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 2024-11-05.

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% 7% 2 6% 4 4 4 4 6% 7% 39%	101293
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	97762
Transparency	Day	41% 6% 12% 22% 16% 2	104838
	Night	42% 5% 12% 23% 16% 2	102306
Seeing	Day	19% 15% 31% 23% 10% 2	15009
	Night	21% 13% 31% 25% 9% 1	24092

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 5% 4 4 5% 3 4 4 46%	2517
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 3 4 6% 35%	2520
	21st to end	Day	22% 11% 3 8% 4 4 4 4 5% 5% 32%	2936
February	1st to 10th	Day	18% 8% 2 6% 4 4 5% 4 5% 6% 38%	1743
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 5% 6% 44%	1848
	21st to end	Day	21% 8% 2 7% 4 4 4 4 4 6% 37%	1570
March	1st to 10th	Day	26% 8% 3 5% 3 3 3 3 5% 5% 35%	3116
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 5% 7% 9% 43%	3294
	21st to end	Day	21% 7% 2 6% 4 3 3 3 5% 6% 41%	4166
April	1st to 10th	Day	19% 9% 3 5% 4 4 4 4 5% 7% 38%	2280
	11th to 20th	Day	18% 6% 3 6% 4 4 3 4 4 7% 41%	2254
	21st to end	Day	19% 7% 2 4 5% 4 4 4 5% 6% 41%	2339
May	1st to 10th	Day	28% 9% 3 5% 3 3 3 3 5% 8% 30%	4106
	11th to 20th	Day	22% 8% 2 7% 4 4 4 4 5% 7% 32%	4325
	21st to end	Day	15% 4 2 5% 3 4 4 4 6% 8% 44%	4837
June	1st to 10th	Day	18% 5% 2 6% 4 4 4 4 7% 9% 37%	2660
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 7% 9% 38%	2660
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 4 6% 9% 39%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 9% 36%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 5% 7% 10% 38%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 5% 6% 8% 32%	4814
August	1st to 10th	Day	19% 8% 2 7% 5% 4 5% 4 6% 8% 32%	4219
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 40%	3730
	21st to end	Day	9% 5% 2 5% 4 5% 4 4 6% 11% 44%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 3 6% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 3 4 5% 7% 50%	2176
	21st to end	Day	19% 6% 2 7% 4 4 5% 4 5% 6% 39%	2090
October	1st to 10th	Day	22% 7% 2 4 4 3 3 3 5% 6% 41%	2019
	11th to 20th	Day	15% 5% 2 6% 3 3 3 3 5% 7% 48%	1900
	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 3 3 3 2 5% 7% 50%	1708
	11th to 20th	Day	19% 6% 2 4 2 3 4 3 5% 7% 46%	1710
	21st to end	Day	16% 6% 3 4 4 3 3 4 6% 6% 45%	1653
December	1st to 10th	Day	19% 7% 2 6% 5% 5% 4 4 6% 6% 37%	1520
	11th to 20th	Day	23% 5% 2 5% 2 3 3 3 4 5% 45%	1906
	21st to end	Day	18% 11% 3 8% 6% 4 4 4 4 6% 32%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 5% 6% 46%	4429
	11th to 20th	Night	16% 9% 3 7% 4 3 3 3 4 5% 43%	4440
	21st to end	Night	24% 12% 3 7% 4 3 3 3 4 4 34%	4715
February	1st to 10th	Night	17% 9% 2 6% 4 4 3 3 4 6% 42%	2436
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 42%	2218
	21st to end	Night	20% 7% 2 5% 5% 3 3 3 5% 5% 42%	1884
March	1st to 10th	Night	24% 11% 2 5% 4 3 3 3 4 5% 36%	3724
	11th to 20th	Night	12% 6% 2 6% 5% 5% 4 3 5% 7% 46%	3663
	21st to end	Night	20% 10% 3 5% 3 3 3 3 4 5% 42%	3466
April	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 39%	1900
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 6% 41%	1875
	21st to end	Night	19% 7% 3 7% 4 4 3 3 4 5% 41%	1838
May	1st to 10th	Night	25% 10% 3 7% 4 3 4 3 5% 7% 29%	2834
	11th to 20th	Night	23% 9% 3 8% 4 4 4 4 5% 6% 31%	2596
	21st to end	Night	18% 7% 2 5% 4 3 3 3 4 7% 44%	2738
June	1st to 10th	Night	18% 6% 2 5% 3 5% 4 3 5% 7% 41%	1520
	11th to 20th	Night	14% 8% 2 5% 4 3 4 4 6% 8% 41%	1520
	21st to end	Night	12% 6% 3 5% 3 4 5% 4 6% 9% 43%	1520
July	1st to 10th	Night	11% 7% 2 8% 6% 5% 5% 5% 7% 8% 36%	2520
	11th to 20th	Night	16% 6% 3 6% 4 4 5% 4 7% 9% 37%	2520
	21st to end	Night	19% 8% 2 5% 3 4 5% 4 7% 8% 34%	2739
August	1st to 10th	Night	23% 7% 3 6% 4 5% 5% 4 5% 7% 31%	2677
	11th to 20th	Night	15% 7% 2 7% 5% 5% 5% 4 6% 6% 39%	3097
	21st to end	Night	10% 4 2 5% 3 3 4 4 5% 8% 52%	3476
September	1st to 10th	Night	15% 5% 2 6% 5% 4 4 3 4 6% 46%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 4 2 6% 6% 52%	2004
	21st to end	Night	17% 7% 3 6% 3 4 4 3 4 6% 44%	2090
October	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 3 5% 42%	2158
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	21% 7% 2 6% 3 2 4 3 4 7% 41%	2691
November	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 50%	2470
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 5% 47%	2470
	21st to end	Night	15% 5% 2 5% 4 3 4 3 5% 7% 47%	2522
December	1st to 10th	Night	20% 7% 3 5% 4 4 4 3 4 7% 39%	2660
	11th to 20th	Night	20% 6% 2 5% 4 3 4 4 5% 6% 42%	3288
	21st to end	Night	21% 9% 3 6% 4 4 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	43% 5% 9% 21% 17% 4	2339
May	1st to 10th	Day	53% 5% 14% 15% 12% 1	4120
	11th to 20th	Day	50% 7% 14% 19% 9% 2	4329
	21st to end	Day	35% 7% 12% 26% 18% 2	4837
June	1st to 10th	Day	42% 8% 12% 22% 14% 1	2660
	11th to 20th	Day	40% 8% 13% 25% 12% 2	2660
	21st to end	Day	39% 8% 14% 23% 14% 1	2660
July	1st to 10th	Day	42% 9% 11% 21% 16% 1	4370
	11th to 20th	Day	39% 8% 16% 22% 13% 1	4365
	21st to end	Day	47% 9% 11% 17% 15% 1	4661
August	1st to 10th	Day	48% 7% 15% 21% 8%	4205
	11th to 20th	Day	37% 7% 14% 30% 11% 1	3766
	21st to end	Day	31% 7% 14% 27% 18% 2	4180
September	1st to 10th	Day	39% 6% 13% 22% 19% 2	2280
	11th to 20th	Day	33% 5% 10% 24% 26% 2	2176
	21st to end	Day	44% 6% 9% 20% 18% 3	2090
October	1st to 10th	Day	33% 7% 11% 20% 23% 6%	3357
	11th to 20th	Day	27% 4 10% 24% 30% 5%	3140
	21st to end	Day	30% 4 15% 23% 25% 4	3117
November	1st to 10th	Day	35% 4 14% 24% 20% 2	1708
	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% 5% 9% 21% 16% 4	1838
May	1st to 10th	Night	56% 5% 11% 17% 10% 1	2840
	11th to 20th	Night	53% 5% 15% 15% 10% 1	2597
	21st to end	Night	41% 5% 14% 24% 13% 3	2739
June	1st to 10th	Night	44% 5% 10% 21% 18% 2	1520
	11th to 20th	Night	39% 6% 15% 27% 12% 1	1520
	21st to end	Night	36% 9% 14% 26% 14% 1	1520
July	1st to 10th	Night	44% 9% 14% 18% 14% 1	2480
	11th to 20th	Night	42% 8% 15% 23% 11% 1	2472
	21st to end	Night	46% 7% 14% 15% 16% 2	2651
August	1st to 10th	Night	51% 7% 14% 16% 10% 1	2667
	11th to 20th	Night	43% 7% 12% 29% 9% 1	3127
	21st to end	Night	28% 6% 13% 33% 18% 1	3476
September	1st to 10th	Night	39% 5% 9% 26% 18% 1	1900
	11th to 20th	Night	33% 4 10% 25% 24% 3	2004
	21st to end	Night	43% 4 8% 22% 20% 3	2090
October	1st to 10th	Night	38% 6% 12% 22% 19% 3	3600
	11th to 20th	Night	25% 4 9% 28% 30% 4	3779
	21st to end	Night	32% 3 14% 28% 20% 3	4429
November	1st to 10th	Night	36% 4 12% 24% 20% 4	2470
	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	26% 16% 36% 17% 5% 1	460
May	1st to 10th	Day	29% 16% 30% 18% 6% 2	463
	11th to 20th	Day	20% 11% 38% 24% 6% 1	487
	21st to end	Day	18% 7% 30% 30% 12% 3	529
June	1st to 10th	Day	17% 11% 28% 30% 12% 2	490
	11th to 20th	Day	16% 10% 33% 28% 12% 1	490
	21st to end	Day	15% 8% 29% 33% 13% 2	490
July	1st to 10th	Day	16% 6% 24% 33% 18% 5%	485
	11th to 20th	Day	9% 6% 26% 36% 20% 3	484
	21st to end	Day	13% 7% 29% 35% 16% 1	532
August	1st to 10th	Day	15% 7% 20% 36% 16% 5%	486
	11th to 20th	Day	15% 5% 20% 36% 18% 5%	479
	21st to end	Day	9% 8% 27% 32% 16% 7%	539
September	1st to 10th	Day	17% 7% 29% 24% 18% 5%	490
	11th to 20th	Day	13% 7% 28% 29% 14% 9%	490
	21st to end	Day	22% 10% 33% 20% 11% 4	490
October	1st to 10th	Day	26% 9% 28% 19% 14% 4	490
	11th to 20th	Day	15% 18% 31% 24% 11% 1	490
	21st to end	Day	24% 16% 31% 21% 7% 1	442
November	1st to 10th	Day	17% 16% 29% 26% 8% 3	378
	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% 34% 23% 5% 1	698
May	1st to 10th	Night	25% 13% 32% 23% 7%	570
	11th to 20th	Night	23% 11% 34% 25% 6%	436
	21st to end	Night	23% 4 21% 38% 13% 1	411
June	1st to 10th	Night	19% 6% 29% 34% 11% 1	380
	11th to 20th	Night	16% 5% 26% 41% 13%	380
	21st to end	Night	11% 4 26% 46% 12% 1	380
July	1st to 10th	Night	13% 4 22% 40% 20% 1	370
	11th to 20th	Night	13% 3 21% 42% 20% 2	368
	21st to end	Night	16% 4 20% 41% 17% 1	416
August	1st to 10th	Night	12% 3 23% 42% 19% 1	471
	11th to 20th	Night	15% 4 18% 37% 24% 2	750
	21st to end	Night	14% 7% 19% 36% 22% 3	836
September	1st to 10th	Night	17% 5% 26% 33% 16% 3	760
	11th to 20th	Night	16% 7% 21% 33% 19% 4	760
	21st to end	Night	21% 5% 27% 30% 14% 2	760
October	1st to 10th	Night	23% 10% 28% 25% 13% 2	760
	11th to 20th	Night	18% 12% 31% 28% 10% 1	760
	21st to end	Night	25% 11% 34% 21% 8%	836
November	1st to 10th	Night	14% 14% 35% 26% 10% 1	760
	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 2024-11-03 02:11:25. Version ac07

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