Gorham Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gorham Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 8% 2 6% 4 4 4 3 5% 7% 38%	137771
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	135248
Transparency	Day	45% 4 12% 22% 14% 3	141293
	Night	46% 3 11% 22% 16% 3	139815
Seeing	Day	22% 15% 33% 20% 8% 2	17175
	Night	22% 11% 31% 27% 9% 1	25923

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% 7% 3 8% 5% 5% 5% 4 6% 6% 39%	4661
	11th to 20th	Day	14% 10% 3 6% 4 4 3 3 4 5% 44%	4650
	21st to end	Day	11% 9% 3 5% 3 3 4 3 5% 5% 49%	5124
February	1st to 10th	Day	16% 9% 3 6% 4 5% 4 4 6% 6% 37%	1875
	11th to 20th	Day	13% 8% 3 7% 5% 5% 4 3 7% 6% 39%	2048
	21st to end	Day	15% 11% 3 8% 4 4 3 3 5% 6% 38%	1730
March	1st to 10th	Day	22% 11% 2 7% 4 4 4 3 4 5% 33%	4576
	11th to 20th	Day	18% 10% 3 6% 4 3 3 3 5% 6% 38%	4967
	21st to end	Day	27% 9% 2 5% 3 3 3 3 5% 6% 33%	5768
April	1st to 10th	Day	20% 10% 3 7% 4 3 4 2 4 5% 36%	2520
	11th to 20th	Day	18% 6% 2 5% 5% 4 5% 4 6% 7% 39%	2581
	21st to end	Day	21% 8% 2 7% 4 4 4 3 4 6% 36%	2730
May	1st to 10th	Day	30% 10% 3 6% 4 4 4 4 5% 5% 26%	7639
	11th to 20th	Day	19% 8% 3 7% 4 5% 4 3 5% 8% 33%	7744
	21st to end	Day	21% 7% 3 7% 5% 5% 5% 4 5% 7% 32%	9060
June	1st to 10th	Day	22% 7% 2 7% 4 4 4 4 6% 7% 33%	2940
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 39%	2940
	21st to end	Day	21% 7% 2 6% 4 4 5% 5% 5% 7% 34%	2940
July	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 9% 33%	6500
	11th to 20th	Day	11% 7% 2 6% 5% 5% 5% 4 7% 9% 39%	6500
	21st to end	Day	16% 6% 2 5% 3 3 4 3 6% 8% 44%	7066
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	4270
	11th to 20th	Day	15% 8% 3 8% 4 5% 4 3 6% 8% 36%	4175
	21st to end	Day	9% 4 2 5% 5% 5% 5% 5% 6% 8% 46%	4427
September	1st to 10th	Day	12% 4 2 5% 3 4 4 4 5% 8% 50%	2362
	11th to 20th	Day	13% 5% 2 4 3 3 3 2 3 5% 55%	2200
	21st to end	Day	16% 6% 2 4 3 4 5% 3 5% 6% 47%	2200
October	1st to 10th	Day	18% 8% 3 6% 4 3 4 3 4 6% 42%	2200
	11th to 20th	Day	17% 5% 2 6% 5% 3 4 3 4 6% 44%	1931
	21st to end	Day	23% 7% 2 4 4 3 4 3 4 4 41%	1980
November	1st to 10th	Day	17% 6% 2 4 3 3 3 3 4 6% 49%	1851
	11th to 20th	Day	18% 6% 1 4 3 3 4 3 5% 6% 47%	1890
	21st to end	Day	22% 6% 2 6% 4 3 3 3 3 6% 42%	1890
December	1st to 10th	Day	15% 10% 3 5% 4 4 4 3 5% 7% 41%	2722
	11th to 20th	Day	18% 7% 2 4 3 3 3 2 4 5% 48%	3066
	21st to end	Day	21% 10% 3 5% 4 3 4 3 5% 5% 36%	4048
January	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 6% 44%	8234
	11th to 20th	Night	15% 10% 3 10% 5% 5% 4 3 5% 6% 34%	8235
	21st to end	Night	13% 9% 3 6% 4 4 4 3 5% 5% 44%	9057
February	1st to 10th	Night	15% 10% 3 7% 4 4 5% 3 5% 5% 39%	2740
	11th to 20th	Night	13% 10% 3 6% 5% 5% 5% 4 5% 5% 41%	2458
	21st to end	Night	15% 8% 3 7% 4 4 4 3 4 6% 41%	2076
March	1st to 10th	Night	26% 9% 2 6% 4 4 4 2 4 5% 34%	5484
	11th to 20th	Night	15% 9% 3 8% 5% 5% 4 4 5% 6% 36%	5212
	21st to end	Night	25% 8% 2 5% 4 3 4 3 6% 6% 33%	5404
April	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 4 6% 36%	2100
	11th to 20th	Night	19% 8% 3 6% 3 4 4 3 5% 6% 39%	1987
	21st to end	Night	21% 8% 2 7% 3 3 4 3 4 6% 37%	1890
May	1st to 10th	Night	30% 9% 3 7% 4 3 4 3 4 6% 28%	5259
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 5% 6% 8% 31%	5158
	21st to end	Night	21% 9% 3 7% 4 4 5% 3 5% 7% 33%	5114
June	1st to 10th	Night	20% 8% 3 7% 5% 4 4 4 5% 7% 33%	1680
	11th to 20th	Night	17% 6% 3 5% 5% 4 4 4 6% 7% 40%	1680
	21st to end	Night	21% 7% 2 5% 5% 4 4 4 5% 8% 36%	1680
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 5% 6% 9% 36%	3680
	11th to 20th	Night	14% 7% 3 8% 4 4 4 4 7% 8% 37%	3680
	21st to end	Night	11% 5% 2 4 4 4 5% 4 6% 7% 48%	4006
August	1st to 10th	Night	20% 6% 3 7% 5% 5% 5% 4 6% 8% 32%	2845
	11th to 20th	Night	15% 9% 3 7% 4 5% 4 4 6% 8% 36%	2872
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 8% 48%	3454
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 4 6% 50%	2036
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 6% 52%	2200
	21st to end	Night	16% 6% 1 5% 3 4 3 3 5% 7% 48%	2200
October	1st to 10th	Night	18% 6% 3 6% 4 4 3 4 5% 5% 42%	2200
	11th to 20th	Night	16% 5% 2 6% 4 4 4 4 5% 6% 43%	2466
	21st to end	Night	23% 7% 2 6% 4 4 3 3 4 6% 38%	2860
November	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 5% 46%	2667
	11th to 20th	Night	18% 6% 2 5% 4 4 3 3 4 5% 46%	2730
	21st to end	Night	17% 7% 2 6% 4 3 4 4 4 6% 44%	2730
December	1st to 10th	Night	16% 10% 3 6% 3 3 4 3 5% 5% 40%	4676
	11th to 20th	Night	23% 8% 2 5% 4 3 3 3 3 4 43%	5348
	21st to end	Night	21% 11% 3 7% 4 4 3 2 4 5% 37%	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	45% 3 11% 24% 16% 1	4662
	11th to 20th	Day	45% 2 11% 24% 15% 3	4680
	21st to end	Day	38% 2 9% 25% 21% 5%	5124
February	1st to 10th	Day	45% 3 11% 22% 14% 4	1875
	11th to 20th	Day	43% 4 12% 22% 15% 4	2057
	21st to end	Day	48% 2 10% 22% 15% 3	1730
March	1st to 10th	Day	55% 2 9% 12% 13% 9%	4579
	11th to 20th	Day	47% 3 15% 19% 13% 4	4967
	21st to end	Day	52% 3 4 16% 18% 6%	5768
April	1st to 10th	Day	50% 3 8% 17% 17% 4	2520
	11th to 20th	Day	44% 4 11% 22% 16% 3	2581
	21st to end	Day	49% 3 9% 18% 17% 4	2730
May	1st to 10th	Day	59% 3 13% 15% 9% 1	7651
	11th to 20th	Day	49% 4 16% 19% 10% 2	7748
	21st to end	Day	50% 5% 12% 22% 11% 1	9061
June	1st to 10th	Day	49% 5% 12% 24% 10%	2940
	11th to 20th	Day	42% 4 13% 22% 17% 3	2940
	21st to end	Day	48% 5% 13% 22% 11% 1	2940
July	1st to 10th	Day	48% 5% 13% 22% 12%	6430
	11th to 20th	Day	40% 6% 15% 24% 13% 1	6425
	21st to end	Day	38% 4 14% 28% 15% 1	6868
August	1st to 10th	Day	44% 5% 16% 25% 9%	4256
	11th to 20th	Day	46% 5% 15% 25% 8% 1	4214
	21st to end	Day	33% 6% 17% 24% 19% 1	4427
September	1st to 10th	Day	33% 5% 13% 29% 18% 1	2362
	11th to 20th	Day	32% 3 9% 30% 23% 3	2200
	21st to end	Day	38% 5% 9% 28% 18% 2	2200
October	1st to 10th	Day	45% 3 11% 23% 17% 2	3590
	11th to 20th	Day	34% 2 9% 31% 21% 2	3121
	21st to end	Day	36% 2 10% 27% 21% 3	3180
November	1st to 10th	Day	38% 4 13% 25% 18% 3	1851
	11th to 20th	Day	38% 3 10% 23% 22% 4	1890
	21st to end	Day	45% 3 15% 22% 13% 2	1890
December	1st to 10th	Day	43% 3 12% 29% 12% 1	2722
	11th to 20th	Day	39% 3 12% 20% 18% 9%	3066
	21st to end	Day	51% 3 9% 18% 13% 5%	4048
January	1st to 10th	Night	42% 2 11% 28% 14% 2	8234
	11th to 20th	Night	52% 3 10% 18% 13% 4	8280
	21st to end	Night	43% 2 7% 22% 20% 6%	9057
February	1st to 10th	Night	47% 3 11% 19% 16% 3	2740
	11th to 20th	Night	45% 4 9% 22% 17% 4	2460
	21st to end	Night	45% 3 9% 24% 15% 4	2076
March	1st to 10th	Night	54% 2 8% 14% 15% 7%	5494
	11th to 20th	Night	49% 3 15% 19% 12% 3	5213
	21st to end	Night	52% 3 5% 14% 21% 5%	5404
April	1st to 10th	Night	50% 4 7% 19% 17% 4	2100
	11th to 20th	Night	47% 3 7% 22% 18% 2	1987
	21st to end	Night	50% 3 9% 18% 17% 4	1890
May	1st to 10th	Night	58% 4 12% 14% 11% 1	5265
	11th to 20th	Night	52% 4 12% 24% 8% 1	5159
	21st to end	Night	51% 5% 11% 22% 10% 1	5115
June	1st to 10th	Night	51% 4 10% 22% 12% 1	1680
	11th to 20th	Night	43% 5% 14% 21% 15% 2	1680
	21st to end	Night	48% 5% 13% 23% 10% 1	1680
July	1st to 10th	Night	44% 5% 14% 23% 13%	3640
	11th to 20th	Night	44% 5% 13% 22% 16%	3632
	21st to end	Night	35% 5% 15% 32% 14% 1	3889
August	1st to 10th	Night	51% 6% 13% 20% 9% 1	2835
	11th to 20th	Night	46% 5% 15% 22% 12%	2899
	21st to end	Night	33% 5% 16% 26% 20% 1	3454
September	1st to 10th	Night	36% 4 11% 29% 18% 1	2036
	11th to 20th	Night	34% 4 10% 29% 21% 2	2200
	21st to end	Night	37% 4 10% 29% 17% 2	2200
October	1st to 10th	Night	44% 3 12% 22% 16% 1	3590
	11th to 20th	Night	32% 3 9% 28% 25% 2	4015
	21st to end	Night	39% 3 10% 28% 18% 3	4610
November	1st to 10th	Night	40% 3 10% 26% 18% 3	2667
	11th to 20th	Night	41% 4 9% 24% 20% 3	2730
	21st to end	Night	42% 3 15% 23% 15% 2	2730
December	1st to 10th	Night	46% 3 9% 29% 12% 1	4676
	11th to 20th	Night	47% 3 10% 17% 20% 3	5348
	21st to end	Night	52% 3 8% 19% 17% 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	26% 20% 29% 21% 3	220
	11th to 20th	Day	25% 24% 28% 18% 4 1	220
	21st to end	Day	19% 22% 40% 13% 6%	242
February	1st to 10th	Day	27% 16% 40% 16% 1	403
	11th to 20th	Day	22% 21% 38% 16% 3	536
	21st to end	Day	24% 18% 37% 17% 4	446
March	1st to 10th	Day	28% 17% 35% 17% 2	540
	11th to 20th	Day	26% 21% 32% 15% 4 1	540
	21st to end	Day	29% 19% 33% 11% 6% 1	594
April	1st to 10th	Day	32% 21% 29% 13% 4 1	540
	11th to 20th	Day	27% 19% 34% 13% 6% 1	532
	21st to end	Day	30% 16% 34% 14% 5% 1	540
May	1st to 10th	Day	31% 19% 31% 13% 4 1	543
	11th to 20th	Day	21% 17% 40% 16% 5% 1	567
	21st to end	Day	22% 16% 34% 21% 6% 1	617
June	1st to 10th	Day	20% 13% 35% 23% 7% 2	570
	11th to 20th	Day	16% 17% 36% 21% 8% 2	570
	21st to end	Day	22% 11% 34% 23% 9% 1	570
July	1st to 10th	Day	19% 7% 35% 27% 9% 3	565
	11th to 20th	Day	14% 8% 36% 27% 10% 4	564
	21st to end	Day	16% 7% 35% 26% 14% 3	616
August	1st to 10th	Day	13% 8% 32% 30% 15% 3	526
	11th to 20th	Day	17% 5% 30% 29% 13% 5%	519
	21st to end	Day	11% 9% 32% 30% 12% 6%	583
September	1st to 10th	Day	15% 8% 33% 26% 14% 4	530
	11th to 20th	Day	16% 8% 32% 24% 12% 8%	530
	21st to end	Day	16% 10% 35% 22% 11% 5%	530
October	1st to 10th	Day	21% 13% 25% 26% 10% 5%	530
	11th to 20th	Day	20% 15% 28% 23% 11% 3	434
	21st to end	Day	27% 13% 30% 22% 8%	462
November	1st to 10th	Day	22% 16% 29% 23% 8% 2	425
	11th to 20th	Day	25% 15% 33% 21% 4 1	430
	21st to end	Day	28% 16% 30% 19% 6% 1	430
December	1st to 10th	Day	24% 22% 33% 16% 6%	249
	11th to 20th	Day	28% 21% 34% 13% 4	220
	21st to end	Day	28% 23% 29% 15% 5%	242
January	1st to 10th	Night	22% 17% 38% 20% 4	840
	11th to 20th	Night	27% 16% 34% 19% 4	840
	21st to end	Night	20% 17% 38% 21% 4	924
February	1st to 10th	Night	23% 17% 38% 19% 3	840
	11th to 20th	Night	22% 16% 38% 22% 2	823
	21st to end	Night	21% 14% 38% 23% 5%	692
March	1st to 10th	Night	25% 15% 39% 18% 2	840
	11th to 20th	Night	24% 15% 33% 21% 7%	840
	21st to end	Night	27% 14% 34% 22% 4	924
April	1st to 10th	Night	28% 12% 33% 23% 4	840
	11th to 20th	Night	24% 13% 32% 25% 6%	743
	21st to end	Night	25% 13% 32% 23% 6% 1	630
May	1st to 10th	Night	27% 8% 30% 26% 7% 1	630
	11th to 20th	Night	21% 10% 31% 30% 8% 1	597
	21st to end	Night	25% 7% 27% 29% 11% 1	455
June	1st to 10th	Night	24% 6% 24% 32% 13% 1	420
	11th to 20th	Night	21% 4 25% 35% 15% 1	420
	21st to end	Night	24% 3 20% 39% 13%	420
July	1st to 10th	Night	20% 2 20% 41% 16% 1	410
	11th to 20th	Night	15% 4 19% 43% 18% 1	408
	21st to end	Night	15% 2 19% 45% 18% 2	458
August	1st to 10th	Night	14% 4 19% 43% 18% 1	553
	11th to 20th	Night	15% 4 22% 38% 19% 2	592
	21st to end	Night	15% 4 23% 38% 18% 3	800
September	1st to 10th	Night	14% 3 28% 37% 18% 1	800
	11th to 20th	Night	17% 5% 25% 28% 20% 4	800
	21st to end	Night	18% 6% 26% 31% 16% 3	800
October	1st to 10th	Night	21% 9% 25% 28% 16% 1	800
	11th to 20th	Night	19% 9% 31% 28% 11% 1	800
	21st to end	Night	27% 10% 30% 27% 6%	880
November	1st to 10th	Night	20% 12% 34% 27% 6% 1	820
	11th to 20th	Night	23% 15% 32% 23% 6% 1	840
	21st to end	Night	22% 18% 32% 22% 5%	840
December	1st to 10th	Night	25% 14% 38% 18% 5%	840
	11th to 20th	Night	25% 18% 34% 21% 3	840
	21st to end	Night	27% 14% 35% 22% 3	924

Page generated at 2026-07-28 03:25:26. Version ac07

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