Cape Cod Imaging Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cape Cod Imaging Station clear sky chart from 2005-11-06 to 2025-08-05.

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% 10% 3 7% 4 4 4 3 5% 7% 37%	125335
	Night	18% 10% 4 7% 4 4 4 3 5% 6% 35%	122686
Transparency	Day	46% 6% 12% 22% 13% 2	128886
	Night	49% 5% 11% 20% 14% 2	127196
Seeing	Day	20% 10% 28% 27% 12% 3	16198
	Night	22% 12% 28% 27% 11% 1	24986

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% 9% 5% 8% 5% 4 4 3 4 5% 41%	3597
	11th to 20th	Day	16% 10% 4 8% 3 3 3 2 5% 5% 40%	3570
	21st to end	Day	19% 10% 3 11% 6% 5% 4 3 4 4 32%	4116
February	1st to 10th	Day	20% 11% 5% 7% 5% 3 3 4 5% 6% 30%	1870
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 3 4 7% 37%	1948
	21st to end	Day	18% 9% 4 8% 4 3 4 3 5% 6% 35%	1650
March	1st to 10th	Day	19% 16% 3 6% 4 4 3 3 4 6% 32%	4476
	11th to 20th	Day	15% 11% 3 7% 4 4 3 3 5% 6% 38%	4905
	21st to end	Day	26% 12% 3 6% 4 3 3 3 4 5% 31%	5570
April	1st to 10th	Day	21% 10% 3 5% 3 3 3 3 5% 6% 38%	2400
	11th to 20th	Day	17% 9% 2 6% 3 3 3 3 6% 6% 42%	2435
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 7% 40%	2600
May	1st to 10th	Day	23% 14% 4 9% 4 4 4 3 5% 7% 23%	5886
	11th to 20th	Day	20% 13% 3 7% 4 4 4 3 5% 7% 31%	5861
	21st to end	Day	19% 10% 3 7% 3 4 3 3 5% 7% 35%	6719
June	1st to 10th	Day	19% 8% 2 5% 5% 3 4 4 6% 8% 34%	2800
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 3 6% 8% 40%	2800
	21st to end	Day	14% 8% 2 6% 4 4 4 4 6% 9% 39%	2800
July	1st to 10th	Day	14% 10% 4 7% 4 4 4 3 5% 9% 34%	6360
	11th to 20th	Day	11% 10% 3 7% 4 4 5% 4 7% 10% 33%	6360
	21st to end	Day	14% 6% 2 6% 4 3 3 4 5% 7% 44%	6727
August	1st to 10th	Day	16% 9% 3 7% 5% 4 4 4 5% 7% 35%	4157
	11th to 20th	Day	12% 8% 2 6% 4 5% 5% 4 7% 7% 40%	4045
	21st to end	Day	8% 4 2 4 3 3 4 4 6% 10% 52%	4230
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	2218
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2090
	21st to end	Day	18% 8% 2 7% 4 3 3 3 5% 7% 40%	2090
October	1st to 10th	Day	21% 6% 1 5% 4 4 4 3 5% 6% 42%	2090
	11th to 20th	Day	15% 6% 2 6% 4 3 3 3 4 7% 46%	1898
	21st to end	Day	18% 7% 2 7% 4 3 4 3 5% 7% 40%	1881
November	1st to 10th	Day	19% 7% 2 7% 4 4 3 3 5% 5% 42%	1761
	11th to 20th	Day	17% 9% 3 5% 4 3 4 3 5% 8% 40%	1800
	21st to end	Day	18% 9% 2 6% 3 4 4 3 5% 6% 40%	1800
December	1st to 10th	Day	15% 9% 3 6% 3 3 4 4 4 6% 43%	2879
	11th to 20th	Day	19% 10% 3 7% 4 4 3 3 3 5% 38%	2986
	21st to end	Day	19% 16% 8% 13% 4 4 4 3 3 5% 22%	3960
January	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 4 6% 45%	6349
	11th to 20th	Night	16% 13% 5% 9% 4 4 4 3 4 4 33%	6315
	21st to end	Night	17% 12% 4 10% 5% 4 4 3 5% 5% 32%	6837
February	1st to 10th	Night	17% 10% 3 9% 5% 4 5% 4 5% 5% 33%	2525
	11th to 20th	Night	20% 10% 3 6% 5% 4 3 4 5% 6% 35%	2338
	21st to end	Night	16% 10% 3 9% 4 4 3 3 5% 6% 37%	1980
March	1st to 10th	Night	25% 12% 2 5% 3 3 3 3 5% 5% 33%	5364
	11th to 20th	Night	15% 13% 5% 7% 5% 4 4 3 4 5% 33%	5053
	21st to end	Night	24% 13% 3 7% 4 4 4 2 4 5% 30%	5358
April	1st to 10th	Night	21% 11% 3 6% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	19% 10% 3 6% 4 4 3 3 5% 5% 37%	1912
	21st to end	Night	18% 10% 3 7% 6% 4 4 3 5% 5% 36%	1800
May	1st to 10th	Night	25% 13% 4 9% 4 3 4 2 4 6% 25%	4054
	11th to 20th	Night	22% 13% 4 7% 5% 4 5% 3 5% 6% 27%	4033
	21st to end	Night	18% 11% 4 9% 5% 5% 5% 4 6% 6% 26%	4184
June	1st to 10th	Night	18% 10% 4 6% 4 4 5% 5% 6% 7% 31%	1600
	11th to 20th	Night	18% 10% 3 5% 4 3 3 3 5% 8% 38%	1600
	21st to end	Night	17% 10% 2 8% 3 3 3 4 7% 10% 33%	1600
July	1st to 10th	Night	17% 16% 4 8% 4 4 4 3 5% 7% 28%	3600
	11th to 20th	Night	17% 10% 3 6% 4 4 5% 4 6% 9% 31%	3600
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 5% 7% 37%	4080
August	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 4 6% 7% 30%	2862
	11th to 20th	Night	10% 10% 3 8% 4 5% 5% 4 6% 8% 37%	2782
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 8% 52%	3409
September	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 5% 6% 45%	1961
	11th to 20th	Night	14% 7% 2 5% 3 3 4 3 4 6% 49%	2090
	21st to end	Night	17% 8% 3 6% 5% 4 4 4 5% 6% 39%	2090
October	1st to 10th	Night	21% 8% 3 6% 5% 3 3 4 4 5% 38%	2090
	11th to 20th	Night	17% 5% 2 5% 3 4 4 3 5% 6% 45%	2279
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 7% 36%	2717
November	1st to 10th	Night	17% 6% 3 6% 5% 4 4 3 5% 6% 42%	2537
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 5% 41%	2600
	21st to end	Night	16% 7% 2 7% 5% 3 3 3 5% 7% 41%	2600
December	1st to 10th	Night	18% 8% 3 6% 4 4 4 3 4 5% 40%	4283
	11th to 20th	Night	18% 11% 4 8% 5% 4 4 3 4 5% 35%	5208
	21st to end	Night	17% 14% 7% 13% 5% 4 4 3 4 5% 24%	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	43% 5% 9% 23% 17% 3	3598
	11th to 20th	Day	45% 5% 13% 21% 11% 6%	3600
	21st to end	Day	55% 4 9% 17% 14% 1	4116
February	1st to 10th	Day	54% 4 12% 16% 12% 2	1870
	11th to 20th	Day	48% 4 11% 22% 12% 2	1957
	21st to end	Day	50% 4 11% 19% 13% 3	1650
March	1st to 10th	Day	54% 3 12% 13% 10% 8%	4479
	11th to 20th	Day	45% 6% 15% 23% 10% 1	4905
	21st to end	Day	57% 4 6% 14% 13% 6%	5570
April	1st to 10th	Day	47% 4 9% 20% 17% 3	2400
	11th to 20th	Day	43% 4 10% 24% 17% 2	2435
	21st to end	Day	45% 5% 9% 22% 16% 4	2600
May	1st to 10th	Day	61% 8% 11% 13% 6% 1	5897
	11th to 20th	Day	52% 7% 12% 17% 10% 1	5865
	21st to end	Day	49% 9% 9% 19% 14% 1	6719
June	1st to 10th	Day	48% 8% 10% 23% 12%	2800
	11th to 20th	Day	43% 6% 13% 24% 13% 1	2800
	21st to end	Day	40% 8% 15% 23% 13% 1	2800
July	1st to 10th	Day	48% 7% 12% 19% 11% 2	6290
	11th to 20th	Day	45% 8% 15% 23% 9% 1	6285
	21st to end	Day	38% 5% 13% 30% 13% 1	6533
August	1st to 10th	Day	49% 6% 16% 21% 7% 1	4144
	11th to 20th	Day	41% 7% 15% 29% 8%	4084
	21st to end	Day	28% 7% 15% 28% 21% 1	4230
September	1st to 10th	Day	39% 6% 8% 25% 20% 2	2218
	11th to 20th	Day	36% 4 9% 28% 20% 3	2090
	21st to end	Day	44% 6% 9% 22% 17% 2	2090
October	1st to 10th	Day	41% 5% 11% 26% 15% 2	3480
	11th to 20th	Day	33% 4 8% 29% 24% 2	3114
	21st to end	Day	33% 4 13% 34% 15% 2	3081
November	1st to 10th	Day	44% 5% 13% 21% 14% 2	1761
	11th to 20th	Day	43% 5% 12% 21% 18% 2	1800
	21st to end	Day	44% 5% 14% 25% 11% 1	1800
December	1st to 10th	Day	42% 5% 14% 27% 11% 1	2879
	11th to 20th	Day	49% 5% 13% 16% 13% 4	2986
	21st to end	Day	64% 5% 9% 15% 6% 2	3960
January	1st to 10th	Night	39% 5% 8% 31% 14% 2	6349
	11th to 20th	Night	52% 4 12% 18% 11% 3	6360
	21st to end	Night	54% 3 9% 18% 13% 2	6837
February	1st to 10th	Night	53% 4 11% 21% 11% 1	2525
	11th to 20th	Night	51% 4 8% 21% 13% 2	2340
	21st to end	Night	47% 5% 9% 24% 12% 3	1980
March	1st to 10th	Night	53% 2 10% 16% 13% 5%	5374
	11th to 20th	Night	52% 5% 13% 16% 12% 2	5054
	21st to end	Night	59% 3 5% 11% 18% 5%	5358
April	1st to 10th	Night	51% 3 7% 19% 16% 3	2000
	11th to 20th	Night	48% 4 9% 20% 16% 3	1912
	21st to end	Night	50% 4 9% 19% 15% 3	1800
May	1st to 10th	Night	60% 6% 12% 13% 6% 2	4060
	11th to 20th	Night	58% 6% 13% 15% 8% 2	4034
	21st to end	Night	55% 9% 9% 15% 11% 1	4185
June	1st to 10th	Night	49% 8% 11% 20% 12% 1	1600
	11th to 20th	Night	46% 6% 10% 23% 15% 1	1600
	21st to end	Night	47% 7% 12% 22% 12% 1	1600
July	1st to 10th	Night	55% 5% 9% 18% 13%	3560
	11th to 20th	Night	49% 6% 15% 20% 9% 1	3552
	21st to end	Night	45% 6% 11% 24% 13% 1	3960
August	1st to 10th	Night	52% 8% 13% 17% 10% 1	2850
	11th to 20th	Night	43% 7% 15% 27% 8%	2809
	21st to end	Night	29% 6% 15% 27% 21% 1	3409
September	1st to 10th	Night	39% 5% 10% 25% 19% 2	1961
	11th to 20th	Night	37% 5% 9% 25% 21% 3	2090
	21st to end	Night	45% 6% 7% 20% 20% 3	2090
October	1st to 10th	Night	44% 5% 9% 28% 13% 2	3480
	11th to 20th	Night	30% 4 8% 24% 31% 2	3776
	21st to end	Night	36% 4 10% 29% 20% 1	4467
November	1st to 10th	Night	43% 5% 11% 22% 15% 2	2537
	11th to 20th	Night	45% 5% 10% 21% 16% 3	2600
	21st to end	Night	43% 5% 14% 26% 11% 2	2600
December	1st to 10th	Night	45% 5% 12% 25% 13%	4283
	11th to 20th	Night	51% 4 14% 16% 15% 1	5208
	21st to end	Night	60% 6% 10% 14% 9%	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	25% 19% 33% 18% 7%	200
	11th to 20th	Day	25% 21% 31% 19% 5% 1	200
	21st to end	Day	21% 20% 33% 19% 5% 2	285
February	1st to 10th	Day	28% 17% 31% 21% 3	435
	11th to 20th	Day	28% 17% 37% 15% 3	496
	21st to end	Day	21% 18% 38% 17% 5% 1	414
March	1st to 10th	Day	25% 17% 33% 21% 4	500
	11th to 20th	Day	20% 15% 36% 23% 5%	500
	21st to end	Day	25% 14% 34% 18% 7% 2	550
April	1st to 10th	Day	28% 14% 31% 20% 7% 1	500
	11th to 20th	Day	23% 10% 36% 24% 6%	492
	21st to end	Day	22% 9% 34% 25% 8% 2	500
May	1st to 10th	Day	28% 10% 28% 25% 7% 1	503
	11th to 20th	Day	20% 9% 31% 28% 10% 2	527
	21st to end	Day	22% 4 24% 32% 15% 3	573
June	1st to 10th	Day	23% 4 23% 32% 17% 1	530
	11th to 20th	Day	16% 6% 28% 35% 12% 3	530
	21st to end	Day	15% 7% 22% 35% 18% 2	530
July	1st to 10th	Day	16% 5% 20% 31% 23% 5%	525
	11th to 20th	Day	12% 6% 18% 34% 24% 6%	524
	21st to end	Day	12% 5% 22% 39% 20% 3	576
August	1st to 10th	Day	13% 4 20% 38% 21% 5%	502
	11th to 20th	Day	15% 6% 15% 36% 23% 5%	479
	21st to end	Day	12% 4 19% 35% 22% 9%	539
September	1st to 10th	Day	17% 4 23% 28% 20% 7%	490
	11th to 20th	Day	17% 6% 22% 27% 19% 9%	490
	21st to end	Day	22% 8% 27% 27% 13% 4	490
October	1st to 10th	Day	24% 9% 19% 26% 17% 5%	490
	11th to 20th	Day	18% 10% 28% 25% 15% 4	409
	21st to end	Day	21% 10% 34% 21% 11% 3	429
November	1st to 10th	Day	18% 14% 28% 26% 11% 3	395
	11th to 20th	Day	21% 17% 32% 23% 7%	400
	21st to end	Day	22% 17% 30% 23% 9% 1	400
December	1st to 10th	Day	23% 15% 34% 22% 6%	375
	11th to 20th	Day	28% 19% 33% 16% 4 1	200
	21st to end	Day	24% 14% 35% 19% 6% 1	220
January	1st to 10th	Night	24% 18% 36% 19% 4	800
	11th to 20th	Night	28% 20% 32% 16% 4	800
	21st to end	Night	22% 18% 35% 20% 3 1	880
February	1st to 10th	Night	22% 17% 35% 23% 2	800
	11th to 20th	Night	27% 19% 36% 17% 2	783
	21st to end	Night	20% 20% 33% 23% 4	660
March	1st to 10th	Night	25% 16% 32% 22% 5%	800
	11th to 20th	Night	22% 15% 36% 21% 6% 1	800
	21st to end	Night	24% 15% 30% 22% 8% 1	880
April	1st to 10th	Night	27% 11% 30% 26% 5%	800
	11th to 20th	Night	24% 9% 32% 29% 6%	728
	21st to end	Night	22% 9% 30% 28% 11% 1	600
May	1st to 10th	Night	26% 8% 28% 28% 9% 2	600
	11th to 20th	Night	22% 7% 32% 30% 8% 1	592
	21st to end	Night	22% 5% 20% 37% 14% 1	548
June	1st to 10th	Night	22% 7% 23% 34% 14% 2	400
	11th to 20th	Night	23% 5% 18% 39% 14% 2	400
	21st to end	Night	20% 7% 15% 40% 19% 1	400
July	1st to 10th	Night	18% 4 18% 38% 21% 1	390
	11th to 20th	Night	18% 4 15% 38% 24% 2	388
	21st to end	Night	17% 4 16% 41% 22% 1	503
August	1st to 10th	Night	11% 3 18% 42% 23% 2	581
	11th to 20th	Night	15% 4 14% 41% 24% 2	562
	21st to end	Night	13% 4 14% 35% 28% 5%	795
September	1st to 10th	Night	16% 3 19% 36% 24% 3	760
	11th to 20th	Night	16% 5% 18% 32% 23% 6%	760
	21st to end	Night	22% 6% 21% 30% 18% 3	760
October	1st to 10th	Night	25% 7% 24% 24% 17% 3	760
	11th to 20th	Night	19% 10% 28% 24% 15% 3	760
	21st to end	Night	23% 11% 30% 26% 8% 1	836
November	1st to 10th	Night	19% 12% 28% 27% 13% 1	780
	11th to 20th	Night	22% 18% 31% 19% 10%	800
	21st to end	Night	21% 17% 30% 24% 7% 1	800
December	1st to 10th	Night	25% 16% 32% 21% 5% 1	800
	11th to 20th	Night	24% 22% 33% 18% 4	800
	21st to end	Night	23% 16% 36% 21% 4	880

Page generated at 2025-08-02 01:32:24. Version ac07

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