Blue Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Sky Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	101302
	Night	18% 8% 2 6% 4 4 4 4 5% 6% 39%	97819
Transparency	Day	25% 47% 6% 13% 8% 1	104840
	Night	25% 47% 6% 12% 9% 1	102357
Seeing	Day	23% 15% 32% 21% 7% 2	15013
	Night	22% 12% 30% 27% 9% 1	23857

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% 3 6% 3 3 3 4 5% 6% 44%	2517
	11th to 20th	Day	18% 6% 3 7% 4 3 4 3 5% 6% 42%	2520
	21st to end	Day	20% 9% 2 6% 5% 4 5% 3 5% 5% 37%	2772
February	1st to 10th	Day	17% 8% 2 6% 4 4 4 4 6% 6% 37%	1728
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 5% 6% 7% 41%	1848
	21st to end	Day	15% 11% 4 6% 6% 3 4 3 5% 6% 38%	1570
March	1st to 10th	Day	26% 13% 2 6% 4 4 4 3 4 5% 29%	3116
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 6% 7% 41%	3369
	21st to end	Day	23% 9% 2 6% 4 4 4 4 5% 6% 34%	3992
April	1st to 10th	Day	19% 9% 2 7% 5% 4 4 3 4 6% 38%	2280
	11th to 20th	Day	17% 6% 3 6% 5% 4 5% 4 6% 7% 37%	2300
	21st to end	Day	20% 7% 3 7% 5% 4 4 3 4 6% 36%	2470
May	1st to 10th	Day	22% 11% 3 8% 5% 5% 4 3 5% 7% 28%	4106
	11th to 20th	Day	23% 7% 2 7% 5% 4 4 3 5% 7% 34%	4164
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 8% 42%	4837
June	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 6% 7% 33%	2660
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 7% 8% 40%	2660
	21st to end	Day	19% 7% 2 7% 5% 5% 5% 5% 5% 8% 33%	2660
July	1st to 10th	Day	13% 7% 2 7% 5% 6% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 10% 40%	4440
	21st to end	Day	18% 6% 3 7% 5% 4 4 4 6% 8% 36%	4814
August	1st to 10th	Day	15% 6% 3 6% 5% 5% 6% 5% 6% 8% 36%	4149
	11th to 20th	Day	16% 8% 3 6% 5% 5% 4 4 6% 7% 36%	4045
	21st to end	Day	8% 4 2 4 4 4 5% 4 7% 10% 47%	4216
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 7% 48%	2261
	11th to 20th	Day	14% 4 2 5% 4 3 3 2 4 5% 53%	2090
	21st to end	Day	16% 6% 2 6% 4 3 4 3 4 6% 46%	2090
October	1st to 10th	Day	21% 7% 3 6% 3 3 4 3 5% 5% 41%	2090
	11th to 20th	Day	15% 5% 2 7% 4 4 4 2 5% 7% 46%	1841
	21st to end	Day	22% 6% 2 5% 4 4 3 4 5% 6% 40%	1881
November	1st to 10th	Day	16% 5% 1 6% 3 4 3 3 4 6% 49%	1734
	11th to 20th	Day	20% 6% 2 4 3 2 3 3 5% 6% 46%	1710
	21st to end	Day	20% 7% 2 5% 3 3 3 3 4 6% 43%	1710
December	1st to 10th	Day	19% 8% 1 7% 4 3 5% 3 4 7% 37%	1544
	11th to 20th	Day	22% 7% 2 5% 3 3 3 3 4 5% 43%	1906
	21st to end	Day	25% 10% 2 5% 5% 4 3 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 5% 6% 45%	4429
	11th to 20th	Night	14% 8% 3 9% 5% 4 4 3 5% 6% 39%	4440
	21st to end	Night	21% 12% 3 6% 5% 4 4 4 5% 5% 32%	4884
February	1st to 10th	Night	15% 10% 2 8% 4 4 4 4 5% 5% 38%	2449
	11th to 20th	Night	14% 9% 2 7% 5% 4 4 4 5% 6% 40%	2218
	21st to end	Night	17% 10% 2 5% 4 3 4 3 5% 6% 42%	1884
March	1st to 10th	Night	29% 10% 2 5% 3 3 3 3 3 5% 34%	3724
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 6% 37%	3589
	21st to end	Night	25% 8% 3 5% 4 4 4 4 7% 6% 32%	3637
April	1st to 10th	Night	20% 8% 2 6% 4 4 5% 3 5% 6% 37%	1900
	11th to 20th	Night	20% 8% 2 6% 3 4 3 4 4 6% 40%	1829
	21st to end	Night	21% 7% 3 6% 4 3 4 3 5% 6% 37%	1710
May	1st to 10th	Night	24% 12% 3 7% 4 4 4 4 5% 7% 25%	2834
	11th to 20th	Night	19% 9% 3 8% 5% 6% 5% 4 6% 8% 26%	2758
	21st to end	Night	17% 7% 2 7% 4 4 4 3 5% 7% 39%	2738
June	1st to 10th	Night	19% 8% 3 6% 5% 5% 4 3 5% 6% 35%	1520
	11th to 20th	Night	20% 6% 2 5% 4 4 5% 4 7% 8% 36%	1520
	21st to end	Night	19% 8% 1 7% 5% 4 5% 3 5% 8% 35%	1520
July	1st to 10th	Night	12% 6% 2 7% 5% 6% 6% 4 8% 9% 34%	2520
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	2520
	21st to end	Night	16% 6% 2 6% 4 5% 5% 4 5% 7% 40%	2739
August	1st to 10th	Night	21% 6% 2 7% 5% 5% 5% 4 5% 6% 34%	2748
	11th to 20th	Night	14% 8% 2 8% 4 6% 5% 4 7% 8% 33%	2782
	21st to end	Night	11% 3 1 5% 4 3 4 4 6% 7% 51%	3437
September	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 7% 49%	1918
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 5% 7% 51%	2090
	21st to end	Night	16% 5% 2 5% 4 3 3 3 5% 7% 46%	2090
October	1st to 10th	Night	20% 6% 3 6% 4 4 3 4 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 6% 3 4 5% 3 5% 6% 44%	2333
	21st to end	Night	23% 6% 2 6% 3 3 3 4 5% 5% 40%	2717
November	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 5% 6% 47%	2505
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 3 6% 46%	2470
	21st to end	Night	17% 7% 3 5% 4 4 4 3 4 7% 44%	2470
December	1st to 10th	Night	20% 10% 2 6% 4 3 4 3 5% 5% 38%	2635
	11th to 20th	Night	20% 8% 2 5% 3 3 4 4 5% 6% 40%	3288
	21st to end	Night	23% 9% 2 7% 4 4 4 3 4 5% 35%	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	25% 38% 6% 22% 8% 1	2518
	11th to 20th	Day	27% 38% 6% 19% 8% 1	2520
	21st to end	Day	36% 41% 5% 13% 5%	2772
February	1st to 10th	Day	17% 65% 5% 7% 5% 1	1728
	11th to 20th	Day	17% 64% 2 6% 9% 1	1857
	21st to end	Day	15% 64% 4 8% 7% 2	1570
March	1st to 10th	Day	41% 41% 3 6% 8% 1	3119
	11th to 20th	Day	24% 42% 9% 12% 9% 4	3369
	21st to end	Day	35% 40% 3 8% 11% 4	3992
April	1st to 10th	Day	20% 64% 2 7% 6% 2	2280
	11th to 20th	Day	16% 66% 4 8% 6% 1	2300
	21st to end	Day	19% 64% 3 6% 7%	2470
May	1st to 10th	Day	36% 43% 6% 9% 5%	4120
	11th to 20th	Day	35% 41% 11% 10% 4	4168
	21st to end	Day	23% 42% 10% 13% 12%	4837
June	1st to 10th	Day	16% 65% 3 10% 6%	2660
	11th to 20th	Day	13% 65% 4 9% 8% 1	2660
	21st to end	Day	17% 64% 3 9% 6%	2660
July	1st to 10th	Day	28% 38% 7% 16% 10% 1	4370
	11th to 20th	Day	23% 38% 11% 18% 9%	4365
	21st to end	Day	32% 37% 8% 14% 9% 1	4661
August	1st to 10th	Day	30% 39% 9% 16% 5%	4135
	11th to 20th	Day	32% 39% 10% 15% 4	4084
	21st to end	Day	18% 40% 10% 19% 12% 1	4216
September	1st to 10th	Day	13% 61% 6% 12% 8% 1	2261
	11th to 20th	Day	17% 59% 3 11% 10% 1	2090
	21st to end	Day	16% 60% 3 13% 8% 1	2090
October	1st to 10th	Day	28% 38% 5% 16% 12% 1	3480
	11th to 20th	Day	17% 37% 5% 21% 18% 2	3031
	21st to end	Day	19% 37% 4 23% 15% 2	3081
November	1st to 10th	Day	18% 56% 6% 12% 7% 1	1734
	11th to 20th	Day	17% 59% 3 8% 12% 2	1710
	21st to end	Day	16% 59% 6% 8% 9% 1	1710
December	1st to 10th	Day	20% 60% 6% 7% 7%	1544
	11th to 20th	Day	25% 48% 5% 9% 9% 4	1906
	21st to end	Day	35% 38% 6% 12% 7% 2	2772
January	1st to 10th	Night	25% 36% 9% 20% 7% 3	4429
	11th to 20th	Night	26% 38% 7% 17% 9% 3	4440
	21st to end	Night	39% 40% 5% 11% 4 1	4884
February	1st to 10th	Night	17% 65% 5% 6% 5% 2	2449
	11th to 20th	Night	16% 63% 3 8% 8% 2	2220
	21st to end	Night	16% 64% 3 9% 6% 3	1884
March	1st to 10th	Night	37% 41% 6% 9% 5% 2	3734
	11th to 20th	Night	27% 42% 6% 10% 13% 2	3590
	21st to end	Night	34% 40% 3 7% 10% 5%	3637
April	1st to 10th	Night	19% 65% 2 6% 7% 1	1900
	11th to 20th	Night	16% 65% 3 7% 7% 1	1829
	21st to end	Night	17% 64% 4 7% 8% 1	1710
May	1st to 10th	Night	38% 43% 7% 8% 4	2840
	11th to 20th	Night	34% 42% 12% 9% 3	2759
	21st to end	Night	27% 41% 11% 12% 9% 1	2739
June	1st to 10th	Night	17% 65% 3 9% 6%	1520
	11th to 20th	Night	14% 66% 4 10% 6% 1	1520
	21st to end	Night	16% 63% 3 10% 6%	1520
July	1st to 10th	Night	27% 39% 9% 15% 10%	2480
	11th to 20th	Night	25% 39% 10% 15% 11%	2472
	21st to end	Night	30% 36% 7% 16% 10%	2651
August	1st to 10th	Night	36% 39% 7% 11% 7% 1	2738
	11th to 20th	Night	31% 40% 10% 12% 7%	2809
	21st to end	Night	17% 39% 10% 18% 16%	3437
September	1st to 10th	Night	14% 60% 6% 12% 8% 1	1918
	11th to 20th	Night	17% 59% 3 11% 10% 1	2090
	21st to end	Night	16% 59% 3 13% 7% 1	2090
October	1st to 10th	Night	27% 38% 9% 16% 8% 1	3480
	11th to 20th	Night	13% 38% 6% 18% 22% 3	3869
	21st to end	Night	20% 37% 6% 22% 13% 2	4467
November	1st to 10th	Night	18% 57% 4 13% 7% 1	2505
	11th to 20th	Night	17% 59% 3 9% 10% 1	2470
	21st to end	Night	15% 59% 6% 11% 7% 1	2470
December	1st to 10th	Night	20% 60% 4 10% 6%	2635
	11th to 20th	Night	24% 49% 5% 9% 13% 1	3288
	21st to end	Night	35% 37% 4 16% 8%	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	26% 24% 29% 18% 3	180
	11th to 20th	Day	27% 24% 33% 12% 3 1	180
	21st to end	Day	18% 23% 38% 16% 5% 1	198
February	1st to 10th	Day	28% 21% 35% 14% 2	350
	11th to 20th	Day	28% 24% 32% 14% 3	456
	21st to end	Day	26% 21% 32% 17% 3	382
March	1st to 10th	Day	29% 21% 33% 15% 2	460
	11th to 20th	Day	26% 25% 33% 12% 4	460
	21st to end	Day	29% 21% 32% 12% 5% 2	506
April	1st to 10th	Day	29% 22% 30% 12% 6% 2	460
	11th to 20th	Day	29% 21% 27% 16% 6% 1	452
	21st to end	Day	28% 15% 35% 14% 6% 2	460
May	1st to 10th	Day	31% 19% 31% 14% 4 1	463
	11th to 20th	Day	21% 16% 39% 18% 6% 1	487
	21st to end	Day	20% 13% 35% 24% 7% 1	529
June	1st to 10th	Day	23% 12% 37% 19% 8% 1	490
	11th to 20th	Day	16% 16% 37% 23% 7% 1	490
	21st to end	Day	20% 12% 36% 24% 6% 2	490
July	1st to 10th	Day	17% 7% 36% 27% 11% 3	485
	11th to 20th	Day	16% 6% 31% 34% 11% 2	484
	21st to end	Day	13% 7% 36% 30% 12% 2	532
August	1st to 10th	Day	15% 7% 29% 31% 14% 4	486
	11th to 20th	Day	18% 7% 32% 27% 11% 6%	479
	21st to end	Day	14% 7% 33% 26% 15% 5%	539
September	1st to 10th	Day	18% 8% 29% 28% 11% 7%	490
	11th to 20th	Day	20% 7% 31% 21% 13% 8%	490
	21st to end	Day	19% 10% 32% 26% 8% 6%	490
October	1st to 10th	Day	25% 11% 27% 22% 11% 4	490
	11th to 20th	Day	20% 14% 29% 24% 11% 3	418
	21st to end	Day	26% 14% 30% 23% 6%	429
November	1st to 10th	Day	18% 18% 31% 22% 10% 2	386
	11th to 20th	Day	29% 21% 25% 19% 5% 1	370
	21st to end	Day	27% 18% 27% 20% 6% 2	370
December	1st to 10th	Day	21% 23% 33% 17% 5% 1	204
	11th to 20th	Day	30% 21% 33% 13% 3	180
	21st to end	Day	29% 21% 29% 16% 5% 1	198
January	1st to 10th	Night	21% 19% 35% 22% 3	760
	11th to 20th	Night	25% 17% 35% 20% 3	760
	21st to end	Night	21% 19% 37% 19% 4	836
February	1st to 10th	Night	22% 17% 36% 23% 2	760
	11th to 20th	Night	21% 19% 39% 17% 3	743
	21st to end	Night	22% 18% 33% 23% 3	628
March	1st to 10th	Night	27% 17% 34% 19% 2	760
	11th to 20th	Night	23% 19% 33% 20% 6%	760
	21st to end	Night	25% 15% 33% 21% 6%	836
April	1st to 10th	Night	25% 16% 32% 22% 4	760
	11th to 20th	Night	24% 13% 32% 23% 8%	705
	21st to end	Night	25% 13% 32% 25% 5%	570
May	1st to 10th	Night	25% 9% 30% 26% 9% 1	570
	11th to 20th	Night	23% 8% 31% 30% 8% 1	533
	21st to end	Night	23% 8% 26% 33% 10% 1	411
June	1st to 10th	Night	23% 5% 27% 31% 12% 1	380
	11th to 20th	Night	23% 4 25% 34% 13% 1	380
	21st to end	Night	22% 3 26% 38% 12%	380
July	1st to 10th	Night	17% 3 19% 44% 16% 1	370
	11th to 20th	Night	14% 3 18% 43% 21% 1	368
	21st to end	Night	15% 3 18% 46% 17% 1	416
August	1st to 10th	Night	15% 3 20% 41% 21% 1	516
	11th to 20th	Night	16% 5% 22% 37% 19% 2	562
	21st to end	Night	17% 3 22% 35% 22% 2	810
September	1st to 10th	Night	15% 4 26% 34% 21% 1	760
	11th to 20th	Night	17% 6% 25% 28% 21% 3	760
	21st to end	Night	19% 4 27% 29% 18% 2	760
October	1st to 10th	Night	23% 8% 24% 27% 15% 2	760
	11th to 20th	Night	18% 10% 29% 28% 13% 1	760
	21st to end	Night	26% 12% 30% 24% 8%	836
November	1st to 10th	Night	19% 12% 32% 29% 8% 1	771
	11th to 20th	Night	22% 16% 31% 23% 7% 1	760
	21st to end	Night	20% 19% 30% 26% 4	760
December	1st to 10th	Night	25% 16% 35% 20% 5%	760
	11th to 20th	Night	23% 19% 31% 23% 3 1	760
	21st to end	Night	25% 16% 34% 22% 3	836

Page generated at 2024-11-05 08:15:50. Version ac07

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