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 2026-02-16.

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	16% 7% 2 6% 4 4 4 4 5% 8% 41%	130732
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	130130
Transparency	Day	41% 5% 13% 23% 16% 3	134259
	Night	42% 4 12% 22% 17% 3	134690
Seeing	Day	18% 13% 30% 25% 12% 3	16565
	Night	20% 12% 31% 27% 9% 1	25784

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	13% 6% 2 5% 3 4 4 4 5% 6% 49%	4661
	11th to 20th	Day	16% 8% 2 5% 4 4 4 3 5% 5% 45%	4650
	21st to end	Day	16% 6% 3 5% 4 4 4 4 5% 7% 42%	5459
February	1st to 10th	Day	16% 7% 3 6% 4 3 5% 4 5% 7% 41%	1929
	11th to 20th	Day	14% 6% 2 7% 4 4 4 3 5% 6% 45%	1998
	21st to end	Day	20% 7% 2 6% 4 3 4 4 5% 7% 39%	1650
March	1st to 10th	Day	21% 7% 2 4 3 3 3 3 4 6% 43%	4476
	11th to 20th	Day	11% 9% 2 4 4 4 5% 4 6% 7% 43%	4710
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 7% 42%	5970
April	1st to 10th	Day	19% 7% 3 5% 5% 4 4 3 6% 7% 38%	2400
	11th to 20th	Day	17% 6% 2 6% 5% 4 3 4 5% 7% 41%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	2460
May	1st to 10th	Day	27% 8% 2 5% 3 3 4 4 5% 8% 30%	5886
	11th to 20th	Day	20% 9% 3 6% 3 3 4 3 5% 7% 37%	6215
	21st to end	Day	20% 6% 2 5% 4 4 5% 4 6% 8% 38%	6949
June	1st to 10th	Day	16% 5% 2 5% 4 5% 4 4 6% 9% 39%	2800
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 4 6% 9% 40%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 6% 9% 42%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 5% 6% 10% 37%	6360
	11th to 20th	Day	9% 7% 2 7% 5% 5% 5% 4 7% 11% 36%	6360
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	6926
August	1st to 10th	Day	17% 7% 2 6% 5% 5% 4 4 6% 9% 35%	4347
	11th to 20th	Day	12% 5% 2 6% 4 4 4 5% 6% 9% 43%	3850
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 7% 47%	2400
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 5% 7% 53%	2290
	21st to end	Day	17% 6% 2 7% 4 4 3 4 5% 6% 40%	2200
October	1st to 10th	Day	19% 6% 2 5% 3 3 3 3 4 7% 45%	2125
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 5% 7% 47%	2000
	21st to end	Day	19% 6% 2 5% 3 4 4 3 5% 6% 43%	2008
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 6% 7% 50%	1851
	11th to 20th	Day	16% 6% 2 4 2 3 4 3 5% 7% 47%	1890
	21st to end	Day	16% 6% 2 5% 4 4 4 4 5% 6% 44%	1832
December	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 7% 43%	2680
	11th to 20th	Day	21% 6% 2 5% 2 3 3 2 4 5% 47%	3066
	21st to end	Day	18% 9% 3 7% 4 4 4 3 5% 6% 38%	4048
January	1st to 10th	Night	13% 8% 3 5% 3 3 3 3 5% 6% 47%	8234
	11th to 20th	Night	16% 9% 3 6% 3 3 3 3 4 5% 45%	8235
	21st to end	Night	17% 8% 2 6% 4 4 4 3 4 5% 43%	8719
February	1st to 10th	Night	16% 9% 2 5% 4 4 4 3 5% 5% 42%	2689
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 5% 7% 41%	2399
	21st to end	Night	19% 8% 2 5% 5% 4 4 3 5% 5% 41%	1980
March	1st to 10th	Night	19% 8% 3 5% 3 3 3 3 4 5% 43%	5364
	11th to 20th	Night	15% 9% 2 6% 4 3 4 3 5% 6% 43%	5246
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 6% 38%	4962
April	1st to 10th	Night	19% 7% 3 5% 5% 4 4 4 5% 7% 37%	2000
	11th to 20th	Night	20% 6% 2 6% 4 3 3 3 5% 6% 40%	1975
	21st to end	Night	18% 7% 3 7% 4 3 3 4 4 6% 42%	1940
May	1st to 10th	Night	27% 10% 3 7% 4 3 4 3 5% 7% 27%	4054
	11th to 20th	Night	22% 9% 3 6% 4 4 3 3 5% 6% 35%	3722
	21st to end	Night	23% 8% 2 6% 4 3 4 3 5% 7% 36%	3926
June	1st to 10th	Night	17% 7% 2 6% 3 4 4 4 6% 7% 41%	1600
	11th to 20th	Night	15% 8% 2 6% 4 3 4 4 7% 8% 40%	1600
	21st to end	Night	10% 6% 2 6% 4 4 5% 5% 6% 10% 42%	1600
July	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 5% 6% 8% 36%	3600
	11th to 20th	Night	13% 8% 3 8% 4 4 6% 4 7% 9% 33%	3600
	21st to end	Night	13% 7% 2 5% 4 4 4 3 6% 8% 42%	3926
August	1st to 10th	Night	22% 7% 2 7% 5% 4 5% 4 6% 7% 32%	2765
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 6% 7% 39%	3197
	21st to end	Night	9% 4 2 4 3 4 4 4 5% 8% 52%	3586
September	1st to 10th	Night	14% 5% 2 6% 4 4 5% 4 5% 6% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 5% 6% 51%	2110
	21st to end	Night	18% 6% 2 6% 4 4 3 3 4 6% 43%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 4 3 3 4 6% 43%	2271
	11th to 20th	Night	15% 6% 1 5% 3 4 3 2 4 6% 49%	2400
	21st to end	Night	20% 6% 3 6% 3 3 3 3 4 6% 42%	2830
November	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 5% 6% 50%	2667
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 4 5% 48%	2730
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 7% 46%	2785
December	1st to 10th	Night	16% 6% 2 5% 3 4 4 3 4 6% 48%	4720
	11th to 20th	Night	24% 8% 2 4 3 2 3 3 4 5% 43%	5348
	21st to end	Night	18% 8% 3 6% 4 3 3 3 5% 5% 42%	7150

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	36% 4 11% 30% 18% 2	4662
	11th to 20th	Day	41% 3 10% 23% 17% 6%	4680
	21st to end	Day	43% 3 11% 26% 16% 1	5459
February	1st to 10th	Day	44% 6% 12% 23% 14% 2	1929
	11th to 20th	Day	41% 4 9% 26% 17% 2	2007
	21st to end	Day	45% 4 11% 23% 15% 3	1650
March	1st to 10th	Day	43% 3 12% 20% 13% 9%	4479
	11th to 20th	Day	37% 7% 14% 28% 11% 3	4710
	21st to end	Day	43% 5% 10% 13% 23% 6%	5970
April	1st to 10th	Day	45% 5% 10% 20% 16% 4	2400
	11th to 20th	Day	41% 5% 12% 21% 18% 3	2374
	21st to end	Day	41% 5% 11% 22% 17% 4	2460
May	1st to 10th	Day	53% 6% 16% 14% 10% 1	5897
	11th to 20th	Day	48% 5% 13% 19% 14% 2	6219
	21st to end	Day	44% 6% 12% 23% 14% 1	6949
June	1st to 10th	Day	40% 8% 13% 22% 15% 1	2800
	11th to 20th	Day	41% 8% 14% 24% 13% 2	2800
	21st to end	Day	38% 8% 15% 23% 15% 1	2800
July	1st to 10th	Day	41% 10% 12% 21% 15% 1	6290
	11th to 20th	Day	39% 8% 19% 23% 10% 1	6285
	21st to end	Day	40% 7% 12% 20% 20% 1	6728
August	1st to 10th	Day	46% 7% 16% 23% 9% 1	4333
	11th to 20th	Day	37% 6% 14% 31% 11% 1	3886
	21st to end	Day	31% 6% 13% 28% 20% 1	4312
September	1st to 10th	Day	37% 6% 14% 23% 18% 2	2400
	11th to 20th	Day	32% 5% 12% 25% 24% 2	2290
	21st to end	Day	44% 5% 11% 19% 18% 3	2200
October	1st to 10th	Day	34% 5% 11% 20% 24% 6%	3463
	11th to 20th	Day	27% 4 11% 23% 31% 5%	3240
	21st to end	Day	29% 4 14% 26% 24% 3	3220
November	1st to 10th	Day	35% 5% 13% 23% 23% 2	1851
	11th to 20th	Day	38% 4 10% 23% 22% 4	1890
	21st to end	Day	40% 4 14% 28% 12% 2	1832
December	1st to 10th	Day	40% 4 16% 28% 12% 1	2680
	11th to 20th	Day	41% 3 14% 22% 17% 3	3066
	21st to end	Day	47% 6% 8% 22% 15% 3	4048
January	1st to 10th	Night	39% 2 12% 24% 20% 3	8234
	11th to 20th	Night	42% 3 12% 20% 17% 6%	8280
	21st to end	Night	43% 3 9% 21% 19% 5%	8719
February	1st to 10th	Night	44% 4 12% 24% 13% 2	2689
	11th to 20th	Night	44% 4 9% 21% 17% 4	2401
	21st to end	Night	45% 4 7% 23% 17% 4	1980
March	1st to 10th	Night	44% 4 11% 20% 13% 9%	5374
	11th to 20th	Night	41% 5% 14% 24% 12% 4	5247
	21st to end	Night	47% 4 7% 14% 24% 5%	4962
April	1st to 10th	Night	47% 4 10% 19% 16% 4	2000
	11th to 20th	Night	44% 5% 10% 19% 21% 2	1975
	21st to end	Night	42% 6% 9% 21% 18% 4	1940
May	1st to 10th	Night	57% 5% 13% 13% 10% 1	4060
	11th to 20th	Night	51% 4 15% 14% 14% 2	3723
	21st to end	Night	48% 5% 13% 21% 10% 2	3927
June	1st to 10th	Night	42% 7% 10% 20% 20% 1	1600
	11th to 20th	Night	40% 6% 15% 26% 12% 1	1600
	21st to end	Night	37% 8% 15% 24% 14% 2	1600
July	1st to 10th	Night	45% 8% 13% 21% 12% 2	3560
	11th to 20th	Night	46% 7% 16% 20% 10% 1	3552
	21st to end	Night	38% 5% 15% 23% 18% 1	3809
August	1st to 10th	Night	51% 7% 15% 17% 10% 1	2755
	11th to 20th	Night	42% 7% 13% 29% 9% 1	3227
	21st to end	Night	30% 6% 12% 32% 18% 1	3586
September	1st to 10th	Night	39% 6% 10% 25% 18% 1	2000
	11th to 20th	Night	34% 4 11% 25% 23% 3	2110
	21st to end	Night	42% 4 10% 21% 20% 2	2200
October	1st to 10th	Night	39% 5% 11% 23% 19% 3	3713
	11th to 20th	Night	25% 4 10% 28% 30% 4	3899
	21st to end	Night	31% 4 13% 29% 22% 2	4568
November	1st to 10th	Night	35% 5% 11% 24% 21% 4	2667
	11th to 20th	Night	39% 3 9% 24% 21% 5%	2730
	21st to end	Night	38% 4 13% 29% 15% 1	2785
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4720
	11th to 20th	Night	46% 3 10% 21% 16% 3	5348
	21st to end	Night	45% 5% 10% 19% 17% 4	7150

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	20% 28% 30% 17% 4	220
	11th to 20th	Day	17% 30% 37% 14% 2	220
	21st to end	Day	17% 18% 45% 15% 4 1	303
February	1st to 10th	Day	22% 16% 40% 20% 2	369
	11th to 20th	Day	21% 19% 40% 18% 2	516
	21st to end	Day	22% 22% 33% 19% 3	414
March	1st to 10th	Day	17% 24% 36% 20% 4	500
	11th to 20th	Day	18% 21% 36% 20% 4 1	500
	21st to end	Day	23% 18% 36% 17% 4 1	550
April	1st to 10th	Day	23% 21% 34% 15% 6% 1	500
	11th to 20th	Day	21% 17% 34% 18% 10% 1	492
	21st to end	Day	23% 15% 34% 20% 6% 1	500
May	1st to 10th	Day	30% 12% 30% 20% 7% 2	503
	11th to 20th	Day	19% 12% 34% 26% 8% 2	527
	21st to end	Day	19% 6% 26% 28% 16% 4	573
June	1st to 10th	Day	16% 8% 27% 31% 16% 2	530
	11th to 20th	Day	16% 8% 28% 32% 13% 2	530
	21st to end	Day	13% 7% 25% 33% 18% 4	530
July	1st to 10th	Day	12% 5% 21% 37% 19% 6%	525
	11th to 20th	Day	10% 3 22% 36% 25% 5%	524
	21st to end	Day	13% 6% 21% 38% 20% 2	576
August	1st to 10th	Day	11% 5% 23% 34% 20% 6%	526
	11th to 20th	Day	15% 4 15% 38% 23% 5%	519
	21st to end	Day	10% 4 28% 33% 17% 8%	583
September	1st to 10th	Day	18% 5% 26% 26% 19% 6%	530
	11th to 20th	Day	13% 4 26% 28% 18% 10%	530
	21st to end	Day	22% 7% 27% 27% 14% 4	530
October	1st to 10th	Day	23% 7% 24% 24% 16% 6%	530
	11th to 20th	Day	17% 14% 29% 25% 13% 2	530
	21st to end	Day	22% 13% 30% 24% 9% 1	476
November	1st to 10th	Day	14% 15% 32% 27% 10% 2	425
	11th to 20th	Day	19% 21% 34% 20% 6%	430
	21st to end	Day	20% 18% 36% 20% 6% 1	372
December	1st to 10th	Day	18% 22% 36% 17% 7%	220
	11th to 20th	Day	25% 26% 33% 14% 2	220
	21st to end	Day	24% 18% 35% 14% 8%	242
January	1st to 10th	Night	19% 19% 39% 19% 3	840
	11th to 20th	Night	23% 19% 34% 21% 3	840
	21st to end	Night	18% 20% 41% 19% 2	924
February	1st to 10th	Night	21% 15% 40% 22% 2	840
	11th to 20th	Night	22% 21% 38% 18% 1	804
	21st to end	Night	23% 17% 36% 20% 3	660
March	1st to 10th	Night	22% 15% 38% 21% 4	800
	11th to 20th	Night	22% 15% 37% 22% 4 1	800
	21st to end	Night	21% 16% 37% 21% 4	880
April	1st to 10th	Night	23% 15% 36% 21% 5%	800
	11th to 20th	Night	20% 14% 34% 26% 6%	791
	21st to end	Night	22% 11% 35% 26% 6% 1	740
May	1st to 10th	Night	25% 10% 31% 27% 7% 1	600
	11th to 20th	Night	22% 11% 34% 26% 7%	463
	21st to end	Night	22% 5% 19% 40% 13% 1	433
June	1st to 10th	Night	18% 5% 28% 36% 13% 1	400
	11th to 20th	Night	17% 3 24% 42% 15%	400
	21st to end	Night	10% 3 23% 51% 14% 1	400
July	1st to 10th	Night	12% 3 19% 44% 21% 1	390
	11th to 20th	Night	13% 4 18% 42% 21% 2	388
	21st to end	Night	13% 5% 16% 46% 18% 1	438
August	1st to 10th	Night	12% 3 21% 43% 20% 2	499
	11th to 20th	Night	14% 4 17% 37% 26% 2	790
	21st to end	Night	14% 5% 19% 35% 25% 3	880
September	1st to 10th	Night	16% 4 26% 33% 19% 3	800
	11th to 20th	Night	15% 4 22% 34% 21% 4	800
	21st to end	Night	21% 6% 22% 33% 16% 2	800
October	1st to 10th	Night	22% 8% 26% 26% 15% 3	800
	11th to 20th	Night	18% 11% 31% 29% 11% 1	800
	21st to end	Night	24% 12% 33% 24% 7%	880
November	1st to 10th	Night	14% 13% 35% 26% 10% 1	820
	11th to 20th	Night	22% 16% 32% 23% 7%	840
	21st to end	Night	18% 18% 33% 25% 5% 1	840
December	1st to 10th	Night	23% 18% 35% 18% 5% 1	840
	11th to 20th	Night	23% 18% 35% 20% 4	840
	21st to end	Night	23% 16% 37% 20% 3	924

Page generated at 2026-02-13 01:42:17. Version ac07

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