Big Blue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Blue 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	12% 7% 3 6% 4 4 4 4 6% 7% 43%	101236
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	97868
Transparency	Day	38% 6% 12% 24% 17% 2	104871
	Night	39% 5% 11% 24% 18% 3	102373
Seeing	Day	17% 13% 28% 25% 13% 3	15638
	Night	18% 13% 30% 27% 12% 1	25442

By Month

Percentage Cloud Cover SE (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% 12% 6% 11% 5% 5% 5% 3 5% 6% 32%	2527
	11th to 20th	Day	12% 10% 4 8% 5% 7% 5% 4 5% 6% 35%	2832
	21st to end	Day	20% 15% 6% 7% 4 4 3 2 4 5% 30%	3135
February	1st to 10th	Day	15% 11% 4 9% 4 4 4 4 4 6% 34%	1710
	11th to 20th	Day	13% 10% 4 7% 5% 4 4 3 5% 6% 39%	1779
	21st to end	Day	14% 11% 3 9% 4 4 4 3 5% 5% 39%	1570
March	1st to 10th	Day	19% 10% 3 6% 4 3 3 2 3 4 42%	3275
	11th to 20th	Day	12% 7% 2 5% 3 3 4 3 5% 6% 49%	3480
	21st to end	Day	16% 9% 3 7% 5% 5% 4 3 6% 7% 33%	3958
April	1st to 10th	Day	11% 7% 3 6% 6% 4 5% 4 4 7% 43%	2280
	11th to 20th	Day	16% 9% 3 5% 4 3 4 4 4 5% 42%	2254
	21st to end	Day	19% 8% 2 7% 5% 4 5% 3 5% 7% 36%	2280
May	1st to 10th	Day	14% 9% 3 7% 5% 5% 5% 4 7% 9% 33%	3829
	11th to 20th	Day	17% 7% 3 7% 4 4 4 3 5% 7% 38%	4387
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 45%	4824
June	1st to 10th	Day	8% 5% 2 4 4 4 5% 4 7% 9% 47%	2660
	11th to 20th	Day	7% 4 2 6% 4 5% 4 3 6% 10% 50%	2660
	21st to end	Day	4 4 2 4 4 5% 6% 5% 7% 11% 48%	2660
July	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 7% 9% 40%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 7% 11% 50%	4415
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	4465
August	1st to 10th	Day	5% 4 1 5% 4 4 5% 5% 7% 10% 50%	4104
	11th to 20th	Day	13% 5% 2 6% 3 5% 5% 4 6% 8% 43%	3794
	21st to end	Day	6% 5% 2 5% 4 4 5% 4 6% 8% 52%	4180
September	1st to 10th	Day	12% 5% 1 5% 4 3 3 4 6% 8% 49%	2280
	11th to 20th	Day	10% 5% 2 5% 4 4 3 4 5% 7% 51%	2280
	21st to end	Day	9% 5% 2 5% 3 3 5% 4 5% 7% 52%	2029
October	1st to 10th	Day	9% 4 1 6% 4 3 3 3 4 7% 55%	1900
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 5% 52%	1900
	21st to end	Day	17% 8% 3 5% 3 4 3 2 4 5% 47%	2090
November	1st to 10th	Day	12% 5% 2 4 3 3 2 3 5% 6% 55%	1746
	11th to 20th	Day	13% 7% 3 6% 4 3 3 2 6% 6% 46%	1710
	21st to end	Day	16% 10% 2 7% 3 3 3 3 4 5% 44%	1582
December	1st to 10th	Day	13% 12% 3 6% 4 4 4 3 4 6% 41%	1520
	11th to 20th	Day	14% 11% 4 8% 4 3 3 3 4 6% 41%	1918
	21st to end	Day	18% 14% 5% 11% 5% 5% 5% 3 5% 5% 25%	2783
January	1st to 10th	Night	13% 11% 7% 10% 5% 4 4 3 4 5% 36%	4428
	11th to 20th	Night	13% 9% 4 12% 6% 7% 6% 4 3 5% 31%	4121
	21st to end	Night	23% 16% 5% 7% 4 4 3 3 3 4 29%	4532
February	1st to 10th	Night	14% 11% 5% 8% 5% 4 4 4 5% 6% 35%	2470
	11th to 20th	Night	15% 8% 3 8% 4 3 4 3 5% 6% 41%	2288
	21st to end	Night	16% 9% 4 8% 4 4 3 3 4 5% 40%	1884
March	1st to 10th	Night	21% 11% 2 4 4 3 2 2 3 4 44%	3607
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	3480
	21st to end	Night	19% 11% 3 8% 5% 4 4 4 5% 5% 32%	3672
April	1st to 10th	Night	10% 7% 3 6% 4 4 3 3 5% 7% 48%	1900
	11th to 20th	Night	14% 7% 2 7% 4 3 4 3 5% 7% 44%	1875
	21st to end	Night	18% 6% 2 6% 4 3 3 4 5% 7% 41%	1900
May	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 5% 8% 40%	3052
	11th to 20th	Night	13% 8% 2 7% 5% 5% 4 3 4 7% 42%	2516
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 7% 9% 43%	2743
June	1st to 10th	Night	8% 6% 2 4 4 4 4 4 6% 8% 50%	1520
	11th to 20th	Night	6% 4 2 4 3 4 4 3 5% 10% 53%	1520
	21st to end	Night	5% 3 2 4 4 4 3 5% 7% 9% 55%	1520
July	1st to 10th	Night	12% 5% 2 7% 4 4 4 3 4 9% 45%	2530
	11th to 20th	Night	5% 3 1 4 3 4 3 4 5% 10% 58%	2551
	21st to end	Night	6% 5% 2 7% 5% 4 5% 4 7% 10% 44%	3098
August	1st to 10th	Night	5% 3 1 4 3 4 3 3 5% 8% 60%	2836
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 5% 7% 50%	3035
	21st to end	Night	6% 3 1 4 4 3 3 3 6% 9% 58%	3476
September	1st to 10th	Night	12% 6% 2 4 4 3 3 3 5% 6% 55%	1900
	11th to 20th	Night	8% 6% 2 5% 4 4 3 3 6% 6% 52%	1900
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 5% 56%	2127
October	1st to 10th	Night	8% 3 1 5% 3 3 4 2 5% 6% 60%	2280
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 53%	2280
	21st to end	Night	18% 6% 2 4 3 3 4 3 4 5% 48%	2508
November	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 4 5% 56%	2430
	11th to 20th	Night	13% 7% 3 5% 4 2 3 3 4 6% 50%	2470
	21st to end	Night	17% 10% 2 6% 3 3 3 3 5% 5% 44%	2596
December	1st to 10th	Night	13% 11% 4 6% 5% 4 4 4 5% 6% 36%	2660
	11th to 20th	Night	16% 10% 3 7% 4 4 3 3 5% 5% 40%	3279
	21st to end	Night	18% 19% 6% 10% 5% 4 4 3 3 4 23%	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	51% 5% 12% 21% 10% 1	2528
	11th to 20th	Day	48% 5% 13% 22% 12%	2832
	21st to end	Day	56% 4 10% 15% 13% 1	3135
February	1st to 10th	Day	51% 5% 10% 21% 11% 1	1710
	11th to 20th	Day	46% 5% 11% 19% 14% 4	1788
	21st to end	Day	46% 4 8% 23% 14% 4	1570
March	1st to 10th	Day	47% 4 7% 20% 18% 4	3275
	11th to 20th	Day	35% 5% 11% 26% 16% 5%	3480
	21st to end	Day	48% 8% 15% 21% 8% 1	3958
April	1st to 10th	Day	40% 5% 11% 24% 18% 1	2280
	11th to 20th	Day	44% 5% 10% 23% 15% 3	2254
	21st to end	Day	48% 6% 11% 21% 13% 1	2280
May	1st to 10th	Day	45% 9% 13% 21% 11% 1	3864
	11th to 20th	Day	44% 8% 10% 24% 13% 1	4391
	21st to end	Day	37% 8% 11% 22% 21% 1	4837
June	1st to 10th	Day	32% 9% 13% 28% 16% 2	2660
	11th to 20th	Day	29% 7% 13% 29% 19% 4	2660
	21st to end	Day	27% 10% 17% 26% 15% 5%	2660
July	1st to 10th	Day	38% 8% 13% 27% 13% 2	4371
	11th to 20th	Day	26% 8% 15% 32% 17% 2	4340
	21st to end	Day	34% 10% 20% 25% 11% 1	4325
August	1st to 10th	Day	27% 7% 16% 35% 14% 1	4090
	11th to 20th	Day	38% 7% 13% 24% 17% 2	3831
	21st to end	Day	29% 7% 15% 32% 14% 3	4179
September	1st to 10th	Day	32% 6% 11% 26% 21% 4	2280
	11th to 20th	Day	32% 5% 8% 27% 25% 2	2280
	21st to end	Day	31% 6% 9% 27% 24% 3	2029
October	1st to 10th	Day	18% 3 6% 19% 47% 7%	3160
	11th to 20th	Day	23% 3 6% 21% 38% 9%	3140
	21st to end	Day	33% 3 7% 28% 26% 2	3428
November	1st to 10th	Day	31% 4 14% 25% 22% 5%	1746
	11th to 20th	Day	38% 4 9% 23% 21% 4	1710
	21st to end	Day	44% 4 9% 27% 16%	1582
December	1st to 10th	Day	44% 5% 11% 27% 12% 1	1520
	11th to 20th	Day	45% 4 12% 22% 15% 2	1915
	21st to end	Day	59% 6% 10% 15% 9% 1	2783
January	1st to 10th	Night	52% 5% 10% 23% 10% 1	4428
	11th to 20th	Night	58% 5% 8% 20% 9% 1	4121
	21st to end	Night	59% 5% 9% 20% 7% 1	4532
February	1st to 10th	Night	51% 6% 10% 19% 13% 1	2470
	11th to 20th	Night	45% 5% 10% 21% 17% 3	2289
	21st to end	Night	47% 4 10% 20% 15% 4	1884
March	1st to 10th	Night	46% 4 8% 23% 17% 3	3607
	11th to 20th	Night	35% 5% 11% 24% 21% 4	3480
	21st to end	Night	52% 8% 10% 24% 6%	3672
April	1st to 10th	Night	36% 5% 12% 26% 18% 4	1900
	11th to 20th	Night	39% 6% 10% 22% 18% 5%	1875
	21st to end	Night	43% 6% 9% 23% 17% 1	1900
May	1st to 10th	Night	41% 8% 17% 18% 14% 2	3082
	11th to 20th	Night	41% 7% 12% 25% 12% 1	2517
	21st to end	Night	35% 8% 16% 21% 18% 2	2751
June	1st to 10th	Night	30% 8% 15% 28% 17% 3	1520
	11th to 20th	Night	25% 7% 12% 28% 24% 4	1520
	21st to end	Night	23% 8% 17% 28% 19% 7%	1520
July	1st to 10th	Night	37% 6% 11% 29% 14% 3	2490
	11th to 20th	Night	21% 7% 13% 36% 22% 2	2503
	21st to end	Night	33% 9% 18% 26% 13% 1	2999
August	1st to 10th	Night	21% 6% 18% 33% 20% 2	2826
	11th to 20th	Night	35% 5% 13% 26% 18% 3	3064
	21st to end	Night	22% 6% 17% 35% 17% 3	3476
September	1st to 10th	Night	31% 5% 10% 25% 24% 6%	1900
	11th to 20th	Night	32% 6% 9% 23% 27% 4	1900
	21st to end	Night	30% 6% 8% 25% 27% 4	2127
October	1st to 10th	Night	16% 3 7% 19% 47% 8%	3800
	11th to 20th	Night	22% 3 7% 23% 35% 11%	3779
	21st to end	Night	32% 4 8% 26% 27% 3	4120
November	1st to 10th	Night	31% 5% 12% 26% 21% 6%	2430
	11th to 20th	Night	36% 4 8% 25% 22% 4	2470
	21st to end	Night	42% 4 11% 24% 17% 2	2596
December	1st to 10th	Night	46% 6% 11% 24% 12% 1	2660
	11th to 20th	Night	47% 4 10% 23% 16% 1	3281
	21st to end	Night	63% 7% 9% 13% 7% 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	26% 27% 31% 13% 3	270
	11th to 20th	Day	33% 23% 33% 11% 1	270
	21st to end	Day	22% 30% 36% 11% 1	297
February	1st to 10th	Day	28% 26% 34% 11% 1	270
	11th to 20th	Day	33% 24% 28% 13% 1 1	270
	21st to end	Day	32% 22% 38% 6% 1	225
March	1st to 10th	Day	23% 30% 39% 7% 1	270
	11th to 20th	Day	32% 23% 33% 10% 1 1	270
	21st to end	Day	34% 24% 30% 11% 1	377
April	1st to 10th	Day	22% 19% 37% 17% 4 1	460
	11th to 20th	Day	27% 20% 30% 17% 5%	452
	21st to end	Day	26% 20% 33% 16% 5%	460
May	1st to 10th	Day	22% 18% 36% 18% 6%	501
	11th to 20th	Day	18% 17% 30% 25% 9% 2	675
	21st to end	Day	13% 12% 33% 26% 11% 4	734
June	1st to 10th	Day	9% 7% 28% 34% 17% 5%	680
	11th to 20th	Day	8% 5% 28% 38% 18% 3	680
	21st to end	Day	6% 9% 34% 35% 13% 3	680
July	1st to 10th	Day	11% 5% 19% 31% 26% 9%	671
	11th to 20th	Day	5% 6% 25% 38% 22% 4	668
	21st to end	Day	8% 5% 20% 35% 27% 5%	739
August	1st to 10th	Day	9% 3 19% 41% 23% 5%	676
	11th to 20th	Day	13% 4 23% 32% 22% 6%	520
	21st to end	Day	10% 2 20% 30% 30% 8%	539
September	1st to 10th	Day	11% 6% 17% 34% 24% 7%	490
	11th to 20th	Day	9% 5% 20% 32% 25% 9%	490
	21st to end	Day	12% 9% 23% 33% 17% 6%	415
October	1st to 10th	Day	12% 11% 34% 26% 12% 4	300
	11th to 20th	Day	22% 19% 32% 19% 7% 1	300
	21st to end	Day	31% 21% 31% 15% 2	330
November	1st to 10th	Day	17% 15% 39% 19% 8% 3	282
	11th to 20th	Day	24% 19% 29% 21% 4 2	270
	21st to end	Day	28% 19% 30% 20% 2 1	270
December	1st to 10th	Day	18% 26% 34% 13% 7% 2	270
	11th to 20th	Day	25% 19% 36% 16% 3	270
	21st to end	Day	27% 28% 32% 10% 2	297
January	1st to 10th	Night	21% 21% 34% 19% 4	950
	11th to 20th	Night	25% 21% 34% 16% 3	950
	21st to end	Night	22% 21% 36% 20% 1	1045
February	1st to 10th	Night	24% 19% 36% 19% 2	950
	11th to 20th	Night	24% 20% 37% 18% 2	813
	21st to end	Night	26% 21% 36% 16% 1	628
March	1st to 10th	Night	25% 17% 38% 18% 2	686
	11th to 20th	Night	24% 14% 37% 21% 4	570
	21st to end	Night	26% 17% 36% 18% 3	627
April	1st to 10th	Night	18% 22% 34% 22% 3 1	570
	11th to 20th	Night	21% 16% 34% 24% 5%	563
	21st to end	Night	22% 13% 38% 22% 5%	570
May	1st to 10th	Night	16% 13% 34% 28% 8%	570
	11th to 20th	Night	17% 10% 29% 30% 13% 1	561
	21st to end	Night	12% 8% 29% 31% 18% 2	617
June	1st to 10th	Night	10% 3 24% 40% 22% 2	570
	11th to 20th	Night	8% 2 28% 42% 19% 1	570
	21st to end	Night	3 4 30% 43% 18% 1	570
July	1st to 10th	Night	12% 2 14% 38% 30% 5%	555
	11th to 20th	Night	6% 2 19% 41% 29% 3	552
	21st to end	Night	7% 2 18% 42% 30% 1	624
August	1st to 10th	Night	8% 3 12% 45% 31% 2	567
	11th to 20th	Night	7% 2 16% 41% 30% 2	564
	21st to end	Night	6% 2 15% 39% 35% 2	627
September	1st to 10th	Night	13% 3 17% 37% 24% 5%	570
	11th to 20th	Night	11% 4 19% 33% 29% 4	570
	21st to end	Night	10% 8% 25% 32% 22% 4	726
October	1st to 10th	Night	10% 7% 29% 28% 23% 3	760
	11th to 20th	Night	16% 15% 32% 27% 10% 1	760
	21st to end	Night	22% 16% 33% 22% 7%	836
November	1st to 10th	Night	14% 14% 38% 25% 10%	760
	11th to 20th	Night	20% 17% 32% 23% 7%	760
	21st to end	Night	23% 18% 32% 21% 5%	886
December	1st to 10th	Night	23% 18% 33% 21% 5% 1	950
	11th to 20th	Night	25% 15% 31% 26% 3	950
	21st to end	Night	28% 20% 32% 18% 2	1045

Page generated at 2024-11-03 01:11:11. Version ac07

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