River House Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the River House Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	137854
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	134862
Transparency	Day	24% 37% 9% 16% 12% 2	141444
	Night	25% 36% 7% 16% 12% 3	139343
Seeing	Day	15% 4 16% 31% 25% 9%	17350
	Night	17% 6% 22% 33% 19% 3	26842

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	13% 4 2 5% 3 4 4 3 5% 7% 51%	4796
	11th to 20th	Day	12% 7% 2 6% 4 3 3 3 4 5% 52%	5280
	21st to end	Day	12% 9% 3 8% 5% 4 4 3 4 5% 44%	5780
February	1st to 10th	Day	12% 9% 2 6% 5% 4 5% 4 5% 7% 41%	1890
	11th to 20th	Day	20% 7% 2 5% 4 3 4 2 4 5% 44%	1984
	21st to end	Day	16% 8% 3 7% 4 5% 5% 3 5% 5% 39%	1730
March	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	4850
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 5% 7% 52%	5090
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 42%	5768
April	1st to 10th	Day	12% 6% 2 6% 3 3 3 3 6% 7% 48%	2520
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 3 5% 6% 49%	2494
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 7% 44%	2520
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 37%	7116
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 5% 8% 41%	8060
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 8% 38%	9045
June	1st to 10th	Day	10% 6% 2 5% 4 5% 4 4 7% 10% 43%	2940
	11th to 20th	Day	10% 4 1 5% 5% 4 4 4 8% 11% 43%	2940
	21st to end	Day	7% 4 2 5% 4 5% 6% 4 7% 10% 44%	2940
July	1st to 10th	Day	6% 5% 2 5% 4 4 5% 5% 8% 12% 42%	6500
	11th to 20th	Day	11% 7% 3 8% 5% 6% 6% 5% 9% 12% 28%	6253
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	6424
August	1st to 10th	Day	13% 8% 3 7% 5% 5% 6% 4 8% 11% 31%	4234
	11th to 20th	Day	9% 6% 2 8% 6% 5% 5% 4 6% 8% 39%	3859
	21st to end	Day	10% 4 1 4 3 4 4 4 6% 10% 50%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 5% 6% 8% 44%	2400
	11th to 20th	Day	13% 6% 3 6% 4 3 5% 3 5% 7% 43%	2400
	21st to end	Day	21% 7% 3 6% 4 3 4 3 4 7% 37%	2138
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 4 6% 46%	2000
	11th to 20th	Day	17% 6% 2 5% 2 3 3 2 4 5% 52%	2000
	21st to end	Day	13% 5% 2 5% 4 3 4 3 4 6% 50%	2200
November	1st to 10th	Day	14% 6% 2 4 3 2 4 2 3 6% 53%	1923
	11th to 20th	Day	13% 6% 2 6% 4 3 4 2 5% 7% 48%	1890
	21st to end	Day	14% 4 2 5% 4 3 4 3 5% 5% 51%	1780
December	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 4 6% 53%	2680
	11th to 20th	Day	18% 8% 2 5% 4 2 3 2 3 5% 47%	3070
	21st to end	Day	13% 6% 1 5% 4 3 4 3 5% 7% 48%	4048
January	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	8095
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 3 4 55%	7680
	21st to end	Night	14% 8% 3 5% 4 3 3 3 4 5% 48%	8401
February	1st to 10th	Night	15% 9% 2 6% 3 4 3 4 5% 6% 43%	2730
	11th to 20th	Night	20% 7% 2 5% 3 3 3 3 4 6% 43%	2520
	21st to end	Night	17% 7% 2 5% 4 2 3 4 5% 5% 46%	2076
March	1st to 10th	Night	17% 7% 2 6% 3 4 4 3 4 5% 45%	5220
	11th to 20th	Night	13% 7% 3 5% 3 3 3 3 5% 6% 50%	5090
	21st to end	Night	15% 8% 3 5% 4 3 3 2 4 6% 46%	5406
April	1st to 10th	Night	13% 8% 2 6% 4 4 5% 3 6% 7% 42%	2100
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 6% 7% 49%	2075
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 6% 46%	2100
May	1st to 10th	Night	14% 6% 3 7% 5% 4 5% 4 6% 9% 36%	5779
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 6% 8% 41%	4799
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 7% 36%	5108
June	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 4 5% 9% 42%	1680
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 8% 10% 40%	1680
	21st to end	Night	8% 5% 1 5% 5% 5% 6% 5% 7% 9% 45%	1680
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 39%	3678
	11th to 20th	Night	10% 6% 2 7% 6% 5% 6% 5% 9% 13% 31%	3810
	21st to end	Night	8% 7% 3 8% 6% 5% 5% 5% 7% 9% 38%	4427
August	1st to 10th	Night	17% 8% 2 7% 5% 5% 5% 4 7% 10% 32%	2915
	11th to 20th	Night	9% 6% 3 8% 4 5% 5% 4 7% 8% 40%	3157
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 4 4 7% 49%	2000
	11th to 20th	Night	11% 5% 3 6% 4 4 5% 4 6% 8% 46%	2000
	21st to end	Night	21% 7% 3 6% 4 4 5% 3 5% 6% 36%	2258
October	1st to 10th	Night	15% 5% 2 5% 4 4 3 3 5% 6% 47%	2400
	11th to 20th	Night	17% 5% 2 5% 4 3 3 2 4 5% 51%	2400
	21st to end	Night	15% 5% 1 5% 3 3 3 3 4 7% 51%	2640
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 2 5% 6% 50%	2592
	11th to 20th	Night	16% 7% 2 5% 3 3 3 4 4 5% 47%	2730
	21st to end	Night	12% 5% 2 4 3 4 4 3 5% 7% 51%	2835
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 4 5% 6% 54%	4720
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 4 6% 39%	5345
	21st to end	Night	12% 7% 3 6% 3 3 4 3 5% 7% 46%	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	26% 22% 7% 22% 17% 6%	4797
	11th to 20th	Day	28% 21% 4 16% 25% 6%	5280
	21st to end	Day	33% 23% 7% 24% 11% 1	5780
February	1st to 10th	Day	18% 59% 5% 11% 5% 1	1890
	11th to 20th	Day	21% 58% 3 8% 8% 2	1993
	21st to end	Day	19% 59% 5% 8% 7% 2	1730
March	1st to 10th	Day	32% 30% 8% 10% 14% 5%	4850
	11th to 20th	Day	21% 29% 6% 19% 18% 6%	5090
	21st to end	Day	29% 30% 9% 13% 14% 6%	5768
April	1st to 10th	Day	16% 59% 5% 10% 8% 1	2520
	11th to 20th	Day	14% 59% 6% 12% 7% 2	2494
	21st to end	Day	16% 60% 6% 12% 5% 2	2520
May	1st to 10th	Day	34% 26% 10% 18% 11% 1	7148
	11th to 20th	Day	30% 25% 10% 22% 12%	8064
	21st to end	Day	36% 25% 8% 16% 13% 2	9061
June	1st to 10th	Day	15% 61% 6% 10% 7% 1	2940
	11th to 20th	Day	11% 61% 8% 11% 8% 1	2940
	21st to end	Day	12% 59% 8% 12% 8%	2940
July	1st to 10th	Day	23% 32% 18% 15% 9% 2	6430
	11th to 20th	Day	35% 32% 18% 13% 2	6178
	21st to end	Day	25% 30% 13% 16% 16%	6240
August	1st to 10th	Day	31% 39% 12% 15% 3	4221
	11th to 20th	Day	24% 40% 9% 19% 8%	3895
	21st to end	Day	14% 38% 10% 21% 16% 1	4312
September	1st to 10th	Day	15% 59% 9% 11% 5% 1	2400
	11th to 20th	Day	17% 58% 5% 11% 7% 2	2400
	21st to end	Day	24% 58% 3 11% 3 1	2138
October	1st to 10th	Day	19% 38% 7% 15% 18% 3	3260
	11th to 20th	Day	14% 36% 3 13% 26% 8%	3240
	21st to end	Day	9% 35% 6% 20% 20% 9%	3538
November	1st to 10th	Day	17% 53% 5% 13% 9% 3	1923
	11th to 20th	Day	17% 55% 5% 9% 12% 3	1890
	21st to end	Day	16% 54% 4 14% 10% 1	1780
December	1st to 10th	Day	21% 36% 6% 22% 13% 3	2680
	11th to 20th	Day	30% 31% 9% 15% 12% 3	3066
	21st to end	Day	27% 28% 11% 23% 11% 1	4048
January	1st to 10th	Night	30% 22% 9% 18% 18% 4	8096
	11th to 20th	Night	27% 21% 3 20% 22% 7%	7680
	21st to end	Night	32% 23% 6% 23% 13% 3	8401
February	1st to 10th	Night	18% 59% 6% 10% 6% 1	2730
	11th to 20th	Night	20% 57% 3 10% 8% 2	2522
	21st to end	Night	16% 59% 4 10% 7% 3	2076
March	1st to 10th	Night	32% 29% 5% 15% 13% 7%	5220
	11th to 20th	Night	23% 29% 7% 17% 18% 6%	5090
	21st to end	Night	28% 29% 5% 19% 13% 5%	5406
April	1st to 10th	Night	18% 60% 4 8% 9% 1	2100
	11th to 20th	Night	15% 60% 4 11% 8% 2	2075
	21st to end	Night	17% 59% 6% 10% 7% 1	2100
May	1st to 10th	Night	35% 25% 11% 20% 7% 1	5809
	11th to 20th	Night	33% 25% 11% 21% 11%	4800
	21st to end	Night	40% 25% 7% 15% 12% 1	5115
June	1st to 10th	Night	18% 60% 4 8% 8% 1	1680
	11th to 20th	Night	14% 61% 7% 10% 7% 1	1680
	21st to end	Night	13% 60% 6% 11% 8% 2	1680
July	1st to 10th	Night	31% 31% 14% 13% 10% 1	3640
	11th to 20th	Night	34% 32% 20% 12% 2	3765
	21st to end	Night	30% 31% 10% 18% 12%	4297
August	1st to 10th	Night	34% 38% 10% 14% 3	2904
	11th to 20th	Night	24% 39% 12% 18% 7%	3187
	21st to end	Night	16% 37% 6% 19% 21% 1	3586
September	1st to 10th	Night	14% 59% 9% 11% 5% 2	2000
	11th to 20th	Night	16% 59% 4 11% 8% 2	2000
	21st to end	Night	26% 57% 5% 8% 3	2258
October	1st to 10th	Night	23% 37% 7% 14% 16% 3	3920
	11th to 20th	Night	13% 36% 5% 15% 24% 7%	3899
	21st to end	Night	9% 35% 4 25% 18% 9%	4252
November	1st to 10th	Night	19% 54% 5% 11% 10% 2	2592
	11th to 20th	Night	18% 54% 4 9% 10% 5%	2730
	21st to end	Night	14% 54% 5% 13% 12% 1	2835
December	1st to 10th	Night	19% 35% 7% 20% 17% 2	4720
	11th to 20th	Night	33% 32% 8% 15% 8% 4	5348
	21st to end	Night	28% 27% 10% 23% 11% 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	17% 8% 26% 40% 8%	244
	11th to 20th	Day	17% 8% 33% 33% 7% 1	330
	21st to end	Day	17% 6% 23% 44% 10% 1	363
February	1st to 10th	Day	17% 6% 24% 39% 14%	330
	11th to 20th	Day	25% 6% 31% 31% 7%	472
	21st to end	Day	23% 5% 27% 36% 10%	446
March	1st to 10th	Day	17% 6% 27% 35% 13% 2	540
	11th to 20th	Day	18% 7% 25% 38% 11% 2	540
	21st to end	Day	18% 4 23% 40% 13% 2	594
April	1st to 10th	Day	14% 5% 26% 36% 16% 3	540
	11th to 20th	Day	9% 4 21% 40% 21% 4	532
	21st to end	Day	16% 5% 17% 34% 26% 3	540
May	1st to 10th	Day	18% 4 17% 35% 20% 6%	543
	11th to 20th	Day	19% 2 13% 32% 28% 6%	567
	21st to end	Day	19% 4 8% 28% 31% 11%	617
June	1st to 10th	Day	11% 2 8% 31% 35% 13%	570
	11th to 20th	Day	11% 2 10% 29% 36% 14%	570
	21st to end	Day	10% 2 6% 31% 37% 14%	570
July	1st to 10th	Day	9% 2 4 23% 40% 21%	565
	11th to 20th	Day	6% 2 2 19% 50% 21%	544
	21st to end	Day	10% 2 4 25% 43% 16%	576
August	1st to 10th	Day	12% 2 6% 27% 40% 13%	526
	11th to 20th	Day	11% 4 5% 24% 39% 18%	519
	21st to end	Day	10% 2 5% 28% 36% 20%	583
September	1st to 10th	Day	15% 1 7% 25% 32% 20%	530
	11th to 20th	Day	15% 2 8% 23% 35% 18%	530
	21st to end	Day	24% 5% 11% 28% 23% 8%	530
October	1st to 10th	Day	18% 3 9% 28% 27% 16%	530
	11th to 20th	Day	18% 5% 19% 27% 24% 8%	530
	21st to end	Day	14% 5% 21% 31% 22% 7%	583
November	1st to 10th	Day	16% 5% 21% 30% 20% 6%	464
	11th to 20th	Day	13% 10% 29% 32% 14% 2	430
	21st to end	Day	15% 7% 26% 32% 18% 3	320
December	1st to 10th	Day	19% 5% 25% 40% 11% 1	220
	11th to 20th	Day	21% 5% 24% 36% 13%	220
	21st to end	Day	17% 6% 24% 41% 12% 1	242
January	1st to 10th	Night	17% 12% 37% 28% 6%	840
	11th to 20th	Night	19% 10% 32% 30% 9%	840
	21st to end	Night	20% 10% 34% 32% 4	924
February	1st to 10th	Night	18% 8% 31% 37% 6%	840
	11th to 20th	Night	22% 11% 36% 27% 4	823
	21st to end	Night	19% 10% 31% 34% 5% 1	692
March	1st to 10th	Night	18% 8% 30% 35% 9% 1	840
	11th to 20th	Night	19% 7% 30% 33% 9% 2	840
	21st to end	Night	18% 6% 30% 38% 7% 1	924
April	1st to 10th	Night	17% 6% 33% 32% 11%	840
	11th to 20th	Night	13% 4 26% 40% 15% 1	831
	21st to end	Night	18% 5% 21% 40% 14% 2	840
May	1st to 10th	Night	18% 5% 23% 39% 14% 2	824
	11th to 20th	Night	17% 4 21% 37% 20% 1	472
	21st to end	Night	16% 2 13% 31% 32% 5%	455
June	1st to 10th	Night	12% 3 14% 35% 32% 4	420
	11th to 20th	Night	11% 3 11% 37% 32% 5%	420
	21st to end	Night	7% 3 10% 39% 35% 5%	420
July	1st to 10th	Night	10% 3 4 36% 40% 8%	410
	11th to 20th	Night	6% 2 4 34% 46% 9%	467
	21st to end	Night	8% 3 6% 37% 41% 5%	657
August	1st to 10th	Night	11% 2 6% 37% 38% 7%	597
	11th to 20th	Night	13% 3 7% 33% 38% 7%	762
	21st to end	Night	11% 2 6% 33% 39% 9%	880
September	1st to 10th	Night	15% 2 8% 31% 34% 10%	800
	11th to 20th	Night	13% 2 9% 29% 39% 9%	800
	21st to end	Night	25% 4 12% 30% 26% 3	800
October	1st to 10th	Night	19% 3 16% 27% 26% 9%	800
	11th to 20th	Night	19% 7% 21% 30% 20% 3	800
	21st to end	Night	18% 7% 24% 29% 19% 4	880
November	1st to 10th	Night	17% 6% 22% 31% 19% 4	820
	11th to 20th	Night	19% 9% 27% 31% 12% 1	840
	21st to end	Night	14% 11% 28% 32% 14% 2	840
December	1st to 10th	Night	20% 9% 32% 27% 10% 2	840
	11th to 20th	Night	18% 8% 36% 29% 9%	840
	21st to end	Night	17% 9% 30% 33% 11%	924

Page generated at 2026-07-13 03:14:50. Version ac07

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