Starry Night Astronomy & Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starry Night Astronomy & Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101586
	Night	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	97942
Transparency	Day	47% 9% 13% 19% 10% 1	105098
	Night	47% 10% 14% 19% 10% 1	102519
Seeing	Day	23% 18% 29% 18% 9% 2	14874
	Night	20% 15% 29% 27% 9% 1	23827

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	10% 12% 4 10% 7% 6% 5% 4 6% 7% 29%	2517
	11th to 20th	Day	14% 11% 5% 11% 9% 6% 5% 3 6% 6% 24%	2520
	21st to end	Day	13% 15% 8% 11% 6% 5% 5% 4 6% 6% 22%	2772
February	1st to 10th	Day	12% 11% 4 9% 7% 6% 6% 4 7% 7% 28%	1676
	11th to 20th	Day	9% 10% 5% 13% 7% 6% 6% 5% 6% 7% 28%	1848
	21st to end	Day	14% 12% 4 8% 7% 6% 5% 4 4 6% 30%	1570
March	1st to 10th	Day	19% 15% 5% 10% 5% 5% 4 4 4 4 25%	3116
	11th to 20th	Day	12% 11% 6% 10% 6% 5% 6% 4 6% 6% 28%	3244
	21st to end	Day	16% 12% 4 10% 5% 4 4 3 4 5% 32%	4166
April	1st to 10th	Day	18% 11% 3 9% 5% 4 4 3 4 6% 33%	2280
	11th to 20th	Day	16% 9% 2 6% 6% 5% 4 4 6% 7% 34%	2254
	21st to end	Day	20% 10% 4 8% 5% 4 4 4 5% 7% 30%	2451
May	1st to 10th	Day	21% 11% 4 8% 6% 5% 4 3 5% 5% 27%	4124
	11th to 20th	Day	17% 7% 3 8% 6% 5% 5% 4 7% 7% 30%	4398
	21st to end	Day	13% 4 2 5% 5% 4 5% 4 8% 9% 41%	4837
June	1st to 10th	Day	18% 8% 2 8% 5% 5% 5% 4 7% 8% 29%	2660
	11th to 20th	Day	17% 6% 2 6% 5% 5% 6% 4 5% 9% 35%	2660
	21st to end	Day	18% 8% 3 7% 5% 6% 6% 4 7% 8% 28%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 10% 37%	4440
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 6% 9% 12% 34%	4440
	21st to end	Day	13% 8% 2 7% 5% 5% 5% 5% 6% 8% 36%	4814
August	1st to 10th	Day	13% 6% 3 7% 6% 6% 5% 5% 7% 9% 33%	4347
	11th to 20th	Day	13% 9% 3 8% 5% 6% 5% 4 6% 9% 32%	3920
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 7% 10% 41%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 5% 6% 48%	2146
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 42%	2090
October	1st to 10th	Day	19% 7% 2 7% 6% 4 5% 3 5% 6% 35%	2052
	11th to 20th	Day	13% 8% 4 8% 6% 4 4 3 5% 7% 37%	1834
	21st to end	Day	22% 11% 3 7% 5% 5% 4 4 5% 6% 30%	1881
November	1st to 10th	Day	11% 9% 2 7% 5% 4 5% 4 7% 7% 38%	1761
	11th to 20th	Day	15% 10% 3 7% 5% 5% 5% 5% 6% 6% 35%	1800
	21st to end	Day	16% 11% 4 10% 4 5% 4 4 6% 6% 30%	1650
December	1st to 10th	Day	15% 14% 4 11% 6% 6% 5% 3 7% 6% 22%	1520
	11th to 20th	Day	19% 12% 3 8% 5% 4 5% 3 5% 7% 29%	1906
	21st to end	Day	19% 14% 4 9% 6% 6% 5% 4 5% 5% 23%	2772
January	1st to 10th	Night	14% 10% 3 8% 5% 5% 5% 4 7% 7% 32%	4429
	11th to 20th	Night	10% 11% 5% 13% 7% 6% 6% 5% 7% 7% 24%	4440
	21st to end	Night	16% 12% 6% 13% 6% 6% 5% 4 5% 4 23%	4884
February	1st to 10th	Night	10% 11% 4 10% 7% 5% 5% 5% 7% 8% 27%	2497
	11th to 20th	Night	9% 9% 4 10% 7% 7% 7% 5% 7% 7% 29%	2218
	21st to end	Night	12% 10% 3 9% 6% 5% 5% 4 7% 7% 32%	1884
March	1st to 10th	Night	21% 13% 2 7% 5% 5% 4 4 5% 5% 28%	3724
	11th to 20th	Night	11% 10% 4 10% 6% 6% 6% 4 5% 6% 31%	3699
	21st to end	Night	16% 12% 3 9% 5% 4 4 3 4 4 35%	3466
April	1st to 10th	Night	16% 8% 3 7% 5% 6% 5% 4 6% 6% 34%	1900
	11th to 20th	Night	16% 9% 3 7% 5% 5% 5% 4 6% 6% 35%	1875
	21st to end	Night	19% 9% 3 6% 4 4 4 3 7% 6% 34%	1729
May	1st to 10th	Night	24% 11% 3 6% 5% 4 5% 4 6% 7% 25%	2834
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 5% 5% 30%	2505
	21st to end	Night	12% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	2738
June	1st to 10th	Night	17% 6% 2 6% 4 5% 6% 5% 6% 9% 34%	1520
	11th to 20th	Night	18% 6% 2 5% 3 4 4 5% 5% 7% 41%	1520
	21st to end	Night	17% 6% 2 5% 5% 5% 6% 4 6% 8% 36%	1520
July	1st to 10th	Night	10% 6% 2 7% 6% 6% 5% 5% 7% 9% 37%	2520
	11th to 20th	Night	13% 4 2 7% 5% 4 4 4 6% 8% 43%	2520
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 7% 6% 41%	2739
August	1st to 10th	Night	17% 8% 2 8% 5% 5% 4 4 5% 6% 36%	2538
	11th to 20th	Night	14% 8% 2 6% 5% 4 4 4 5% 8% 39%	2903
	21st to end	Night	9% 4 2 5% 4 3 4 4 7% 10% 48%	3476
September	1st to 10th	Night	12% 5% 1 5% 4 4 4 4 6% 8% 48%	1900
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 5% 6% 51%	2033
	21st to end	Night	13% 6% 2 5% 3 4 5% 4 6% 7% 46%	2090
October	1st to 10th	Night	18% 7% 3 8% 4 4 4 3 6% 7% 35%	2125
	11th to 20th	Night	15% 7% 3 6% 4 5% 5% 4 6% 7% 37%	2346
	21st to end	Night	20% 9% 3 7% 4 4 5% 4 6% 8% 30%	2717
November	1st to 10th	Night	12% 8% 3 9% 6% 5% 5% 3 6% 6% 38%	2537
	11th to 20th	Night	14% 7% 3 7% 5% 5% 6% 5% 6% 7% 36%	2600
	21st to end	Night	13% 10% 3 8% 5% 4 5% 4 6% 7% 33%	2684
December	1st to 10th	Night	15% 11% 4 11% 7% 6% 6% 4 6% 7% 22%	2660
	11th to 20th	Night	17% 9% 5% 8% 6% 5% 4 4 6% 7% 29%	3288
	21st to end	Night	19% 13% 5% 12% 6% 5% 4 4 5% 6% 22%	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	48% 12% 14% 20% 4	2518
	11th to 20th	Day	60% 10% 10% 14% 4 1	2520
	21st to end	Day	60% 9% 11% 12% 6% 1	2772
February	1st to 10th	Day	54% 9% 14% 15% 8% 1	1676
	11th to 20th	Day	53% 10% 12% 16% 8% 2	1857
	21st to end	Day	53% 8% 12% 15% 11% 1	1570
March	1st to 10th	Day	61% 9% 9% 10% 10% 2	3119
	11th to 20th	Day	50% 14% 13% 12% 9% 2	3244
	21st to end	Day	51% 9% 7% 12% 17% 3	4166
April	1st to 10th	Day	51% 6% 10% 20% 10% 3	2280
	11th to 20th	Day	47% 9% 11% 17% 14% 1	2254
	21st to end	Day	53% 8% 9% 16% 11% 2	2451
May	1st to 10th	Day	56% 9% 10% 17% 7% 1	4138
	11th to 20th	Day	47% 14% 15% 15% 8% 1	4402
	21st to end	Day	35% 12% 15% 25% 11% 1	4837
June	1st to 10th	Day	50% 10% 15% 15% 9%	2660
	11th to 20th	Day	45% 8% 13% 19% 13% 2	2660
	21st to end	Day	52% 10% 11% 17% 9% 1	2660
July	1st to 10th	Day	39% 8% 16% 24% 13%	4370
	11th to 20th	Day	38% 10% 19% 25% 8%	4365
	21st to end	Day	43% 9% 15% 22% 12% 1	4661
August	1st to 10th	Day	44% 10% 16% 20% 10%	4333
	11th to 20th	Day	46% 9% 16% 24% 5%	3959
	21st to end	Day	34% 8% 18% 27% 12% 1	4180
September	1st to 10th	Day	35% 9% 15% 28% 13% 1	2280
	11th to 20th	Day	36% 7% 11% 28% 17% 1	2146
	21st to end	Day	38% 9% 15% 25% 11% 2	2090
October	1st to 10th	Day	43% 7% 12% 21% 16% 1	3416
	11th to 20th	Day	41% 5% 14% 25% 14% 1	3024
	21st to end	Day	40% 7% 10% 29% 13% 1	3081
November	1st to 10th	Day	41% 9% 15% 21% 13%	1761
	11th to 20th	Day	46% 9% 13% 16% 14% 2	1800
	21st to end	Day	53% 10% 16% 16% 5% 1	1650
December	1st to 10th	Day	59% 11% 13% 12% 5% 1	1520
	11th to 20th	Day	55% 8% 11% 16% 8% 1	1906
	21st to end	Day	63% 9% 9% 12% 8%	2772
January	1st to 10th	Night	47% 13% 16% 16% 7% 1	4429
	11th to 20th	Night	56% 15% 12% 12% 5% 1	4440
	21st to end	Night	60% 11% 9% 12% 6% 1	4884
February	1st to 10th	Night	51% 12% 14% 15% 8% 1	2497
	11th to 20th	Night	48% 11% 13% 17% 10% 1	2220
	21st to end	Night	48% 10% 14% 18% 8% 3	1884
March	1st to 10th	Night	54% 9% 12% 15% 8% 1	3734
	11th to 20th	Night	48% 12% 12% 16% 10% 1	3700
	21st to end	Night	50% 8% 8% 13% 17% 3	3466
April	1st to 10th	Night	49% 8% 11% 19% 12% 2	1900
	11th to 20th	Night	47% 8% 11% 21% 12% 1	1875
	21st to end	Night	49% 9% 10% 17% 12% 3	1729
May	1st to 10th	Night	53% 11% 11% 15% 8% 1	2840
	11th to 20th	Night	50% 14% 12% 15% 8% 1	2506
	21st to end	Night	37% 13% 17% 19% 13% 1	2739
June	1st to 10th	Night	43% 13% 15% 17% 11%	1520
	11th to 20th	Night	42% 10% 15% 18% 12% 2	1520
	21st to end	Night	44% 11% 13% 22% 9% 1	1520
July	1st to 10th	Night	39% 9% 18% 20% 13%	2480
	11th to 20th	Night	37% 9% 17% 26% 11%	2472
	21st to end	Night	40% 7% 13% 29% 10% 1	2651
August	1st to 10th	Night	48% 9% 13% 21% 9%	2528
	11th to 20th	Night	42% 7% 20% 22% 9%	2930
	21st to end	Night	27% 8% 21% 32% 12% 1	3476
September	1st to 10th	Night	33% 9% 16% 28% 13% 1	1900
	11th to 20th	Night	33% 7% 14% 29% 16% 1	2033
	21st to end	Night	36% 9% 14% 24% 15% 2	2090
October	1st to 10th	Night	46% 7% 13% 22% 10% 1	3541
	11th to 20th	Night	35% 6% 17% 25% 15% 3	3895
	21st to end	Night	42% 7% 12% 29% 10% 1	4467
November	1st to 10th	Night	45% 9% 14% 21% 10% 2	2537
	11th to 20th	Night	44% 10% 14% 19% 12% 2	2600
	21st to end	Night	47% 11% 17% 18% 7%	2684
December	1st to 10th	Night	59% 11% 13% 13% 4	2660
	11th to 20th	Night	54% 10% 14% 13% 10% 1	3288
	21st to end	Night	60% 11% 11% 14% 4	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	24% 24% 28% 19% 4 1	180
	11th to 20th	Day	31% 25% 27% 13% 3 1	180
	21st to end	Day	19% 26% 33% 17% 5% 1	198
February	1st to 10th	Day	31% 30% 29% 8% 2	261
	11th to 20th	Day	27% 26% 31% 14% 2	456
	21st to end	Day	29% 23% 30% 16% 3	382
March	1st to 10th	Day	28% 20% 35% 15% 3	460
	11th to 20th	Day	32% 25% 27% 12% 4 1	460
	21st to end	Day	32% 24% 24% 12% 6% 3	506
April	1st to 10th	Day	35% 18% 27% 13% 5% 1	460
	11th to 20th	Day	34% 23% 23% 11% 8% 1	452
	21st to end	Day	30% 23% 27% 11% 6% 2	460
May	1st to 10th	Day	32% 22% 24% 14% 6% 2	463
	11th to 20th	Day	18% 21% 34% 21% 5% 2	487
	21st to end	Day	19% 16% 33% 22% 8% 1	529
June	1st to 10th	Day	22% 18% 37% 15% 8% 1	490
	11th to 20th	Day	19% 17% 33% 20% 10% 1	490
	21st to end	Day	21% 19% 32% 18% 8% 2	490
July	1st to 10th	Day	16% 13% 33% 25% 11% 3	485
	11th to 20th	Day	13% 10% 37% 26% 12% 3	484
	21st to end	Day	14% 10% 40% 24% 10% 2	532
August	1st to 10th	Day	13% 12% 31% 26% 16% 3	486
	11th to 20th	Day	16% 12% 27% 25% 13% 7%	479
	21st to end	Day	11% 16% 27% 26% 14% 6%	539
September	1st to 10th	Day	14% 14% 27% 23% 15% 6%	490
	11th to 20th	Day	13% 14% 26% 17% 19% 11%	490
	21st to end	Day	18% 12% 29% 23% 12% 7%	490
October	1st to 10th	Day	24% 14% 24% 19% 13% 4	490
	11th to 20th	Day	21% 20% 24% 22% 12% 2	455
	21st to end	Day	33% 17% 24% 19% 7% 1	429
November	1st to 10th	Day	23% 19% 23% 23% 11% 1	395
	11th to 20th	Day	28% 20% 28% 17% 8% 1	400
	21st to end	Day	32% 19% 27% 16% 4	268
December	1st to 10th	Day	28% 21% 33% 14% 4 1	180
	11th to 20th	Day	31% 24% 32% 11% 3	180
	21st to end	Day	33% 19% 28% 17% 3	198
January	1st to 10th	Night	20% 19% 37% 21% 3	760
	11th to 20th	Night	23% 20% 32% 22% 3	760
	21st to end	Night	16% 19% 36% 25% 4	836
February	1st to 10th	Night	18% 22% 38% 21% 2	760
	11th to 20th	Night	19% 21% 36% 21% 3	743
	21st to end	Night	21% 20% 33% 22% 3	628
March	1st to 10th	Night	22% 16% 35% 24% 3	760
	11th to 20th	Night	23% 20% 30% 21% 5%	760
	21st to end	Night	26% 17% 29% 21% 7%	836
April	1st to 10th	Night	23% 21% 29% 23% 5%	760
	11th to 20th	Night	21% 24% 27% 21% 7%	751
	21st to end	Night	27% 17% 28% 21% 6%	589
May	1st to 10th	Night	27% 14% 29% 22% 8%	570
	11th to 20th	Night	15% 15% 31% 32% 5% 1	384
	21st to end	Night	15% 11% 27% 35% 12%	411
June	1st to 10th	Night	17% 10% 31% 32% 10%	380
	11th to 20th	Night	18% 10% 24% 36% 12%	380
	21st to end	Night	17% 7% 27% 36% 12% 1	380
July	1st to 10th	Night	15% 5% 19% 45% 13% 2	370
	11th to 20th	Night	15% 3 25% 38% 18% 1	368
	21st to end	Night	13% 6% 23% 41% 17%	416
August	1st to 10th	Night	15% 4 18% 42% 21%	397
	11th to 20th	Night	15% 7% 23% 35% 17% 3	631
	21st to end	Night	13% 7% 21% 36% 22% 2	836
September	1st to 10th	Night	14% 8% 26% 32% 19% 2	760
	11th to 20th	Night	14% 9% 20% 29% 22% 5%	760
	21st to end	Night	16% 9% 25% 30% 19% 2	760
October	1st to 10th	Night	24% 12% 19% 29% 15% 2	760
	11th to 20th	Night	20% 14% 24% 30% 11% 1	760
	21st to end	Night	25% 16% 27% 25% 6%	836
November	1st to 10th	Night	19% 17% 28% 27% 8%	780
	11th to 20th	Night	19% 18% 28% 25% 8% 1	800
	21st to end	Night	21% 20% 28% 25% 5%	789
December	1st to 10th	Night	24% 19% 35% 19% 4	760
	11th to 20th	Night	23% 20% 29% 23% 4	760
	21st to end	Night	26% 19% 34% 19% 3	836

Page generated at 2024-11-25 00:22:18. Version ac07

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