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 2024-04-25.

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% 6% 2 5% 5% 3 4 5% 5% 7% 40%	83204
	Night	18% 7% 3 5% 4 3 4 4 5% 6% 42%	85895
Transparency	Day	42% 6% 11% 23% 16% 2	82909
	Night	43% 5% 11% 23% 15% 2	85724
Seeing	Day	20% 15% 30% 23% 10% 2	14247
	Night	21% 13% 31% 25% 9% 1	23505

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	16% 7% 2 4 5% 3 4 4 4 5% 46%	2517
	11th to 20th	Day	16% 9% 3 6% 7% 3 5% 5% 4 6% 36%	2520
	21st to end	Day	22% 10% 3 7% 5% 3 4 4 4 6% 32%	2936
February	1st to 10th	Day	18% 7% 2 6% 4 3 5% 4 6% 6% 38%	1743
	11th to 20th	Day	15% 6% 3 5% 6% 3 4 4 4 7% 44%	1848
	21st to end	Day	21% 8% 2 6% 4 3 4 5% 4 6% 37%	1570
March	1st to 10th	Day	26% 8% 3 5% 4 2 3 4 5% 5% 35%	3116
	11th to 20th	Day	9% 4 2 4 6% 4 5% 6% 7% 9% 43%	3294
	21st to end	Day	21% 6% 2 5% 4 2 3 3 5% 6% 41%	4166
April	1st to 10th	Day	19% 8% 4 4 4 3 4 4 5% 7% 38%	2280
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 4 7% 41%	2254
	21st to end	Day	20% 6% 2 4 5% 3 4 4 5% 6% 41%	2265
May	1st to 10th	Day	23% 7% 3 4 4 2 4 5% 5% 7% 34%	2340
	11th to 20th	Day	19% 6% 2 5% 4 3 5% 5% 5% 8% 38%	2433
	21st to end	Day	16% 5% 2 5% 4 3 4 5% 6% 8% 42%	2725
June	1st to 10th	Day	19% 5% 2 5% 5% 3 4 5% 6% 9% 36%	2520
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 5% 6% 8% 37%	2520
	21st to end	Day	12% 6% 3 5% 6% 4 5% 5% 6% 10% 39%	2520
July	1st to 10th	Day	12% 6% 2 5% 6% 3 5% 6% 7% 9% 39%	2520
	11th to 20th	Day	11% 5% 2 5% 5% 4 4 6% 7% 10% 41%	2520
	21st to end	Day	13% 6% 3 5% 5% 3 6% 6% 7% 8% 38%	2744
August	1st to 10th	Day	11% 6% 2 6% 6% 3 6% 6% 6% 11% 38%	2399
	11th to 20th	Day	13% 6% 2 5% 6% 4 5% 5% 6% 9% 39%	2126
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 6% 9% 42%	2376
September	1st to 10th	Day	13% 6% 2 5% 4 3 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	14% 4 2 4 3 2 3 4 5% 7% 50%	2062
	21st to end	Day	18% 5% 2 5% 5% 3 5% 5% 5% 6% 41%	1980
October	1st to 10th	Day	23% 6% 2 4 4 3 3 4 5% 6% 41%	1913
	11th to 20th	Day	16% 5% 2 5% 4 3 3 4 6% 8% 45%	1800
	21st to end	Day	22% 6% 1 4 4 2 4 5% 5% 6% 39%	1805
November	1st to 10th	Day	15% 6% 2 4 4 2 3 3 5% 7% 50%	1671
	11th to 20th	Day	19% 5% 2 3 3 2 4 4 5% 7% 47%	1710
	21st to end	Day	16% 5% 3 4 4 3 4 5% 5% 6% 45%	1653
December	1st to 10th	Day	19% 7% 2 5% 6% 4 4 5% 6% 7% 37%	1520
	11th to 20th	Day	23% 5% 2 4 3 2 3 3 4 5% 45%	1906
	21st to end	Day	18% 10% 4 7% 6% 3 5% 5% 4 7% 32%	2772
January	1st to 10th	Night	17% 6% 2 5% 4 2 3 3 4 6% 46%	4429
	11th to 20th	Night	17% 9% 3 6% 4 3 3 4 4 5% 43%	4440
	21st to end	Night	24% 11% 3 6% 5% 2 3 3 4 4 34%	4715
February	1st to 10th	Night	17% 8% 3 5% 5% 3 3 4 4 6% 42%	2436
	11th to 20th	Night	17% 7% 3 5% 5% 2 4 4 4 6% 42%	2218
	21st to end	Night	20% 7% 3 5% 5% 3 3 4 4 5% 42%	1884
March	1st to 10th	Night	24% 11% 2 5% 4 3 3 4 4 5% 36%	3724
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 4 5% 7% 46%	3663
	21st to end	Night	20% 9% 3 5% 4 2 3 3 4 5% 42%	3466
April	1st to 10th	Night	19% 7% 3 5% 5% 3 4 5% 5% 6% 39%	1900
	11th to 20th	Night	20% 6% 2 5% 5% 2 3 4 5% 6% 41%	1875
	21st to end	Night	19% 6% 3 6% 5% 3 3 4 4 5% 42%	1792
May	1st to 10th	Night	23% 7% 3 6% 5% 3 4 4 4 7% 34%	1620
	11th to 20th	Night	20% 8% 2 6% 4 3 4 4 6% 6% 37%	1474
	21st to end	Night	18% 7% 2 4 5% 3 3 4 5% 6% 42%	1550
June	1st to 10th	Night	19% 5% 2 5% 4 3 5% 4 4 8% 41%	1440
	11th to 20th	Night	15% 7% 3 5% 5% 2 5% 4 5% 9% 40%	1440
	21st to end	Night	12% 5% 3 4 4 2 5% 5% 6% 10% 44%	1440
July	1st to 10th	Night	13% 5% 3 6% 6% 4 5% 6% 6% 8% 39%	1440
	11th to 20th	Night	13% 5% 3 6% 5% 3 6% 4 7% 9% 40%	1440
	21st to end	Night	12% 5% 3 4 4 4 6% 5% 7% 10% 41%	1574
August	1st to 10th	Night	8% 6% 3 5% 5% 3 6% 6% 6% 8% 44%	1519
	11th to 20th	Night	12% 6% 2 6% 7% 4 5% 7% 7% 7% 37%	1774
	21st to end	Night	14% 5% 2 5% 4 3 4 5% 5% 8% 44%	1980
September	1st to 10th	Night	15% 5% 2 5% 5% 3 4 4 4 6% 45%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 4 3 5% 5% 52%	1898
	21st to end	Night	17% 6% 3 5% 4 3 4 3 4 6% 45%	1980
October	1st to 10th	Night	20% 7% 3 5% 5% 2 4 3 3 5% 42%	2044
	11th to 20th	Night	16% 6% 2 5% 4 3 4 4 5% 6% 47%	2160
	21st to end	Night	22% 7% 2 5% 4 2 4 3 4 6% 41%	2549
November	1st to 10th	Night	13% 5% 2 5% 4 2 4 4 5% 6% 50%	2407
	11th to 20th	Night	21% 6% 2 4 3 2 3 3 4 6% 47%	2470
	21st to end	Night	15% 5% 2 4 4 2 4 4 5% 7% 47%	2522
December	1st to 10th	Night	20% 7% 3 5% 5% 3 4 3 4 7% 39%	2660
	11th to 20th	Night	20% 6% 2 4 4 3 4 4 5% 6% 42%	3288
	21st to end	Night	21% 9% 3 5% 4 2 4 3 5% 6% 37%	4884

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	39% 5% 8% 32% 12% 3	2518
	11th to 20th	Day	47% 7% 10% 23% 11% 1	2520
	21st to end	Day	55% 4 9% 20% 11% 2	2936
February	1st to 10th	Day	46% 5% 12% 21% 15% 2	1743
	11th to 20th	Day	40% 5% 9% 24% 18% 3	1857
	21st to end	Day	48% 4 11% 20% 14% 3	1570
March	1st to 10th	Day	50% 5% 11% 20% 11% 2	3119
	11th to 20th	Day	32% 9% 13% 31% 14% 2	3294
	21st to end	Day	45% 4 6% 17% 22% 6%	4166
April	1st to 10th	Day	45% 5% 9% 21% 16% 4	2280
	11th to 20th	Day	43% 5% 10% 20% 19% 3	2254
	21st to end	Day	44% 5% 9% 21% 17% 4	2265
May	1st to 10th	Day	48% 6% 9% 21% 14% 2	2340
	11th to 20th	Day	43% 6% 10% 20% 17% 3	2437
	21st to end	Day	39% 8% 17% 21% 14% 2	2725
June	1st to 10th	Day	43% 8% 13% 22% 13% 1	2520
	11th to 20th	Day	42% 8% 12% 24% 12% 1	2520
	21st to end	Day	39% 8% 13% 24% 14% 1	2520
July	1st to 10th	Day	40% 7% 12% 22% 17% 2	2450
	11th to 20th	Day	37% 7% 15% 25% 14% 2	2445
	21st to end	Day	40% 8% 12% 20% 17% 2	2591
August	1st to 10th	Day	38% 8% 15% 25% 13% 1	2385
	11th to 20th	Day	39% 8% 14% 24% 14% 1	2126
	21st to end	Day	38% 7% 11% 23% 19% 2	2376
September	1st to 10th	Day	39% 6% 13% 22% 17% 2	2160
	11th to 20th	Day	34% 5% 11% 24% 25% 2	2062
	21st to end	Day	43% 6% 9% 21% 19% 3	1980
October	1st to 10th	Day	43% 7% 8% 23% 17% 3	1913
	11th to 20th	Day	37% 5% 13% 26% 17% 2	1800
	21st to end	Day	44% 5% 10% 21% 17% 3	1805
November	1st to 10th	Day	36% 4 13% 24% 20% 3	1671
	11th to 20th	Day	39% 4 10% 22% 21% 3	1710
	21st to end	Day	39% 4 13% 28% 13% 3	1653
December	1st to 10th	Day	45% 6% 15% 18% 14% 1	1520
	11th to 20th	Day	42% 4 15% 20% 16% 2	1906
	21st to end	Day	51% 6% 10% 22% 9% 1	2772
January	1st to 10th	Night	40% 3 11% 30% 12% 4	4429
	11th to 20th	Night	44% 4 12% 24% 12% 3	4440
	21st to end	Night	55% 3 8% 21% 11% 2	4715
February	1st to 10th	Night	44% 4 13% 24% 13% 2	2436
	11th to 20th	Night	44% 4 10% 20% 18% 5%	2220
	21st to end	Night	45% 4 7% 24% 16% 4	1884
March	1st to 10th	Night	52% 3 10% 21% 12% 2	3734
	11th to 20th	Night	38% 5% 15% 25% 16% 3	3664
	21st to end	Night	46% 3 7% 15% 23% 6%	3466
April	1st to 10th	Night	46% 4 10% 20% 16% 3	1900
	11th to 20th	Night	45% 3 10% 19% 21% 1	1875
	21st to end	Night	45% 4 9% 21% 16% 4	1792
May	1st to 10th	Night	51% 5% 10% 18% 14% 2	1620
	11th to 20th	Night	47% 5% 10% 18% 18% 2	1475
	21st to end	Night	44% 6% 15% 21% 12% 2	1551
June	1st to 10th	Night	44% 5% 10% 20% 18% 2	1440
	11th to 20th	Night	41% 7% 15% 25% 12% 1	1440
	21st to end	Night	35% 9% 14% 26% 15% 1	1440
July	1st to 10th	Night	41% 6% 15% 20% 15% 2	1400
	11th to 20th	Night	39% 8% 14% 27% 11% 1	1392
	21st to end	Night	36% 8% 17% 22% 15% 2	1486
August	1st to 10th	Night	36% 8% 16% 24% 16% 1	1509
	11th to 20th	Night	43% 8% 13% 24% 11% 1	1774
	21st to end	Night	36% 8% 13% 23% 19% 2	1980
September	1st to 10th	Night	40% 6% 10% 27% 16% 1	1800
	11th to 20th	Night	34% 4 10% 24% 25% 3	1898
	21st to end	Night	41% 4 8% 23% 21% 3	1980
October	1st to 10th	Night	45% 5% 11% 22% 13% 3	2044
	11th to 20th	Night	38% 5% 12% 29% 15% 1	2160
	21st to end	Night	46% 3 12% 22% 15% 1	2549
November	1st to 10th	Night	36% 4 12% 24% 20% 4	2407
	11th to 20th	Night	40% 3 9% 25% 19% 4	2470
	21st to end	Night	38% 4 13% 30% 13% 2	2522
December	1st to 10th	Night	46% 4 12% 22% 15% 1	2660
	11th to 20th	Night	43% 4 12% 19% 18% 3	3288
	21st to end	Night	48% 5% 11% 23% 13% 1	4884

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% 28% 27% 17% 3	180
	11th to 20th	Day	19% 32% 37% 10% 3	180
	21st to end	Day	19% 24% 39% 13% 5%	245
February	1st to 10th	Day	24% 27% 36% 12% 1	303
	11th to 20th	Day	24% 25% 38% 12% 1	456
	21st to end	Day	27% 21% 34% 15% 3	382
March	1st to 10th	Day	18% 25% 37% 17% 3	460
	11th to 20th	Day	20% 25% 36% 16% 2 1	460
	21st to end	Day	25% 22% 34% 14% 4 1	506
April	1st to 10th	Day	23% 25% 30% 16% 5%	460
	11th to 20th	Day	23% 18% 33% 16% 8% 1	452
	21st to end	Day	27% 16% 36% 16% 4 1	437
May	1st to 10th	Day	29% 16% 30% 17% 5% 2	423
	11th to 20th	Day	19% 12% 39% 23% 6% 1	447
	21st to end	Day	18% 7% 28% 30% 13% 3	485
June	1st to 10th	Day	19% 10% 27% 30% 13% 2	450
	11th to 20th	Day	17% 10% 32% 27% 11% 2	450
	21st to end	Day	15% 6% 28% 34% 14% 2	450
July	1st to 10th	Day	16% 6% 22% 33% 18% 5%	445
	11th to 20th	Day	10% 5% 23% 37% 22% 3	444
	21st to end	Day	12% 6% 28% 36% 16% 1	488
August	1st to 10th	Day	13% 7% 20% 38% 17% 5%	446
	11th to 20th	Day	16% 4 19% 37% 19% 5%	439
	21st to end	Day	10% 8% 27% 32% 16% 7%	495
September	1st to 10th	Day	18% 6% 28% 25% 18% 5%	450
	11th to 20th	Day	14% 7% 29% 28% 13% 9%	450
	21st to end	Day	19% 10% 33% 22% 12% 4	450
October	1st to 10th	Day	28% 8% 27% 19% 14% 4	450
	11th to 20th	Day	16% 18% 29% 25% 11% 1	450
	21st to end	Day	25% 16% 31% 20% 7%	408
November	1st to 10th	Day	17% 16% 29% 27% 9% 3	365
	11th to 20th	Day	23% 20% 33% 19% 5%	370
	21st to end	Day	21% 19% 32% 23% 5% 1	313
December	1st to 10th	Day	23% 23% 33% 16% 5%	180
	11th to 20th	Day	35% 21% 31% 11% 3	180
	21st to end	Day	24% 20% 36% 16% 4	198
January	1st to 10th	Night	20% 21% 38% 18% 3	760
	11th to 20th	Night	25% 20% 33% 20% 3	760
	21st to end	Night	20% 20% 39% 18% 3	836
February	1st to 10th	Night	22% 18% 39% 20% 2	760
	11th to 20th	Night	23% 23% 37% 16% 1	743
	21st to end	Night	25% 21% 34% 18% 3	628
March	1st to 10th	Night	23% 19% 34% 20% 3	760
	11th to 20th	Night	21% 19% 38% 18% 3 1	760
	21st to end	Night	22% 18% 37% 18% 4	836
April	1st to 10th	Night	22% 19% 35% 19% 5%	760
	11th to 20th	Night	23% 13% 36% 24% 5%	751
	21st to end	Night	23% 14% 35% 22% 5% 1	682
May	1st to 10th	Night	26% 12% 32% 24% 7%	540
	11th to 20th	Night	23% 12% 35% 24% 6%	413
	21st to end	Night	23% 4 21% 37% 13% 1	389
June	1st to 10th	Night	19% 6% 29% 33% 11% 1	360
	11th to 20th	Night	16% 4 26% 40% 13%	360
	21st to end	Night	11% 3 26% 48% 12% 1	360
July	1st to 10th	Night	14% 3 21% 39% 21% 1	350
	11th to 20th	Night	12% 2 22% 42% 20% 2	348
	21st to end	Night	15% 4 20% 42% 18% 1	394
August	1st to 10th	Night	10% 3 24% 42% 20% 1	445
	11th to 20th	Night	15% 4 18% 37% 25% 2	710
	21st to end	Night	15% 6% 19% 35% 22% 3	792
September	1st to 10th	Night	17% 5% 26% 34% 16% 3	720
	11th to 20th	Night	17% 7% 22% 33% 18% 4	720
	21st to end	Night	21% 5% 27% 31% 15% 2	720
October	1st to 10th	Night	24% 10% 28% 24% 12% 3	720
	11th to 20th	Night	19% 12% 30% 29% 10% 1	720
	21st to end	Night	27% 12% 35% 20% 7%	792
November	1st to 10th	Night	14% 14% 35% 26% 10% 1	740
	11th to 20th	Night	24% 14% 35% 19% 7%	760
	21st to end	Night	19% 19% 30% 25% 6% 1	760
December	1st to 10th	Night	25% 18% 33% 18% 5% 1	760
	11th to 20th	Night	24% 19% 34% 20% 3	760
	21st to end	Night	26% 18% 36% 18% 3	836

Page generated at 2024-04-22 09:25:42. Version ac07

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