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 2024-02-15.

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% 3 5% 5% 3 4 4 5% 7% 40%	78196
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	80876
Transparency	Day	44% 4 12% 23% 14% 2	77901
	Night	46% 4 11% 23% 15% 2	80705
Seeing	Day	22% 14% 33% 21% 8% 2	13953
	Night	22% 11% 31% 27% 9% 1	22928

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	16% 8% 3 5% 6% 2 4 3 5% 5% 44%	2517
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 4 5% 42%	2520
	21st to end	Day	17% 11% 4 6% 4 2 4 4 5% 6% 37%	2772
February	1st to 10th	Day	18% 8% 4 5% 4 3 5% 5% 5% 6% 36%	1695
	11th to 20th	Day	14% 8% 3 6% 6% 3 5% 4 6% 7% 39%	1788
	21st to end	Day	15% 11% 5% 7% 5% 3 4 4 4 6% 36%	1480
March	1st to 10th	Day	17% 8% 3 6% 6% 3 5% 4 5% 6% 38%	1797
	11th to 20th	Day	17% 7% 3 6% 4 3 4 3 5% 6% 43%	1930
	21st to end	Day	17% 8% 4 6% 5% 2 4 3 3 5% 42%	2245
April	1st to 10th	Day	20% 9% 4 6% 5% 2 4 3 4 6% 37%	2160
	11th to 20th	Day	18% 5% 3 5% 5% 3 4 5% 5% 7% 39%	2208
	21st to end	Day	23% 8% 2 6% 5% 3 4 4 5% 6% 34%	2340
May	1st to 10th	Day	24% 6% 4 6% 5% 2 4 4 5% 6% 34%	2340
	11th to 20th	Day	18% 7% 3 6% 4 3 4 4 5% 8% 38%	2336
	21st to end	Day	18% 5% 3 6% 5% 3 5% 5% 5% 9% 38%	2725
June	1st to 10th	Day	23% 7% 2 6% 5% 3 5% 5% 5% 7% 33%	2520
	11th to 20th	Day	17% 6% 3 5% 4 3 5% 4 5% 8% 41%	2520
	21st to end	Day	22% 6% 3 5% 5% 3 5% 6% 5% 7% 34%	2520
July	1st to 10th	Day	16% 5% 3 5% 4 3 6% 5% 6% 8% 39%	2520
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 6% 8% 9% 37%	2520
	21st to end	Day	13% 6% 2 5% 5% 4 6% 7% 6% 8% 40%	2744
August	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 6% 7% 9% 40%	2347
	11th to 20th	Day	13% 6% 3 6% 6% 3 4 5% 7% 9% 39%	2304
	21st to end	Day	13% 5% 1 4 4 3 5% 5% 6% 8% 44%	2445
September	1st to 10th	Day	12% 4 2 4 4 3 5% 5% 5% 8% 48%	2126
	11th to 20th	Day	14% 5% 2 4 4 2 3 3 3 5% 55%	1980
	21st to end	Day	16% 6% 2 4 4 3 5% 4 5% 6% 47%	1980
October	1st to 10th	Day	19% 8% 3 5% 5% 2 4 4 4 6% 40%	1980
	11th to 20th	Day	17% 5% 2 6% 5% 2 4 5% 4 6% 43%	1739
	21st to end	Day	24% 7% 3 3 4 3 4 3 5% 4 39%	1782
November	1st to 10th	Day	18% 5% 2 4 3 2 3 4 4 6% 49%	1671
	11th to 20th	Day	20% 5% 2 4 4 2 4 4 5% 6% 46%	1710
	21st to end	Day	22% 6% 2 5% 4 2 3 4 4 6% 43%	1710
December	1st to 10th	Day	22% 9% 3 4 5% 2 3 4 4 6% 38%	1547
	11th to 20th	Day	22% 7% 2 4 4 3 3 3 4 4 43%	1906
	21st to end	Day	25% 10% 3 4 5% 2 4 4 5% 5% 32%	2772
January	1st to 10th	Night	17% 9% 3 4 4 2 3 4 4 5% 44%	4429
	11th to 20th	Night	14% 7% 3 8% 7% 4 3 4 5% 6% 39%	4440
	21st to end	Night	20% 11% 3 7% 5% 2 4 4 4 5% 35%	4884
February	1st to 10th	Night	16% 9% 3 6% 5% 3 5% 4 5% 5% 38%	2480
	11th to 20th	Night	14% 9% 4 6% 5% 4 5% 5% 4 5% 40%	2153
	21st to end	Night	16% 7% 3 6% 5% 4 5% 3 4 6% 40%	1776
March	1st to 10th	Night	15% 10% 3 5% 5% 3 5% 4 5% 5% 40%	2150
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 5% 5% 6% 39%	2029
	21st to end	Night	19% 8% 3 4 4 2 4 4 4 5% 43%	2109
April	1st to 10th	Night	22% 8% 3 4 4 3 4 4 3 6% 37%	1800
	11th to 20th	Night	18% 7% 3 5% 4 3 4 5% 5% 6% 40%	1701
	21st to end	Night	23% 7% 3 5% 4 3 5% 4 5% 7% 35%	1620
May	1st to 10th	Night	22% 8% 3 5% 5% 3 4 5% 6% 6% 34%	1620
	11th to 20th	Night	18% 7% 3 5% 6% 4 6% 6% 5% 8% 32%	1572
	21st to end	Night	20% 9% 3 6% 5% 2 4 5% 4 7% 36%	1550
June	1st to 10th	Night	22% 6% 4 7% 6% 3 4 4 5% 6% 33%	1440
	11th to 20th	Night	19% 6% 2 4 6% 2 5% 5% 5% 7% 40%	1440
	21st to end	Night	22% 6% 2 4 5% 3 4 5% 5% 8% 37%	1440
July	1st to 10th	Night	16% 5% 2 5% 6% 4 5% 5% 5% 7% 39%	1440
	11th to 20th	Night	15% 5% 2 7% 5% 3 4 5% 8% 10% 36%	1440
	21st to end	Night	14% 5% 2 3 5% 3 6% 5% 6% 9% 44%	1574
August	1st to 10th	Night	13% 4 2 5% 5% 3 5% 5% 6% 10% 42%	1571
	11th to 20th	Night	14% 6% 2 6% 6% 2 5% 4 7% 9% 39%	1596
	21st to end	Night	15% 4 2 5% 4 3 4 5% 5% 7% 46%	1908
September	1st to 10th	Night	13% 5% 2 4 5% 3 4 4 4 7% 49%	1832
	11th to 20th	Night	15% 5% 2 4 3 2 3 4 5% 6% 51%	1980
	21st to end	Night	16% 5% 2 4 4 3 3 3 5% 7% 49%	1980
October	1st to 10th	Night	20% 6% 3 5% 5% 3 4 4 5% 6% 40%	1980
	11th to 20th	Night	17% 5% 2 5% 5% 3 4 5% 5% 7% 42%	2218
	21st to end	Night	24% 7% 2 5% 4 3 4 4 4 6% 37%	2574
November	1st to 10th	Night	17% 5% 2 4 5% 3 4 5% 5% 5% 45%	2407
	11th to 20th	Night	19% 6% 2 4 4 3 4 4 4 6% 45%	2470
	21st to end	Night	17% 6% 3 5% 4 3 4 4 4 6% 45%	2470
December	1st to 10th	Night	21% 8% 4 6% 4 3 4 5% 4 5% 37%	2631
	11th to 20th	Night	22% 8% 2 4 4 2 4 4 4 5% 42%	3288
	21st to end	Night	24% 10% 3 6% 5% 3 4 3 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	45% 5% 11% 22% 13% 5%	1797
	21st to end	Day	50% 2 9% 21% 14% 4	1480
March	1st to 10th	Day	47% 3 12% 18% 15% 5%	1800
	11th to 20th	Day	43% 2 10% 22% 17% 5%	1930
	21st to end	Day	46% 3 7% 20% 18% 6%	2245
April	1st to 10th	Day	49% 3 8% 19% 17% 4	2160
	11th to 20th	Day	43% 4 11% 23% 16% 3	2208
	21st to end	Day	51% 3 10% 17% 15% 4	2340
May	1st to 10th	Day	50% 3 8% 20% 16% 2	2340
	11th to 20th	Day	44% 4 10% 20% 17% 4	2340
	21st to end	Day	44% 6% 14% 22% 11% 3	2725
June	1st to 10th	Day	50% 5% 12% 22% 10%	2520
	11th to 20th	Day	42% 4 12% 23% 17% 3	2520
	21st to end	Day	48% 6% 13% 22% 11% 1	2520
July	1st to 10th	Day	41% 5% 14% 25% 15% 1	2450
	11th to 20th	Day	40% 5% 16% 27% 11% 1	2445
	21st to end	Day	38% 6% 16% 25% 12% 2	2591
August	1st to 10th	Day	38% 6% 19% 26% 11% 1	2333
	11th to 20th	Day	40% 5% 16% 27% 11% 1	2304
	21st to end	Day	35% 5% 14% 25% 19% 2	2445
September	1st to 10th	Day	35% 5% 14% 28% 17% 1	2126
	11th to 20th	Day	33% 3 9% 29% 23% 3	1980
	21st to end	Day	38% 6% 9% 27% 18% 2	1980
October	1st to 10th	Day	46% 3 9% 23% 16% 2	1980
	11th to 20th	Day	42% 4 11% 30% 12% 1	1739
	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	47% 4 8% 22% 16% 4	2155
	21st to end	Night	46% 3 9% 23% 15% 4	1776
March	1st to 10th	Night	47% 4 7% 22% 17% 4	2160
	11th to 20th	Night	46% 3 10% 20% 16% 5%	2030
	21st to end	Night	44% 3 8% 18% 21% 6%	2109
April	1st to 10th	Night	49% 4 7% 19% 16% 4	1800
	11th to 20th	Night	45% 3 8% 23% 19% 2	1701
	21st to end	Night	51% 4 9% 19% 14% 4	1620
May	1st to 10th	Night	50% 4 8% 21% 13% 2	1620
	11th to 20th	Night	49% 6% 9% 19% 12% 4	1573
	21st to end	Night	50% 5% 12% 19% 13% 2	1551
June	1st to 10th	Night	52% 4 10% 21% 12% 1	1440
	11th to 20th	Night	44% 5% 12% 21% 15% 2	1440
	21st to end	Night	46% 6% 13% 23% 10% 1	1440
July	1st to 10th	Night	43% 6% 14% 21% 14% 1	1400
	11th to 20th	Night	41% 5% 14% 28% 12%	1392
	21st to end	Night	36% 7% 15% 29% 11% 1	1486
August	1st to 10th	Night	38% 7% 15% 28% 12% 1	1561
	11th to 20th	Night	41% 6% 16% 22% 15%	1596
	21st to end	Night	38% 5% 10% 27% 19% 1	1908
September	1st to 10th	Night	37% 4 11% 29% 18% 1	1832
	11th to 20th	Night	35% 4 9% 29% 20% 2	1980
	21st to end	Night	36% 4 10% 30% 17% 2	1980
October	1st to 10th	Night	45% 4 11% 23% 16% 2	1980
	11th to 20th	Night	42% 4 12% 28% 13% 1	2218
	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% 21% 37% 17% 3	432
	21st to end	Day	25% 17% 36% 18% 4	346
March	1st to 10th	Day	27% 16% 36% 18% 3	420
	11th to 20th	Day	24% 20% 35% 15% 4 1	420
	21st to end	Day	25% 18% 35% 13% 7% 2	462
April	1st to 10th	Day	30% 20% 30% 13% 5% 2	420
	11th to 20th	Day	24% 18% 34% 15% 7% 1	412
	21st to end	Day	32% 15% 34% 14% 5% 1	420
May	1st to 10th	Day	30% 19% 32% 13% 4 2	423
	11th to 20th	Day	19% 17% 42% 16% 5% 1	447
	21st to end	Day	21% 15% 33% 22% 7% 2	485
June	1st to 10th	Day	21% 14% 34% 22% 6% 2	450
	11th to 20th	Day	14% 17% 38% 21% 8% 2	450
	21st to end	Day	22% 10% 34% 23% 10% 1	450
July	1st to 10th	Day	20% 7% 33% 27% 10% 3	445
	11th to 20th	Day	14% 6% 34% 30% 11% 5%	444
	21st to end	Day	14% 5% 37% 26% 16% 2	488
August	1st to 10th	Day	13% 8% 31% 31% 15% 2	446
	11th to 20th	Day	17% 5% 29% 29% 14% 5%	439
	21st to end	Day	13% 8% 32% 29% 13% 5%	495
September	1st to 10th	Day	16% 7% 31% 27% 15% 4	450
	11th to 20th	Day	18% 10% 34% 20% 12% 7%	450
	21st to end	Day	16% 12% 34% 22% 11% 6%	450
October	1st to 10th	Day	23% 13% 24% 26% 10% 4	450
	11th to 20th	Day	19% 15% 29% 24% 11% 3	372
	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% 16% 37% 22% 2	722
	21st to end	Night	21% 14% 37% 22% 5%	592
March	1st to 10th	Night	23% 15% 41% 19% 2	720
	11th to 20th	Night	23% 14% 34% 21% 7%	720
	21st to end	Night	25% 14% 34% 22% 4 1	792
April	1st to 10th	Night	28% 12% 33% 23% 4	720
	11th to 20th	Night	23% 12% 32% 27% 6%	637
	21st to end	Night	26% 13% 31% 24% 6% 1	540
May	1st to 10th	Night	26% 8% 31% 26% 8% 1	540
	11th to 20th	Night	22% 10% 31% 29% 8% 1	511
	21st to end	Night	26% 7% 25% 29% 12% 1	389
June	1st to 10th	Night	25% 6% 25% 31% 12% 1	360
	11th to 20th	Night	22% 3 24% 34% 16% 1	360
	21st to end	Night	25% 3 20% 38% 14% 1	360
July	1st to 10th	Night	19% 2 21% 39% 18% 1	350
	11th to 20th	Night	16% 3 17% 44% 19% 1	348
	21st to end	Night	15% 2 18% 46% 18% 2	394
August	1st to 10th	Night	13% 4 19% 44% 18% 1	497
	11th to 20th	Night	16% 3 21% 38% 19% 2	532
	21st to end	Night	16% 4 22% 38% 18% 3	720
September	1st to 10th	Night	14% 2 27% 37% 19% 1	720
	11th to 20th	Night	19% 6% 26% 27% 19% 4	720
	21st to end	Night	18% 7% 26% 30% 16% 3	720
October	1st to 10th	Night	23% 10% 24% 27% 16% 1	720
	11th to 20th	Night	19% 9% 31% 28% 12% 1	720
	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 2024-02-12 06:35:35. Version ac07

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