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 2024-11-30.

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 6% 8% 41%	101527
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	98087
Transparency	Day	41% 6% 13% 23% 16% 2	105072
	Night	42% 5% 12% 23% 16% 2	102631
Seeing	Day	18% 12% 30% 25% 12% 3	15092
	Night	20% 12% 31% 27% 9% 1	24203

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% 6% 2 5% 4 3 3 3 4 5% 47%	2517
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 6% 6% 37%	2520
	21st to end	Day	23% 8% 3 6% 4 4 5% 3 5% 6% 32%	2941
February	1st to 10th	Day	17% 7% 3 6% 4 3 5% 4 5% 7% 40%	1745
	11th to 20th	Day	15% 5% 2 7% 4 4 4 3 5% 6% 46%	1848
	21st to end	Day	20% 7% 2 6% 4 3 4 4 5% 7% 38%	1570
March	1st to 10th	Day	26% 8% 2 5% 4 3 3 3 4 6% 37%	3116
	11th to 20th	Day	9% 5% 1 4 4 5% 5% 5% 7% 9% 45%	3294
	21st to end	Day	20% 7% 2 5% 4 3 3 3 4 5% 43%	4166
April	1st to 10th	Day	18% 7% 3 5% 5% 3 4 3 6% 7% 39%	2280
	11th to 20th	Day	17% 6% 2 6% 5% 3 3 4 6% 7% 42%	2254
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	2337
May	1st to 10th	Day	26% 9% 2 5% 3 3 3 3 5% 8% 32%	4106
	11th to 20th	Day	21% 8% 3 7% 4 4 4 4 6% 7% 34%	4323
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 9% 47%	4837
June	1st to 10th	Day	16% 5% 2 5% 4 5% 4 4 6% 10% 39%	2660
	11th to 20th	Day	13% 7% 2 5% 5% 4 4 4 7% 9% 40%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 6% 10% 42%	2660
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 7% 10% 40%	4440
	21st to end	Day	14% 8% 2 7% 5% 5% 6% 4 6% 7% 35%	4814
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 4 6% 9% 34%	4214
	11th to 20th	Day	12% 6% 2 6% 4 4 4 5% 6% 8% 43%	3730
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 47%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 5% 4 4 5% 6% 47%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2176
	21st to end	Day	18% 6% 2 6% 4 4 3 4 5% 6% 41%	2090
October	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 4 7% 43%	2019
	11th to 20th	Day	14% 5% 2 6% 3 3 4 3 5% 7% 48%	1900
	21st to end	Day	20% 6% 2 5% 3 3 4 3 5% 6% 42%	1908
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 6% 7% 50%	1761
	11th to 20th	Day	17% 6% 3 4 2 3 4 3 4 7% 48%	1800
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 6% 45%	1743
December	1st to 10th	Day	17% 7% 3 6% 4 5% 4 3 7% 7% 37%	1520
	11th to 20th	Day	23% 5% 2 4 2 3 3 2 5% 6% 45%	1906
	21st to end	Day	18% 10% 3 8% 5% 5% 4 4 5% 6% 32%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 5% 6% 46%	4429
	11th to 20th	Night	17% 8% 3 7% 4 3 3 3 4 5% 44%	4440
	21st to end	Night	24% 11% 3 6% 4 3 3 3 4 5% 33%	4712
February	1st to 10th	Night	17% 10% 2 5% 4 4 3 3 5% 5% 42%	2433
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 4 6% 41%	2218
	21st to end	Night	20% 8% 2 5% 4 4 4 3 5% 5% 41%	1884
March	1st to 10th	Night	24% 10% 3 6% 3 3 4 3 4 5% 36%	3724
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 5% 6% 46%	3662
	21st to end	Night	21% 8% 3 5% 3 3 4 3 5% 5% 40%	3466
April	1st to 10th	Night	18% 8% 3 5% 5% 3 4 4 5% 7% 38%	1900
	11th to 20th	Night	19% 6% 3 6% 4 4 4 3 5% 6% 40%	1875
	21st to end	Night	18% 7% 3 7% 4 2 3 4 4 6% 41%	1843
May	1st to 10th	Night	24% 10% 4 7% 4 3 4 3 5% 7% 29%	2834
	11th to 20th	Night	23% 9% 3 6% 5% 4 4 3 5% 6% 32%	2600
	21st to end	Night	17% 7% 2 5% 3 3 4 3 5% 8% 43%	2738
June	1st to 10th	Night	17% 7% 2 6% 3 4 4 3 6% 7% 41%	1520
	11th to 20th	Night	13% 8% 2 5% 4 3 4 4 7% 9% 41%	1520
	21st to end	Night	10% 6% 2 6% 4 4 5% 5% 6% 11% 42%	1520
July	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 5% 7% 8% 36%	2520
	11th to 20th	Night	13% 6% 3 7% 3 5% 6% 4 7% 9% 37%	2520
	21st to end	Night	17% 9% 3 6% 4 4 4 4 6% 8% 34%	2739
August	1st to 10th	Night	22% 6% 2 7% 5% 4 5% 4 6% 6% 32%	2679
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 6% 7% 38%	3097
	21st to end	Night	9% 5% 2 4 3 4 4 4 6% 8% 51%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 4 5% 4 5% 6% 45%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 5% 52%	2004
	21st to end	Night	18% 6% 2 6% 4 4 3 3 4 6% 44%	2090
October	1st to 10th	Night	19% 8% 2 6% 5% 4 3 3 4 6% 41%	2157
	11th to 20th	Night	14% 6% 1 5% 3 4 3 2 4 7% 50%	2280
	21st to end	Night	21% 6% 3 6% 3 3 3 3 4 6% 42%	2688
November	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 6% 50%	2537
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 4 5% 48%	2600
	21st to end	Night	15% 5% 2 5% 3 4 4 3 6% 7% 46%	2650
December	1st to 10th	Night	20% 7% 3 5% 4 4 4 3 5% 6% 39%	2660
	11th to 20th	Night	20% 7% 2 4 4 3 3 4 5% 6% 42%	3288
	21st to end	Night	20% 9% 3 6% 4 3 4 4 5% 5% 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% 4 11% 31% 13% 3	2518
	11th to 20th	Day	46% 6% 11% 25% 11% 1	2520
	21st to end	Day	55% 4 10% 21% 9% 2	2941
February	1st to 10th	Day	45% 6% 12% 22% 13% 2	1745
	11th to 20th	Day	41% 5% 9% 25% 18% 2	1857
	21st to end	Day	46% 4 11% 23% 14% 3	1570
March	1st to 10th	Day	50% 4 12% 23% 10% 2	3119
	11th to 20th	Day	32% 9% 14% 31% 13% 2	3294
	21st to end	Day	44% 4 8% 17% 22% 5%	4166
April	1st to 10th	Day	44% 5% 10% 21% 16% 4	2280
	11th to 20th	Day	41% 5% 12% 21% 18% 3	2254
	21st to end	Day	42% 6% 11% 21% 17% 4	2337
May	1st to 10th	Day	52% 5% 15% 14% 12% 1	4120
	11th to 20th	Day	50% 5% 16% 19% 8% 2	4327
	21st to end	Day	34% 7% 14% 25% 18% 2	4837
June	1st to 10th	Day	40% 8% 13% 23% 15% 1	2660
	11th to 20th	Day	40% 8% 14% 24% 12% 2	2660
	21st to end	Day	38% 8% 15% 23% 15% 1	2660
July	1st to 10th	Day	40% 10% 12% 20% 16% 1	4370
	11th to 20th	Day	37% 8% 17% 23% 13% 2	4365
	21st to end	Day	47% 8% 13% 16% 15% 1	4661
August	1st to 10th	Day	46% 7% 16% 22% 8%	4200
	11th to 20th	Day	37% 6% 14% 31% 11% 1	3766
	21st to end	Day	32% 6% 13% 28% 20% 1	4180
September	1st to 10th	Day	38% 6% 14% 22% 18% 2	2280
	11th to 20th	Day	33% 5% 12% 25% 24% 2	2176
	21st to end	Day	44% 5% 11% 20% 18% 3	2090
October	1st to 10th	Day	35% 5% 11% 20% 24% 6%	3357
	11th to 20th	Day	27% 4 11% 23% 31% 5%	3140
	21st to end	Day	29% 4 14% 25% 23% 3	3120
November	1st to 10th	Day	34% 5% 13% 23% 23% 2	1761
	11th to 20th	Day	38% 4 10% 23% 22% 4	1800
	21st to end	Day	40% 4 14% 27% 12% 2	1743
December	1st to 10th	Day	45% 5% 15% 19% 14% 1	1520
	11th to 20th	Day	42% 4 15% 24% 14% 2	1906
	21st to end	Day	51% 7% 10% 23% 9% 1	2772
January	1st to 10th	Night	40% 3 12% 30% 12% 4	4429
	11th to 20th	Night	44% 4 13% 24% 13% 2	4440
	21st to end	Night	53% 4 8% 21% 11% 2	4712
February	1st to 10th	Night	44% 4 13% 24% 13% 2	2433
	11th to 20th	Night	44% 5% 10% 20% 17% 5%	2220
	21st to end	Night	44% 4 8% 24% 16% 4	1884
March	1st to 10th	Night	51% 4 10% 21% 12% 2	3734
	11th to 20th	Night	37% 5% 14% 25% 15% 3	3663
	21st to end	Night	46% 4 8% 15% 23% 6%	3466
April	1st to 10th	Night	46% 4 10% 19% 16% 3	1900
	11th to 20th	Night	44% 5% 10% 19% 21% 1	1875
	21st to end	Night	43% 6% 9% 21% 17% 4	1843
May	1st to 10th	Night	55% 6% 12% 16% 10% 1	2840
	11th to 20th	Night	54% 5% 16% 14% 10% 1	2601
	21st to end	Night	40% 6% 15% 23% 13% 3	2739
June	1st to 10th	Night	42% 7% 10% 21% 19% 2	1520
	11th to 20th	Night	39% 6% 16% 27% 12% 1	1520
	21st to end	Night	36% 8% 15% 25% 14% 1	1520
July	1st to 10th	Night	44% 9% 15% 18% 13% 1	2480
	11th to 20th	Night	42% 7% 16% 23% 11% 1	2472
	21st to end	Night	45% 6% 16% 15% 16% 2	2651
August	1st to 10th	Night	51% 7% 16% 16% 9% 1	2669
	11th to 20th	Night	42% 7% 12% 28% 10% 1	3127
	21st to end	Night	30% 6% 12% 32% 18% 1	3476
September	1st to 10th	Night	39% 6% 10% 25% 18% 1	1900
	11th to 20th	Night	34% 4 11% 25% 22% 3	2004
	21st to end	Night	42% 4 10% 21% 20% 2	2090
October	1st to 10th	Night	40% 5% 11% 23% 19% 3	3599
	11th to 20th	Night	24% 4 10% 28% 30% 4	3779
	21st to end	Night	31% 4 13% 29% 21% 2	4426
November	1st to 10th	Night	35% 5% 11% 24% 21% 4	2537
	11th to 20th	Night	40% 3 9% 24% 20% 5%	2600
	21st to end	Night	37% 4 13% 30% 14% 2	2650
December	1st to 10th	Night	45% 5% 13% 21% 15% 1	2660
	11th to 20th	Night	43% 5% 13% 19% 18% 3	3288
	21st to end	Night	47% 6% 12% 22% 12% 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	21% 29% 26% 19% 4 1	180
	11th to 20th	Day	17% 30% 36% 14% 3	180
	21st to end	Day	19% 17% 41% 17% 5% 1	247
February	1st to 10th	Day	25% 17% 39% 17% 2	305
	11th to 20th	Day	21% 20% 41% 17% 2	456
	21st to end	Day	23% 23% 32% 18% 4	382
March	1st to 10th	Day	17% 23% 35% 20% 4	460
	11th to 20th	Day	17% 23% 36% 20% 4 1	460
	21st to end	Day	24% 19% 35% 17% 4 1	506
April	1st to 10th	Day	22% 22% 34% 15% 6% 1	460
	11th to 20th	Day	21% 16% 33% 19% 10% 1	452
	21st to end	Day	24% 15% 34% 19% 6% 1	460
May	1st to 10th	Day	29% 13% 30% 19% 7% 2	463
	11th to 20th	Day	19% 12% 33% 27% 8% 2	487
	21st to end	Day	18% 6% 26% 29% 17% 4	529
June	1st to 10th	Day	16% 9% 27% 31% 16% 2	490
	11th to 20th	Day	14% 9% 28% 33% 13% 2	490
	21st to end	Day	13% 8% 26% 32% 17% 4	490
July	1st to 10th	Day	12% 5% 21% 36% 19% 7%	485
	11th to 20th	Day	10% 3 22% 36% 25% 4	484
	21st to end	Day	13% 6% 21% 37% 20% 2	532
August	1st to 10th	Day	11% 5% 23% 35% 20% 6%	486
	11th to 20th	Day	15% 4 16% 38% 23% 4	479
	21st to end	Day	10% 4 27% 33% 17% 9%	539
September	1st to 10th	Day	17% 5% 27% 26% 19% 6%	490
	11th to 20th	Day	13% 4 27% 27% 18% 11%	490
	21st to end	Day	22% 7% 28% 26% 13% 4	490
October	1st to 10th	Day	25% 7% 24% 23% 15% 5%	490
	11th to 20th	Day	15% 13% 30% 26% 13% 2	490
	21st to end	Day	23% 12% 30% 23% 10% 2	442
November	1st to 10th	Day	15% 13% 32% 27% 10% 3	395
	11th to 20th	Day	20% 19% 35% 20% 6% 1	400
	21st to end	Day	19% 18% 35% 21% 6% 1	345
December	1st to 10th	Day	19% 22% 36% 17% 6% 1	180
	11th to 20th	Day	26% 27% 32% 13% 2	180
	21st to end	Day	26% 17% 36% 14% 7% 1	198
January	1st to 10th	Night	20% 18% 39% 20% 3	760
	11th to 20th	Night	23% 18% 35% 21% 3	760
	21st to end	Night	19% 19% 40% 20% 2	836
February	1st to 10th	Night	22% 15% 40% 22% 2	760
	11th to 20th	Night	22% 20% 38% 18% 1	743
	21st to end	Night	24% 18% 35% 20% 4	628
March	1st to 10th	Night	22% 14% 38% 21% 4	760
	11th to 20th	Night	21% 16% 38% 21% 4 1	760
	21st to end	Night	21% 16% 37% 21% 4	836
April	1st to 10th	Night	22% 15% 36% 22% 6%	760
	11th to 20th	Night	20% 14% 34% 26% 6%	751
	21st to end	Night	22% 11% 34% 26% 6% 1	703
May	1st to 10th	Night	24% 10% 32% 26% 7% 1	570
	11th to 20th	Night	21% 11% 35% 25% 8%	440
	21st to end	Night	21% 4 18% 41% 14% 1	411
June	1st to 10th	Night	19% 5% 27% 36% 13% 1	380
	11th to 20th	Night	16% 3 23% 43% 15%	380
	21st to end	Night	9% 3 23% 52% 12% 1	380
July	1st to 10th	Night	12% 3 20% 44% 20% 1	370
	11th to 20th	Night	13% 4 18% 43% 21% 1	368
	21st to end	Night	14% 5% 17% 47% 17% 1	416
August	1st to 10th	Night	12% 3 21% 43% 19% 2	473
	11th to 20th	Night	14% 4 18% 36% 26% 2	750
	21st to end	Night	14% 5% 18% 35% 25% 3	836
September	1st to 10th	Night	16% 3 26% 34% 19% 3	760
	11th to 20th	Night	16% 4 22% 33% 20% 4	760
	21st to end	Night	21% 6% 23% 33% 16% 2	760
October	1st to 10th	Night	23% 8% 27% 26% 14% 2	760
	11th to 20th	Night	17% 10% 32% 29% 12% 1	760
	21st to end	Night	25% 11% 33% 23% 7%	836
November	1st to 10th	Night	14% 13% 35% 27% 10% 1	780
	11th to 20th	Night	22% 14% 33% 23% 7%	800
	21st to end	Night	18% 18% 34% 24% 6% 1	800
December	1st to 10th	Night	24% 17% 35% 18% 5% 1	760
	11th to 20th	Night	23% 17% 34% 21% 4	760
	21st to end	Night	24% 15% 37% 20% 3	836

Page generated at 2024-11-27 17:30:52. Version ac07

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