Stamford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stamford Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 7% 2 6% 4 4 4 4 6% 7% 39%	126559
	Night	18% 7% 2 6% 4 3 4 3 5% 6% 42%	123168
Transparency	Day	41% 6% 12% 22% 16% 3	130086
	Night	42% 5% 11% 22% 16% 3	127728
Seeing	Day	20% 15% 31% 23% 9% 2	16245
	Night	21% 13% 31% 25% 9% 1	25293

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 4 5% 4 5% 3 5% 6% 48%	3597
	11th to 20th	Day	14% 9% 3 6% 5% 4 4 3 4 6% 41%	3570
	21st to end	Day	18% 8% 2 8% 4 4 4 3 4 5% 39%	4208
February	1st to 10th	Day	17% 8% 2 6% 4 4 5% 4 5% 7% 38%	1835
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 3 5% 6% 43%	1948
	21st to end	Day	20% 8% 2 7% 4 4 4 4 4 6% 38%	1650
March	1st to 10th	Day	22% 6% 2 5% 4 3 4 3 5% 5% 41%	4476
	11th to 20th	Day	13% 7% 3 5% 4 4 4 4 6% 7% 42%	4710
	21st to end	Day	19% 8% 2 6% 4 4 4 3 5% 6% 39%	5970
April	1st to 10th	Day	19% 9% 3 5% 4 4 4 4 5% 7% 37%	2400
	11th to 20th	Day	18% 6% 3 6% 4 4 3 4 4 7% 41%	2374
	21st to end	Day	19% 6% 2 4 5% 4 4 4 5% 6% 41%	2462
May	1st to 10th	Day	30% 8% 2 5% 3 3 4 4 5% 8% 27%	5886
	11th to 20th	Day	22% 9% 2 6% 3 4 4 3 5% 7% 35%	6217
	21st to end	Day	21% 6% 2 6% 3 4 4 4 7% 8% 35%	6949
June	1st to 10th	Day	18% 5% 2 6% 5% 4 4 5% 7% 8% 36%	2800
	11th to 20th	Day	16% 7% 3 6% 4 4 5% 4 7% 8% 37%	2800
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 4 6% 10% 39%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 5% 8% 11% 33%	6360
	21st to end	Day	13% 7% 3 6% 4 4 5% 5% 6% 8% 39%	6926
August	1st to 10th	Day	19% 7% 2 7% 5% 4 5% 4 6% 8% 32%	4352
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 40%	3850
	21st to end	Day	9% 5% 2 5% 4 5% 4 4 6% 10% 45%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 3 6% 6% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 7% 51%	2290
	21st to end	Day	18% 6% 3 7% 4 4 5% 4 5% 6% 39%	2200
October	1st to 10th	Day	21% 6% 2 4 3 3 3 3 5% 6% 43%	2125
	11th to 20th	Day	15% 5% 2 6% 3 3 3 3 6% 7% 47%	1980
	21st to end	Day	21% 6% 1 5% 3 4 4 4 5% 6% 41%	1905
November	1st to 10th	Day	14% 6% 2 4 4 3 3 2 5% 7% 51%	1761
	11th to 20th	Day	18% 5% 2 4 2 3 4 3 5% 7% 48%	1800
	21st to end	Day	17% 6% 2 4 4 3 3 4 5% 6% 44%	1740
December	1st to 10th	Day	16% 7% 2 6% 4 4 3 3 5% 7% 44%	2600
	11th to 20th	Day	23% 6% 2 5% 3 3 2 2 4 5% 46%	2986
	21st to end	Day	18% 10% 3 7% 4 4 4 3 4 6% 38%	3960
January	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 49%	6349
	11th to 20th	Night	14% 8% 3 6% 4 3 3 3 4 5% 47%	6315
	21st to end	Night	18% 9% 3 6% 4 4 4 3 5% 5% 39%	6743
February	1st to 10th	Night	17% 9% 2 6% 4 4 3 3 4 6% 41%	2564
	11th to 20th	Night	17% 7% 3 6% 4 3 4 3 5% 6% 41%	2338
	21st to end	Night	19% 7% 3 6% 5% 4 3 3 5% 5% 41%	1980
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 6% 43%	5364
	11th to 20th	Night	15% 9% 3 5% 4 4 4 3 4 6% 43%	5247
	21st to end	Night	19% 10% 3 6% 4 3 3 3 4 5% 40%	4962
April	1st to 10th	Night	20% 7% 3 6% 4 5% 4 3 5% 6% 38%	2000
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 5% 6% 40%	1975
	21st to end	Night	19% 7% 2 7% 4 3 3 4 4 5% 42%	1935
May	1st to 10th	Night	28% 10% 3 7% 4 3 4 2 5% 7% 27%	4054
	11th to 20th	Night	22% 10% 3 6% 3 3 4 4 5% 6% 35%	3718
	21st to end	Night	23% 8% 2 5% 4 3 3 3 4 6% 37%	3926
June	1st to 10th	Night	18% 6% 2 5% 3 5% 4 3 5% 7% 41%	1600
	11th to 20th	Night	15% 8% 2 6% 5% 3 4 4 6% 8% 40%	1600
	21st to end	Night	12% 6% 3 5% 3 4 5% 4 6% 9% 43%	1600
July	1st to 10th	Night	12% 8% 2 8% 6% 4 5% 4 6% 8% 37%	3600
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 7% 8% 33%	3600
	21st to end	Night	14% 7% 2 5% 3 4 5% 3 6% 8% 43%	3926
August	1st to 10th	Night	23% 7% 3 6% 4 5% 5% 4 5% 7% 32%	2763
	11th to 20th	Night	15% 7% 2 6% 5% 5% 5% 4 6% 6% 39%	3197
	21st to end	Night	10% 4 2 5% 3 3 4 4 5% 8% 52%	3586
September	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 52%	2110
	21st to end	Night	18% 7% 3 6% 3 4 4 3 4 6% 44%	2200
October	1st to 10th	Night	18% 7% 3 6% 4 3 4 3 3 5% 44%	2272
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 49%	2377
	21st to end	Night	21% 7% 2 6% 3 2 4 3 4 7% 41%	2691
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 6% 50%	2537
	11th to 20th	Night	20% 6% 2 4 3 2 2 3 4 5% 48%	2600
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 7% 47%	2655
December	1st to 10th	Night	17% 6% 2 5% 4 3 3 2 4 5% 49%	4580
	11th to 20th	Night	25% 6% 2 4 3 3 3 3 4 4 42%	5208
	21st to end	Night	19% 8% 3 6% 4 3 3 2 4 5% 41%	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	34% 6% 8% 30% 19% 3	3598
	11th to 20th	Day	42% 6% 9% 22% 15% 7%	3600
	21st to end	Day	48% 4 9% 23% 15% 1	4208
February	1st to 10th	Day	45% 5% 12% 21% 15% 2	1835
	11th to 20th	Day	41% 5% 9% 24% 18% 3	1957
	21st to end	Day	47% 4 11% 20% 15% 3	1650
March	1st to 10th	Day	43% 5% 11% 18% 13% 9%	4479
	11th to 20th	Day	37% 7% 13% 28% 12% 3	4710
	21st to end	Day	45% 5% 8% 14% 22% 6%	5970
April	1st to 10th	Day	46% 5% 9% 20% 16% 4	2400
	11th to 20th	Day	43% 5% 10% 20% 19% 3	2374
	21st to end	Day	43% 5% 9% 22% 18% 4	2462
May	1st to 10th	Day	54% 6% 15% 14% 10% 1	5897
	11th to 20th	Day	48% 7% 11% 19% 14% 2	6221
	21st to end	Day	44% 7% 11% 23% 13% 1	6949
June	1st to 10th	Day	42% 8% 12% 22% 14% 1	2800
	11th to 20th	Day	41% 8% 13% 24% 12% 2	2800
	21st to end	Day	40% 8% 14% 23% 14% 1	2800
July	1st to 10th	Day	44% 9% 11% 21% 14% 1	6290
	11th to 20th	Day	40% 9% 18% 22% 10% 1	6285
	21st to end	Day	40% 8% 11% 22% 19% 1	6728
August	1st to 10th	Day	47% 7% 15% 22% 9%	4338
	11th to 20th	Day	36% 8% 14% 30% 12% 1	3886
	21st to end	Day	31% 7% 14% 27% 19% 2	4312
September	1st to 10th	Day	38% 6% 12% 23% 19% 2	2400
	11th to 20th	Day	33% 5% 11% 24% 26% 2	2290
	21st to end	Day	45% 6% 10% 20% 17% 3	2200
October	1st to 10th	Day	33% 6% 11% 20% 24% 6%	3463
	11th to 20th	Day	27% 4 10% 24% 30% 5%	3220
	21st to end	Day	30% 4 15% 23% 25% 4	3117
November	1st to 10th	Day	35% 4 14% 24% 21% 2	1761
	11th to 20th	Day	37% 4 10% 23% 23% 4	1800
	21st to end	Day	39% 4 13% 28% 12% 3	1740
December	1st to 10th	Day	39% 5% 15% 27% 12% 1	2600
	11th to 20th	Day	42% 4 14% 20% 18% 3	2986
	21st to end	Day	47% 6% 8% 21% 15% 3	3960
January	1st to 10th	Night	35% 3 10% 27% 23% 3	6349
	11th to 20th	Night	40% 4 14% 21% 15% 6%	6360
	21st to end	Night	47% 3 10% 20% 16% 5%	6743
February	1st to 10th	Night	45% 4 12% 24% 13% 2	2564
	11th to 20th	Night	44% 4 10% 20% 18% 5%	2340
	21st to end	Night	46% 4 7% 23% 17% 4	1980
March	1st to 10th	Night	44% 3 10% 20% 14% 8%	5374
	11th to 20th	Night	42% 4 14% 23% 12% 4	5248
	21st to end	Night	48% 4 6% 14% 23% 5%	4962
April	1st to 10th	Night	47% 4 10% 20% 16% 3	2000
	11th to 20th	Night	46% 3 10% 19% 21% 1	1975
	21st to end	Night	44% 4 9% 22% 17% 4	1935
May	1st to 10th	Night	58% 5% 12% 14% 10% 1	4060
	11th to 20th	Night	50% 6% 14% 15% 13% 3	3719
	21st to end	Night	49% 5% 12% 21% 11% 2	3927
June	1st to 10th	Night	44% 5% 10% 21% 19% 2	1600
	11th to 20th	Night	41% 6% 15% 26% 12% 1	1600
	21st to end	Night	37% 8% 14% 25% 14% 2	1600
July	1st to 10th	Night	45% 8% 13% 21% 12% 2	3560
	11th to 20th	Night	47% 8% 15% 19% 10% 1	3552
	21st to end	Night	37% 7% 14% 23% 18% 1	3809
August	1st to 10th	Night	51% 7% 14% 17% 10% 1	2753
	11th to 20th	Night	42% 7% 12% 29% 9% 1	3227
	21st to end	Night	28% 6% 13% 33% 18% 1	3586
September	1st to 10th	Night	39% 6% 9% 27% 18% 1	2000
	11th to 20th	Night	34% 4 10% 24% 25% 3	2110
	21st to end	Night	43% 4 9% 22% 19% 3	2200
October	1st to 10th	Night	38% 6% 12% 23% 19% 3	3714
	11th to 20th	Night	26% 4 9% 28% 29% 4	3876
	21st to end	Night	32% 3 14% 28% 20% 3	4429
November	1st to 10th	Night	35% 4 12% 24% 21% 4	2537
	11th to 20th	Night	40% 3 9% 24% 20% 5%	2600
	21st to end	Night	38% 4 13% 30% 14% 2	2655
December	1st to 10th	Night	39% 3 11% 31% 14% 2	4580
	11th to 20th	Night	47% 3 10% 20% 17% 3	5208
	21st to end	Night	45% 4 10% 19% 17% 5%	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	27% 31% 25% 15% 3	200
	11th to 20th	Day	20% 33% 36% 10% 3	200
	21st to end	Day	21% 26% 37% 12% 5%	273
February	1st to 10th	Day	25% 27% 37% 10% 1	335
	11th to 20th	Day	27% 24% 36% 11% 1	496
	21st to end	Day	27% 23% 34% 14% 2	414
March	1st to 10th	Day	19% 27% 36% 15% 3	500
	11th to 20th	Day	21% 24% 36% 16% 2 1	500
	21st to end	Day	26% 23% 34% 13% 3 1	550
April	1st to 10th	Day	26% 26% 29% 15% 5%	500
	11th to 20th	Day	25% 19% 33% 15% 8% 1	492
	21st to end	Day	26% 15% 36% 17% 4 1	500
May	1st to 10th	Day	29% 16% 31% 17% 6% 1	503
	11th to 20th	Day	20% 14% 36% 23% 6% 1	527
	21st to end	Day	20% 9% 30% 28% 12% 3	573
June	1st to 10th	Day	18% 11% 28% 29% 12% 2	530
	11th to 20th	Day	18% 11% 33% 26% 11% 1	530
	21st to end	Day	14% 8% 29% 35% 12% 2	530
July	1st to 10th	Day	15% 7% 24% 32% 17% 4	525
	11th to 20th	Day	9% 6% 28% 34% 20% 3	524
	21st to end	Day	13% 7% 30% 35% 15% 1	576
August	1st to 10th	Day	14% 7% 22% 35% 16% 6%	526
	11th to 20th	Day	15% 5% 20% 37% 18% 5%	519
	21st to end	Day	9% 9% 29% 31% 15% 7%	583
September	1st to 10th	Day	18% 7% 30% 24% 17% 5%	530
	11th to 20th	Day	13% 7% 29% 29% 14% 8%	530
	21st to end	Day	22% 9% 33% 22% 10% 4	530
October	1st to 10th	Day	25% 9% 28% 20% 14% 5%	530
	11th to 20th	Day	17% 18% 30% 23% 11% 1	522
	21st to end	Day	24% 16% 31% 21% 7% 1	442
November	1st to 10th	Day	16% 17% 30% 26% 8% 3	395
	11th to 20th	Day	22% 21% 34% 19% 5%	400
	21st to end	Day	23% 20% 31% 21% 4 1	340
December	1st to 10th	Day	24% 24% 32% 17% 5%	200
	11th to 20th	Day	35% 23% 29% 11% 3	200
	21st to end	Day	24% 19% 36% 16% 5%	220
January	1st to 10th	Night	21% 21% 37% 18% 3	800
	11th to 20th	Night	25% 20% 33% 20% 3	800
	21st to end	Night	20% 20% 39% 18% 2	880
February	1st to 10th	Night	22% 18% 39% 19% 2	800
	11th to 20th	Night	25% 22% 36% 15% 1	783
	21st to end	Night	24% 22% 34% 17% 3	660
March	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	22% 18% 38% 18% 4 1	800
	21st to end	Night	22% 18% 37% 18% 4	880
April	1st to 10th	Night	23% 19% 35% 18% 5%	800
	11th to 20th	Night	24% 14% 36% 23% 4	791
	21st to end	Night	23% 14% 35% 23% 5% 1	735
May	1st to 10th	Night	26% 13% 32% 23% 7%	600
	11th to 20th	Night	24% 12% 34% 25% 6%	459
	21st to end	Night	24% 4 22% 37% 12% 1	433
June	1st to 10th	Night	18% 7% 29% 34% 11% 1	400
	11th to 20th	Night	16% 5% 28% 40% 12%	400
	21st to end	Night	11% 5% 26% 46% 12% 1	400
July	1st to 10th	Night	13% 4 22% 41% 19% 1	390
	11th to 20th	Night	14% 3 21% 41% 19% 2	388
	21st to end	Night	15% 4 20% 42% 17% 1	438
August	1st to 10th	Night	12% 3 22% 41% 20% 1	497
	11th to 20th	Night	15% 4 19% 37% 23% 2	790
	21st to end	Night	14% 6% 21% 35% 21% 3	880
September	1st to 10th	Night	17% 6% 26% 33% 16% 3	800
	11th to 20th	Night	15% 7% 21% 34% 19% 4	800
	21st to end	Night	21% 5% 27% 30% 15% 2	800
October	1st to 10th	Night	22% 10% 27% 25% 14% 2	800
	11th to 20th	Night	19% 13% 31% 27% 10% 1	793
	21st to end	Night	25% 11% 34% 21% 8%	836
November	1st to 10th	Night	14% 14% 35% 26% 10% 1	780
	11th to 20th	Night	24% 15% 36% 19% 7%	800
	21st to end	Night	20% 19% 30% 24% 6% 1	800
December	1st to 10th	Night	25% 18% 33% 18% 5% 1	800
	11th to 20th	Night	25% 18% 34% 20% 3	800
	21st to end	Night	26% 18% 35% 18% 3	880

Page generated at 2025-10-16 01:50:29. Version ac07

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