Metacom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Metacom Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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% 39%	126667
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	124037
Transparency	Day	44% 6% 12% 21% 15% 3	130214
	Night	44% 6% 12% 21% 16% 3	128577
Seeing	Day	24% 19% 33% 17% 6% 2	16508
	Night	22% 16% 33% 22% 6% 1	25413

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	11% 9% 3 6% 3 3 4 3 5% 6% 47%	3597
	11th to 20th	Day	16% 8% 2 5% 3 3 3 3 4 4 48%	3570
	21st to end	Day	17% 9% 2 6% 4 4 4 3 4 5% 42%	3960
February	1st to 10th	Day	19% 8% 2 5% 4 5% 5% 4 5% 6% 37%	1915
	11th to 20th	Day	14% 8% 2 5% 5% 4 4 4 4 7% 42%	1948
	21st to end	Day	19% 8% 2 6% 5% 4 4 3 5% 6% 38%	1650
March	1st to 10th	Day	22% 11% 3 5% 3 3 3 2 4 5% 38%	4476
	11th to 20th	Day	16% 9% 2 5% 4 3 4 4 5% 7% 41%	4786
	21st to end	Day	25% 10% 2 7% 4 3 4 3 5% 5% 32%	5756
April	1st to 10th	Day	21% 8% 3 5% 4 4 3 4 5% 7% 37%	2400
	11th to 20th	Day	19% 7% 2 5% 4 4 4 3 5% 7% 40%	2379
	21st to end	Day	20% 8% 2 6% 4 4 4 4 5% 5% 39%	2600
May	1st to 10th	Day	28% 10% 3 7% 4 3 3 3 4 7% 27%	5886
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 6% 33%	5861
	21st to end	Day	21% 7% 2 6% 4 4 5% 4 5% 8% 35%	6946
June	1st to 10th	Day	19% 6% 3 5% 4 4 5% 4 7% 8% 34%	2800
	11th to 20th	Day	16% 6% 3 5% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	15% 6% 2 5% 4 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	17% 7% 3 7% 4 4 4 4 7% 9% 33%	6360
	11th to 20th	Day	14% 7% 2 6% 4 5% 4 4 8% 10% 35%	6360
	21st to end	Day	17% 6% 2 6% 4 4 4 3 6% 7% 41%	6913
August	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	4235
	11th to 20th	Day	13% 5% 2 7% 6% 6% 5% 4 5% 8% 38%	4087
	21st to end	Day	7% 4 2 5% 4 5% 6% 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 3 4 4 5% 6% 46%	2388
	11th to 20th	Day	13% 5% 2 5% 3 4 3 2 5% 6% 52%	2200
	21st to end	Day	20% 5% 2 6% 3 3 5% 3 5% 6% 42%	2200
October	1st to 10th	Day	22% 6% 2 5% 4 3 3 2 4 5% 45%	2200
	11th to 20th	Day	17% 5% 2 5% 4 2 3 3 5% 7% 47%	1975
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 6% 44%	1980
November	1st to 10th	Day	15% 8% 2 6% 3 3 3 3 5% 5% 47%	1851
	11th to 20th	Day	17% 6% 2 5% 4 3 3 2 5% 6% 47%	1890
	21st to end	Day	18% 8% 3 4 4 4 3 3 4 6% 43%	1890
December	1st to 10th	Day	16% 8% 2 5% 3 3 3 2 4 6% 48%	2750
	11th to 20th	Day	19% 7% 3 4 3 2 2 2 4 5% 49%	2986
	21st to end	Day	20% 10% 4 7% 4 3 3 2 4 4 38%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 6% 51%	6349
	11th to 20th	Night	15% 10% 3 8% 4 4 4 2 4 5% 42%	6315
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 4 5% 40%	2478
	11th to 20th	Night	18% 8% 3 5% 4 4 3 3 5% 6% 40%	2338
	21st to end	Night	18% 9% 2 6% 4 3 5% 3 4 5% 41%	1980
March	1st to 10th	Night	24% 8% 1 5% 3 3 4 3 4 6% 37%	5364
	11th to 20th	Night	18% 10% 3 6% 4 3 4 3 4 5% 39%	5172
	21st to end	Night	24% 10% 2 6% 4 4 4 4 5% 5% 33%	5174
April	1st to 10th	Night	20% 9% 2 7% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 38%	1970
	21st to end	Night	21% 7% 3 7% 3 4 4 3 4 5% 40%	1800
May	1st to 10th	Night	29% 8% 4 7% 4 4 4 3 4 6% 26%	4054
	11th to 20th	Night	21% 10% 3 7% 5% 5% 5% 3 6% 7% 28%	4033
	21st to end	Night	22% 8% 3 7% 4 4 4 4 5% 6% 30%	3971
June	1st to 10th	Night	17% 8% 3 7% 4 5% 4 3 6% 7% 36%	1600
	11th to 20th	Night	19% 7% 2 6% 4 5% 4 3 5% 8% 37%	1600
	21st to end	Night	18% 7% 2 5% 3 3 4 3 6% 9% 39%	1600
July	1st to 10th	Night	20% 8% 3 7% 4 4 5% 4 6% 8% 32%	3600
	11th to 20th	Night	19% 9% 3 6% 4 4 4 4 6% 9% 33%	3600
	21st to end	Night	14% 7% 3 6% 4 4 5% 3 5% 7% 41%	3926
August	1st to 10th	Night	24% 8% 3 6% 4 4 5% 5% 6% 8% 29%	2883
	11th to 20th	Night	10% 7% 3 7% 5% 6% 6% 5% 6% 8% 38%	2956
	21st to end	Night	9% 4 2 5% 4 4 3 3 5% 8% 54%	3586
September	1st to 10th	Night	14% 6% 2 7% 4 4 4 4 5% 6% 43%	2011
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 52%	2200
	21st to end	Night	20% 8% 3 6% 4 4 4 3 4 5% 41%	2200
October	1st to 10th	Night	20% 6% 2 6% 4 4 4 2 4 6% 43%	2200
	11th to 20th	Night	16% 6% 1 4 4 3 4 3 5% 8% 47%	2420
	21st to end	Night	20% 8% 3 6% 4 3 4 3 4 6% 40%	2860
November	1st to 10th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 46%	2730
	21st to end	Night	16% 7% 2 6% 3 3 4 3 5% 7% 45%	2730
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 2 4 4 47%	4470
	11th to 20th	Night	23% 9% 2 5% 3 3 3 2 3 5% 41%	5208
	21st to end	Night	18% 11% 3 6% 3 3 3 3 4 5% 40%	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	38% 4 7% 26% 21% 3	3598
	11th to 20th	Day	39% 6% 10% 21% 17% 7%	3600
	21st to end	Day	46% 4 9% 20% 19% 2	3960
February	1st to 10th	Day	46% 6% 14% 16% 14% 3	1915
	11th to 20th	Day	42% 6% 12% 21% 16% 3	1957
	21st to end	Day	47% 5% 11% 19% 15% 3	1650
March	1st to 10th	Day	49% 4 11% 13% 12% 11%	4479
	11th to 20th	Day	40% 8% 14% 26% 10% 2	4786
	21st to end	Day	54% 4 7% 13% 14% 7%	5756
April	1st to 10th	Day	47% 6% 11% 18% 16% 3	2400
	11th to 20th	Day	43% 7% 9% 20% 18% 3	2379
	21st to end	Day	46% 6% 10% 18% 16% 4	2600
May	1st to 10th	Day	58% 7% 14% 13% 7% 1	5897
	11th to 20th	Day	51% 7% 15% 15% 11% 2	5865
	21st to end	Day	46% 8% 12% 20% 13% 1	6946
June	1st to 10th	Day	45% 9% 12% 22% 12% 1	2800
	11th to 20th	Day	42% 8% 14% 21% 13% 2	2800
	21st to end	Day	41% 9% 15% 21% 14% 1	2800
July	1st to 10th	Day	47% 8% 13% 19% 12% 2	6290
	11th to 20th	Day	42% 8% 17% 22% 10% 1	6285
	21st to end	Day	42% 6% 12% 24% 15% 1	6715
August	1st to 10th	Day	49% 8% 16% 19% 8% 1	4222
	11th to 20th	Day	42% 8% 13% 26% 10% 1	4126
	21st to end	Day	32% 7% 15% 26% 18% 2	4312
September	1st to 10th	Day	39% 7% 11% 22% 21% 1	2388
	11th to 20th	Day	34% 5% 10% 24% 24% 3	2200
	21st to end	Day	44% 5% 9% 20% 21% 2	2200
October	1st to 10th	Day	38% 5% 10% 22% 20% 4	3590
	11th to 20th	Day	32% 4 7% 26% 27% 3	3191
	21st to end	Day	31% 4 14% 27% 21% 3	3180
November	1st to 10th	Day	40% 5% 13% 21% 19% 2	1851
	11th to 20th	Day	39% 4 10% 22% 21% 4	1890
	21st to end	Day	42% 5% 12% 26% 12% 3	1890
December	1st to 10th	Day	39% 4 14% 27% 15% 1	2750
	11th to 20th	Day	40% 4 14% 17% 18% 7%	2986
	21st to end	Day	49% 4 9% 20% 13% 4	3960
January	1st to 10th	Night	34% 3 10% 33% 17% 2	6349
	11th to 20th	Night	45% 4 11% 21% 13% 5%	6360
	21st to end	Night	49% 5% 9% 16% 16% 5%	6996
February	1st to 10th	Night	46% 5% 13% 21% 13% 2	2478
	11th to 20th	Night	46% 5% 9% 19% 17% 4	2340
	21st to end	Night	45% 4 9% 23% 15% 4	1980
March	1st to 10th	Night	48% 4 11% 16% 15% 7%	5374
	11th to 20th	Night	45% 7% 16% 17% 12% 2	5173
	21st to end	Night	54% 4 6% 11% 19% 6%	5174
April	1st to 10th	Night	48% 6% 10% 18% 15% 3	2000
	11th to 20th	Night	46% 7% 9% 18% 18% 3	1970
	21st to end	Night	46% 7% 9% 18% 16% 4	1800
May	1st to 10th	Night	57% 8% 15% 11% 7% 2	4060
	11th to 20th	Night	54% 8% 14% 14% 7% 2	4034
	21st to end	Night	51% 8% 15% 16% 9% 1	3972
June	1st to 10th	Night	45% 8% 13% 20% 13% 1	1600
	11th to 20th	Night	45% 7% 12% 22% 13% 1	1600
	21st to end	Night	41% 8% 16% 22% 13% 1	1600
July	1st to 10th	Night	48% 7% 12% 21% 13%	3560
	11th to 20th	Night	46% 7% 16% 21% 9% 1	3552
	21st to end	Night	43% 6% 12% 24% 14% 1	3809
August	1st to 10th	Night	52% 9% 12% 18% 9% 1	2872
	11th to 20th	Night	42% 9% 14% 26% 8%	2983
	21st to end	Night	28% 6% 16% 27% 22% 1	3586
September	1st to 10th	Night	40% 7% 11% 23% 17% 2	2011
	11th to 20th	Night	35% 6% 11% 23% 25% 2	2200
	21st to end	Night	46% 6% 9% 19% 18% 3	2200
October	1st to 10th	Night	39% 5% 12% 24% 16% 3	3590
	11th to 20th	Night	27% 4 9% 26% 31% 3	3943
	21st to end	Night	34% 4 11% 28% 21% 2	4610
November	1st to 10th	Night	37% 6% 12% 22% 19% 3	2667
	11th to 20th	Night	40% 4 10% 22% 20% 4	2730
	21st to end	Night	39% 5% 13% 27% 13% 2	2730
December	1st to 10th	Night	42% 4 12% 27% 15% 1	4470
	11th to 20th	Night	46% 4 14% 14% 19% 2	5208
	21st to end	Night	46% 5% 9% 20% 17% 3	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	30% 28% 27% 14% 2	200
	11th to 20th	Day	29% 30% 33% 7% 2	200
	21st to end	Day	26% 27% 33% 10% 2 1	220
February	1st to 10th	Day	33% 24% 31% 12% 1	445
	11th to 20th	Day	31% 31% 30% 6% 1	496
	21st to end	Day	28% 26% 32% 11% 3	414
March	1st to 10th	Day	34% 22% 30% 13% 2	500
	11th to 20th	Day	30% 26% 29% 11% 3	500
	21st to end	Day	31% 24% 31% 9% 4 1	550
April	1st to 10th	Day	34% 23% 27% 10% 5% 1	500
	11th to 20th	Day	29% 26% 31% 11% 2 1	492
	21st to end	Day	27% 23% 32% 14% 2 2	500
May	1st to 10th	Day	34% 23% 29% 11% 2 1	503
	11th to 20th	Day	25% 19% 40% 12% 4 1	527
	21st to end	Day	21% 16% 37% 20% 6% 1	573
June	1st to 10th	Day	21% 16% 36% 20% 6% 1	530
	11th to 20th	Day	17% 20% 39% 19% 4 1	530
	21st to end	Day	18% 15% 37% 23% 6% 1	530
July	1st to 10th	Day	18% 10% 34% 27% 9% 2	525
	11th to 20th	Day	16% 9% 35% 27% 12% 2	524
	21st to end	Day	14% 12% 40% 26% 8% 1	576
August	1st to 10th	Day	13% 11% 36% 29% 10% 2	526
	11th to 20th	Day	15% 11% 34% 27% 11% 3	519
	21st to end	Day	12% 13% 36% 23% 12% 5%	583
September	1st to 10th	Day	20% 12% 30% 22% 12% 4	530
	11th to 20th	Day	16% 12% 29% 26% 10% 6%	530
	21st to end	Day	25% 12% 32% 20% 6% 4	530
October	1st to 10th	Day	27% 13% 27% 20% 9% 5%	530
	11th to 20th	Day	21% 18% 33% 16% 10% 1	472
	21st to end	Day	26% 19% 29% 17% 7% 2	462
November	1st to 10th	Day	24% 16% 34% 16% 8% 2	425
	11th to 20th	Day	26% 24% 32% 14% 4	430
	21st to end	Day	28% 23% 30% 13% 6%	430
December	1st to 10th	Day	23% 30% 31% 12% 4	286
	11th to 20th	Day	34% 25% 32% 7% 2 1	200
	21st to end	Day	27% 24% 33% 11% 3 2	220
January	1st to 10th	Night	23% 23% 34% 16% 3	800
	11th to 20th	Night	29% 22% 33% 14% 2	800
	21st to end	Night	22% 24% 35% 17% 3	880
February	1st to 10th	Night	25% 21% 35% 18% 1	800
	11th to 20th	Night	28% 23% 35% 13% 1	783
	21st to end	Night	24% 24% 33% 16% 3	660
March	1st to 10th	Night	26% 20% 35% 16% 4	800
	11th to 20th	Night	22% 23% 35% 17% 3	800
	21st to end	Night	27% 21% 32% 16% 3	880
April	1st to 10th	Night	25% 22% 34% 17% 3	800
	11th to 20th	Night	24% 21% 34% 17% 3	786
	21st to end	Night	24% 17% 39% 17% 3	600
May	1st to 10th	Night	30% 13% 34% 19% 4	600
	11th to 20th	Night	21% 17% 39% 20% 3	592
	21st to end	Night	22% 9% 36% 28% 6%	452
June	1st to 10th	Night	18% 10% 32% 32% 8%	400
	11th to 20th	Night	20% 7% 35% 30% 7%	400
	21st to end	Night	20% 7% 32% 36% 5% 1	400
July	1st to 10th	Night	18% 5% 33% 36% 8%	390
	11th to 20th	Night	16% 5% 33% 36% 10% 1	388
	21st to end	Night	15% 5% 32% 39% 8%	438
August	1st to 10th	Night	14% 6% 36% 36% 8%	578
	11th to 20th	Night	13% 6% 30% 38% 12%	637
	21st to end	Night	14% 6% 28% 36% 15% 1	880
September	1st to 10th	Night	17% 5% 33% 32% 12% 1	800
	11th to 20th	Night	17% 8% 28% 28% 16% 3	800
	21st to end	Night	25% 8% 27% 26% 12% 2	800
October	1st to 10th	Night	23% 12% 27% 25% 12% 2	800
	11th to 20th	Night	20% 11% 34% 22% 11% 1	800
	21st to end	Night	25% 16% 33% 19% 7%	880
November	1st to 10th	Night	19% 16% 33% 22% 9% 1	820
	11th to 20th	Night	23% 20% 33% 16% 6%	840
	21st to end	Night	23% 19% 33% 20% 4 1	840
December	1st to 10th	Night	26% 19% 34% 16% 4	809
	11th to 20th	Night	28% 19% 32% 18% 2 1	800
	21st to end	Night	26% 21% 33% 17% 3	880

Page generated at 2025-11-30 01:00:57. Version ac07

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