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 2026-04-17.

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% 4 9% 5% 5% 5% 4 5% 7% 32%	131725
	Night	15% 11% 4 8% 5% 4 4 4 5% 7% 33%	130414
Transparency	Day	50% 5% 13% 20% 11% 1	135320
	Night	50% 4 12% 20% 12% 1	134905
Seeing	Day	20% 11% 28% 23% 13% 5%	16249
	Night	20% 9% 28% 29% 12% 1	25863

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% 16% 6% 11% 5% 4 4 3 4 5% 24%	4699
	11th to 20th	Day	11% 22% 9% 18% 8% 6% 4 3 4 4 11%	5246
	21st to end	Day	15% 15% 7% 18% 9% 7% 5% 3 4 4 15%	5780
February	1st to 10th	Day	16% 17% 6% 14% 7% 6% 4 4 4 5% 19%	1890
	11th to 20th	Day	14% 15% 7% 13% 7% 4 5% 3 5% 5% 23%	1843
	21st to end	Day	18% 15% 5% 11% 7% 4 4 3 4 5% 23%	1646
March	1st to 10th	Day	18% 10% 6% 11% 5% 4 4 3 4 5% 30%	5042
	11th to 20th	Day	11% 14% 5% 9% 5% 5% 5% 3 5% 7% 32%	5090
	21st to end	Day	19% 12% 4 8% 5% 4 4 3 4 5% 31%	5586
April	1st to 10th	Day	18% 13% 4 8% 5% 4 4 3 5% 6% 30%	2460
	11th to 20th	Day	17% 9% 4 7% 5% 4 4 3 5% 8% 33%	2520
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 5% 5% 35%	2600
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 3 5% 6% 33%	5888
	11th to 20th	Day	16% 9% 4 9% 5% 5% 4 3 5% 8% 32%	6152
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 7% 38%	6949
June	1st to 10th	Day	13% 6% 3 5% 5% 5% 5% 5% 6% 8% 40%	2800
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 7% 8% 40%	2800
	21st to end	Day	12% 5% 2 6% 4 4 4 5% 7% 9% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	6360
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 7% 9% 42%	6234
	21st to end	Day	8% 4 2 6% 4 4 5% 4 6% 9% 47%	6424
August	1st to 10th	Day	16% 6% 2 5% 4 5% 4 4 7% 9% 38%	4234
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 5% 8% 10% 35%	4174
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	4490
September	1st to 10th	Day	12% 6% 2 6% 3 5% 4 5% 6% 8% 42%	2400
	11th to 20th	Day	9% 5% 1 5% 5% 5% 5% 3 5% 7% 49%	2255
	21st to end	Day	14% 7% 3 7% 5% 4 5% 5% 7% 9% 34%	2173
October	1st to 10th	Day	17% 9% 3 6% 4 5% 5% 3 5% 7% 39%	2000
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 3 5% 7% 32%	2000
	21st to end	Day	22% 11% 3 9% 6% 4 5% 4 5% 6% 26%	2200
November	1st to 10th	Day	14% 8% 3 8% 4 5% 5% 4 6% 7% 35%	1836
	11th to 20th	Day	13% 14% 3 9% 5% 5% 5% 4 6% 7% 29%	1680
	21st to end	Day	19% 15% 3 8% 5% 5% 4 4 5% 7% 24%	1680
December	1st to 10th	Day	19% 15% 5% 10% 6% 6% 6% 4 7% 6% 15%	2680
	11th to 20th	Day	21% 17% 4 7% 5% 4 5% 3 5% 4 25%	3066
	21st to end	Day	26% 18% 6% 11% 5% 4 4 3 4 5% 14%	4048
January	1st to 10th	Night	15% 16% 4 10% 5% 5% 4 3 5% 6% 25%	8192
	11th to 20th	Night	11% 18% 7% 13% 7% 5% 4 4 5% 6% 20%	7639
	21st to end	Night	16% 14% 5% 14% 7% 5% 4 3 4 5% 22%	8401
February	1st to 10th	Night	15% 16% 4 9% 7% 5% 5% 3 5% 5% 23%	2730
	11th to 20th	Night	15% 13% 5% 10% 6% 6% 5% 4 6% 5% 26%	2663
	21st to end	Night	17% 13% 5% 10% 6% 5% 5% 5% 5% 7% 23%	2155
March	1st to 10th	Night	16% 10% 4 8% 7% 5% 5% 4 4 6% 33%	5018
	11th to 20th	Night	13% 9% 3 7% 4 4 5% 4 5% 8% 38%	5089
	21st to end	Night	19% 9% 3 6% 4 3 3 3 4 6% 38%	5588
April	1st to 10th	Night	19% 10% 4 9% 6% 4 4 4 5% 5% 30%	2158
	11th to 20th	Night	18% 8% 3 8% 5% 4 5% 4 5% 6% 36%	1978
	21st to end	Night	15% 8% 3 8% 5% 4 6% 4 5% 6% 37%	1800
May	1st to 10th	Night	22% 9% 4 8% 5% 4 3 2 5% 5% 32%	4053
	11th to 20th	Night	13% 7% 3 7% 5% 5% 6% 4 5% 7% 36%	3741
	21st to end	Night	18% 6% 2 6% 3 3 4 4 6% 8% 41%	3926
June	1st to 10th	Night	11% 6% 2 7% 4 5% 6% 5% 7% 8% 40%	1600
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 7% 7% 44%	1600
	21st to end	Night	11% 5% 2 7% 5% 4 5% 3 6% 8% 45%	1600
July	1st to 10th	Night	7% 5% 2 7% 4 4 4 4 6% 9% 47%	3600
	11th to 20th	Night	9% 5% 2 7% 4 4 5% 3 6% 9% 47%	3709
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 8% 50%	4428
August	1st to 10th	Night	10% 6% 2 6% 5% 4 3 4 6% 8% 45%	2915
	11th to 20th	Night	12% 5% 2 7% 4 5% 5% 4 6% 8% 44%	2875
	21st to end	Night	7% 4 2 5% 4 5% 5% 3 6% 9% 50%	3374
September	1st to 10th	Night	12% 7% 2 5% 5% 3 5% 3 4 7% 47%	2000
	11th to 20th	Night	9% 5% 1 4 4 3 3 4 5% 6% 56%	2140
	21st to end	Night	15% 7% 3 8% 5% 4 5% 4 5% 7% 39%	2225
October	1st to 10th	Night	14% 6% 3 6% 5% 4 3 3 4 7% 45%	2400
	11th to 20th	Night	18% 9% 3 7% 5% 4 5% 3 6% 7% 35%	2400
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 5% 5% 33%	2640
November	1st to 10th	Night	15% 8% 2 6% 4 3 5% 4 6% 8% 40%	2679
	11th to 20th	Night	15% 11% 3 7% 5% 5% 5% 4 6% 8% 31%	2940
	21st to end	Night	19% 12% 4 7% 5% 5% 4 4 5% 7% 28%	2940
December	1st to 10th	Night	15% 17% 4 9% 6% 5% 5% 4 7% 7% 22%	4720
	11th to 20th	Night	17% 18% 5% 9% 4 4 4 3 4 5% 25%	5348
	21st to end	Night	25% 17% 6% 10% 5% 4 4 3 4 5% 18%	7150

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% 3 12% 11% 9% 1	4700
	11th to 20th	Day	79% 2 7% 6% 5% 1	5280
	21st to end	Day	73% 4 10% 8% 6%	5780
February	1st to 10th	Day	67% 4 9% 14% 5%	1890
	11th to 20th	Day	63% 5% 10% 16% 6% 1	1852
	21st to end	Day	63% 5% 10% 15% 6% 1	1646
March	1st to 10th	Day	57% 4 14% 17% 8%	5045
	11th to 20th	Day	51% 6% 16% 19% 7% 2	5090
	21st to end	Day	55% 5% 7% 22% 11% 1	5586
April	1st to 10th	Day	55% 4 11% 17% 12% 1	2460
	11th to 20th	Day	49% 5% 12% 21% 13% 1	2520
	21st to end	Day	50% 6% 10% 19% 13% 2	2600
May	1st to 10th	Day	51% 4 12% 17% 10% 5%	5897
	11th to 20th	Day	51% 7% 12% 19% 11% 1	6156
	21st to end	Day	45% 4 13% 22% 13% 2	6949
June	1st to 10th	Day	40% 6% 12% 24% 16% 2	2800
	11th to 20th	Day	39% 7% 15% 24% 13% 1	2800
	21st to end	Day	37% 7% 14% 26% 15% 1	2800
July	1st to 10th	Day	36% 4 13% 28% 18% 2	6290
	11th to 20th	Day	37% 4 14% 28% 15% 1	6159
	21st to end	Day	33% 5% 15% 26% 19% 1	6240
August	1st to 10th	Day	42% 5% 16% 23% 14% 1	4221
	11th to 20th	Day	41% 5% 17% 26% 10% 1	4213
	21st to end	Day	37% 3 12% 34% 12% 1	4490
September	1st to 10th	Day	38% 6% 15% 27% 13% 1	2400
	11th to 20th	Day	34% 6% 13% 31% 16%	2255
	21st to end	Day	44% 6% 16% 24% 9% 1	2173
October	1st to 10th	Day	36% 4 15% 27% 17% 1	3260
	11th to 20th	Day	42% 4 14% 26% 14% 1	3240
	21st to end	Day	49% 3 16% 23% 8%	3538
November	1st to 10th	Day	47% 6% 14% 23% 8% 2	1836
	11th to 20th	Day	53% 6% 14% 16% 10% 1	1680
	21st to end	Day	60% 5% 16% 14% 5%	1680
December	1st to 10th	Day	68% 4 15% 10% 3	2680
	11th to 20th	Day	63% 4 12% 11% 9% 1	3066
	21st to end	Day	74% 4 7% 10% 5%	4048
January	1st to 10th	Night	60% 4 11% 18% 6%	8192
	11th to 20th	Night	65% 4 12% 10% 8%	7680
	21st to end	Night	66% 3 8% 12% 11% 1	8401
February	1st to 10th	Night	62% 5% 12% 15% 6% 1	2730
	11th to 20th	Night	58% 5% 12% 17% 7% 1	2665
	21st to end	Night	60% 5% 12% 16% 7%	2155
March	1st to 10th	Night	53% 4 12% 18% 10% 2	5028
	11th to 20th	Night	45% 4 17% 20% 12% 1	5090
	21st to end	Night	48% 4 9% 21% 17% 2	5588
April	1st to 10th	Night	54% 5% 10% 18% 12% 1	2158
	11th to 20th	Night	50% 5% 8% 20% 15% 2	1978
	21st to end	Night	46% 5% 10% 19% 16% 3	1800
May	1st to 10th	Night	55% 3 9% 15% 12% 6%	4060
	11th to 20th	Night	47% 6% 13% 22% 10% 2	3742
	21st to end	Night	41% 4 11% 25% 17% 2	3927
June	1st to 10th	Night	40% 7% 10% 24% 17% 3	1600
	11th to 20th	Night	37% 6% 13% 26% 17% 2	1600
	21st to end	Night	38% 5% 13% 24% 18% 2	1600
July	1st to 10th	Night	35% 3 14% 23% 22% 3	3560
	11th to 20th	Night	35% 4 12% 30% 17% 2	3661
	21st to end	Night	33% 4 13% 23% 24% 2	4297
August	1st to 10th	Night	37% 5% 14% 26% 16% 2	2904
	11th to 20th	Night	38% 4 16% 31% 10%	2902
	21st to end	Night	31% 4 14% 34% 16% 1	3374
September	1st to 10th	Night	37% 5% 12% 28% 17% 2	2000
	11th to 20th	Night	28% 5% 11% 37% 18% 1	2140
	21st to end	Night	44% 6% 11% 23% 14% 2	2225
October	1st to 10th	Night	34% 3 8% 28% 25% 1	3920
	11th to 20th	Night	44% 3 13% 26% 14%	3899
	21st to end	Night	40% 4 15% 26% 14% 1	4252
November	1st to 10th	Night	42% 6% 13% 25% 13% 1	2679
	11th to 20th	Night	51% 5% 13% 20% 10% 1	2940
	21st to end	Night	57% 5% 17% 16% 5%	2940
December	1st to 10th	Night	62% 4 17% 14% 3	4720
	11th to 20th	Night	62% 5% 9% 13% 10% 1	5348
	21st to end	Night	69% 4 10% 11% 5%	7150

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% 22% 30% 13% 7% 1	227
	11th to 20th	Day	32% 23% 30% 10% 4 1	330
	21st to end	Day	25% 21% 32% 17% 5% 1	363
February	1st to 10th	Day	24% 19% 37% 15% 4 1	330
	11th to 20th	Day	26% 21% 38% 14% 2	330
	21st to end	Day	29% 25% 34% 12%	289
March	1st to 10th	Day	25% 13% 34% 19% 8% 1	540
	11th to 20th	Day	26% 16% 35% 17% 6% 1	540
	21st to end	Day	30% 12% 31% 18% 8% 1	594
April	1st to 10th	Day	24% 15% 33% 21% 7% 1	540
	11th to 20th	Day	25% 16% 28% 19% 9% 3	516
	21st to end	Day	25% 8% 32% 23% 9% 2	500
May	1st to 10th	Day	25% 7% 30% 24% 9% 5%	503
	11th to 20th	Day	18% 9% 27% 27% 15% 4	527
	21st to end	Day	14% 6% 27% 27% 19% 7%	573
June	1st to 10th	Day	13% 12% 28% 31% 14% 2	530
	11th to 20th	Day	12% 8% 26% 30% 20% 3	530
	21st to end	Day	17% 5% 29% 31% 15% 4	530
July	1st to 10th	Day	10% 3 21% 31% 26% 9%	525
	11th to 20th	Day	15% 7% 22% 27% 20% 10%	524
	21st to end	Day	10% 3 27% 33% 19% 7%	576
August	1st to 10th	Day	10% 3 22% 29% 24% 12%	526
	11th to 20th	Day	11% 5% 21% 32% 18% 12%	519
	21st to end	Day	10% 5% 21% 31% 22% 10%	583
September	1st to 10th	Day	14% 6% 23% 25% 20% 12%	530
	11th to 20th	Day	9% 4 16% 25% 27% 18%	530
	21st to end	Day	17% 8% 25% 27% 15% 8%	530
October	1st to 10th	Day	19% 8% 24% 23% 18% 7%	530
	11th to 20th	Day	20% 14% 33% 23% 8% 3	530
	21st to end	Day	29% 11% 29% 21% 9%	583
November	1st to 10th	Day	20% 19% 34% 18% 6% 3	349
	11th to 20th	Day	24% 21% 28% 18% 8% 1	220
	21st to end	Day	30% 16% 36% 15% 2	220
December	1st to 10th	Day	27% 19% 32% 17% 5% 1	220
	11th to 20th	Day	29% 24% 30% 15% 4	220
	21st to end	Day	30% 20% 32% 15% 2	242
January	1st to 10th	Night	25% 15% 35% 19% 5% 1	840
	11th to 20th	Night	28% 16% 32% 19% 4	840
	21st to end	Night	22% 13% 38% 21% 6%	924
February	1st to 10th	Night	25% 15% 34% 23% 4	840
	11th to 20th	Night	21% 14% 35% 25% 4	823
	21st to end	Night	24% 14% 34% 25% 3	692
March	1st to 10th	Night	21% 11% 33% 29% 7%	840
	11th to 20th	Night	24% 12% 34% 23% 7%	840
	21st to end	Night	22% 9% 33% 27% 8% 1	924
April	1st to 10th	Night	24% 10% 34% 28% 4	840
	11th to 20th	Night	20% 11% 32% 27% 9% 1	758
	21st to end	Night	18% 5% 33% 35% 9% 1	600
May	1st to 10th	Night	21% 4 27% 32% 15% 2	600
	11th to 20th	Night	16% 3 25% 42% 14% 1	469
	21st to end	Night	15% 5% 22% 36% 21% 2	433
June	1st to 10th	Night	13% 3 30% 39% 15% 1	400
	11th to 20th	Night	12% 3 26% 40% 19% 1	400
	21st to end	Night	13% 3 22% 45% 17% 1	400
July	1st to 10th	Night	6% 2 18% 47% 24% 2	390
	11th to 20th	Night	10% 3 20% 43% 22% 3	432
	21st to end	Night	10% 2 17% 45% 25% 2	657
August	1st to 10th	Night	9% 2 12% 42% 31% 3	597
	11th to 20th	Night	11% 2 14% 35% 35% 3	594
	21st to end	Night	10% 2 16% 39% 29% 3	746
September	1st to 10th	Night	15% 5% 17% 34% 25% 5%	800
	11th to 20th	Night	13% 2 15% 32% 33% 6%	800
	21st to end	Night	19% 5% 19% 32% 21% 5%	800
October	1st to 10th	Night	18% 6% 25% 29% 20% 3	800
	11th to 20th	Night	23% 9% 28% 29% 9% 2	800
	21st to end	Night	24% 9% 32% 29% 6%	880
November	1st to 10th	Night	20% 10% 29% 28% 11% 2	820
	11th to 20th	Night	23% 13% 31% 23% 9% 2	840
	21st to end	Night	27% 11% 33% 22% 7%	840
December	1st to 10th	Night	28% 16% 29% 20% 6%	840
	11th to 20th	Night	25% 17% 35% 19% 5%	840
	21st to end	Night	32% 11% 31% 23% 2	924

Page generated at 2026-04-14 02:44:23. Version ac07

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