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 2026-09-14.

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% 11% 4 10% 5% 4 4 3 5% 6% 32%	148596
	Night	14% 14% 5% 12% 6% 5% 4 3 4 5% 28%	142764
Transparency	Day	51% 6% 10% 21% 11% 1	152076
	Night	58% 6% 10% 17% 8% 1	147358
Seeing	Day	20% 15% 30% 22% 10% 3	18189
	Night	21% 13% 29% 27% 9% 1	27744

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	12% 21% 12% 30% 6% 4 2 2 2 2 7%	4691
	11th to 20th	Day	9% 21% 8% 26% 12% 11% 4 2 2 2 3	4679
	21st to end	Day	10% 15% 8% 21% 10% 8% 6% 3 3 3 11%	5156
February	1st to 10th	Day	8% 20% 7% 16% 15% 7% 4 2 3 5% 13%	1811
	11th to 20th	Day	12% 17% 7% 16% 10% 6% 4 3 3 4 18%	2015
	21st to end	Day	12% 19% 7% 15% 7% 5% 4 3 5% 5% 19%	1730
March	1st to 10th	Day	13% 11% 4 12% 7% 5% 4 4 6% 5% 31%	4578
	11th to 20th	Day	11% 13% 6% 13% 5% 4 5% 4 5% 7% 26%	4991
	21st to end	Day	21% 12% 8% 15% 6% 4 4 3 3 5% 20%	5743
April	1st to 10th	Day	19% 16% 5% 9% 5% 4 4 3 5% 5% 27%	2520
	11th to 20th	Day	21% 15% 3 5% 4 3 3 3 5% 5% 33%	2624
	21st to end	Day	16% 11% 4 8% 4 4 5% 4 6% 7% 31%	2730
May	1st to 10th	Day	14% 9% 2 7% 4 4 4 3 5% 7% 42%	7642
	11th to 20th	Day	15% 9% 2 7% 5% 4 4 3 4 5% 42%	7861
	21st to end	Day	13% 7% 3 6% 4 4 4 3 5% 7% 45%	9061
June	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 8% 42%	3134
	11th to 20th	Day	15% 7% 2 6% 4 5% 4 5% 6% 9% 38%	3150
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 6% 8% 40%	3123
July	1st to 10th	Day	17% 5% 2 5% 3 3 4 3 6% 9% 42%	8279
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 6% 9% 48%	8280
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 9% 47%	8992
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	6201
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 7% 46%	5951
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 5% 6% 38%	6389
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 4 5% 8% 42%	2477
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 7% 45%	2233
	21st to end	Day	17% 8% 2 7% 4 3 4 4 5% 9% 37%	2200
October	1st to 10th	Day	19% 9% 3 9% 4 3 3 3 5% 6% 35%	2195
	11th to 20th	Day	25% 16% 3 9% 6% 4 4 3 5% 6% 20%	1895
	21st to end	Day	22% 18% 5% 10% 5% 4 5% 3 6% 5% 17%	1980
November	1st to 10th	Day	17% 21% 5% 8% 4 4 3 3 5% 6% 24%	1852
	11th to 20th	Day	13% 26% 7% 12% 5% 4 4 3 5% 5% 15%	1890
	21st to end	Day	15% 27% 11% 12% 5% 4 2 2 4 4 12%	1696
December	1st to 10th	Day	11% 32% 12% 19% 6% 3 3 2 3 2 7%	2350
	11th to 20th	Day	11% 24% 10% 22% 9% 5% 4 3 3 2 6%	2691
	21st to end	Day	20% 23% 8% 16% 6% 5% 3 2 3 4 9%	3806
January	1st to 10th	Night	12% 19% 9% 30% 8% 4 3 2 2 3 8%	8233
	11th to 20th	Night	10% 19% 9% 22% 16% 10% 4 2 2 2 5%	8236
	21st to end	Night	8% 16% 8% 21% 12% 9% 6% 3 3 3 10%	9058
February	1st to 10th	Night	9% 19% 7% 17% 13% 7% 5% 3 4 3 14%	2809
	11th to 20th	Night	11% 15% 7% 17% 11% 5% 5% 3 3 5% 19%	2489
	21st to end	Night	13% 21% 5% 14% 6% 5% 4 3 4 5% 21%	2076
March	1st to 10th	Night	16% 11% 4 9% 6% 4 3 3 4 6% 34%	5482
	11th to 20th	Night	11% 15% 5% 9% 5% 4 4 4 5% 7% 30%	5188
	21st to end	Night	20% 13% 6% 10% 4 3 3 3 4 5% 27%	5461
April	1st to 10th	Night	19% 13% 4 10% 5% 4 4 3 4 6% 27%	2100
	11th to 20th	Night	21% 12% 4 6% 4 3 4 4 3 6% 33%	1944
	21st to end	Night	17% 11% 3 8% 5% 4 3 3 5% 6% 35%	1890
May	1st to 10th	Night	13% 8% 2 6% 4 5% 5% 4 6% 7% 40%	5257
	11th to 20th	Night	13% 8% 3 6% 4 4 4 3 6% 7% 41%	5009
	21st to end	Night	15% 8% 2 6% 3 4 3 3 4 6% 46%	5148
June	1st to 10th	Night	15% 5% 3 6% 4 4 5% 3 5% 8% 42%	1481
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 6% 9% 41%	1470
	21st to end	Night	14% 8% 2 5% 4 5% 4 4 4 7% 43%	1489
July	1st to 10th	Night	14% 6% 3 6% 4 4 5% 4 5% 8% 41%	4710
	11th to 20th	Night	9% 5% 2 5% 4 5% 4 4 6% 7% 50%	4710
	21st to end	Night	10% 6% 2 5% 3 4 4 3 5% 7% 49%	5118
August	1st to 10th	Night	15% 8% 3 7% 4 4 3 3 5% 6% 42%	3955
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 8% 49%	4095
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	4720
September	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 7% 48%	2121
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 4 8% 47%	2234
	21st to end	Night	19% 8% 3 5% 5% 4 4 3 6% 7% 38%	2200
October	1st to 10th	Night	18% 9% 3 7% 4 4 3 2 5% 6% 39%	2205
	11th to 20th	Night	22% 15% 4 8% 4 4 4 3 5% 6% 24%	2500
	21st to end	Night	26% 16% 3 7% 5% 4 5% 2 4 5% 23%	2860
November	1st to 10th	Night	17% 20% 5% 8% 5% 4 3 3 4 6% 26%	2666
	11th to 20th	Night	14% 23% 6% 12% 6% 4 4 3 4 5% 18%	2730
	21st to end	Night	17% 25% 10% 12% 6% 4 4 3 4 4 13%	2916
December	1st to 10th	Night	11% 28% 13% 18% 7% 5% 4 2 3 3 8%	5060
	11th to 20th	Night	11% 23% 9% 21% 8% 5% 3 3 3 3 10%	5736
	21st to end	Night	20% 21% 9% 16% 6% 4 4 3 3 4 10%	7408

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	86% 3 6% 4 1	4692
	11th to 20th	Day	90% 3 3 2 1	4710
	21st to end	Day	77% 6% 6% 7% 2	5156
February	1st to 10th	Day	76% 5% 6% 10% 2	1811
	11th to 20th	Day	69% 6% 9% 10% 6%	2025
	21st to end	Day	67% 6% 9% 14% 4	1730
March	1st to 10th	Day	52% 6% 15% 15% 10% 2	4580
	11th to 20th	Day	57% 5% 13% 17% 7% 1	4991
	21st to end	Day	67% 5% 8% 12% 7%	5743
April	1st to 10th	Day	59% 5% 9% 18% 9% 1	2520
	11th to 20th	Day	52% 4 10% 20% 13% 1	2624
	21st to end	Day	51% 7% 11% 15% 14% 3	2730
May	1st to 10th	Day	39% 6% 9% 24% 18% 3	7651
	11th to 20th	Day	44% 5% 7% 24% 18% 2	7865
	21st to end	Day	38% 5% 9% 24% 20% 4	9061
June	1st to 10th	Day	39% 7% 9% 27% 16% 2	3134
	11th to 20th	Day	41% 7% 12% 23% 16% 1	3150
	21st to end	Day	42% 7% 13% 25% 13% 1	3123
July	1st to 10th	Day	38% 6% 16% 25% 13% 1	8211
	11th to 20th	Day	31% 7% 13% 37% 13% 1	8205
	21st to end	Day	33% 7% 13% 34% 13% 1	8794
August	1st to 10th	Day	36% 8% 13% 28% 15% 1	6186
	11th to 20th	Day	37% 6% 13% 33% 10%	5990
	21st to end	Day	45% 7% 13% 26% 8% 1	6388
September	1st to 10th	Day	40% 7% 12% 24% 15% 1	2477
	11th to 20th	Day	37% 8% 14% 29% 12% 1	2233
	21st to end	Day	44% 7% 14% 24% 11%	2200
October	1st to 10th	Day	37% 5% 9% 33% 16%	3572
	11th to 20th	Day	57% 5% 9% 19% 10%	3059
	21st to end	Day	63% 6% 12% 15% 3	3180
November	1st to 10th	Day	60% 7% 9% 18% 6%	1852
	11th to 20th	Day	71% 6% 10% 9% 4	1890
	21st to end	Day	77% 5% 9% 7% 2	1696
December	1st to 10th	Day	85% 5% 6% 4 1	2350
	11th to 20th	Day	85% 5% 5% 4 1	2691
	21st to end	Day	80% 5% 6% 6% 3	3806
January	1st to 10th	Night	83% 4 6% 5% 2	8233
	11th to 20th	Night	87% 4 5% 3 1	8280
	21st to end	Night	78% 6% 6% 7% 3	9058
February	1st to 10th	Night	76% 5% 6% 9% 3 1	2809
	11th to 20th	Night	69% 5% 8% 15% 3	2490
	21st to end	Night	66% 5% 10% 13% 5%	2076
March	1st to 10th	Night	51% 5% 17% 18% 9%	5493
	11th to 20th	Night	53% 5% 16% 19% 7% 1	5189
	21st to end	Night	58% 5% 10% 17% 9%	5461
April	1st to 10th	Night	58% 5% 8% 17% 10% 1	2100
	11th to 20th	Night	53% 4 7% 21% 12% 2	1944
	21st to end	Night	50% 6% 9% 19% 14% 2	1890
May	1st to 10th	Night	40% 7% 11% 24% 16% 3	5264
	11th to 20th	Night	41% 6% 8% 26% 18% 1	5010
	21st to end	Night	39% 5% 10% 22% 17% 7%	5149
June	1st to 10th	Night	40% 6% 11% 25% 18% 1	1481
	11th to 20th	Night	40% 7% 12% 25% 14% 2	1470
	21st to end	Night	41% 7% 11% 25% 14% 2	1489
July	1st to 10th	Night	40% 7% 13% 21% 14% 4	4670
	11th to 20th	Night	32% 7% 11% 34% 15% 1	4662
	21st to end	Night	33% 7% 15% 30% 16%	5002
August	1st to 10th	Night	42% 6% 14% 22% 15% 1	3946
	11th to 20th	Night	33% 5% 19% 32% 10%	4122
	21st to end	Night	40% 7% 14% 27% 12% 1	4720
September	1st to 10th	Night	36% 7% 15% 24% 17% 1	2121
	11th to 20th	Night	37% 7% 14% 29% 13% 1	2234
	21st to end	Night	46% 7% 14% 23% 10%	2200
October	1st to 10th	Night	41% 6% 11% 29% 13%	3608
	11th to 20th	Night	51% 5% 10% 20% 13%	4062
	21st to end	Night	58% 6% 13% 18% 4	4610
November	1st to 10th	Night	60% 6% 10% 17% 6% 1	2666
	11th to 20th	Night	70% 5% 10% 9% 5% 1	2730
	21st to end	Night	76% 6% 10% 7% 1	2916
December	1st to 10th	Night	82% 6% 6% 5%	5060
	11th to 20th	Night	79% 6% 7% 6% 1	5735
	21st to end	Night	79% 7% 7% 5% 2	7408

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	21% 19% 41% 14% 5%	330
	11th to 20th	Day	25% 20% 37% 15% 3 1	330
	21st to end	Day	23% 17% 37% 17% 5%	363
February	1st to 10th	Day	19% 33% 32% 11% 5% 1	330
	11th to 20th	Day	22% 25% 35% 14% 3	330
	21st to end	Day	27% 28% 34% 10% 1	273
March	1st to 10th	Day	23% 26% 34% 15% 2	330
	11th to 20th	Day	24% 19% 34% 18% 5%	493
	21st to end	Day	29% 17% 26% 20% 7% 1	594
April	1st to 10th	Day	28% 17% 32% 16% 6% 1	540
	11th to 20th	Day	33% 15% 29% 14% 8% 2	532
	21st to end	Day	21% 17% 30% 21% 9% 2	540
May	1st to 10th	Day	24% 16% 32% 18% 8% 2	543
	11th to 20th	Day	19% 16% 32% 23% 7% 3	646
	21st to end	Day	16% 14% 31% 26% 11% 2	844
June	1st to 10th	Day	15% 14% 32% 26% 11% 2	780
	11th to 20th	Day	19% 12% 30% 28% 9% 2	780
	21st to end	Day	20% 8% 33% 27% 10% 2	780
July	1st to 10th	Day	11% 7% 32% 30% 15% 5%	771
	11th to 20th	Day	12% 10% 29% 30% 14% 5%	768
	21st to end	Day	11% 7% 31% 33% 14% 4	849
August	1st to 10th	Day	9% 8% 26% 29% 21% 7%	650
	11th to 20th	Day	15% 5% 22% 32% 17% 10%	559
	21st to end	Day	19% 9% 24% 26% 17% 5%	627
September	1st to 10th	Day	19% 10% 25% 24% 16% 6%	570
	11th to 20th	Day	13% 8% 20% 30% 20% 9%	542
	21st to end	Day	17% 12% 20% 31% 15% 4	530
October	1st to 10th	Day	22% 17% 27% 21% 12% 2	530
	11th to 20th	Day	30% 16% 32% 18% 3 1	425
	21st to end	Day	29% 21% 31% 13% 4 1	363
November	1st to 10th	Day	32% 16% 32% 14% 6%	330
	11th to 20th	Day	26% 23% 34% 15% 3	330
	21st to end	Day	28% 27% 28% 14% 2	247
December	1st to 10th	Day	30% 18% 36% 10% 4 2	220
	11th to 20th	Day	25% 28% 34% 12% 1 1	220
	21st to end	Day	33% 28% 29% 8% 2 1	300
January	1st to 10th	Night	21% 19% 33% 21% 6%	1050
	11th to 20th	Night	23% 20% 31% 21% 5%	1050
	21st to end	Night	19% 15% 35% 25% 5%	1155
February	1st to 10th	Night	18% 19% 35% 23% 5% 1	919
	11th to 20th	Night	20% 21% 31% 24% 5%	823
	21st to end	Night	25% 16% 33% 23% 3	692
March	1st to 10th	Night	19% 13% 34% 25% 8% 1	840
	11th to 20th	Night	21% 16% 33% 24% 5%	840
	21st to end	Night	21% 12% 30% 29% 7% 1	874
April	1st to 10th	Night	27% 13% 30% 24% 5% 1	630
	11th to 20th	Night	27% 11% 28% 25% 8%	623
	21st to end	Night	21% 9% 32% 30% 8% 1	630
May	1st to 10th	Night	17% 8% 30% 36% 9%	630
	11th to 20th	Night	20% 7% 30% 33% 10%	621
	21st to end	Night	16% 7% 26% 37% 13% 1	683
June	1st to 10th	Night	17% 6% 29% 38% 9% 1	431
	11th to 20th	Night	12% 8% 24% 43% 13%	420
	21st to end	Night	16% 6% 28% 36% 13% 1	439
July	1st to 10th	Night	14% 3 17% 45% 19% 2	615
	11th to 20th	Night	10% 5% 21% 38% 24% 2	612
	21st to end	Night	9% 3 24% 41% 22% 1	690
August	1st to 10th	Night	11% 3 20% 42% 24% 1	627
	11th to 20th	Night	13% 6% 15% 37% 28% 3	624
	21st to end	Night	18% 5% 20% 37% 19% 1	693
September	1st to 10th	Night	14% 6% 23% 34% 21% 3	651
	11th to 20th	Night	17% 5% 19% 34% 22% 3	813
	21st to end	Night	22% 8% 20% 32% 15% 3	800
October	1st to 10th	Night	22% 11% 25% 29% 12% 1	800
	11th to 20th	Night	30% 13% 28% 23% 6% 1	800
	21st to end	Night	35% 11% 27% 21% 6%	880
November	1st to 10th	Night	28% 14% 31% 21% 6%	820
	11th to 20th	Night	27% 19% 30% 21% 3	840
	21st to end	Night	30% 18% 31% 18% 3	874
December	1st to 10th	Night	25% 21% 32% 19% 3	1050
	11th to 20th	Night	23% 22% 33% 17% 3 1	1050
	21st to end	Night	29% 16% 34% 17% 3	1155

Page generated at 2026-09-11 04:11:56. Version ac07

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