Stamford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stamford 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	15% 12% 4 10% 6% 5% 5% 4 5% 7% 28%	127199
	Night	14% 13% 5% 12% 6% 5% 4 3 5% 6% 28%	123500
Transparency	Day	53% 7% 11% 18% 10% 1	130675
	Night	55% 6% 11% 17% 10% 1	128098
Seeing	Day	27% 19% 28% 16% 7% 2	16159
	Night	26% 19% 25% 21% 8% 1	25286

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	9% 14% 12% 25% 8% 3 3 2 3 4 17%	3597
	11th to 20th	Day	10% 16% 8% 21% 10% 6% 4 2 3 4 15%	3570
	21st to end	Day	14% 15% 8% 12% 7% 5% 4 3 4 4 24%	4253
February	1st to 10th	Day	13% 16% 6% 14% 10% 5% 5% 4 5% 4 19%	1800
	11th to 20th	Day	9% 13% 6% 16% 9% 5% 4 4 5% 6% 22%	1880
	21st to end	Day	15% 14% 4 13% 7% 4 5% 4 4 5% 24%	1650
March	1st to 10th	Day	17% 16% 6% 13% 5% 5% 3 3 4 4 24%	4476
	11th to 20th	Day	11% 15% 7% 10% 5% 4 3 3 4 5% 34%	4842
	21st to end	Day	17% 10% 5% 11% 5% 5% 4 3 4 5% 30%	5873
April	1st to 10th	Day	15% 17% 4 10% 5% 4 4 3 4 5% 29%	2400
	11th to 20th	Day	15% 13% 3 7% 5% 5% 4 3 5% 5% 34%	2374
	21st to end	Day	19% 14% 4 7% 5% 4 3 3 4 5% 32%	2435
May	1st to 10th	Day	28% 13% 4 7% 4 4 4 3 4 6% 23%	6110
	11th to 20th	Day	16% 12% 3 9% 6% 5% 5% 4 6% 6% 28%	6318
	21st to end	Day	19% 10% 3 8% 5% 4 5% 4 6% 7% 27%	6949
June	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 3 7% 9% 29%	2800
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 5% 7% 10% 28%	2800
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 5% 6% 8% 30%	2800
July	1st to 10th	Day	10% 7% 3 9% 5% 6% 6% 5% 6% 9% 33%	6360
	11th to 20th	Day	8% 6% 3 8% 6% 7% 7% 5% 9% 11% 30%	6360
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	6926
August	1st to 10th	Day	17% 9% 4 8% 5% 5% 5% 4 6% 8% 29%	4418
	11th to 20th	Day	12% 9% 3 8% 6% 6% 7% 5% 7% 9% 27%	3895
	21st to end	Day	13% 8% 3 6% 5% 5% 5% 4 6% 9% 34%	4312
September	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 4 5% 7% 39%	2400
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 5% 7% 44%	2345
	21st to end	Day	16% 7% 3 6% 5% 5% 5% 4 6% 8% 35%	2200
October	1st to 10th	Day	17% 11% 3 7% 5% 4 4 4 5% 6% 34%	2031
	11th to 20th	Day	15% 13% 2 7% 4 5% 5% 4 6% 7% 34%	2000
	21st to end	Day	20% 16% 3 8% 4 4 4 3 5% 6% 27%	2039
November	1st to 10th	Day	12% 14% 3 8% 4 4 4 4 5% 6% 36%	1851
	11th to 20th	Day	13% 18% 5% 10% 6% 4 4 3 4 4 28%	1890
	21st to end	Day	14% 17% 5% 10% 6% 5% 4 4 4 5% 23%	1683
December	1st to 10th	Day	10% 19% 7% 22% 8% 5% 4 3 5% 5% 13%	2616
	11th to 20th	Day	15% 16% 9% 13% 6% 4 4 3 3 4 24%	2986
	21st to end	Day	13% 16% 8% 14% 6% 4 4 4 5% 6% 20%	3960
January	1st to 10th	Night	11% 13% 9% 25% 7% 4 4 3 4 5% 16%	6349
	11th to 20th	Night	8% 16% 8% 24% 10% 5% 4 3 3 4 15%	6315
	21st to end	Night	12% 14% 5% 14% 8% 7% 5% 4 5% 6% 21%	6702
February	1st to 10th	Night	11% 16% 6% 13% 9% 6% 5% 4 5% 5% 21%	2600
	11th to 20th	Night	9% 12% 6% 16% 11% 7% 5% 5% 4 6% 19%	2404
	21st to end	Night	16% 13% 5% 12% 8% 5% 4 4 5% 7% 22%	1980
March	1st to 10th	Night	14% 11% 6% 15% 8% 6% 4 3 4 5% 24%	5364
	11th to 20th	Night	12% 15% 6% 11% 6% 4 3 3 4 6% 31%	5101
	21st to end	Night	17% 13% 5% 12% 5% 4 4 3 4 4 28%	5059
April	1st to 10th	Night	15% 16% 5% 9% 5% 5% 3 4 5% 6% 27%	2000
	11th to 20th	Night	16% 15% 3 7% 5% 3 3 3 4 7% 33%	1975
	21st to end	Night	19% 12% 4 6% 4 4 4 3 4 6% 33%	1964
May	1st to 10th	Night	30% 14% 3 7% 3 3 3 3 4 5% 24%	3873
	11th to 20th	Night	16% 15% 4 9% 5% 4 4 4 4 6% 30%	3575
	21st to end	Night	22% 12% 3 7% 4 4 4 4 5% 6% 28%	3926
June	1st to 10th	Night	15% 8% 3 8% 5% 4 4 4 6% 7% 37%	1600
	11th to 20th	Night	16% 10% 2 6% 4 4 4 4 6% 8% 37%	1600
	21st to end	Night	12% 6% 2 6% 4 6% 6% 4 7% 8% 40%	1600
July	1st to 10th	Night	10% 8% 3 7% 5% 5% 5% 4 5% 8% 38%	3600
	11th to 20th	Night	11% 8% 3 8% 5% 5% 4 4 7% 8% 36%	3600
	21st to end	Night	11% 8% 3 7% 4 5% 4 3 5% 6% 45%	3926
August	1st to 10th	Night	20% 11% 2 6% 5% 4 4 3 5% 6% 34%	2715
	11th to 20th	Night	13% 9% 4 8% 5% 4 5% 3 5% 6% 38%	3121
	21st to end	Night	11% 10% 3 6% 5% 4 4 3 4 7% 43%	3586
September	1st to 10th	Night	14% 7% 3 6% 3 3 4 3 6% 7% 43%	2000
	11th to 20th	Night	14% 6% 1 5% 4 3 3 3 4 7% 50%	2055
	21st to end	Night	15% 9% 2 8% 4 5% 4 3 4 7% 40%	2200
October	1st to 10th	Night	19% 10% 3 6% 4 4 4 3 4 7% 36%	2365
	11th to 20th	Night	16% 11% 3 7% 4 4 3 3 6% 7% 36%	2400
	21st to end	Night	20% 14% 3 8% 4 4 4 4 5% 7% 27%	2800
November	1st to 10th	Night	12% 14% 4 8% 5% 4 4 3 4 6% 36%	2667
	11th to 20th	Night	14% 15% 6% 10% 5% 4 5% 3 5% 5% 28%	2730
	21st to end	Night	12% 15% 7% 11% 5% 4 5% 4 5% 6% 27%	2935
December	1st to 10th	Night	11% 17% 8% 18% 6% 5% 5% 4 5% 5% 15%	4609
	11th to 20th	Night	18% 16% 8% 12% 5% 4 3 3 3 4 25%	5208
	21st to end	Night	15% 17% 7% 15% 6% 5% 4 3 4 4 21%	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	71% 3 7% 14% 4 1	3598
	11th to 20th	Day	72% 5% 6% 10% 5% 2	3600
	21st to end	Day	62% 5% 8% 18% 6% 2	4253
February	1st to 10th	Day	66% 5% 9% 13% 6% 1	1800
	11th to 20th	Day	62% 4 8% 16% 9% 1	1889
	21st to end	Day	60% 5% 10% 15% 9%	1650
March	1st to 10th	Day	61% 5% 10% 11% 9% 2	4479
	11th to 20th	Day	52% 4 15% 18% 7% 4	4842
	21st to end	Day	55% 5% 8% 12% 18% 3	5873
April	1st to 10th	Day	58% 4 7% 19% 10% 2	2400
	11th to 20th	Day	51% 6% 11% 18% 12% 1	2374
	21st to end	Day	54% 5% 9% 18% 12% 2	2435
May	1st to 10th	Day	62% 6% 9% 14% 9% 1	6121
	11th to 20th	Day	53% 9% 10% 19% 9% 1	6322
	21st to end	Day	53% 8% 11% 20% 8% 1	6949
June	1st to 10th	Day	50% 8% 12% 18% 11%	2800
	11th to 20th	Day	49% 9% 13% 19% 9% 1	2800
	21st to end	Day	50% 8% 13% 19% 9% 1	2800
July	1st to 10th	Day	45% 9% 14% 21% 10%	6290
	11th to 20th	Day	42% 11% 15% 21% 11%	6285
	21st to end	Day	42% 7% 12% 27% 12%	6728
August	1st to 10th	Day	52% 8% 13% 17% 9%	4404
	11th to 20th	Day	47% 11% 16% 22% 5%	3932
	21st to end	Day	42% 8% 14% 27% 8% 1	4312
September	1st to 10th	Day	42% 6% 12% 26% 13% 1	2400
	11th to 20th	Day	39% 6% 10% 24% 20% 1	2345
	21st to end	Day	45% 6% 12% 21% 14% 2	2200
October	1st to 10th	Day	47% 7% 9% 21% 14% 1	3304
	11th to 20th	Day	44% 5% 9% 20% 16% 5%	3240
	21st to end	Day	42% 7% 14% 25% 11% 2	3264
November	1st to 10th	Day	48% 5% 12% 20% 13% 2	1851
	11th to 20th	Day	59% 5% 9% 15% 11% 2	1890
	21st to end	Day	61% 5% 12% 16% 5% 1	1683
December	1st to 10th	Day	70% 8% 10% 9% 3	2616
	11th to 20th	Day	64% 5% 9% 14% 8%	2986
	21st to end	Day	63% 6% 8% 10% 11% 2	3960
January	1st to 10th	Night	69% 5% 10% 10% 6% 1	6349
	11th to 20th	Night	73% 5% 9% 8% 5% 1	6360
	21st to end	Night	62% 6% 10% 10% 10% 1	6702
February	1st to 10th	Night	63% 6% 10% 13% 7% 1	2600
	11th to 20th	Night	65% 6% 9% 13% 7% 1	2406
	21st to end	Night	60% 5% 11% 14% 8% 2	1980
March	1st to 10th	Night	60% 5% 12% 14% 9% 1	5374
	11th to 20th	Night	55% 5% 16% 15% 7% 3	5102
	21st to end	Night	57% 5% 6% 14% 16% 1	5059
April	1st to 10th	Night	58% 5% 10% 16% 8% 1	2000
	11th to 20th	Night	52% 5% 11% 18% 14%	1975
	21st to end	Night	51% 5% 10% 18% 12% 2	1964
May	1st to 10th	Night	62% 6% 8% 13% 9% 2	3879
	11th to 20th	Night	54% 8% 11% 19% 8% 1	3576
	21st to end	Night	54% 8% 11% 17% 9% 1	3927
June	1st to 10th	Night	45% 6% 11% 24% 13% 1	1600
	11th to 20th	Night	44% 8% 14% 25% 9%	1600
	21st to end	Night	39% 9% 16% 24% 10% 1	1600
July	1st to 10th	Night	42% 8% 15% 20% 13% 1	3560
	11th to 20th	Night	42% 11% 14% 21% 12% 1	3552
	21st to end	Night	38% 7% 13% 29% 12% 1	3809
August	1st to 10th	Night	51% 7% 13% 21% 8% 1	2705
	11th to 20th	Night	44% 8% 16% 24% 8%	3150
	21st to end	Night	39% 7% 13% 32% 9%	3586
September	1st to 10th	Night	39% 7% 13% 26% 14% 1	2000
	11th to 20th	Night	35% 5% 11% 27% 21% 1	2055
	21st to end	Night	46% 5% 10% 22% 15% 1	2200
October	1st to 10th	Night	50% 8% 12% 20% 10% 1	3872
	11th to 20th	Night	42% 7% 14% 17% 16% 5%	3899
	21st to end	Night	44% 7% 14% 24% 10% 1	4512
November	1st to 10th	Night	49% 5% 11% 19% 14% 2	2667
	11th to 20th	Night	55% 6% 10% 16% 10% 3	2730
	21st to end	Night	57% 5% 14% 17% 6%	2935
December	1st to 10th	Night	65% 9% 12% 12% 2	4609
	11th to 20th	Night	64% 5% 9% 12% 11%	5208
	21st to end	Night	66% 5% 7% 10% 9% 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	36% 30% 22% 10% 3	200
	11th to 20th	Day	38% 30% 23% 8% 1 1	200
	21st to end	Day	34% 27% 29% 7% 3	283
February	1st to 10th	Day	43% 28% 25% 5%	300
	11th to 20th	Day	41% 32% 18% 6% 2	428
	21st to end	Day	36% 27% 26% 9% 2	414
March	1st to 10th	Day	34% 26% 26% 11% 3	500
	11th to 20th	Day	37% 23% 26% 10% 3	500
	21st to end	Day	35% 27% 22% 9% 4 2	550
April	1st to 10th	Day	37% 26% 21% 12% 4	500
	11th to 20th	Day	33% 27% 23% 10% 5% 2	492
	21st to end	Day	34% 21% 27% 11% 5% 2	500
May	1st to 10th	Day	38% 19% 25% 10% 5% 3	503
	11th to 20th	Day	22% 21% 31% 18% 6% 2	527
	21st to end	Day	21% 14% 32% 20% 10% 3	573
June	1st to 10th	Day	23% 18% 29% 21% 7% 2	530
	11th to 20th	Day	18% 18% 31% 23% 8% 2	530
	21st to end	Day	22% 19% 31% 19% 7% 2	530
July	1st to 10th	Day	14% 12% 34% 24% 14% 3	525
	11th to 20th	Day	12% 11% 34% 29% 11% 3	524
	21st to end	Day	15% 12% 39% 23% 9% 2	576
August	1st to 10th	Day	10% 14% 30% 25% 16% 5%	526
	11th to 20th	Day	15% 10% 30% 26% 12% 6%	519
	21st to end	Day	16% 13% 33% 22% 10% 6%	583
September	1st to 10th	Day	21% 12% 25% 22% 13% 7%	530
	11th to 20th	Day	15% 10% 27% 21% 16% 11%	530
	21st to end	Day	21% 15% 27% 22% 10% 5%	530
October	1st to 10th	Day	25% 15% 25% 18% 13% 5%	530
	11th to 20th	Day	28% 19% 27% 17% 7% 2	530
	21st to end	Day	30% 22% 27% 15% 5%	494
November	1st to 10th	Day	31% 19% 26% 15% 8%	425
	11th to 20th	Day	33% 25% 23% 13% 5% 1	430
	21st to end	Day	43% 23% 26% 9%	223
December	1st to 10th	Day	39% 25% 23% 8% 4	204
	11th to 20th	Day	41% 23% 27% 8% 2	200
	21st to end	Day	35% 28% 28% 7% 2	220
January	1st to 10th	Night	28% 31% 29% 11% 2	800
	11th to 20th	Night	31% 25% 28% 13% 2 1	800
	21st to end	Night	26% 30% 27% 15% 2	880
February	1st to 10th	Night	31% 24% 32% 11% 1	800
	11th to 20th	Night	31% 28% 28% 11% 1	783
	21st to end	Night	35% 24% 26% 12% 3	660
March	1st to 10th	Night	31% 26% 28% 13% 2	800
	11th to 20th	Night	30% 27% 27% 14% 2 1	800
	21st to end	Night	29% 24% 26% 15% 6%	880
April	1st to 10th	Night	29% 23% 28% 17% 3	800
	11th to 20th	Night	28% 24% 26% 16% 7%	791
	21st to end	Night	30% 19% 23% 21% 6% 1	764
May	1st to 10th	Night	31% 16% 25% 20% 8% 1	523
	11th to 20th	Night	21% 14% 29% 28% 7% 1	394
	21st to end	Night	17% 10% 29% 30% 13% 2	433
June	1st to 10th	Night	18% 11% 27% 31% 13% 1	400
	11th to 20th	Night	19% 12% 21% 35% 13%	400
	21st to end	Night	15% 8% 27% 38% 12% 1	400
July	1st to 10th	Night	14% 6% 24% 41% 15% 1	390
	11th to 20th	Night	13% 5% 25% 37% 19% 1	388
	21st to end	Night	13% 7% 22% 44% 13% 1	438
August	1st to 10th	Night	14% 6% 24% 33% 23% 1	475
	11th to 20th	Night	16% 7% 19% 34% 21% 3	738
	21st to end	Night	18% 8% 19% 33% 21% 2	880
September	1st to 10th	Night	18% 10% 24% 29% 17% 2	800
	11th to 20th	Night	17% 9% 21% 27% 22% 5%	800
	21st to end	Night	20% 13% 24% 26% 15% 3	800
October	1st to 10th	Night	27% 14% 20% 22% 15% 3	800
	11th to 20th	Night	25% 17% 27% 22% 8% 1	800
	21st to end	Night	30% 18% 28% 20% 4	880
November	1st to 10th	Night	25% 21% 28% 21% 6%	820
	11th to 20th	Night	30% 24% 24% 15% 6% 1	840
	21st to end	Night	28% 26% 24% 17% 4 1	840
December	1st to 10th	Night	31% 29% 24% 12% 3	809
	11th to 20th	Night	34% 24% 26% 15% 2	800
	21st to end	Night	33% 29% 23% 14% 3	880

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