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 2025-03-12.

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%	108761
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	110269
Transparency	Day	21% 44% 8% 14% 10% 2	112394
	Night	23% 42% 6% 15% 11% 2	114775
Seeing	Day	15% 4 15% 31% 26% 9%	15532
	Night	17% 6% 22% 33% 19% 3	25169

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% 3 1 4 3 3 3 3 4 6% 55%	3704
	11th to 20th	Day	12% 8% 2 6% 4 3 3 3 4 5% 49%	4060
	21st to end	Day	12% 9% 3 8% 5% 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	13% 9% 2 6% 5% 4 5% 4 6% 7% 40%	1800
	11th to 20th	Day	20% 7% 2 5% 4 3 4 2 4 5% 45%	1887
	21st to end	Day	16% 8% 3 7% 4 5% 5% 3 5% 6% 40%	1650
March	1st to 10th	Day	21% 9% 3 6% 4 3 4 3 5% 6% 35%	3407
	11th to 20th	Day	10% 4 1 4 3 3 3 4 5% 8% 55%	3491
	21st to end	Day	19% 8% 2 5% 3 3 3 3 5% 7% 40%	3937
April	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 48%	2280
	11th to 20th	Day	10% 6% 2 5% 5% 5% 4 3 5% 6% 49%	2254
	21st to end	Day	14% 7% 3 5% 3 4 3 4 5% 6% 45%	2280
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 3 6% 10% 30%	3819
	11th to 20th	Day	20% 7% 2 7% 4 5% 5% 4 5% 7% 33%	4319
	21st to end	Day	9% 4 2 5% 4 4 4 4 7% 10% 46%	4823
June	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 8% 10% 43%	2660
	11th to 20th	Day	10% 4 1 5% 5% 4 4 5% 8% 10% 43%	2660
	21st to end	Day	7% 4 2 5% 4 6% 6% 5% 8% 10% 44%	2660
July	1st to 10th	Day	5% 5% 2 5% 4 4 6% 6% 8% 13% 42%	4440
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 5% 8% 11% 35%	4319
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 6% 8% 9% 34%	4466
August	1st to 10th	Day	13% 8% 3 7% 5% 5% 6% 4 8% 11% 30%	4104
	11th to 20th	Day	9% 6% 2 8% 6% 5% 5% 4 6% 8% 39%	3739
	21st to end	Day	10% 4 1 4 3 4 4 4 6% 10% 50%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 5% 6% 7% 44%	2280
	11th to 20th	Day	12% 6% 3 6% 4 3 5% 3 5% 7% 44%	2280
	21st to end	Day	21% 7% 3 6% 4 3 4 3 4 7% 38%	2031
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Day	16% 6% 2 4 2 3 4 2 4 5% 52%	1900
	21st to end	Day	12% 5% 2 5% 4 3 4 3 4 6% 50%	2090
November	1st to 10th	Day	14% 6% 2 4 3 3 4 3 3 6% 53%	1830
	11th to 20th	Day	13% 6% 2 6% 4 3 4 2 5% 7% 47%	1800
	21st to end	Day	14% 4 2 5% 4 3 4 3 5% 5% 51%	1695
December	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 4 7% 54%	2600
	11th to 20th	Day	18% 9% 2 5% 4 2 2 2 3 5% 47%	2990
	21st to end	Day	13% 6% 1 5% 4 3 4 3 5% 8% 48%	3960
January	1st to 10th	Night	12% 6% 2 6% 4 3 3 2 4 5% 52%	6239
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 3 5% 54%	5900
	21st to end	Night	14% 8% 3 5% 4 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 9% 2 5% 3 4 3 4 5% 6% 43%	2600
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 4 6% 43%	2397
	21st to end	Night	17% 7% 2 5% 4 2 3 4 5% 5% 46%	1980
March	1st to 10th	Night	23% 8% 2 6% 3 4 3 3 4 5% 37%	3664
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 5% 7% 50%	3498
	21st to end	Night	17% 9% 3 6% 3 2 3 2 4 5% 47%	3695
April	1st to 10th	Night	14% 8% 2 6% 4 4 5% 3 5% 8% 41%	1900
	11th to 20th	Night	12% 6% 2 6% 3 4 3 3 5% 8% 49%	1875
	21st to end	Night	16% 6% 2 5% 4 4 3 3 6% 6% 45%	1900
May	1st to 10th	Night	20% 7% 2 8% 5% 4 5% 4 5% 7% 33%	3103
	11th to 20th	Night	22% 10% 2 6% 4 4 4 3 5% 6% 33%	2573
	21st to end	Night	9% 6% 1 6% 4 4 4 3 6% 10% 45%	2732
June	1st to 10th	Night	12% 6% 2 5% 5% 5% 5% 4 5% 9% 41%	1520
	11th to 20th	Night	10% 4 2 6% 5% 4 5% 4 8% 11% 40%	1520
	21st to end	Night	8% 5% 1 5% 5% 5% 6% 5% 7% 9% 44%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	2520
	11th to 20th	Night	8% 6% 2 6% 5% 5% 6% 5% 8% 12% 38%	2637
	21st to end	Night	11% 8% 3 9% 6% 5% 5% 4 6% 8% 33%	3087
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 7% 10% 31%	2825
	11th to 20th	Night	9% 6% 3 8% 4 5% 5% 4 7% 8% 41%	3058
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 4 7% 48%	1900
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 5% 8% 47%	1900
	21st to end	Night	20% 7% 3 6% 4 4 5% 3 5% 6% 37%	2145
October	1st to 10th	Night	15% 5% 2 5% 5% 4 4 3 5% 7% 46%	2280
	11th to 20th	Night	16% 5% 1 5% 4 3 3 2 4 5% 52%	2280
	21st to end	Night	15% 5% 1 5% 3 3 3 3 4 7% 51%	2508
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 2 5% 6% 50%	2466
	11th to 20th	Night	16% 7% 2 5% 3 3 3 4 4 5% 47%	2600
	21st to end	Night	12% 5% 2 4 3 4 4 3 5% 7% 51%	2700
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 4 5% 6% 55%	4580
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 4 6% 39%	5205
	21st to end	Night	12% 7% 3 6% 3 3 4 3 5% 7% 46%	6996

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	22% 26% 3 21% 21% 7%	3705
	11th to 20th	Day	27% 27% 4 16% 21% 6%	4060
	21st to end	Day	32% 29% 7% 21% 9% 1	4466
February	1st to 10th	Day	17% 62% 5% 11% 4 1	1800
	11th to 20th	Day	19% 61% 3 8% 7% 2	1896
	21st to end	Day	17% 62% 5% 7% 7% 2	1650
March	1st to 10th	Day	34% 41% 7% 8% 8% 2	3407
	11th to 20th	Day	13% 40% 4 21% 18% 4	3491
	21st to end	Day	28% 40% 6% 11% 12% 3	3937
April	1st to 10th	Day	14% 65% 4 9% 7% 1	2280
	11th to 20th	Day	12% 65% 5% 10% 7% 1	2254
	21st to end	Day	14% 65% 5% 10% 5% 2	2280
May	1st to 10th	Day	33% 43% 7% 13% 3 1	3854
	11th to 20th	Day	31% 41% 9% 16% 3	4323
	21st to end	Day	17% 42% 9% 17% 12% 2	4837
June	1st to 10th	Day	13% 67% 5% 9% 5% 1	2660
	11th to 20th	Day	9% 66% 7% 10% 8%	2660
	21st to end	Day	10% 64% 7% 10% 7%	2660
July	1st to 10th	Day	19% 43% 19% 11% 6% 2	4370
	11th to 20th	Day	28% 39% 15% 15% 3	4244
	21st to end	Day	29% 39% 9% 12% 10% 1	4324
August	1st to 10th	Day	31% 40% 12% 14% 2	4091
	11th to 20th	Day	23% 41% 9% 19% 8%	3775
	21st to end	Day	14% 39% 10% 21% 16% 1	4180
September	1st to 10th	Day	14% 62% 8% 9% 5% 2	2280
	11th to 20th	Day	16% 61% 4 11% 7% 2	2280
	21st to end	Day	23% 60% 2 11% 3 1	2031
October	1st to 10th	Day	19% 39% 7% 14% 18% 3	3160
	11th to 20th	Day	13% 37% 3 13% 26% 8%	3140
	21st to end	Day	8% 36% 6% 21% 20% 9%	3428
November	1st to 10th	Day	17% 55% 5% 11% 8% 3	1830
	11th to 20th	Day	16% 57% 4 7% 12% 3	1800
	21st to end	Day	15% 57% 3 13% 10% 1	1695
December	1st to 10th	Day	20% 37% 5% 22% 14% 3	2600
	11th to 20th	Day	29% 31% 9% 15% 12% 3	2986
	21st to end	Day	26% 28% 11% 23% 11% 1	3960
January	1st to 10th	Night	27% 26% 3 16% 22% 5%	6239
	11th to 20th	Night	24% 26% 3 20% 21% 5%	5900
	21st to end	Night	30% 29% 5% 22% 13% 2	6490
February	1st to 10th	Night	17% 62% 7% 10% 5% 1	2600
	11th to 20th	Night	18% 60% 3 8% 8% 2	2399
	21st to end	Night	15% 62% 4 9% 7% 3	1980
March	1st to 10th	Night	36% 40% 4 12% 7% 1	3664
	11th to 20th	Night	17% 40% 4 17% 17% 3	3498
	21st to end	Night	25% 40% 3 16% 12% 4	3695
April	1st to 10th	Night	17% 65% 2 6% 8% 1	1900
	11th to 20th	Night	13% 65% 3 10% 7% 2	1875
	21st to end	Night	16% 65% 5% 7% 5% 1	1900
May	1st to 10th	Night	35% 41% 7% 13% 4	3133
	11th to 20th	Night	34% 41% 7% 14% 3	2574
	21st to end	Night	19% 43% 9% 16% 12%	2739
June	1st to 10th	Night	16% 66% 4 8% 6% 1	1520
	11th to 20th	Night	11% 67% 6% 9% 6% 1	1520
	21st to end	Night	11% 65% 5% 10% 6% 2	1520
July	1st to 10th	Night	26% 43% 12% 11% 7% 1	2480
	11th to 20th	Night	25% 39% 18% 15% 2	2589
	21st to end	Night	33% 39% 9% 12% 7%	2988
August	1st to 10th	Night	34% 39% 10% 14% 3	2814
	11th to 20th	Night	23% 40% 12% 18% 7%	3088
	21st to end	Night	16% 38% 6% 20% 21% 1	3476
September	1st to 10th	Night	13% 62% 9% 9% 4 2	1900
	11th to 20th	Night	15% 61% 4 10% 7% 2	1900
	21st to end	Night	24% 60% 4 8% 3	2145
October	1st to 10th	Night	23% 38% 8% 14% 15% 3	3800
	11th to 20th	Night	11% 37% 5% 15% 24% 7%	3779
	21st to end	Night	8% 36% 4 25% 18% 9%	4120
November	1st to 10th	Night	19% 56% 5% 11% 8% 2	2466
	11th to 20th	Night	17% 57% 4 9% 10% 5%	2600
	21st to end	Night	13% 57% 5% 12% 12% 1	2700
December	1st to 10th	Night	18% 36% 7% 20% 17% 3	4580
	11th to 20th	Night	34% 33% 8% 14% 8% 4	5208
	21st to end	Night	28% 28% 10% 23% 11%	6996

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% 25% 41% 9%	222
	11th to 20th	Day	17% 9% 33% 33% 8% 1	300
	21st to end	Day	17% 4 23% 45% 11% 1	330
February	1st to 10th	Day	17% 6% 22% 40% 14%	300
	11th to 20th	Day	25% 6% 32% 30% 7%	435
	21st to end	Day	22% 5% 27% 36% 10%	414
March	1st to 10th	Day	17% 6% 27% 35% 12% 2	500
	11th to 20th	Day	18% 6% 25% 39% 11% 2	464
	21st to end	Day	20% 4 22% 38% 14% 2	506
April	1st to 10th	Day	15% 4 26% 37% 16% 2	460
	11th to 20th	Day	9% 3 21% 39% 23% 5%	452
	21st to end	Day	17% 5% 17% 33% 25% 3	460
May	1st to 10th	Day	18% 4 16% 34% 22% 6%	463
	11th to 20th	Day	20% 2 13% 32% 29% 6%	487
	21st to end	Day	17% 3 6% 27% 34% 13%	529
June	1st to 10th	Day	11% 2 7% 30% 37% 13%	490
	11th to 20th	Day	10% 1 9% 29% 36% 15%	490
	21st to end	Day	10% 2 6% 31% 39% 12%	490
July	1st to 10th	Day	9% 2 4 24% 39% 21%	485
	11th to 20th	Day	6% 2 2 20% 49% 21%	484
	21st to end	Day	11% 2 4 27% 43% 14%	532
August	1st to 10th	Day	10% 2 6% 27% 41% 14%	486
	11th to 20th	Day	10% 4 5% 25% 38% 18%	479
	21st to end	Day	10% 1 5% 27% 37% 21%	539
September	1st to 10th	Day	15% 1 6% 24% 32% 21%	490
	11th to 20th	Day	14% 1 8% 23% 34% 18%	490
	21st to end	Day	24% 5% 11% 28% 24% 8%	490
October	1st to 10th	Day	19% 3 9% 28% 27% 14%	490
	11th to 20th	Day	16% 4 19% 27% 25% 9%	490
	21st to end	Day	13% 6% 19% 31% 23% 7%	539
November	1st to 10th	Day	16% 5% 21% 30% 21% 7%	431
	11th to 20th	Day	14% 9% 29% 32% 15% 2	400
	21st to end	Day	16% 7% 26% 29% 19% 3	295
December	1st to 10th	Day	17% 5% 27% 39% 12% 1	200
	11th to 20th	Day	23% 5% 23% 36% 14% 1	200
	21st to end	Day	18% 6% 21% 41% 13% 1	220
January	1st to 10th	Night	18% 13% 37% 27% 6%	800
	11th to 20th	Night	20% 10% 31% 31% 9%	800
	21st to end	Night	21% 10% 34% 32% 4	880
February	1st to 10th	Night	19% 7% 31% 37% 5% 1	800
	11th to 20th	Night	22% 11% 36% 27% 4	783
	21st to end	Night	19% 10% 30% 35% 5% 1	660
March	1st to 10th	Night	18% 9% 31% 34% 8% 1	800
	11th to 20th	Night	20% 6% 30% 33% 9% 3	767
	21st to end	Night	19% 5% 29% 38% 8% 1	836
April	1st to 10th	Night	17% 6% 34% 32% 11% 1	760
	11th to 20th	Night	13% 5% 26% 39% 16% 1	751
	21st to end	Night	19% 4 21% 39% 14% 2	760
May	1st to 10th	Night	19% 5% 21% 39% 14% 2	746
	11th to 20th	Night	18% 4 19% 37% 20% 2	426
	21st to end	Night	15% 2 10% 33% 35% 6%	411
June	1st to 10th	Night	13% 2 14% 33% 33% 4	380
	11th to 20th	Night	11% 3 9% 38% 33% 6%	380
	21st to end	Night	7% 3 10% 40% 35% 4	380
July	1st to 10th	Night	9% 3 4 37% 38% 8%	370
	11th to 20th	Night	5% 3 3 33% 47% 9%	433
	21st to end	Night	8% 3 6% 38% 40% 5%	624
August	1st to 10th	Night	10% 2 5% 37% 40% 7%	567
	11th to 20th	Night	12% 3 7% 33% 38% 7%	723
	21st to end	Night	12% 2 6% 31% 40% 9%	836
September	1st to 10th	Night	15% 2 8% 30% 35% 11%	760
	11th to 20th	Night	13% 2 9% 29% 39% 9%	760
	21st to end	Night	25% 4 12% 30% 26% 4	760
October	1st to 10th	Night	19% 3 17% 27% 26% 8%	760
	11th to 20th	Night	18% 6% 21% 31% 21% 3	760
	21st to end	Night	18% 7% 23% 29% 19% 4	836
November	1st to 10th	Night	18% 6% 21% 31% 19% 4	780
	11th to 20th	Night	20% 8% 27% 32% 13% 1	800
	21st to end	Night	14% 11% 28% 32% 14% 2	800
December	1st to 10th	Night	20% 9% 32% 27% 10% 2	800
	11th to 20th	Night	18% 8% 35% 29% 9%	800
	21st to end	Night	17% 8% 29% 34% 11%	880

Page generated at 2025-03-09 08:08:47. Version ac07

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