Red Barn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Red Barn Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	10% 6% 2 6% 5% 5% 5% 4 7% 9% 41%	101264
	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 43%	98450
Transparency	Day	38% 6% 13% 19% 16% 7%	105094
	Night	37% 6% 12% 18% 18% 9%	102759
Seeing	Day	12% 5% 18% 25% 24% 17%	15192
	Night	12% 5% 20% 33% 25% 5%	25223

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	12% 7% 2 6% 5% 5% 5% 4 5% 6% 45%	2836
	11th to 20th	Day	12% 7% 2 4 4 3 3 3 6% 9% 48%	2840
	21st to end	Day	13% 8% 3 6% 5% 4 4 3 5% 6% 44%	3325
February	1st to 10th	Day	15% 7% 2 7% 5% 3 4 4 5% 7% 41%	1900
	11th to 20th	Day	16% 7% 2 5% 4 4 3 3 5% 6% 46%	1848
	21st to end	Day	15% 8% 2 5% 4 4 3 3 6% 7% 44%	1570
March	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 4 7% 36%	3312
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 4 6% 9% 49%	3480
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 6% 44%	3815
April	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 6% 45%	2090
	11th to 20th	Day	11% 4 2 5% 5% 3 4 4 5% 8% 48%	2180
	21st to end	Day	7% 4 2 5% 4 4 5% 4 5% 7% 55%	2418
May	1st to 10th	Day	8% 4 2 5% 4 5% 5% 4 6% 9% 48%	4082
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 7% 8% 38%	4081
	21st to end	Day	5% 3 2 5% 4 5% 5% 5% 8% 11% 48%	4476
June	1st to 10th	Day	9% 7% 3 7% 6% 6% 7% 6% 8% 10% 33%	2470
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 6% 8% 12% 39%	2470
	21st to end	Day	6% 6% 3 8% 6% 7% 7% 7% 10% 11% 30%	2470
July	1st to 10th	Day	7% 7% 3 9% 7% 8% 9% 8% 10% 12% 20%	4120
	11th to 20th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 13% 37%	4120
	21st to end	Day	6% 6% 4 9% 8% 8% 7% 7% 10% 14% 22%	4466
August	1st to 10th	Day	11% 5% 2 7% 6% 6% 7% 6% 9% 12% 28%	4088
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 42%	3729
	21st to end	Day	7% 6% 3 8% 6% 6% 7% 5% 8% 11% 33%	4062
September	1st to 10th	Day	11% 7% 3 7% 6% 7% 7% 5% 7% 10% 31%	2090
	11th to 20th	Day	12% 8% 3 7% 6% 5% 5% 4 7% 9% 35%	2090
	21st to end	Day	11% 5% 2 5% 4 5% 5% 5% 6% 8% 45%	2090
October	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 5% 6% 50%	2090
	11th to 20th	Day	8% 4 2 4 3 3 3 3 5% 7% 58%	1981
	21st to end	Day	8% 4 2 5% 3 4 4 4 4 6% 56%	2090
November	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 7% 52%	1926
	11th to 20th	Day	10% 6% 2 4 4 3 3 3 5% 4 55%	1800
	21st to end	Day	12% 6% 2 5% 4 4 3 3 5% 8% 47%	1800
December	1st to 10th	Day	13% 6% 2 5% 5% 4 4 3 6% 7% 45%	1782
	11th to 20th	Day	16% 7% 2 5% 4 4 4 3 5% 6% 42%	2153
	21st to end	Day	17% 10% 3 7% 5% 3 3 2 3 5% 41%	3124
January	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	4109
	11th to 20th	Night	8% 7% 3 7% 5% 4 5% 4 6% 7% 46%	4120
	21st to end	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	4326
February	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 5% 7% 44%	2280
	11th to 20th	Night	15% 7% 2 6% 4 4 3 3 4 6% 47%	2218
	21st to end	Night	12% 7% 2 5% 3 5% 4 3 5% 7% 47%	1884
March	1st to 10th	Night	21% 11% 3 7% 4 3 3 3 4 5% 36%	3527
	11th to 20th	Night	11% 5% 2 5% 3 4 5% 4 6% 8% 47%	3480
	21st to end	Night	15% 5% 2 5% 3 4 4 4 5% 7% 45%	3817
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 7% 49%	2090
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 5% 8% 49%	1963
	21st to end	Night	6% 4 1 4 4 4 4 3 5% 7% 59%	1762
May	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 5% 6% 9% 46%	2813
	11th to 20th	Night	12% 7% 3 7% 4 5% 5% 3 6% 6% 43%	2812
	21st to end	Night	5% 4 2 5% 4 4 4 3 6% 10% 53%	3079
June	1st to 10th	Night	7% 5% 3 7% 6% 6% 6% 6% 10% 12% 32%	1710
	11th to 20th	Night	5% 5% 2 5% 5% 5% 6% 6% 10% 12% 40%	1710
	21st to end	Night	6% 5% 2 8% 7% 6% 7% 5% 9% 11% 35%	1710
July	1st to 10th	Night	10% 10% 5% 12% 7% 7% 7% 5% 7% 11% 20%	2840
	11th to 20th	Night	4 4 2 6% 6% 5% 6% 6% 11% 14% 35%	2840
	21st to end	Night	6% 7% 3 8% 7% 7% 8% 6% 11% 12% 26%	3087
August	1st to 10th	Night	11% 5% 2 8% 6% 6% 6% 5% 8% 12% 29%	2825
	11th to 20th	Night	5% 5% 2 6% 4 5% 7% 5% 7% 12% 41%	3098
	21st to end	Night	5% 5% 3 6% 6% 5% 5% 5% 8% 11% 39%	3560
September	1st to 10th	Night	10% 7% 3 9% 5% 5% 5% 4 8% 12% 31%	2090
	11th to 20th	Night	11% 7% 3 8% 5% 5% 5% 5% 6% 8% 37%	2090
	21st to end	Night	9% 5% 2 7% 5% 4 5% 4 6% 9% 43%	2090
October	1st to 10th	Night	9% 6% 2 5% 3 4 3 3 6% 8% 51%	2090
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 4 6% 59%	2198
	21st to end	Night	8% 5% 1 5% 4 4 4 4 4 5% 56%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 4 6% 54%	2369
	11th to 20th	Night	11% 5% 2 5% 3 3 3 4 4 6% 52%	2600
	21st to end	Night	11% 7% 2 5% 3 3 4 3 6% 7% 49%	2600
December	1st to 10th	Night	11% 6% 2 6% 4 5% 3 3 6% 7% 47%	2581
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 5% 49%	3042
	21st to end	Night	16% 10% 3 8% 5% 4 4 3 5% 6% 37%	4532

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	40% 3 6% 18% 22% 11%	2837
	11th to 20th	Day	35% 3 6% 18% 24% 14%	2840
	21st to end	Day	42% 3 7% 13% 22% 13%	3325
February	1st to 10th	Day	43% 3 8% 16% 20% 9%	1900
	11th to 20th	Day	40% 3 5% 16% 26% 11%	1858
	21st to end	Day	40% 3 9% 18% 19% 12%	1570
March	1st to 10th	Day	48% 4 8% 18% 14% 7%	3312
	11th to 20th	Day	32% 4 14% 14% 23% 13%	3480
	21st to end	Day	40% 4 10% 23% 15% 8%	3815
April	1st to 10th	Day	37% 5% 9% 21% 17% 11%	2090
	11th to 20th	Day	35% 4 7% 20% 23% 10%	2180
	21st to end	Day	29% 4 9% 25% 25% 8%	2418
May	1st to 10th	Day	31% 6% 14% 27% 17% 5%	4120
	11th to 20th	Day	43% 5% 12% 22% 15% 2	4085
	21st to end	Day	28% 6% 20% 27% 18% 2	4489
June	1st to 10th	Day	43% 7% 16% 22% 10% 1	2470
	11th to 20th	Day	34% 8% 19% 24% 14% 1	2470
	21st to end	Day	41% 9% 22% 17% 9% 2	2470
July	1st to 10th	Day	48% 15% 25% 9% 3	4055
	11th to 20th	Day	35% 12% 32% 19% 3	4051
	21st to end	Day	46% 16% 24% 12% 2	4324
August	1st to 10th	Day	44% 10% 22% 21% 4	4075
	11th to 20th	Day	31% 8% 19% 30% 12% 1	3765
	21st to end	Day	42% 8% 20% 25% 4 1	4062
September	1st to 10th	Day	46% 8% 16% 17% 11% 3	2090
	11th to 20th	Day	46% 6% 12% 17% 15% 3	2090
	21st to end	Day	36% 4 12% 23% 19% 7%	2090
October	1st to 10th	Day	40% 3 6% 11% 20% 20%	3480
	11th to 20th	Day	17% 3 4 18% 25% 33%	3273
	21st to end	Day	19% 2 4 20% 38% 17%	3428
November	1st to 10th	Day	32% 4 8% 18% 24% 15%	1926
	11th to 20th	Day	33% 3 6% 17% 26% 16%	1800
	21st to end	Day	36% 3 7% 19% 24% 11%	1800
December	1st to 10th	Day	39% 4 11% 17% 20% 10%	1782
	11th to 20th	Day	42% 3 6% 17% 25% 8%	2150
	21st to end	Day	48% 2 4 16% 24% 6%	3124
January	1st to 10th	Night	41% 4 6% 13% 21% 15%	4109
	11th to 20th	Night	37% 3 6% 15% 28% 11%	4120
	21st to end	Night	45% 4 6% 13% 23% 9%	4326
February	1st to 10th	Night	39% 4 8% 16% 21% 12%	2280
	11th to 20th	Night	40% 4 6% 13% 22% 15%	2219
	21st to end	Night	37% 4 7% 19% 17% 15%	1884
March	1st to 10th	Night	51% 4 9% 15% 13% 8%	3527
	11th to 20th	Night	33% 5% 10% 18% 22% 13%	3480
	21st to end	Night	38% 3 9% 17% 21% 11%	3817
April	1st to 10th	Night	34% 5% 11% 17% 20% 13%	2090
	11th to 20th	Night	33% 4 9% 19% 24% 11%	1963
	21st to end	Night	26% 4 9% 27% 25% 9%	1762
May	1st to 10th	Night	32% 7% 13% 25% 16% 6%	2840
	11th to 20th	Night	40% 6% 14% 24% 13% 3	2813
	21st to end	Night	26% 6% 16% 29% 21% 3	3087
June	1st to 10th	Night	39% 9% 20% 20% 12% 1	1710
	11th to 20th	Night	32% 8% 24% 22% 11% 1	1710
	21st to end	Night	39% 11% 21% 18% 8% 2	1710
July	1st to 10th	Night	56% 12% 19% 10% 3 1	2795
	11th to 20th	Night	32% 15% 31% 18% 3	2786
	21st to end	Night	45% 15% 24% 13% 2 1	2988
August	1st to 10th	Night	44% 10% 27% 16% 3	2814
	11th to 20th	Night	32% 11% 21% 26% 8% 1	3128
	21st to end	Night	35% 9% 27% 24% 4 1	3560
September	1st to 10th	Night	44% 10% 18% 15% 11% 2	2090
	11th to 20th	Night	43% 8% 14% 16% 16% 3	2090
	21st to end	Night	37% 6% 14% 19% 17% 8%	2090
October	1st to 10th	Night	37% 4 9% 13% 19% 17%	3480
	11th to 20th	Night	17% 2 3 15% 29% 34%	3645
	21st to end	Night	19% 3 4 17% 38% 18%	4120
November	1st to 10th	Night	31% 3 8% 18% 23% 16%	2369
	11th to 20th	Night	33% 3 6% 15% 26% 15%	2600
	21st to end	Night	34% 3 9% 20% 21% 13%	2600
December	1st to 10th	Night	36% 3 14% 16% 23% 8%	2581
	11th to 20th	Night	38% 3 6% 16% 25% 11%	3044
	21st to end	Night	49% 3 5% 16% 22% 4	4532

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	11% 14% 43% 24% 7% 1	270
	11th to 20th	Day	11% 12% 31% 30% 11% 4	270
	21st to end	Day	22% 14% 43% 16% 4 1	297
February	1st to 10th	Day	20% 11% 34% 24% 10%	270
	11th to 20th	Day	23% 10% 34% 26% 7%	270
	21st to end	Day	15% 14% 36% 28% 6%	225
March	1st to 10th	Day	15% 7% 31% 30% 16% 1	384
	11th to 20th	Day	14% 6% 33% 32% 13% 2	460
	21st to end	Day	14% 8% 32% 31% 13% 1	506
April	1st to 10th	Day	15% 11% 26% 27% 18% 3	460
	11th to 20th	Day	13% 7% 26% 30% 19% 5%	452
	21st to end	Day	10% 3 20% 35% 23% 8%	460
May	1st to 10th	Day	6% 6% 21% 29% 25% 12%	463
	11th to 20th	Day	10% 3 14% 28% 27% 19%	487
	21st to end	Day	9% 1 9% 25% 32% 24%	529
June	1st to 10th	Day	11% 1 8% 24% 32% 25%	490
	11th to 20th	Day	7% 1 10% 21% 29% 32%	490
	21st to end	Day	9% 1 4 19% 37% 31%	490
July	1st to 10th	Day	8% 1 4 12% 31% 44%	485
	11th to 20th	Day	9% 2 1 8% 30% 51%	484
	21st to end	Day	7% 2 4 10% 35% 42%	532
August	1st to 10th	Day	12% 2 3 14% 30% 38%	486
	11th to 20th	Day	7% 1 2 12% 30% 47%	479
	21st to end	Day	11% 2 4 18% 35% 30%	539
September	1st to 10th	Day	16% 3 3 21% 29% 29%	490
	11th to 20th	Day	15% 1 5% 28% 36% 15%	490
	21st to end	Day	12% 1 9% 24% 31% 23%	490
October	1st to 10th	Day	14% 2 17% 35% 24% 8%	490
	11th to 20th	Day	8% 3 21% 31% 26% 12%	490
	21st to end	Day	10% 5% 19% 35% 26% 5%	539
November	1st to 10th	Day	14% 3 22% 31% 22% 8%	464
	11th to 20th	Day	15% 6% 33% 30% 11% 5%	300
	21st to end	Day	10% 10% 31% 29% 16% 4	300
December	1st to 10th	Day	16% 7% 31% 28% 13% 4	294
	11th to 20th	Day	24% 6% 32% 26% 10% 2	270
	21st to end	Day	21% 6% 30% 27% 15% 1	297
January	1st to 10th	Night	16% 11% 35% 33% 6% 1	760
	11th to 20th	Night	13% 10% 31% 33% 12% 1	760
	21st to end	Night	19% 9% 37% 31% 4	836
February	1st to 10th	Night	16% 9% 30% 37% 8%	760
	11th to 20th	Night	18% 8% 33% 34% 7%	743
	21st to end	Night	15% 8% 35% 37% 4	628
March	1st to 10th	Night	16% 7% 30% 39% 9%	760
	11th to 20th	Night	13% 7% 32% 34% 13% 1	760
	21st to end	Night	11% 9% 33% 34% 13%	836
April	1st to 10th	Night	13% 6% 29% 37% 13% 1	760
	11th to 20th	Night	10% 6% 27% 41% 14% 2	651
	21st to end	Night	8% 3 26% 40% 22% 2	570
May	1st to 10th	Night	8% 4 23% 38% 24% 3	570
	11th to 20th	Night	9% 2 19% 39% 26% 5%	561
	21st to end	Night	9% 1 9% 38% 35% 8%	617
June	1st to 10th	Night	10% 1 5% 38% 38% 8%	570
	11th to 20th	Night	7% 2 6% 31% 41% 13%	570
	21st to end	Night	7% 1 3 32% 47% 11%	570
July	1st to 10th	Night	10% 2 3 24% 43% 19%	555
	11th to 20th	Night	8% 1 1 15% 53% 23%	552
	21st to end	Night	6% 1 2 20% 57% 14%	624
August	1st to 10th	Night	9% 1 1 24% 51% 14%	567
	11th to 20th	Night	9% 2 2 18% 51% 19%	564
	21st to end	Night	9% 2 4 27% 41% 17%	672
September	1st to 10th	Night	12% 2 5% 24% 43% 12%	760
	11th to 20th	Night	15% 2 5% 35% 38% 5%	760
	21st to end	Night	11% 2 11% 30% 41% 5%	760
October	1st to 10th	Night	12% 2 16% 38% 29% 3	760
	11th to 20th	Night	11% 2 19% 37% 27% 3	760
	21st to end	Night	10% 4 21% 38% 25% 1	836
November	1st to 10th	Night	13% 4 21% 39% 21% 2	780
	11th to 20th	Night	14% 7% 27% 35% 17% 2	800
	21st to end	Night	13% 5% 30% 35% 17% 1	800
December	1st to 10th	Night	14% 6% 32% 33% 15% 1	795
	11th to 20th	Night	16% 5% 31% 31% 16% 1	760
	21st to end	Night	22% 8% 25% 32% 13%	836

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