Red n Blue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Red n Blue Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	13% 8% 3 7% 5% 4 5% 4 5% 7% 38%	131754
	Night	15% 9% 3 7% 4 4 4 3 5% 6% 40%	131571
Transparency	Day	43% 5% 11% 23% 15% 2	135371
	Night	43% 5% 11% 23% 17% 2	136027
Seeing	Day	16% 9% 24% 24% 18% 8%	17578
	Night	18% 8% 26% 31% 16% 2	27437

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	14% 11% 7% 11% 5% 4 5% 4 4 5% 29%	4691
	11th to 20th	Day	11% 11% 4 12% 7% 5% 4 3 4 5% 33%	4710
	21st to end	Day	19% 13% 5% 9% 6% 4 4 3 4 5% 27%	5680
February	1st to 10th	Day	16% 12% 5% 8% 4 3 4 3 5% 6% 33%	2100
	11th to 20th	Day	16% 10% 3 9% 5% 5% 4 3 5% 6% 35%	2048
	21st to end	Day	15% 11% 4 9% 5% 4 4 4 4 6% 34%	1730
March	1st to 10th	Day	19% 11% 4 7% 4 3 3 2 3 4 41%	4580
	11th to 20th	Day	12% 10% 3 6% 4 3 3 3 5% 7% 43%	4807
	21st to end	Day	14% 10% 3 7% 5% 5% 5% 3 5% 7% 36%	5998
April	1st to 10th	Day	14% 8% 3 6% 5% 4 4 4 5% 8% 39%	2520
	11th to 20th	Day	17% 8% 2 6% 5% 3 4 3 6% 6% 40%	2494
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 8% 40%	2606
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 7% 9% 27%	5992
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	5991
	21st to end	Day	13% 7% 2 7% 5% 5% 6% 5% 7% 9% 34%	6717
June	1st to 10th	Day	11% 6% 2 6% 4 3 4 4 5% 9% 46%	2801
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 8% 46%	2800
	21st to end	Day	8% 5% 2 5% 4 4 5% 5% 7% 10% 44%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 43%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	6360
	21st to end	Day	9% 7% 2 8% 6% 7% 7% 5% 8% 10% 31%	6749
August	1st to 10th	Day	7% 5% 1 5% 4 4 4 4 6% 10% 51%	4151
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	3849
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 9% 54%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 3 5% 7% 48%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 8% 50%	2205
	21st to end	Day	12% 6% 2 5% 3 3 4 3 4 6% 52%	2200
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 4 6% 56%	2200
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 6% 49%	2070
	21st to end	Day	19% 8% 2 6% 4 3 4 3 4 6% 42%	1980
November	1st to 10th	Day	9% 5% 2 6% 4 3 4 3 5% 6% 52%	1852
	11th to 20th	Day	16% 9% 3 6% 4 3 4 3 4 7% 42%	1890
	21st to end	Day	20% 10% 4 6% 4 4 3 3 4 5% 38%	1890
December	1st to 10th	Day	15% 11% 3 6% 3 3 4 3 3 4 45%	3020
	11th to 20th	Day	16% 12% 6% 10% 5% 4 4 3 4 6% 30%	3131
	21st to end	Day	26% 17% 5% 9% 6% 4 3 2 3 5% 20%	4070
January	1st to 10th	Night	16% 13% 5% 9% 4 4 4 4 5% 5% 32%	8232
	11th to 20th	Night	13% 13% 4 11% 5% 6% 5% 3 4 5% 32%	8280
	21st to end	Night	18% 12% 4 7% 4 4 4 3 4 6% 34%	8506
February	1st to 10th	Night	21% 10% 4 9% 5% 5% 4 3 4 5% 29%	2520
	11th to 20th	Night	17% 10% 2 8% 5% 4 3 3 5% 6% 36%	2458
	21st to end	Night	19% 12% 3 7% 4 4 3 3 5% 5% 35%	2076
March	1st to 10th	Night	21% 11% 3 6% 4 4 3 3 4 5% 35%	5493
	11th to 20th	Night	13% 6% 2 6% 3 4 4 4 4 6% 48%	5372
	21st to end	Night	17% 8% 4 9% 5% 5% 5% 4 5% 5% 34%	5216
April	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 6% 7% 42%	2100
	11th to 20th	Night	16% 7% 2 6% 4 3 4 4 5% 7% 43%	2075
	21st to end	Night	17% 6% 2 6% 5% 5% 4 4 7% 7% 37%	2012
May	1st to 10th	Night	18% 10% 2 8% 5% 5% 5% 3 5% 8% 31%	4123
	11th to 20th	Night	15% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	4122
	21st to end	Night	13% 6% 3 7% 4 5% 5% 4 7% 9% 35%	4368
June	1st to 10th	Night	10% 5% 1 6% 4 3 4 4 7% 8% 48%	1600
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 3 6% 10% 48%	1600
	21st to end	Night	5% 4 2 4 4 4 6% 4 7% 9% 52%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 6% 8% 47%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	3617
	21st to end	Night	7% 6% 2 8% 5% 6% 6% 5% 8% 11% 35%	4112
August	1st to 10th	Night	7% 4 2 4 4 3 3 3 5% 8% 57%	2980
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 6% 8% 50%	3200
	21st to end	Night	3 3 2 4 3 3 4 3 6% 9% 60%	3586
September	1st to 10th	Night	10% 4 2 6% 4 4 4 3 6% 7% 51%	2000
	11th to 20th	Night	6% 5% 2 6% 3 3 4 3 6% 6% 54%	2175
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 7% 50%	2200
October	1st to 10th	Night	8% 4 2 5% 3 3 3 3 5% 5% 59%	2200
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	2325
	21st to end	Night	18% 8% 3 5% 4 3 3 2 4 5% 45%	2860
November	1st to 10th	Night	11% 6% 2 6% 4 3 4 3 5% 5% 51%	2666
	11th to 20th	Night	14% 7% 3 6% 3 3 3 3 5% 6% 46%	2730
	21st to end	Night	20% 9% 3 5% 4 3 3 2 4 5% 41%	2730
December	1st to 10th	Night	17% 11% 3 7% 4 3 3 3 3 5% 41%	4380
	11th to 20th	Night	19% 10% 3 7% 3 3 4 3 5% 6% 37%	5289
	21st to end	Night	27% 15% 4 8% 5% 4 3 3 4 4 22%	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	57% 4 12% 21% 6%	4692
	11th to 20th	Day	54% 2 10% 21% 11% 1	4710
	21st to end	Day	60% 3 9% 16% 8% 4	5680
February	1st to 10th	Day	52% 4 11% 21% 11% 1	2100
	11th to 20th	Day	51% 4 10% 21% 12% 2	2058
	21st to end	Day	52% 3 10% 20% 12% 3	1730
March	1st to 10th	Day	50% 3 4 19% 19% 4	4580
	11th to 20th	Day	40% 4 10% 28% 16% 2	4807
	21st to end	Day	47% 5% 13% 23% 12% 1	6000
April	1st to 10th	Day	43% 7% 13% 21% 16% 1	2520
	11th to 20th	Day	44% 5% 11% 23% 14% 3	2494
	21st to end	Day	44% 5% 11% 26% 11% 2	2606
May	1st to 10th	Day	51% 7% 13% 18% 10% 1	6027
	11th to 20th	Day	47% 6% 12% 23% 11% 1	5995
	21st to end	Day	44% 7% 13% 20% 15% 2	6731
June	1st to 10th	Day	35% 5% 11% 27% 18% 3	2801
	11th to 20th	Day	33% 8% 13% 24% 18% 4	2800
	21st to end	Day	33% 8% 14% 22% 18% 4	2800
July	1st to 10th	Day	36% 6% 10% 26% 19% 2	6291
	11th to 20th	Day	37% 8% 14% 26% 13% 1	6285
	21st to end	Day	45% 9% 16% 21% 8% 1	6555
August	1st to 10th	Day	27% 7% 14% 35% 15% 1	4138
	11th to 20th	Day	35% 7% 13% 26% 18% 1	3885
	21st to end	Day	26% 6% 13% 35% 16% 4	4311
September	1st to 10th	Day	35% 6% 10% 24% 21% 5%	2400
	11th to 20th	Day	31% 5% 10% 29% 21% 3	2205
	21st to end	Day	34% 5% 8% 24% 25% 4	2200
October	1st to 10th	Day	20% 4 5% 27% 42% 3	3590
	11th to 20th	Day	25% 3 9% 26% 33% 4	3350
	21st to end	Day	35% 3 8% 24% 26% 4	3180
November	1st to 10th	Day	33% 4 10% 29% 17% 5%	1852
	11th to 20th	Day	44% 3 9% 23% 18% 4	1890
	21st to end	Day	49% 3 9% 22% 15% 1	1890
December	1st to 10th	Day	44% 3 13% 26% 14%	3020
	11th to 20th	Day	57% 3 9% 20% 9% 1	3128
	21st to end	Day	68% 3 12% 12% 5%	4070
January	1st to 10th	Night	54% 4 12% 22% 7%	8233
	11th to 20th	Night	56% 3 6% 18% 15% 3	8280
	21st to end	Night	51% 3 11% 21% 13% 1	8506
February	1st to 10th	Night	57% 4 10% 19% 8% 2	2520
	11th to 20th	Night	49% 5% 9% 22% 14% 1	2459
	21st to end	Night	51% 4 11% 21% 11% 3	2076
March	1st to 10th	Night	52% 4 6% 20% 15% 4	5493
	11th to 20th	Night	37% 4 10% 25% 21% 2	5372
	21st to end	Night	51% 5% 9% 24% 10% 1	5216
April	1st to 10th	Night	40% 6% 11% 26% 15% 3	2100
	11th to 20th	Night	41% 4 12% 20% 18% 5%	2075
	21st to end	Night	45% 6% 10% 23% 15% 2	2012
May	1st to 10th	Night	50% 7% 14% 21% 8% 1	4150
	11th to 20th	Night	45% 8% 11% 24% 11%	4123
	21st to end	Night	42% 7% 15% 20% 13% 3	4376
June	1st to 10th	Night	32% 7% 12% 24% 22% 4	1600
	11th to 20th	Night	32% 6% 13% 21% 22% 7%	1600
	21st to end	Night	26% 8% 16% 25% 19% 7%	1600
July	1st to 10th	Night	35% 6% 10% 25% 21% 3	3580
	11th to 20th	Night	32% 8% 15% 28% 15% 2	3572
	21st to end	Night	39% 8% 17% 23% 11% 1	3993
August	1st to 10th	Night	25% 6% 15% 38% 14% 2	2969
	11th to 20th	Night	29% 7% 14% 30% 19% 1	3230
	21st to end	Night	21% 5% 14% 31% 26% 4	3586
September	1st to 10th	Night	31% 6% 8% 27% 22% 6%	2000
	11th to 20th	Night	29% 5% 9% 25% 27% 5%	2175
	21st to end	Night	34% 5% 9% 21% 24% 6%	2200
October	1st to 10th	Night	18% 3 6% 17% 50% 5%	3590
	11th to 20th	Night	25% 2 5% 26% 37% 4	3784
	21st to end	Night	33% 3 8% 23% 29% 6%	4610
November	1st to 10th	Night	35% 4 10% 24% 23% 5%	2666
	11th to 20th	Night	39% 3 9% 23% 21% 5%	2730
	21st to end	Night	47% 3 10% 23% 15% 2	2730
December	1st to 10th	Night	48% 3 11% 24% 15%	4380
	11th to 20th	Night	49% 4 12% 22% 12% 1	5291
	21st to end	Night	67% 4 9% 14% 5% 1	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	24% 17% 31% 22% 6% 1	330
	11th to 20th	Day	24% 23% 34% 16% 3	330
	21st to end	Day	25% 18% 39% 14% 4	363
February	1st to 10th	Day	25% 17% 35% 18% 5% 1	330
	11th to 20th	Day	25% 20% 40% 12% 3 1	330
	21st to end	Day	25% 14% 41% 15% 5%	273
March	1st to 10th	Day	25% 16% 39% 16% 4	330
	11th to 20th	Day	25% 18% 36% 15% 5% 1	413
	21st to end	Day	25% 15% 37% 17% 5% 1	594
April	1st to 10th	Day	20% 15% 32% 24% 9% 1	540
	11th to 20th	Day	21% 16% 26% 23% 11% 2	532
	21st to end	Day	20% 11% 33% 24% 10% 3	540
May	1st to 10th	Day	19% 12% 28% 26% 10% 4	543
	11th to 20th	Day	19% 8% 25% 26% 14% 7%	567
	21st to end	Day	14% 5% 27% 25% 18% 11%	680
June	1st to 10th	Day	14% 5% 18% 29% 24% 9%	731
	11th to 20th	Day	8% 4 17% 34% 24% 12%	730
	21st to end	Day	10% 4 22% 29% 24% 12%	730
July	1st to 10th	Day	12% 2 12% 25% 27% 21%	721
	11th to 20th	Day	9% 3 13% 33% 29% 14%	718
	21st to end	Day	10% 3 11% 29% 33% 15%	719
August	1st to 10th	Day	11% 2 11% 28% 30% 18%	526
	11th to 20th	Day	14% 3 11% 30% 30% 13%	519
	21st to end	Day	7% 2 9% 25% 36% 21%	583
September	1st to 10th	Day	9% 3 16% 26% 32% 14%	530
	11th to 20th	Day	7% 3 10% 27% 35% 18%	530
	21st to end	Day	15% 6% 19% 24% 24% 12%	530
October	1st to 10th	Day	10% 4 19% 32% 25% 9%	530
	11th to 20th	Day	13% 10% 32% 23% 16% 5%	410
	21st to end	Day	27% 13% 30% 23% 6% 1	363
November	1st to 10th	Day	12% 12% 35% 26% 11% 5%	330
	11th to 20th	Day	19% 16% 32% 22% 9% 2	330
	21st to end	Day	28% 9% 30% 24% 7% 1	330
December	1st to 10th	Day	18% 20% 36% 17% 7% 2	330
	11th to 20th	Day	24% 13% 36% 20% 7% 1	330
	21st to end	Day	28% 18% 31% 19% 3	363
January	1st to 10th	Night	22% 13% 32% 28% 3	1050
	11th to 20th	Night	24% 15% 36% 22% 3	1050
	21st to end	Night	21% 11% 40% 25% 2	1088
February	1st to 10th	Night	25% 11% 35% 26% 3	840
	11th to 20th	Night	24% 13% 37% 24% 3	823
	21st to end	Night	25% 12% 38% 23% 2	692
March	1st to 10th	Night	24% 12% 33% 27% 4	840
	11th to 20th	Night	23% 10% 34% 25% 6% 1	840
	21st to end	Night	23% 9% 30% 32% 5% 1	759
April	1st to 10th	Night	16% 10% 33% 33% 7%	630
	11th to 20th	Night	20% 11% 28% 29% 12%	623
	21st to end	Night	21% 6% 29% 33% 8% 1	630
May	1st to 10th	Night	17% 6% 29% 33% 13% 1	630
	11th to 20th	Night	16% 6% 21% 36% 20% 2	621
	21st to end	Night	11% 4 18% 37% 27% 3	683
June	1st to 10th	Night	12% 3 16% 41% 27% 2	600
	11th to 20th	Night	9% 2 16% 44% 26% 2	600
	21st to end	Night	7% 1 16% 41% 33% 3	600
July	1st to 10th	Night	11% 2 8% 31% 40% 9%	585
	11th to 20th	Night	6% 4 7% 39% 39% 5%	582
	21st to end	Night	9% 2 9% 33% 40% 6%	657
August	1st to 10th	Night	8% 2 9% 36% 40% 6%	597
	11th to 20th	Night	7% 1 9% 39% 39% 6%	594
	21st to end	Night	6% 1 7% 37% 42% 7%	660
September	1st to 10th	Night	11% 3 10% 33% 38% 5%	600
	11th to 20th	Night	8% 2 11% 32% 40% 6%	775
	21st to end	Night	14% 4 19% 31% 29% 4	800
October	1st to 10th	Night	10% 4 17% 37% 26% 6%	800
	11th to 20th	Night	16% 7% 26% 33% 17% 2	800
	21st to end	Night	23% 7% 33% 26% 11%	880
November	1st to 10th	Night	15% 8% 29% 36% 12% 1	820
	11th to 20th	Night	17% 12% 32% 31% 9%	840
	21st to end	Night	24% 12% 26% 29% 9%	840
December	1st to 10th	Night	25% 7% 33% 30% 5% 1	840
	11th to 20th	Night	25% 8% 33% 28% 6%	1013
	21st to end	Night	29% 12% 33% 24% 3	1155

Page generated at 2026-05-29 01:53:30. Version ac07

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