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 2024-11-10.

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	15% 12% 4 9% 5% 5% 5% 4 5% 7% 29%	101463
	Night	15% 13% 5% 10% 5% 5% 4 3 5% 6% 29%	97680
Transparency	Day	52% 7% 11% 19% 10% 1	104957
	Night	54% 6% 12% 18% 9% 1	102262
Seeing	Day	26% 19% 28% 16% 8% 3	14820
	Night	25% 19% 26% 21% 8% 1	23935

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	11% 16% 13% 16% 5% 3 3 3 4 4 22%	2517
	11th to 20th	Day	12% 17% 9% 15% 8% 7% 4 3 4 4 19%	2520
	21st to end	Day	17% 19% 9% 11% 7% 6% 4 3 4 4 17%	2967
February	1st to 10th	Day	14% 16% 6% 13% 9% 5% 4 4 5% 5% 19%	1710
	11th to 20th	Day	9% 13% 7% 16% 8% 5% 4 4 5% 6% 23%	1783
	21st to end	Day	15% 14% 4 13% 7% 4 5% 4 4 5% 24%	1570
March	1st to 10th	Day	22% 16% 5% 11% 5% 3 3 3 4 4 25%	3116
	11th to 20th	Day	11% 17% 8% 12% 5% 4 4 3 4 4 26%	3384
	21st to end	Day	19% 11% 5% 11% 4 4 3 3 4 5% 31%	4097
April	1st to 10th	Day	14% 16% 4 10% 5% 4 4 3 4 5% 30%	2280
	11th to 20th	Day	15% 12% 3 7% 5% 5% 4 3 5% 5% 34%	2254
	21st to end	Day	19% 14% 4 7% 5% 4 3 3 4 5% 31%	2313
May	1st to 10th	Day	24% 12% 3 6% 4 4 4 3 4 6% 29%	4260
	11th to 20th	Day	17% 11% 3 8% 5% 5% 6% 4 6% 7% 28%	4398
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	4837
June	1st to 10th	Day	17% 9% 3 7% 6% 5% 5% 3 7% 9% 28%	2660
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 5% 7% 10% 29%	2660
	21st to end	Day	17% 8% 3 6% 5% 5% 6% 5% 6% 8% 30%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 6% 6% 6% 7% 10% 33%	4440
	11th to 20th	Day	9% 6% 3 8% 6% 6% 7% 5% 8% 12% 30%	4440
	21st to end	Day	13% 8% 3 9% 6% 6% 5% 5% 7% 8% 31%	4814
August	1st to 10th	Day	18% 9% 4 8% 5% 5% 5% 4 6% 8% 28%	4282
	11th to 20th	Day	12% 9% 3 8% 6% 6% 7% 5% 7% 10% 26%	3773
	21st to end	Day	13% 8% 3 6% 5% 5% 5% 4 6% 8% 34%	4180
September	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 4 5% 8% 38%	2280
	11th to 20th	Day	13% 8% 2 7% 4 3 4 3 5% 7% 44%	2228
	21st to end	Day	16% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	2090
October	1st to 10th	Day	18% 11% 3 7% 6% 4 4 4 5% 6% 32%	1930
	11th to 20th	Day	15% 13% 2 7% 4 5% 4 4 5% 7% 34%	1900
	21st to end	Day	21% 16% 3 8% 4 4 4 3 5% 6% 26%	1937
November	1st to 10th	Day	13% 13% 3 7% 4 4 3 4 5% 6% 38%	1752
	11th to 20th	Day	15% 17% 4 10% 6% 4 5% 3 4 4 28%	1710
	21st to end	Day	15% 16% 5% 10% 6% 5% 4 4 5% 6% 25%	1523
December	1st to 10th	Day	13% 21% 6% 16% 6% 4 5% 3 5% 5% 16%	1520
	11th to 20th	Day	18% 17% 9% 11% 6% 3 3 2 3 4 24%	1906
	21st to end	Day	17% 20% 9% 12% 5% 4 4 4 5% 5% 15%	2772
January	1st to 10th	Night	13% 14% 9% 14% 5% 4 5% 3 5% 6% 21%	4429
	11th to 20th	Night	11% 18% 8% 16% 10% 6% 4 3 3 4 18%	4440
	21st to end	Night	16% 18% 6% 15% 8% 6% 4 3 4 4 16%	4688
February	1st to 10th	Night	11% 16% 6% 13% 9% 6% 5% 4 5% 5% 21%	2470
	11th to 20th	Night	9% 13% 7% 15% 10% 7% 5% 5% 5% 6% 19%	2281
	21st to end	Night	17% 12% 5% 12% 7% 5% 4 4 5% 7% 24%	1884
March	1st to 10th	Night	19% 13% 6% 12% 6% 5% 4 3 4 5% 25%	3724
	11th to 20th	Night	10% 17% 8% 13% 7% 5% 4 3 4 4 26%	3559
	21st to end	Night	15% 14% 6% 11% 4 4 4 3 4 4 31%	3535
April	1st to 10th	Night	15% 16% 5% 9% 5% 5% 4 4 5% 6% 28%	1900
	11th to 20th	Night	16% 14% 3 7% 5% 3 4 3 4 7% 34%	1875
	21st to end	Night	20% 12% 4 6% 4 3 4 3 4 6% 33%	1866
May	1st to 10th	Night	25% 12% 3 7% 4 3 3 3 4 6% 31%	2709
	11th to 20th	Night	16% 12% 4 9% 5% 4 4 4 5% 6% 31%	2495
	21st to end	Night	13% 9% 3 7% 4 5% 5% 4 6% 8% 36%	2738
June	1st to 10th	Night	15% 7% 3 8% 5% 4 4 4 6% 7% 37%	1520
	11th to 20th	Night	16% 9% 2 5% 4 4 4 4 6% 9% 38%	1520
	21st to end	Night	12% 6% 2 6% 4 6% 6% 4 7% 8% 40%	1520
July	1st to 10th	Night	10% 7% 3 7% 6% 6% 6% 5% 5% 8% 37%	2520
	11th to 20th	Night	11% 8% 3 7% 5% 5% 4 4 7% 9% 39%	2520
	21st to end	Night	13% 9% 3 7% 4 5% 5% 4 5% 6% 40%	2739
August	1st to 10th	Night	21% 11% 2 6% 5% 4 4 3 5% 7% 33%	2631
	11th to 20th	Night	14% 9% 4 8% 5% 4 4 3 5% 6% 38%	3024
	21st to end	Night	11% 10% 3 6% 5% 4 4 3 4 7% 43%	3476
September	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 6% 7% 43%	1900
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 7% 49%	1952
	21st to end	Night	15% 8% 2 8% 4 5% 4 3 4 7% 40%	2090
October	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 4 6% 34%	2247
	11th to 20th	Night	16% 11% 3 7% 4 4 4 3 6% 7% 36%	2280
	21st to end	Night	21% 14% 3 8% 4 4 4 4 5% 7% 26%	2660
November	1st to 10th	Night	11% 14% 5% 8% 5% 4 4 3 4 6% 37%	2531
	11th to 20th	Night	15% 14% 6% 9% 5% 4 5% 3 5% 6% 28%	2470
	21st to end	Night	12% 14% 6% 10% 6% 4 5% 4 5% 6% 28%	2655
December	1st to 10th	Night	14% 19% 7% 13% 6% 5% 5% 3 5% 5% 18%	2660
	11th to 20th	Night	17% 15% 7% 13% 6% 5% 4 4 4 5% 22%	3288
	21st to end	Night	19% 17% 7% 14% 6% 5% 4 3 4 4 18%	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	65% 4 10% 16% 4 1	2518
	11th to 20th	Day	69% 4 8% 13% 4 2	2520
	21st to end	Day	70% 4 8% 12% 4 1	2967
February	1st to 10th	Day	66% 5% 10% 13% 6% 1	1710
	11th to 20th	Day	60% 4 8% 17% 9% 1	1792
	21st to end	Day	60% 5% 10% 15% 9%	1570
March	1st to 10th	Day	63% 4 10% 13% 9% 1	3119
	11th to 20th	Day	59% 5% 13% 15% 8% 1	3384
	21st to end	Day	56% 3 8% 14% 17% 2	4097
April	1st to 10th	Day	57% 4 7% 20% 10% 2	2280
	11th to 20th	Day	51% 6% 11% 18% 12% 1	2254
	21st to end	Day	55% 5% 8% 17% 12% 2	2313
May	1st to 10th	Day	56% 5% 10% 18% 10% 1	4274
	11th to 20th	Day	52% 9% 11% 20% 7% 1	4402
	21st to end	Day	41% 8% 13% 25% 12% 1	4837
June	1st to 10th	Day	51% 8% 12% 17% 11%	2660
	11th to 20th	Day	48% 9% 13% 20% 10% 1	2660
	21st to end	Day	50% 8% 13% 19% 9% 1	2660
July	1st to 10th	Day	43% 10% 15% 22% 10% 1	4370
	11th to 20th	Day	42% 10% 16% 21% 10%	4365
	21st to end	Day	48% 8% 14% 19% 10%	4661
August	1st to 10th	Day	53% 8% 13% 17% 9%	4268
	11th to 20th	Day	48% 11% 16% 21% 4	3810
	21st to end	Day	43% 8% 14% 27% 8% 1	4180
September	1st to 10th	Day	43% 6% 12% 25% 13% 1	2280
	11th to 20th	Day	40% 5% 10% 23% 20% 1	2228
	21st to end	Day	46% 6% 12% 21% 14% 2	2090
October	1st to 10th	Day	48% 7% 9% 20% 14% 1	3203
	11th to 20th	Day	44% 5% 9% 20% 17% 5%	3140
	21st to end	Day	42% 7% 14% 25% 10% 2	3162
November	1st to 10th	Day	47% 5% 13% 20% 13% 2	1752
	11th to 20th	Day	59% 5% 9% 15% 10% 2	1710
	21st to end	Day	59% 5% 13% 17% 6% 1	1523
December	1st to 10th	Day	69% 6% 10% 10% 6%	1520
	11th to 20th	Day	66% 4 11% 15% 4	1906
	21st to end	Day	70% 6% 8% 9% 7%	2772
January	1st to 10th	Night	60% 6% 13% 13% 7% 1	4429
	11th to 20th	Night	69% 5% 9% 11% 5% 1	4440
	21st to end	Night	71% 4 7% 11% 5% 1	4688
February	1st to 10th	Night	63% 6% 10% 13% 7% 1	2470
	11th to 20th	Night	64% 6% 9% 13% 7% 1	2283
	21st to end	Night	59% 5% 11% 15% 8% 2	1884
March	1st to 10th	Night	61% 5% 9% 15% 9% 1	3734
	11th to 20th	Night	60% 5% 11% 17% 6% 1	3560
	21st to end	Night	56% 4 7% 15% 16% 2	3535
April	1st to 10th	Night	57% 5% 11% 17% 9% 1	1900
	11th to 20th	Night	51% 6% 11% 18% 14%	1875
	21st to end	Night	52% 5% 10% 18% 12% 2	1866
May	1st to 10th	Night	55% 6% 10% 18% 10% 2	2715
	11th to 20th	Night	53% 8% 12% 19% 7% 1	2496
	21st to end	Night	43% 8% 14% 20% 13% 1	2739
June	1st to 10th	Night	45% 6% 11% 25% 12% 1	1520
	11th to 20th	Night	43% 8% 15% 25% 10%	1520
	21st to end	Night	39% 9% 16% 25% 10% 1	1520
July	1st to 10th	Night	43% 9% 15% 19% 13% 1	2480
	11th to 20th	Night	39% 9% 15% 25% 11%	2472
	21st to end	Night	44% 8% 12% 25% 11% 1	2651
August	1st to 10th	Night	51% 7% 13% 21% 7%	2621
	11th to 20th	Night	45% 8% 16% 23% 8%	3053
	21st to end	Night	38% 7% 13% 32% 9%	3476
September	1st to 10th	Night	40% 7% 13% 26% 13% 1	1900
	11th to 20th	Night	36% 5% 11% 25% 22% 1	1952
	21st to end	Night	46% 5% 10% 22% 15% 1	2090
October	1st to 10th	Night	51% 8% 12% 20% 9% 1	3754
	11th to 20th	Night	42% 7% 14% 17% 16% 5%	3779
	21st to end	Night	44% 7% 15% 24% 9% 1	4372
November	1st to 10th	Night	48% 5% 11% 18% 15% 3	2531
	11th to 20th	Night	55% 6% 11% 16% 9% 3	2470
	21st to end	Night	56% 5% 15% 17% 6% 1	2655
December	1st to 10th	Night	67% 6% 12% 12% 4	2660
	11th to 20th	Night	65% 5% 10% 12% 7%	3288
	21st to end	Night	69% 6% 8% 12% 6%	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	34% 27% 24% 11% 3	180
	11th to 20th	Day	38% 28% 24% 8% 1 1	180
	21st to end	Day	30% 28% 31% 8% 4	254
February	1st to 10th	Day	40% 29% 26% 6%	270
	11th to 20th	Day	39% 33% 20% 7% 2	391
	21st to end	Day	37% 25% 27% 9% 2	382
March	1st to 10th	Day	33% 25% 28% 12% 3	460
	11th to 20th	Day	38% 23% 25% 10% 3	460
	21st to end	Day	35% 27% 22% 10% 4 2	506
April	1st to 10th	Day	34% 26% 22% 13% 4	460
	11th to 20th	Day	31% 26% 24% 11% 6% 2	452
	21st to end	Day	34% 20% 28% 11% 5% 2	460
May	1st to 10th	Day	38% 19% 25% 11% 6% 3	463
	11th to 20th	Day	21% 20% 32% 18% 7% 3	487
	21st to end	Day	19% 13% 33% 21% 10% 3	529
June	1st to 10th	Day	22% 18% 30% 20% 8% 2	490
	11th to 20th	Day	18% 17% 31% 24% 9% 2	490
	21st to end	Day	22% 19% 31% 19% 7% 2	490
July	1st to 10th	Day	14% 11% 32% 25% 15% 3	485
	11th to 20th	Day	12% 11% 34% 29% 11% 3	484
	21st to end	Day	15% 12% 38% 23% 9% 2	532
August	1st to 10th	Day	11% 14% 31% 23% 15% 5%	486
	11th to 20th	Day	15% 10% 31% 25% 12% 6%	479
	21st to end	Day	16% 12% 32% 23% 10% 7%	539
September	1st to 10th	Day	22% 12% 25% 21% 14% 7%	490
	11th to 20th	Day	16% 10% 27% 20% 17% 11%	490
	21st to end	Day	21% 15% 28% 22% 9% 6%	490
October	1st to 10th	Day	26% 16% 26% 16% 12% 4	490
	11th to 20th	Day	27% 19% 28% 18% 7% 2	490
	21st to end	Day	31% 22% 27% 15% 5%	458
November	1st to 10th	Day	30% 17% 28% 16% 9%	392
	11th to 20th	Day	34% 26% 22% 12% 5% 1	370
	21st to end	Day	37% 22% 30% 10% 1	183
December	1st to 10th	Day	40% 24% 22% 8% 5%	180
	11th to 20th	Day	41% 22% 29% 7% 1	180
	21st to end	Day	37% 28% 26% 7% 2	198
January	1st to 10th	Night	28% 29% 30% 11% 2	760
	11th to 20th	Night	31% 25% 28% 13% 2 1	760
	21st to end	Night	25% 30% 28% 16% 2	836
February	1st to 10th	Night	29% 24% 34% 11% 1	760
	11th to 20th	Night	29% 28% 29% 11% 1	743
	21st to end	Night	36% 23% 25% 13% 3	628
March	1st to 10th	Night	30% 26% 28% 13% 2	760
	11th to 20th	Night	29% 27% 27% 14% 2 1	760
	21st to end	Night	29% 24% 25% 16% 6%	836
April	1st to 10th	Night	28% 23% 28% 17% 3	760
	11th to 20th	Night	27% 22% 26% 17% 7%	751
	21st to end	Night	30% 19% 23% 21% 7% 1	726
May	1st to 10th	Night	30% 15% 25% 21% 8% 1	497
	11th to 20th	Night	20% 15% 29% 28% 7% 1	374
	21st to end	Night	15% 10% 28% 31% 14% 2	411
June	1st to 10th	Night	17% 12% 27% 30% 13% 1	380
	11th to 20th	Night	19% 11% 20% 37% 14%	380
	21st to end	Night	14% 7% 28% 38% 12% 1	380
July	1st to 10th	Night	14% 5% 24% 40% 15% 1	370
	11th to 20th	Night	13% 5% 25% 37% 18% 1	368
	21st to end	Night	13% 7% 22% 43% 14% 1	416
August	1st to 10th	Night	14% 6% 24% 33% 22% 1	451
	11th to 20th	Night	16% 7% 20% 34% 21% 3	701
	21st to end	Night	18% 8% 18% 33% 22% 2	836
September	1st to 10th	Night	17% 10% 25% 29% 17% 2	760
	11th to 20th	Night	18% 9% 22% 25% 21% 5%	760
	21st to end	Night	20% 13% 24% 26% 15% 3	760
October	1st to 10th	Night	27% 14% 20% 22% 14% 2	760
	11th to 20th	Night	24% 17% 27% 23% 8% 1	760
	21st to end	Night	31% 18% 28% 19% 4	836
November	1st to 10th	Night	23% 22% 28% 21% 6%	779
	11th to 20th	Night	29% 24% 25% 15% 6% 1	760
	21st to end	Night	26% 27% 25% 17% 4 1	760
December	1st to 10th	Night	30% 29% 24% 12% 4	760
	11th to 20th	Night	33% 25% 26% 15% 2	760
	21st to end	Night	33% 29% 22% 13% 2	836

Page generated at 2024-11-07 06:55:43. Version ac07

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