Night	22% 18% 33% 23% 3	660
March	1st to 10th	Night	28% 17% 34% 19% 2	800
	11th to 20th	Night	23% 19% 33% 20% 6%	800
	21st to end	Night	26% 15% 32% 21% 5%	880
April	1st to 10th	Night	26% 17% 32% 21% 4	800
	11th to 20th	Night	24% 14% 32% 22% 7%	742
	21st to end	Night	25% 13% 33% 25% 5%	600
May	1st to 10th	Night	27% 9% 30% 25% 9% 1	600
	11th to 20th	Night	22% 9% 32% 29% 8% 1	556
	21st to end	Night	23% 7% 27% 33% 10% 1	433
June	1st to 10th	Night	24% 5% 27% 31% 12% 1	400
	11th to 20th	Night	23% 4 25% 34% 13% 1	380
	21st to end	Night	22% 3 26% 38% 12%	380
July	1st to 10th	Night	17% 3 19% 44% 16% 1	370
	11th to 20th	Night	14% 3 18% 43% 21% 1	368
	21st to end	Night	15% 3 18% 46% 17% 1	416
August	1st to 10th	Night	15% 3 19% 41% 20% 1	513
	11th to 20th	Night	16% 5% 22% 37% 19% 2	562
	21st to end	Night	17% 3 22% 35% 22% 2	812
September	1st to 10th	Night	15% 4 26% 34% 21% 1	760
	11th to 20th	Night	17% 6% 25% 28% 21% 3	760
	21st to end	Night	19% 4 27% 29% 18% 2	760
October	1st to 10th	Night	23% 8% 24% 27% 15% 2	760
	11th to 20th	Night	18% 10% 29% 28% 13% 1	760
	21st to end	Night	26% 12% 30% 24% 8%	836
November	1st to 10th	Night	19% 11% 32% 29% 7% 1	780
	11th to 20th	Night	22% 16% 32% 22% 7% 1	800
	21st to end	Night	22% 19% 30% 26% 4	800
December	1st to 10th	Night	25% 16% 35% 20% 5%	800
	11th to 20th	Night	24% 19% 30% 23% 3 1	800
	21st to end	Night	24% 16% 33% 23% 3	880

Page generated at 2025-06-07 10:28:16. Version ac07

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