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 2025-03-27.

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% 8% 2 6% 4 4 4 3 5% 7% 39%	108735
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	110577
Transparency	Day	44% 4 12% 23% 15% 3	112303
	Night	45% 4 11% 22% 15% 2	115173
Seeing	Day	22% 14% 33% 21% 8% 2	15394
	Night	22% 11% 31% 27% 9% 1	24345

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% 7% 3 7% 6% 5% 5% 3 5% 6% 40%	3597
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 4 5% 46%	3570
	21st to end	Day	14% 10% 3 5% 3 3 4 3 4 5% 47%	3960
February	1st to 10th	Day	17% 9% 3 6% 4 5% 4 4 6% 6% 37%	1785
	11th to 20th	Day	13% 8% 3 7% 5% 5% 4 3 6% 7% 39%	1948
	21st to end	Day	15% 11% 4 8% 4 4 4 3 4 6% 37%	1650
March	1st to 10th	Day	26% 12% 2 7% 4 4 4 3 4 4 30%	3216
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 5% 6% 40%	3509
	21st to end	Day	24% 10% 3 6% 3 3 4 3 4 6% 36%	4003
April	1st to 10th	Day	20% 10% 3 7% 4 3 4 2 5% 6% 36%	2280
	11th to 20th	Day	18% 6% 2 5% 5% 4 5% 4 6% 7% 39%	2333
	21st to end	Day	22% 8% 2 7% 4 4 4 3 5% 6% 35%	2470
May	1st to 10th	Day	23% 12% 3 7% 4 4 4 4 5% 6% 28%	4106
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 4 7% 34%	4144
	21st to end	Day	17% 5% 2 6% 4 4 5% 4 6% 9% 39%	4837
June	1st to 10th	Day	23% 7% 2 6% 4 4 4 4 6% 7% 33%	2660
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 8% 40%	2660
	21st to end	Day	22% 7% 2 6% 4 4 5% 5% 6% 7% 33%	2660
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 4 6% 9% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	4440
	21st to end	Day	19% 7% 2 5% 4 4 4 4 6% 7% 37%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 4 5% 4 6% 8% 37%	4139
	11th to 20th	Day	16% 9% 3 8% 4 5% 4 3 6% 8% 35%	4045
	21st to end	Day	9% 4 2 5% 5% 5% 5% 5% 6% 9% 45%	4291
September	1st to 10th	Day	12% 4 2 5% 4 4 5% 3 5% 8% 49%	2244
	11th to 20th	Day	13% 5% 2 4 4 3 3 2 3 5% 55%	2090
	21st to end	Day	16% 6% 2 4 3 4 5% 3 5% 6% 46%	2090
October	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Day	17% 6% 2 6% 5% 3 4 3 4 6% 44%	1835
	21st to end	Day	23% 7% 3 4 4 3 4 3 4 4 41%	1881
November	1st to 10th	Day	17% 6% 2 4 3 3 3 3 4 6% 49%	1761
	11th to 20th	Day	19% 5% 1 4 3 3 4 3 5% 5% 47%	1800
	21st to end	Day	23% 6% 2 5% 3 3 3 3 4 6% 42%	1800
December	1st to 10th	Day	15% 10% 3 5% 4 4 4 3 5% 7% 41%	2641
	11th to 20th	Day	19% 7% 2 4 3 3 3 2 4 5% 48%	2986
	21st to end	Day	21% 10% 3 5% 4 3 4 3 5% 5% 36%	3960
January	1st to 10th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 44%	6349
	11th to 20th	Night	13% 8% 4 10% 5% 5% 4 3 5% 5% 36%	6315
	21st to end	Night	15% 10% 3 7% 4 3 4 3 5% 5% 42%	6996
February	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 5% 5% 39%	2610
	11th to 20th	Night	14% 10% 3 6% 5% 5% 5% 4 4 5% 41%	2338
	21st to end	Night	16% 8% 3 7% 4 5% 4 3 4 6% 40%	1980
March	1st to 10th	Night	28% 10% 2 5% 4 3 4 2 4 5% 33%	3844
	11th to 20th	Night	14% 7% 3 8% 5% 5% 4 5% 5% 7% 36%	3670
	21st to end	Night	25% 8% 2 5% 4 4 4 3 6% 5% 33%	3766
April	1st to 10th	Night	21% 9% 3 5% 4 4 4 3 4 6% 37%	1900
	11th to 20th	Night	18% 8% 3 6% 3 4 4 4 5% 6% 39%	1796
	21st to end	Night	22% 8% 2 6% 3 3 4 3 4 7% 37%	1710
May	1st to 10th	Night	24% 10% 3 7% 4 3 5% 4 5% 7% 28%	2834
	11th to 20th	Night	17% 8% 3 7% 6% 6% 5% 6% 6% 8% 26%	2764
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 7% 39%	2738
June	1st to 10th	Night	21% 8% 3 8% 5% 4 4 4 5% 6% 32%	1520
	11th to 20th	Night	18% 6% 3 5% 5% 4 4 4 6% 7% 39%	1520
	21st to end	Night	22% 7% 2 5% 5% 4 4 4 5% 8% 35%	1520
July	1st to 10th	Night	12% 6% 2 7% 6% 6% 6% 5% 7% 9% 34%	2520
	11th to 20th	Night	12% 5% 3 8% 5% 4 4 4 8% 8% 38%	2520
	21st to end	Night	15% 7% 2 5% 4 5% 5% 4 6% 7% 40%	2739
August	1st to 10th	Night	21% 6% 3 7% 5% 5% 5% 4 6% 8% 32%	2757
	11th to 20th	Night	15% 9% 3 7% 4 5% 4 3 6% 8% 36%	2782
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 8% 48%	3348
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 4 7% 50%	1934
	11th to 20th	Night	15% 5% 2 4 3 2 3 2 5% 6% 51%	2090
	21st to end	Night	16% 6% 1 5% 3 3 3 3 5% 7% 48%	2090
October	1st to 10th	Night	19% 6% 3 6% 4 4 3 4 5% 6% 40%	2090
	11th to 20th	Night	16% 5% 2 6% 4 4 4 4 5% 6% 44%	2342
	21st to end	Night	24% 8% 2 6% 3 4 3 3 4 6% 38%	2717
November	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 5% 46%	2537
	11th to 20th	Night	18% 6% 2 4 4 4 3 3 4 5% 46%	2600
	21st to end	Night	17% 7% 2 6% 4 3 4 3 4 6% 44%	2600
December	1st to 10th	Night	17% 10% 3 6% 3 3 4 3 5% 5% 40%	4537
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 3 4 43%	5208
	21st to end	Night	21% 11% 3 7% 4 3 3 2 4 5% 37%	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	45% 3 8% 26% 17% 1	3598
	11th to 20th	Day	41% 2 13% 27% 14% 3	3600
	21st to end	Day	41% 2 9% 22% 21% 6%	3960
February	1st to 10th	Day	46% 3 12% 22% 14% 4	1785
	11th to 20th	Day	44% 4 12% 21% 15% 4	1957
	21st to end	Day	49% 2 10% 21% 14% 3	1650
March	1st to 10th	Day	59% 3 9% 12% 14% 4	3219
	11th to 20th	Day	42% 4 13% 20% 16% 5%	3509
	21st to end	Day	50% 4 4 15% 19% 7%	4003
April	1st to 10th	Day	50% 3 8% 18% 17% 4	2280
	11th to 20th	Day	43% 4 11% 23% 16% 3	2333
	21st to end	Day	50% 3 9% 18% 16% 4	2470
May	1st to 10th	Day	56% 3 14% 17% 10% 1	4120
	11th to 20th	Day	50% 4 17% 18% 10% 2	4148
	21st to end	Day	41% 5% 16% 23% 14% 2	4837
June	1st to 10th	Day	50% 5% 12% 23% 9%	2660
	11th to 20th	Day	42% 4 13% 22% 16% 3	2660
	21st to end	Day	48% 5% 13% 21% 11% 1	2660
July	1st to 10th	Day	44% 5% 13% 25% 13% 1	4370
	11th to 20th	Day	38% 6% 17% 27% 12%	4365
	21st to end	Day	45% 5% 14% 23% 13% 1	4661
August	1st to 10th	Day	45% 5% 16% 24% 9%	4125
	11th to 20th	Day	47% 5% 15% 24% 8% 1	4084
	21st to end	Day	33% 6% 17% 24% 19% 1	4291
September	1st to 10th	Day	34% 5% 13% 29% 18% 1	2244
	11th to 20th	Day	33% 3 9% 30% 23% 3	2090
	21st to end	Day	38% 5% 9% 27% 18% 2	2090
October	1st to 10th	Day	46% 3 11% 22% 16% 1	3480
	11th to 20th	Day	34% 2 9% 31% 22% 2	3025
	21st to end	Day	36% 2 10% 27% 21% 3	3081
November	1st to 10th	Day	37% 4 13% 26% 18% 2	1761
	11th to 20th	Day	38% 3 10% 23% 22% 4	1800
	21st to end	Day	45% 3 15% 22% 13% 2	1800
December	1st to 10th	Day	43% 3 12% 29% 11% 1	2641
	11th to 20th	Day	40% 3 12% 19% 18% 9%	2986
	21st to end	Day	51% 3 9% 18% 13% 5%	3960
January	1st to 10th	Night	41% 2 11% 30% 13% 2	6349
	11th to 20th	Night	50% 4 12% 20% 11% 3	6360
	21st to end	Night	45% 2 8% 20% 18% 7%	6996
February	1st to 10th	Night	48% 3 12% 18% 15% 3	2610
	11th to 20th	Night	46% 4 9% 22% 17% 4	2340
	21st to end	Night	46% 3 9% 23% 14% 4	1980
March	1st to 10th	Night	56% 3 9% 17% 12% 4	3854
	11th to 20th	Night	47% 3 11% 21% 15% 3	3671
	21st to end	Night	51% 4 5% 16% 18% 6%	3766
April	1st to 10th	Night	49% 4 7% 19% 17% 4	1900
	11th to 20th	Night	46% 3 7% 22% 19% 2	1796
	21st to end	Night	50% 3 9% 18% 16% 4	1710
May	1st to 10th	Night	55% 4 14% 17% 8% 1	2840
	11th to 20th	Night	54% 5% 15% 17% 7% 2	2765
	21st to end	Night	46% 5% 15% 19% 14% 2	2739
June	1st to 10th	Night	52% 5% 10% 20% 12% 1	1520
	11th to 20th	Night	43% 6% 14% 21% 15% 2	1520
	21st to end	Night	48% 6% 13% 22% 10% 1	1520
July	1st to 10th	Night	45% 6% 14% 20% 13% 1	2480
	11th to 20th	Night	41% 6% 13% 24% 15%	2472
	21st to end	Night	42% 5% 12% 26% 13% 1	2651
August	1st to 10th	Night	51% 6% 13% 20% 9% 1	2747
	11th to 20th	Night	46% 6% 15% 21% 12%	2809
	21st to end	Night	33% 5% 16% 25% 20% 1	3348
September	1st to 10th	Night	37% 4 11% 29% 18% 1	1934
	11th to 20th	Night	34% 4 10% 29% 20% 2	2090
	21st to end	Night	37% 4 10% 29% 17% 2	2090
October	1st to 10th	Night	45% 3 13% 22% 15% 1	3480
	11th to 20th	Night	32% 3 9% 28% 26% 3	3891
	21st to end	Night	39% 2 10% 27% 19% 3	4467
November	1st to 10th	Night	40% 3 11% 26% 17% 3	2537
	11th to 20th	Night	40% 4 9% 24% 20% 3	2600
	21st to end	Night	42% 3 15% 23% 15% 2	2600
December	1st to 10th	Night	46% 3 9% 29% 12% 1	4537
	11th to 20th	Night	47% 3 10% 17% 20% 3	5208
	21st to end	Night	52% 3 8% 19% 17% 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	27% 21% 30% 19% 3 1	200
	11th to 20th	Day	24% 24% 28% 20% 5% 1	200
	21st to end	Day	18% 21% 41% 14% 6%	220
February	1st to 10th	Day	28% 17% 40% 14% 1	373
	11th to 20th	Day	23% 22% 37% 16% 2	496
	21st to end	Day	25% 18% 36% 17% 4	414
March	1st to 10th	Day	29% 16% 36% 16% 3	500
	11th to 20th	Day	25% 22% 33% 16% 4 1	500
	21st to end	Day	28% 18% 34% 12% 6% 2	530
April	1st to 10th	Day	31% 20% 28% 13% 5% 2	460
	11th to 20th	Day	25% 19% 34% 15% 7% 1	452
	21st to end	Day	31% 15% 35% 13% 5% 2	460
May	1st to 10th	Day	30% 19% 33% 13% 4 2	463
	11th to 20th	Day	20% 16% 41% 17% 5% 1	487
	21st to end	Day	20% 16% 33% 22% 7% 2	529
June	1st to 10th	Day	21% 14% 34% 23% 6% 1	490
	11th to 20th	Day	13% 17% 37% 22% 8% 3	490
	21st to end	Day	23% 11% 34% 23% 9% 1	490
July	1st to 10th	Day	19% 7% 34% 26% 10% 3	485
	11th to 20th	Day	13% 7% 35% 29% 12% 4	484
	21st to end	Day	16% 5% 36% 25% 15% 2	532
August	1st to 10th	Day	14% 8% 30% 30% 15% 2	486
	11th to 20th	Day	18% 5% 30% 29% 13% 5%	479
	21st to end	Day	12% 8% 32% 29% 13% 6%	539
September	1st to 10th	Day	15% 8% 32% 27% 15% 4	490
	11th to 20th	Day	17% 9% 32% 23% 12% 8%	490
	21st to end	Day	17% 11% 33% 23% 11% 6%	490
October	1st to 10th	Day	22% 13% 25% 25% 11% 4	490
	11th to 20th	Day	19% 15% 28% 24% 11% 3	403
	21st to end	Day	28% 13% 29% 22% 8%	429
November	1st to 10th	Day	21% 14% 30% 24% 9% 2	395
	11th to 20th	Day	26% 15% 34% 21% 4 1	400
	21st to end	Day	28% 15% 29% 20% 6% 1	400
December	1st to 10th	Day	23% 21% 34% 16% 6%	228
	11th to 20th	Day	27% 22% 34% 14% 4	200
	21st to end	Day	26% 21% 30% 16% 6%	220
January	1st to 10th	Night	22% 17% 38% 20% 4	800
	11th to 20th	Night	26% 16% 34% 20% 4	800
	21st to end	Night	19% 17% 38% 21% 4	880
February	1st to 10th	Night	25% 17% 37% 19% 3	800
	11th to 20th	Night	23% 16% 37% 22% 2	783
	21st to end	Night	21% 14% 37% 24% 5%	660
March	1st to 10th	Night	25% 15% 39% 19% 2	800
	11th to 20th	Night	23% 14% 34% 21% 7%	800
	21st to end	Night	26% 14% 34% 21% 4	863
April	1st to 10th	Night	28% 11% 33% 24% 4	760
	11th to 20th	Night	23% 12% 32% 26% 6%	672
	21st to end	Night	25% 12% 32% 24% 6% 1	570
May	1st to 10th	Night	25% 8% 31% 27% 8% 1	570
	11th to 20th	Night	21% 9% 30% 30% 9% 1	539
	21st to end	Night	25% 8% 25% 29% 12% 1	411
June	1st to 10th	Night	24% 6% 25% 32% 12% 1	380
	11th to 20th	Night	21% 3 24% 35% 16% 1	380
	21st to end	Night	26% 3 21% 37% 13% 1	380
July	1st to 10th	Night	19% 2 21% 40% 17% 1	370
	11th to 20th	Night	15% 4 17% 44% 18% 1	368
	21st to end	Night	15% 2 19% 45% 18% 2	416
August	1st to 10th	Night	15% 4 19% 43% 18% 1	525
	11th to 20th	Night	16% 3 22% 38% 19% 2	562
	21st to end	Night	15% 4 22% 38% 19% 3	760
September	1st to 10th	Night	14% 3 28% 37% 18% 1	760
	11th to 20th	Night	18% 6% 25% 27% 20% 4	760
	21st to end	Night	18% 6% 26% 31% 17% 3	760
October	1st to 10th	Night	22% 9% 24% 27% 16% 1	760
	11th to 20th	Night	19% 9% 31% 28% 12% 1	760
	21st to end	Night	28% 10% 29% 26% 6%	836
November	1st to 10th	Night	19% 12% 34% 28% 6% 2	780
	11th to 20th	Night	23% 15% 33% 23% 6% 1	800
	21st to end	Night	22% 19% 32% 23% 5%	800
December	1st to 10th	Night	25% 14% 38% 18% 6%	800
	11th to 20th	Night	24% 18% 34% 21% 3	800
	21st to end	Night	26% 13% 36% 22% 3	880

Page generated at 2025-03-24 09:04:27. Version ac07

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