Raven Hill Discovery Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Raven Hill Discovery Center clear sky chart from 2005-11-06 to 2025-05-11.

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	14% 12% 5% 11% 5% 4 4 3 5% 6% 30%	113094
	Night	14% 15% 6% 13% 6% 5% 4 3 4 5% 26%	115992
Transparency	Day	54% 6% 10% 19% 10% 1	116622
	Night	60% 6% 10% 16% 8% 1	120614
Seeing	Day	20% 14% 30% 23% 10% 3	16247
	Night	21% 12% 29% 27% 9% 1	26056

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	9% 20% 12% 30% 6% 5% 3 2 2 3 8%	3617
	11th to 20th	Day	10% 19% 7% 27% 12% 11% 4 2 3 2 4	3589
	21st to end	Day	13% 19% 9% 22% 7% 6% 6% 3 3 3 10%	3982
February	1st to 10th	Day	9% 20% 7% 16% 15% 6% 3 2 3 5% 13%	1725
	11th to 20th	Day	12% 18% 7% 16% 10% 6% 4 3 3 4 17%	1916
	21st to end	Day	12% 19% 7% 14% 7% 4 4 3 5% 5% 20%	1650
March	1st to 10th	Day	13% 11% 4 12% 6% 5% 4 4 6% 5% 31%	4478
	11th to 20th	Day	11% 13% 6% 13% 5% 4 5% 4 5% 7% 26%	4883
	21st to end	Day	21% 12% 8% 15% 6% 4 4 2 3 5% 20%	5619
April	1st to 10th	Day	19% 16% 4 9% 5% 4 4 3 5% 5% 27%	2400
	11th to 20th	Day	21% 14% 3 5% 4 3 3 3 5% 6% 33%	2498
	21st to end	Day	16% 11% 4 8% 4 4 5% 4 6% 7% 31%	2600
May	1st to 10th	Day	15% 9% 2 7% 4 4 4 4 5% 6% 41%	4238
	11th to 20th	Day	11% 8% 2 6% 4 4 4 4 4 6% 47%	4204
	21st to end	Day	13% 6% 2 5% 4 4 5% 3 6% 8% 43%	4837
June	1st to 10th	Day	14% 6% 2 6% 4 5% 4 4 6% 8% 41%	2836
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 5% 6% 9% 40%	2850
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 6% 8% 40%	2826
July	1st to 10th	Day	19% 6% 2 5% 3 4 4 3 6% 8% 39%	4440
	11th to 20th	Day	11% 4 2 4 3 4 4 3 6% 9% 49%	4440
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 9% 47%	4814
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 8% 40%	4231
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 46%	4044
	21st to end	Day	15% 7% 2 7% 4 4 4 3 5% 5% 44%	4387
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 5% 8% 42%	2241
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2090
	21st to end	Day	17% 8% 2 7% 4 3 4 4 5% 9% 37%	2090
October	1st to 10th	Day	19% 10% 3 9% 4 3 3 3 5% 6% 35%	2085
	11th to 20th	Day	26% 16% 3 10% 6% 4 4 3 4 5% 19%	1800
	21st to end	Day	23% 19% 5% 10% 5% 4 5% 3 5% 5% 15%	1881
November	1st to 10th	Day	17% 21% 5% 8% 4 4 3 3 5% 5% 24%	1762
	11th to 20th	Day	13% 26% 8% 12% 5% 4 5% 3 5% 5% 15%	1800
	21st to end	Day	16% 27% 11% 12% 5% 4 2 2 4 4 13%	1616
December	1st to 10th	Day	11% 32% 12% 18% 6% 3 3 2 3 2 7%	2280
	11th to 20th	Day	11% 25% 9% 22% 9% 5% 4 3 3 2 6%	2621
	21st to end	Day	20% 23% 8% 16% 6% 5% 3 2 3 4 9%	3724
January	1st to 10th	Night	10% 19% 10% 31% 7% 4 3 2 2 3 9%	6348
	11th to 20th	Night	11% 16% 8% 22% 17% 9% 4 2 3 2 6%	6316
	21st to end	Night	11% 20% 10% 21% 9% 7% 4 2 3 3 9%	6996
February	1st to 10th	Night	9% 19% 7% 18% 12% 7% 4 3 4 3 13%	2675
	11th to 20th	Night	11% 16% 7% 17% 12% 6% 5% 2 3 5% 18%	2368
	21st to end	Night	13% 20% 4 14% 6% 5% 4 3 4 5% 22%	1980
March	1st to 10th	Night	16% 11% 4 9% 6% 4 3 3 4 6% 34%	5362
	11th to 20th	Night	11% 15% 4 9% 5% 4 4 4 5% 7% 31%	5076
	21st to end	Night	20% 13% 6% 10% 4 3 3 3 4 5% 27%	5342
April	1st to 10th	Night	19% 13% 4 10% 5% 4 4 3 4 6% 28%	2000
	11th to 20th	Night	21% 11% 4 6% 4 3 4 4 3 6% 33%	1850
	21st to end	Night	17% 11% 3 8% 5% 4 3 3 6% 6% 34%	1800
May	1st to 10th	Night	15% 9% 2 6% 5% 5% 4 3 5% 6% 40%	2923
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 6% 6% 41%	2697
	21st to end	Night	18% 7% 2 6% 4 4 3 3 5% 7% 42%	2750
June	1st to 10th	Night	15% 5% 3 6% 4 4 5% 3 5% 8% 42%	1340
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 9% 43%	1330
	21st to end	Night	13% 8% 1 4 5% 4 4 4 4 7% 44%	1347
July	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 43%	2530
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 3 5% 5% 51%	2530
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	2750
August	1st to 10th	Night	13% 6% 3 6% 4 5% 4 4 6% 7% 43%	2701
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 5% 7% 49%	2785
	21st to end	Night	12% 6% 2 6% 4 4 4 4 5% 6% 47%	3252
September	1st to 10th	Night	13% 6% 2 6% 3 3 4 3 5% 7% 49%	1919
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 4 8% 47%	2090
	21st to end	Night	19% 8% 3 4 5% 4 4 4 6% 7% 38%	2090
October	1st to 10th	Night	19% 9% 3 7% 4 4 3 2 5% 6% 38%	2095
	11th to 20th	Night	22% 16% 4 8% 4 4 4 3 5% 5% 24%	2375
	21st to end	Night	26% 16% 3 7% 5% 4 5% 2 4 5% 22%	2717
November	1st to 10th	Night	17% 20% 5% 8% 5% 4 3 3 4 6% 26%	2536
	11th to 20th	Night	15% 24% 6% 13% 6% 4 4 3 4 4 18%	2600
	21st to end	Night	17% 24% 10% 12% 6% 4 4 3 4 4 13%	2777
December	1st to 10th	Night	11% 28% 13% 18% 6% 5% 4 2 3 3 8%	4910
	11th to 20th	Night	11% 23% 9% 21% 8% 5% 3 3 3 3 10%	5586
	21st to end	Night	20% 21% 9% 16% 6% 4 4 3 3 4 10%	7249

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	84% 3 5% 6% 1	3618
	11th to 20th	Day	89% 4 4 3 1	3620
	21st to end	Day	79% 7% 8% 4 2	3982
February	1st to 10th	Day	76% 6% 6% 10% 2	1725
	11th to 20th	Day	70% 6% 9% 9% 6%	1926
	21st to end	Day	66% 6% 9% 14% 4	1650
March	1st to 10th	Day	52% 6% 15% 15% 10% 2	4480
	11th to 20th	Day	56% 5% 13% 18% 7% 1	4883
	21st to end	Day	67% 5% 8% 13% 7%	5619
April	1st to 10th	Day	59% 5% 9% 18% 9% 1	2400
	11th to 20th	Day	52% 4 10% 20% 13% 1	2498
	21st to end	Day	51% 7% 10% 15% 14% 3	2600
May	1st to 10th	Day	41% 7% 10% 21% 19% 2	4250
	11th to 20th	Day	37% 7% 9% 28% 18% 2	4208
	21st to end	Day	37% 7% 12% 22% 20% 2	4837
June	1st to 10th	Day	40% 7% 10% 27% 15% 2	2836
	11th to 20th	Day	39% 8% 12% 23% 17% 1	2850
	21st to end	Day	42% 7% 12% 25% 13% 1	2826
July	1st to 10th	Day	43% 6% 14% 24% 12% 1	4371
	11th to 20th	Day	29% 7% 13% 36% 14% 1	4365
	21st to end	Day	32% 8% 12% 30% 18%	4662
August	1st to 10th	Day	38% 9% 12% 27% 13% 1	4216
	11th to 20th	Day	38% 6% 11% 31% 13% 1	4083
	21st to end	Day	41% 7% 12% 29% 10% 1	4386
September	1st to 10th	Day	40% 7% 11% 24% 16% 1	2241
	11th to 20th	Day	38% 8% 13% 28% 12% 1	2090
	21st to end	Day	44% 7% 14% 24% 10%	2090
October	1st to 10th	Day	37% 5% 9% 33% 16%	3462
	11th to 20th	Day	58% 5% 9% 18% 10%	2964
	21st to end	Day	64% 6% 12% 14% 3	3081
November	1st to 10th	Day	61% 6% 9% 18% 6%	1762
	11th to 20th	Day	72% 6% 9% 9% 4	1800
	21st to end	Day	77% 5% 10% 7% 1	1616
December	1st to 10th	Day	85% 5% 6% 4 1	2280
	11th to 20th	Day	85% 5% 5% 4 1	2621
	21st to end	Day	80% 5% 6% 6% 3	3724
January	1st to 10th	Night	81% 5% 6% 7% 2	6348
	11th to 20th	Night	85% 4 6% 3 1	6360
	21st to end	Night	81% 6% 6% 7% 1	6996
February	1st to 10th	Night	77% 5% 6% 8% 3 1	2675
	11th to 20th	Night	70% 5% 8% 13% 3	2369
	21st to end	Night	66% 5% 10% 13% 6%	1980
March	1st to 10th	Night	51% 5% 17% 18% 9%	5373
	11th to 20th	Night	52% 5% 16% 19% 7% 1	5077
	21st to end	Night	58% 5% 10% 17% 9% 1	5342
April	1st to 10th	Night	58% 5% 9% 18% 10% 1	2000
	11th to 20th	Night	53% 4 8% 20% 12% 2	1850
	21st to end	Night	50% 6% 9% 18% 15% 3	1800
May	1st to 10th	Night	44% 6% 11% 20% 16% 3	2930
	11th to 20th	Night	42% 5% 9% 25% 17% 1	2698
	21st to end	Night	42% 6% 13% 20% 15% 5%	2751
June	1st to 10th	Night	40% 6% 11% 25% 16% 1	1340
	11th to 20th	Night	38% 7% 13% 25% 15% 3	1330
	21st to end	Night	40% 7% 11% 25% 15% 2	1347
July	1st to 10th	Night	40% 6% 14% 23% 13% 3	2490
	11th to 20th	Night	32% 8% 13% 29% 17% 1	2482
	21st to end	Night	33% 8% 15% 24% 19% 1	2662
August	1st to 10th	Night	39% 8% 15% 25% 13% 1	2692
	11th to 20th	Night	36% 5% 15% 30% 14% 1	2812
	21st to end	Night	35% 7% 14% 27% 16% 1	3252
September	1st to 10th	Night	35% 7% 14% 25% 18% 2	1919
	11th to 20th	Night	38% 7% 13% 29% 12% 1	2090
	21st to end	Night	46% 7% 15% 23% 9%	2090
October	1st to 10th	Night	41% 6% 11% 29% 13% 1	3498
	11th to 20th	Night	51% 5% 10% 20% 13%	3937
	21st to end	Night	59% 6% 13% 17% 4	4467
November	1st to 10th	Night	59% 7% 11% 17% 6% 1	2536
	11th to 20th	Night	70% 5% 9% 9% 6% 1	2600
	21st to end	Night	76% 7% 10% 7% 1	2777
December	1st to 10th	Night	82% 6% 6% 5%	4910
	11th to 20th	Night	79% 6% 8% 6% 1	5585
	21st to end	Night	79% 7% 7% 5% 2	7249

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	20% 17% 42% 15% 6%	300
	11th to 20th	Day	25% 18% 37% 15% 3 1	300
	21st to end	Day	23% 17% 36% 17% 5%	330
February	1st to 10th	Day	18% 33% 33% 11% 4 1	300
	11th to 20th	Day	22% 27% 35% 14% 3	300
	21st to end	Day	25% 27% 36% 10% 1	249
March	1st to 10th	Day	22% 26% 35% 15% 2	300
	11th to 20th	Day	22% 18% 34% 20% 5%	455
	21st to end	Day	28% 16% 26% 21% 7% 1	550
April	1st to 10th	Day	27% 17% 32% 16% 7% 1	500
	11th to 20th	Day	32% 16% 29% 13% 8% 2	492
	21st to end	Day	21% 17% 31% 21% 9% 2	500
May	1st to 10th	Day	24% 15% 33% 17% 9% 3	503
	11th to 20th	Day	16% 15% 33% 25% 8% 4	558
	21st to end	Day	15% 12% 30% 28% 12% 2	734
June	1st to 10th	Day	15% 14% 32% 27% 10% 2	680
	11th to 20th	Day	17% 11% 29% 30% 11% 3	680
	21st to end	Day	20% 8% 31% 28% 11% 2	680
July	1st to 10th	Day	12% 7% 30% 30% 16% 5%	671
	11th to 20th	Day	12% 10% 27% 32% 15% 5%	668
	21st to end	Day	11% 6% 29% 34% 16% 4	739
August	1st to 10th	Day	9% 7% 26% 29% 22% 8%	562
	11th to 20th	Day	14% 4 20% 33% 18% 10%	479
	21st to end	Day	19% 8% 23% 27% 18% 5%	539
September	1st to 10th	Day	18% 10% 25% 23% 17% 6%	490
	11th to 20th	Day	14% 8% 21% 30% 19% 7%	490
	21st to end	Day	18% 12% 20% 30% 16% 4	490
October	1st to 10th	Day	22% 17% 27% 20% 12% 2	490
	11th to 20th	Day	31% 17% 31% 18% 3 1	390
	21st to end	Day	32% 20% 31% 12% 3 1	330
November	1st to 10th	Day	31% 16% 32% 14% 6%	300
	11th to 20th	Day	24% 23% 35% 15% 2	300
	21st to end	Day	28% 26% 29% 14% 3	225
December	1st to 10th	Day	32% 16% 37% 10% 4 2	200
	11th to 20th	Day	23% 27% 35% 13% 1 1	200
	21st to end	Day	32% 28% 29% 8% 2 1	273
January	1st to 10th	Night	21% 19% 33% 21% 6% 1	1000
	11th to 20th	Night	22% 20% 31% 21% 5%	1000
	21st to end	Night	20% 15% 34% 25% 5%	1100
February	1st to 10th	Night	18% 19% 35% 23% 4 1	875
	11th to 20th	Night	20% 21% 31% 23% 5%	783
	21st to end	Night	25% 17% 33% 23% 3	660
March	1st to 10th	Night	19% 13% 34% 25% 8% 1	800
	11th to 20th	Night	20% 16% 34% 24% 6%	800
	21st to end	Night	22% 12% 29% 30% 7% 1	832
April	1st to 10th	Night	26% 12% 31% 25% 5% 1	600
	11th to 20th	Night	26% 11% 29% 25% 8%	593
	21st to end	Night	21% 9% 32% 30% 8% 1	600
May	1st to 10th	Night	18% 8% 29% 35% 9% 1	600
	11th to 20th	Night	20% 7% 30% 33% 10% 1	563
	21st to end	Night	16% 6% 25% 37% 14% 1	617
June	1st to 10th	Night	17% 6% 28% 37% 10% 1	390
	11th to 20th	Night	11% 7% 24% 44% 14%	380
	21st to end	Night	15% 6% 28% 36% 15% 1	397
July	1st to 10th	Night	14% 3 17% 46% 18% 2	555
	11th to 20th	Night	10% 5% 22% 36% 24% 3	552
	21st to end	Night	9% 3 25% 38% 23% 1	624
August	1st to 10th	Night	10% 3 20% 41% 24% 1	567
	11th to 20th	Night	13% 6% 15% 34% 30% 3	564
	21st to end	Night	18% 5% 21% 37% 19% 1	627
September	1st to 10th	Night	15% 4 23% 36% 20% 3	589
	11th to 20th	Night	18% 6% 20% 34% 21% 3	760
	21st to end	Night	22% 8% 20% 33% 14% 3	760
October	1st to 10th	Night	23% 11% 25% 29% 12% 1	760
	11th to 20th	Night	31% 13% 28% 22% 6% 1	760
	21st to end	Night	36% 11% 28% 19% 5%	836
November	1st to 10th	Night	27% 14% 32% 21% 6%	780
	11th to 20th	Night	28% 19% 30% 20% 3	800
	21st to end	Night	30% 18% 31% 18% 3	832
December	1st to 10th	Night	25% 21% 33% 18% 3	1000
	11th to 20th	Night	23% 21% 34% 17% 3 1	1000
	21st to end	Night	28% 16% 35% 18% 3	1100

Page generated at 2025-05-08 09:33:59. Version ac07

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