The Blue Marble Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Blue Marble Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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% 10% 3 8% 5% 4 5% 4 6% 7% 33%	101786
	Night	16% 10% 3 8% 5% 4 4 4 5% 7% 34%	98331
Transparency	Day	48% 5% 13% 21% 11% 1	105392
	Night	49% 5% 13% 21% 12% 1	102813
Seeing	Day	19% 11% 28% 24% 13% 5%	14596
	Night	20% 9% 28% 29% 13% 1	24145

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	17% 18% 6% 10% 5% 4 4 3 5% 5% 24%	2527
	11th to 20th	Day	14% 20% 7% 15% 9% 5% 5% 3 4 5% 14%	2840
	21st to end	Day	22% 19% 7% 12% 7% 5% 4 3 3 3 13%	3124
February	1st to 10th	Day	17% 17% 6% 14% 7% 6% 4 4 4 5% 17%	1710
	11th to 20th	Day	13% 15% 7% 13% 6% 4 5% 3 5% 5% 23%	1663
	21st to end	Day	20% 14% 5% 11% 6% 4 4 3 4 4 24%	1493
March	1st to 10th	Day	20% 11% 4 9% 5% 5% 4 4 4 5% 28%	3432
	11th to 20th	Day	11% 17% 5% 9% 5% 4 5% 3 5% 5% 30%	3480
	21st to end	Day	18% 10% 3 8% 6% 4 4 3 4 7% 32%	3815
April	1st to 10th	Day	18% 14% 4 8% 5% 4 4 3 5% 6% 30%	2226
	11th to 20th	Day	18% 8% 4 7% 5% 3 4 4 5% 8% 34%	2324
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 5% 5% 34%	2470
May	1st to 10th	Day	18% 8% 2 7% 5% 5% 5% 4 5% 6% 34%	4108
	11th to 20th	Day	15% 9% 4 9% 5% 5% 5% 4 6% 8% 31%	4288
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 7% 47%	4837
June	1st to 10th	Day	13% 6% 3 5% 5% 5% 5% 5% 6% 8% 40%	2660
	11th to 20th	Day	11% 6% 3 7% 5% 3 5% 5% 7% 8% 40%	2660
	21st to end	Day	13% 6% 2 6% 4 4 4 4 7% 8% 42%	2660
July	1st to 10th	Day	8% 5% 1 6% 5% 4 5% 4 7% 11% 45%	4440
	11th to 20th	Day	11% 6% 2 5% 4 5% 5% 5% 7% 10% 41%	4356
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	4466
August	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 7% 9% 38%	4104
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 5% 8% 11% 35%	4044
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	4353
September	1st to 10th	Day	12% 7% 2 6% 3 5% 4 5% 6% 8% 42%	2280
	11th to 20th	Day	9% 5% 1 5% 5% 5% 5% 3 5% 7% 48%	2142
	21st to end	Day	14% 7% 3 7% 5% 4 5% 5% 7% 9% 34%	2065
October	1st to 10th	Day	17% 9% 3 7% 4 5% 5% 3 5% 6% 37%	1900
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 32%	1900
	21st to end	Day	22% 11% 3 9% 6% 4 5% 4 5% 6% 25%	2090
November	1st to 10th	Day	14% 8% 3 8% 4 5% 5% 4 6% 7% 36%	1747
	11th to 20th	Day	14% 14% 3 9% 5% 5% 5% 4 6% 7% 29%	1600
	21st to end	Day	20% 15% 3 8% 5% 5% 4 4 5% 7% 24%	1600
December	1st to 10th	Day	18% 15% 5% 10% 6% 6% 5% 3 7% 6% 19%	1600
	11th to 20th	Day	20% 15% 5% 9% 5% 4 5% 4 6% 5% 21%	1986
	21st to end	Day	28% 20% 6% 11% 5% 4 3 3 4 4 12%	2796
January	1st to 10th	Night	17% 17% 5% 9% 6% 5% 4 3 5% 5% 24%	4415
	11th to 20th	Night	15% 16% 6% 11% 6% 6% 5% 4 5% 5% 20%	4120
	21st to end	Night	24% 17% 6% 12% 4 4 4 3 4 5% 18%	4532
February	1st to 10th	Night	16% 17% 4 9% 7% 6% 5% 3 5% 5% 22%	2470
	11th to 20th	Night	15% 13% 5% 10% 6% 6% 5% 4 6% 6% 26%	2403
	21st to end	Night	18% 12% 5% 10% 6% 4 4 5% 5% 7% 24%	1957
March	1st to 10th	Night	19% 10% 3 8% 6% 5% 4 3 4 5% 33%	3408
	11th to 20th	Night	16% 11% 3 7% 4 3 4 3 5% 7% 36%	3479
	21st to end	Night	17% 8% 3 6% 4 4 3 3 5% 6% 42%	3817
April	1st to 10th	Night	19% 10% 4 8% 6% 4 3 4 5% 6% 32%	1952
	11th to 20th	Night	18% 8% 3 7% 5% 4 5% 4 5% 6% 36%	1820
	21st to end	Night	15% 8% 3 8% 5% 4 5% 4 5% 6% 36%	1710
May	1st to 10th	Night	20% 9% 3 8% 4 4 3 2 5% 6% 36%	2833
	11th to 20th	Night	12% 7% 3 7% 6% 5% 6% 5% 6% 8% 35%	2605
	21st to end	Night	11% 5% 2 6% 4 3 4 4 6% 8% 48%	2738
June	1st to 10th	Night	11% 6% 2 7% 4 5% 6% 6% 7% 8% 40%	1520
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 7% 7% 45%	1520
	21st to end	Night	11% 5% 2 7% 5% 4 5% 3 6% 8% 45%	1520
July	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 6% 9% 49%	2520
	11th to 20th	Night	10% 4 2 6% 3 3 4 3 6% 9% 48%	2601
	21st to end	Night	8% 4 2 5% 4 4 5% 4 6% 8% 49%	3087
August	1st to 10th	Night	10% 6% 2 6% 5% 4 3 4 6% 8% 45%	2825
	11th to 20th	Night	12% 5% 2 7% 4 5% 5% 4 6% 8% 43%	2785
	21st to end	Night	7% 4 2 5% 4 5% 5% 3 6% 9% 50%	3270
September	1st to 10th	Night	13% 7% 2 5% 5% 3 5% 3 4 7% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 4 3 3 4 5% 6% 55%	2033
	21st to end	Night	15% 7% 3 8% 5% 4 5% 4 5% 7% 38%	2114
October	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 4 7% 43%	2280
	11th to 20th	Night	18% 9% 2 7% 5% 4 5% 3 6% 7% 34%	2280
	21st to end	Night	21% 9% 3 7% 4 4 4 4 5% 5% 32%	2508
November	1st to 10th	Night	15% 8% 2 6% 4 3 5% 4 6% 8% 40%	2548
	11th to 20th	Night	16% 11% 3 7% 5% 5% 5% 4 6% 8% 32%	2800
	21st to end	Night	19% 12% 4 7% 5% 5% 4 4 5% 7% 28%	2800
December	1st to 10th	Night	19% 15% 4 10% 5% 5% 4 4 6% 6% 22%	2800
	11th to 20th	Night	16% 18% 5% 10% 5% 5% 4 3 5% 6% 24%	3428
	21st to end	Night	28% 18% 6% 9% 5% 5% 4 3 4 4 14%	4933

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	63% 5% 13% 10% 8%	2528
	11th to 20th	Day	75% 3 11% 9% 2	2840
	21st to end	Day	75% 6% 9% 8% 3	3124
February	1st to 10th	Day	68% 5% 10% 13% 4	1710
	11th to 20th	Day	62% 5% 9% 15% 7% 1	1672
	21st to end	Day	62% 5% 10% 15% 7% 1	1493
March	1st to 10th	Day	57% 5% 11% 21% 5% 1	3435
	11th to 20th	Day	52% 8% 14% 18% 7% 1	3480
	21st to end	Day	51% 6% 8% 22% 11% 1	3815
April	1st to 10th	Day	55% 4 11% 18% 12%	2226
	11th to 20th	Day	48% 5% 12% 20% 13% 2	2324
	21st to end	Day	50% 6% 11% 19% 12% 2	2470
May	1st to 10th	Day	48% 5% 13% 19% 12% 2	4120
	11th to 20th	Day	49% 8% 16% 19% 8% 1	4292
	21st to end	Day	35% 6% 14% 25% 17% 3	4837
June	1st to 10th	Day	40% 7% 12% 24% 15% 2	2660
	11th to 20th	Day	38% 7% 15% 24% 14% 1	2660
	21st to end	Day	37% 8% 14% 26% 15% 1	2660
July	1st to 10th	Day	34% 5% 13% 29% 17% 2	4370
	11th to 20th	Day	36% 5% 14% 31% 13%	4281
	21st to end	Day	35% 7% 17% 21% 19% 1	4324
August	1st to 10th	Day	42% 5% 16% 23% 13% 1	4091
	11th to 20th	Day	41% 5% 17% 26% 9% 1	4083
	21st to end	Day	37% 4 12% 33% 13% 1	4353
September	1st to 10th	Day	38% 6% 16% 25% 13% 1	2280
	11th to 20th	Day	34% 6% 13% 29% 17% 1	2142
	21st to end	Day	44% 6% 15% 24% 9% 1	2065
October	1st to 10th	Day	37% 4 16% 26% 17% 1	3160
	11th to 20th	Day	42% 4 14% 26% 14% 1	3140
	21st to end	Day	49% 3 16% 24% 7%	3428
November	1st to 10th	Day	46% 7% 14% 23% 8% 2	1747
	11th to 20th	Day	53% 7% 14% 16% 10% 1	1600
	21st to end	Day	59% 5% 17% 14% 5%	1600
December	1st to 10th	Day	64% 6% 14% 13% 3	1600
	11th to 20th	Day	63% 6% 15% 10% 6%	1986
	21st to end	Day	77% 5% 8% 8% 2	2796
January	1st to 10th	Night	62% 6% 13% 13% 5%	4415
	11th to 20th	Night	65% 6% 17% 8% 3	4120
	21st to end	Night	70% 5% 11% 10% 4 1	4532
February	1st to 10th	Night	63% 5% 12% 15% 5% 1	2470
	11th to 20th	Night	58% 6% 11% 17% 8% 1	2405
	21st to end	Night	59% 5% 12% 16% 7%	1957
March	1st to 10th	Night	53% 5% 12% 22% 7% 1	3418
	11th to 20th	Night	48% 5% 15% 18% 12% 2	3480
	21st to end	Night	42% 5% 9% 24% 18% 2	3817
April	1st to 10th	Night	53% 5% 10% 19% 13% 1	1952
	11th to 20th	Night	49% 5% 8% 20% 15% 2	1820
	21st to end	Night	47% 6% 10% 18% 15% 3	1710
May	1st to 10th	Night	49% 4 10% 19% 13% 4	2840
	11th to 20th	Night	45% 8% 15% 23% 8% 2	2606
	21st to end	Night	33% 6% 12% 25% 22% 2	2739
June	1st to 10th	Night	40% 7% 10% 24% 17% 2	1520
	11th to 20th	Night	36% 6% 13% 25% 18% 2	1520
	21st to end	Night	38% 5% 13% 25% 17% 2	1520
July	1st to 10th	Night	34% 3 12% 24% 22% 5%	2480
	11th to 20th	Night	33% 4 13% 32% 16% 1	2553
	21st to end	Night	32% 5% 16% 24% 19% 2	2988
August	1st to 10th	Night	37% 5% 14% 27% 15% 2	2814
	11th to 20th	Night	39% 5% 16% 30% 10%	2812
	21st to end	Night	31% 4 14% 33% 17% 1	3270
September	1st to 10th	Night	38% 5% 12% 26% 17% 2	1900
	11th to 20th	Night	29% 5% 11% 35% 19% 1	2033
	21st to end	Night	45% 6% 12% 23% 13% 1	2114
October	1st to 10th	Night	34% 4 8% 28% 25% 1	3800
	11th to 20th	Night	44% 3 13% 26% 14%	3779
	21st to end	Night	40% 4 16% 26% 14% 1	4120
November	1st to 10th	Night	41% 6% 13% 25% 13% 1	2548
	11th to 20th	Night	51% 5% 13% 20% 10% 1	2800
	21st to end	Night	56% 5% 18% 17% 5%	2800
December	1st to 10th	Night	62% 5% 15% 13% 4	2800
	11th to 20th	Night	62% 6% 12% 14% 5%	3428
	21st to end	Night	73% 6% 11% 8% 2	4933

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% 20% 29% 15% 9% 1	185
	11th to 20th	Day	29% 22% 32% 11% 4 1	270
	21st to end	Day	23% 22% 31% 19% 5% 1	297
February	1st to 10th	Day	25% 18% 40% 13% 3 1	270
	11th to 20th	Day	24% 21% 40% 14% 1	270
	21st to end	Day	29% 25% 33% 13%	239
March	1st to 10th	Day	24% 13% 33% 21% 8% 1	460
	11th to 20th	Day	23% 15% 38% 18% 6% 1	460
	21st to end	Day	30% 11% 31% 19% 9% 1	506
April	1st to 10th	Day	22% 14% 33% 22% 8% 1	460
	11th to 20th	Day	25% 15% 28% 19% 9% 4	452
	21st to end	Day	25% 8% 32% 23% 9% 3	460
May	1st to 10th	Day	24% 7% 29% 25% 10% 5%	463
	11th to 20th	Day	17% 8% 26% 28% 15% 5%	487
	21st to end	Day	12% 6% 26% 29% 20% 8%	529
June	1st to 10th	Day	13% 11% 28% 32% 15% 2	490
	11th to 20th	Day	12% 8% 24% 30% 21% 4	490
	21st to end	Day	17% 5% 29% 30% 15% 4	490
July	1st to 10th	Day	10% 3 20% 31% 27% 10%	485
	11th to 20th	Day	15% 6% 21% 27% 20% 10%	484
	21st to end	Day	10% 3 27% 33% 20% 8%	532
August	1st to 10th	Day	11% 3 23% 28% 23% 12%	486
	11th to 20th	Day	11% 5% 21% 32% 18% 13%	479
	21st to end	Day	11% 4 22% 30% 22% 11%	539
September	1st to 10th	Day	14% 6% 23% 26% 19% 12%	490
	11th to 20th	Day	10% 4 17% 24% 26% 18%	490
	21st to end	Day	17% 9% 25% 27% 14% 8%	490
October	1st to 10th	Day	20% 9% 24% 23% 18% 6%	490
	11th to 20th	Day	20% 13% 32% 23% 8% 3	490
	21st to end	Day	30% 12% 28% 20% 10%	539
November	1st to 10th	Day	19% 20% 33% 19% 7% 3	320
	11th to 20th	Day	25% 19% 29% 19% 8% 2	200
	21st to end	Day	29% 16% 37% 17% 2 1	200
December	1st to 10th	Day	26% 18% 34% 17% 5% 1	200
	11th to 20th	Day	28% 24% 30% 14% 4	200
	21st to end	Day	27% 20% 34% 16% 2	204
January	1st to 10th	Night	25% 15% 34% 19% 6% 1	760
	11th to 20th	Night	29% 15% 32% 20% 4	760
	21st to end	Night	22% 12% 37% 23% 6%	836
February	1st to 10th	Night	26% 15% 34% 22% 4	760
	11th to 20th	Night	21% 14% 35% 26% 4	743
	21st to end	Night	24% 14% 33% 25% 3	628
March	1st to 10th	Night	21% 10% 33% 29% 7%	760
	11th to 20th	Night	23% 11% 34% 24% 8%	760
	21st to end	Night	22% 8% 33% 28% 9% 1	836
April	1st to 10th	Night	24% 9% 33% 30% 4	760
	11th to 20th	Night	20% 10% 32% 27% 10% 1	696
	21st to end	Night	18% 5% 32% 35% 9% 1	570
May	1st to 10th	Night	21% 4 26% 32% 15% 2	570
	11th to 20th	Night	16% 3 24% 43% 14% 1	445
	21st to end	Night	14% 5% 21% 36% 22% 2	411
June	1st to 10th	Night	13% 3 30% 38% 16% 1	380
	11th to 20th	Night	12% 3 25% 40% 19% 1	380
	21st to end	Night	13% 3 22% 45% 17% 1	380
July	1st to 10th	Night	6% 2 18% 46% 25% 2	370
	11th to 20th	Night	11% 2 20% 42% 21% 3	410
	21st to end	Night	10% 2 17% 45% 24% 2	624
August	1st to 10th	Night	10% 2 13% 41% 31% 3	567
	11th to 20th	Night	11% 2 15% 34% 36% 3	564
	21st to end	Night	10% 2 16% 38% 30% 3	708
September	1st to 10th	Night	15% 5% 17% 33% 24% 5%	760
	11th to 20th	Night	13% 2 15% 32% 32% 6%	760
	21st to end	Night	19% 5% 19% 31% 21% 5%	760
October	1st to 10th	Night	18% 6% 26% 29% 19% 3	760
	11th to 20th	Night	23% 9% 27% 29% 10% 2	760
	21st to end	Night	25% 9% 31% 28% 6%	836
November	1st to 10th	Night	20% 10% 29% 28% 11% 2	780
	11th to 20th	Night	23% 12% 31% 23% 9% 2	800
	21st to end	Night	26% 11% 33% 22% 7%	800
December	1st to 10th	Night	29% 16% 30% 19% 6% 1	800
	11th to 20th	Night	26% 16% 34% 19% 5%	800
	21st to end	Night	33% 11% 31% 23% 2	851

Page generated at 2024-12-21 04:13:51. Version ac07

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