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 2024-10-20.

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%	101134
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 38%	97541
Transparency	Day	45% 4 12% 22% 14% 2	100892
	Night	47% 4 11% 22% 14% 2	97403
Seeing	Day	22% 14% 33% 21% 8% 2	14937
	Night	22% 11% 31% 27% 9% 1	23713

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	15% 9% 2 6% 5% 4 3 2 5% 5% 44%	2517
	11th to 20th	Day	17% 7% 3 6% 5% 5% 4 3 4 5% 42%	2520
	21st to end	Day	17% 11% 4 7% 3 3 4 4 5% 5% 37%	2772
February	1st to 10th	Day	18% 9% 3 6% 4 5% 3 4 6% 6% 36%	1695
	11th to 20th	Day	13% 8% 3 7% 5% 5% 4 3 6% 6% 39%	1848
	21st to end	Day	15% 12% 4 8% 4 4 4 3 5% 6% 37%	1570
March	1st to 10th	Day	27% 13% 2 6% 4 4 4 3 4 4 29%	3116
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 5% 6% 40%	3402
	21st to end	Day	24% 10% 3 6% 3 3 4 3 4 6% 36%	3937
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%	1826
	21st to end	Day	24% 7% 3 4 4 3 4 3 5% 4 39%	1782
November	1st to 10th	Day	18% 6% 2 4 3 3 3 3 4 6% 49%	1671
	11th to 20th	Day	20% 5% 1 4 3 3 4 3 5% 6% 45%	1710
	21st to end	Day	22% 6% 2 5% 3 3 3 3 4 6% 43%	1710
December	1st to 10th	Day	22% 9% 3 5% 4 3 3 3 4 6% 38%	1547
	11th to 20th	Day	22% 8% 2 4 4 3 3 2 4 4 43%	1906
	21st to end	Day	25% 10% 3 5% 4 3 5% 3 5% 5% 32%	2772
January	1st to 10th	Night	17% 9% 2 5% 3 3 3 3 5% 5% 44%	4429
	11th to 20th	Night	14% 7% 3 9% 5% 4 4 3 5% 6% 39%	4440
	21st to end	Night	20% 12% 3 7% 4 3 4 3 4 5% 35%	4884
February	1st to 10th	Night	16% 10% 3 7% 4 4 5% 3 5% 5% 38%	2480
	11th to 20th	Night	14% 10% 3 6% 5% 5% 5% 3 4 5% 40%	2218
	21st to end	Night	16% 8% 3 7% 4 4 5% 3 5% 5% 41%	1884
March	1st to 10th	Night	29% 10% 2 4 4 3 4 2 4 5% 34%	3724
	11th to 20th	Night	14% 7% 3 8% 5% 5% 4 5% 5% 7% 36%	3557
	21st to end	Night	25% 9% 2 5% 4 4 4 3 6% 5% 33%	3693
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% 43%	2336
	21st to end	Night	24% 8% 2 6% 4 4 3 3 4 6% 37%	2574
November	1st to 10th	Night	17% 6% 2 5% 4 4 4 4 5% 5% 45%	2407
	11th to 20th	Night	19% 6% 2 4 4 4 3 3 5% 5% 45%	2470
	21st to end	Night	17% 7% 2 5% 4 3 4 3 4 6% 44%	2470
December	1st to 10th	Night	21% 9% 3 6% 4 4 4 3 4 4 37%	2631
	11th to 20th	Night	22% 8% 2 4 4 3 4 3 4 5% 41%	3288
	21st to end	Night	24% 10% 3 7% 4 4 3 2 4 5% 34%	4884

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	44% 3 9% 30% 13% 1	2518
	11th to 20th	Day	45% 3 13% 25% 11% 3	2520
	21st to end	Day	49% 2 11% 24% 13% 1	2772
February	1st to 10th	Day	47% 3 12% 22% 13% 3	1695
	11th to 20th	Day	44% 4 11% 21% 15% 4	1857
	21st to end	Day	50% 2 10% 21% 14% 4	1570
March	1st to 10th	Day	59% 2 9% 12% 14% 3	3119
	11th to 20th	Day	42% 4 13% 20% 16% 5%	3402
	21st to end	Day	50% 4 4 15% 20% 7%	3937
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 10% 22% 17% 2	2090
	11th to 20th	Day	41% 4 11% 30% 13% 1	1826
	21st to end	Day	48% 4 10% 22% 14% 2	1782
November	1st to 10th	Day	38% 4 13% 26% 17% 3	1671
	11th to 20th	Day	39% 4 10% 23% 20% 4	1710
	21st to end	Day	44% 3 16% 21% 14% 2	1710
December	1st to 10th	Day	49% 3 11% 22% 13% 2	1547
	11th to 20th	Day	45% 4 9% 21% 16% 5%	1906
	21st to end	Day	55% 4 12% 18% 9% 2	2772
January	1st to 10th	Night	43% 3 13% 28% 11% 3	4429
	11th to 20th	Night	46% 4 12% 22% 13% 4	4440
	21st to end	Night	53% 3 10% 22% 12% 2	4884
February	1st to 10th	Night	48% 3 12% 19% 15% 3	2480
	11th to 20th	Night	46% 4 8% 22% 17% 3	2220
	21st to end	Night	46% 3 9% 23% 14% 5%	1884
March	1st to 10th	Night	56% 3 9% 17% 12% 3	3734
	11th to 20th	Night	47% 3 10% 21% 15% 3	3558
	21st to end	Night	52% 4 5% 16% 17% 6%	3693
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% 4 11% 23% 15% 2	2090
	11th to 20th	Night	41% 4 12% 28% 15% 1	2336
	21st to end	Night	49% 3 10% 22% 14% 2	2574
November	1st to 10th	Night	42% 3 10% 25% 17% 2	2407
	11th to 20th	Night	42% 4 9% 24% 19% 3	2470
	21st to end	Night	42% 3 15% 23% 15% 2	2470
December	1st to 10th	Night	51% 3 10% 23% 12% 1	2631
	11th to 20th	Night	46% 3 11% 19% 20% 1	3288
	21st to end	Night	55% 3 9% 21% 11% 1	4884

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	29% 18% 28% 21% 3 1	180
	11th to 20th	Day	26% 23% 27% 18% 4 1	180
	21st to end	Day	18% 19% 41% 15% 7%	198
February	1st to 10th	Day	29% 16% 40% 14% 1	343
	11th to 20th	Day	22% 22% 37% 16% 3	456
	21st to end	Day	25% 18% 34% 18% 4	382
March	1st to 10th	Day	29% 15% 35% 18% 3	460
	11th to 20th	Day	25% 21% 34% 16% 4 1	460
	21st to end	Day	28% 18% 34% 12% 7% 2	506
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	400
	21st to end	Day	29% 13% 28% 21% 8%	396
November	1st to 10th	Day	21% 14% 30% 25% 9% 2	365
	11th to 20th	Day	26% 14% 32% 22% 4 1	370
	21st to end	Day	26% 15% 29% 21% 7% 1	370
December	1st to 10th	Day	24% 21% 33% 16% 6%	207
	11th to 20th	Day	28% 21% 34% 14% 3	180
	21st to end	Day	28% 22% 30% 14% 6% 1	198
January	1st to 10th	Night	22% 16% 38% 20% 4	760
	11th to 20th	Night	27% 16% 34% 19% 4	760
	21st to end	Night	20% 17% 37% 22% 4	836
February	1st to 10th	Night	24% 16% 38% 19% 3	760
	11th to 20th	Night	22% 17% 37% 22% 2	743
	21st to end	Night	21% 14% 36% 23% 5%	628
March	1st to 10th	Night	24% 15% 40% 19% 2	760
	11th to 20th	Night	23% 14% 34% 21% 7%	760
	21st to end	Night	26% 13% 34% 22% 4	836
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	759
	21st to end	Night	29% 10% 28% 26% 6% 1	792
November	1st to 10th	Night	20% 11% 33% 28% 6% 2	740
	11th to 20th	Night	24% 14% 31% 23% 7% 1	760
	21st to end	Night	22% 18% 32% 23% 5%	760
December	1st to 10th	Night	25% 14% 38% 18% 5%	760
	11th to 20th	Night	23% 18% 34% 22% 3	760
	21st to end	Night	27% 13% 36% 21% 3	836

Page generated at 2024-10-17 13:09:40. Version ac07

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