Red Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Red Hill Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	6% 6% 2 6% 4 3 4 3 5% 6% 55%	139451
	Night	6% 7% 3 6% 4 4 4 4 5% 7% 51%	139730
Transparency	Day	28% 6% 10% 22% 20% 13%	134184
	Night	32% 6% 12% 22% 17% 10%	136013
Seeing	Day	9% 8% 24% 29% 21% 8%	16560
	Night	10% 6% 23% 40% 19% 2	26425

By Month

Percentage Cloud Cover SW (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% 16% 5% 10% 6% 5% 4 4 6% 7% 29%	4722
	11th to 20th	Day	5% 8% 5% 11% 4 4 4 3 4 6% 46%	4740
	21st to end	Day	7% 10% 4 8% 4 4 4 4 5% 7% 43%	5699
February	1st to 10th	Day	8% 11% 4 9% 5% 5% 5% 4 6% 7% 37%	1890
	11th to 20th	Day	8% 12% 4 10% 5% 4 4 5% 6% 8% 35%	1967
	21st to end	Day	6% 13% 3 10% 5% 4 4 4 5% 8% 38%	1730
March	1st to 10th	Day	9% 12% 4 9% 5% 4 4 3 4 6% 40%	4721
	11th to 20th	Day	6% 10% 2 6% 4 4 4 4 6% 6% 48%	5110
	21st to end	Day	7% 10% 3 8% 6% 5% 6% 4 7% 8% 36%	5607
April	1st to 10th	Day	9% 9% 2 6% 5% 5% 4 4 7% 7% 43%	2325
	11th to 20th	Day	7% 8% 2 5% 4 4 4 3 5% 7% 48%	2535
	21st to end	Day	7% 6% 2 6% 4 4 4 4 7% 8% 48%	2600
May	1st to 10th	Day	6% 6% 2 6% 3 4 3 3 5% 6% 55%	5880
	11th to 20th	Day	8% 5% 2 5% 4 3 4 3 5% 6% 55%	5841
	21st to end	Day	5% 4 1 4 3 3 4 3 5% 8% 61%	6460
June	1st to 10th	Day	6% 4 1 4 3 4 4 4 7% 9% 54%	2895
	11th to 20th	Day	5% 5% 1 3 3 3 4 2 5% 8% 61%	3000
	21st to end	Day	2 2 1 2 2 2 3 3 4 6% 73%	2855
July	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 77%	6319
	11th to 20th	Day	1 1 1 3 2 3 4 4 5% 5% 72%	6320
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 69%	6538
August	1st to 10th	Day	3 3 1 3 3 3 3 3 6% 7% 64%	5779
	11th to 20th	Day	2 1 1 3 2 2 3 2 4 5% 75%	5798
	21st to end	Day	4 3 1 4 3 3 3 2 4 5% 67%	6394
September	1st to 10th	Day	3 2 1 2 3 3 2 3 4 7% 71%	2284
	11th to 20th	Day	4 3 1 3 3 2 3 3 5% 6% 68%	2200
	21st to end	Day	6% 3 1 3 2 3 2 3 4 5% 67%	2200
October	1st to 10th	Day	5% 3 1 4 3 2 3 2 4 5% 68%	3537
	11th to 20th	Day	6% 4 2 4 2 3 3 2 4 6% 63%	3250
	21st to end	Day	5% 6% 2 5% 3 3 4 4 6% 8% 57%	3270
November	1st to 10th	Day	7% 9% 3 6% 5% 4 5% 3 6% 6% 47%	1854
	11th to 20th	Day	8% 7% 2 7% 4 5% 5% 4 6% 9% 43%	1890
	21st to end	Day	7% 10% 4 8% 5% 4 5% 4 5% 8% 42%	1760
December	1st to 10th	Day	6% 6% 2 8% 3 4 3 3 4 6% 55%	2700
	11th to 20th	Day	10% 13% 4 8% 4 5% 5% 5% 7% 9% 29%	2801
	21st to end	Day	12% 13% 5% 11% 6% 4 4 4 5% 6% 31%	3980
January	1st to 10th	Night	11% 13% 4 12% 6% 5% 5% 3 5% 7% 29%	8172
	11th to 20th	Night	5% 9% 4 12% 5% 4 3 3 5% 7% 43%	8220
	21st to end	Night	6% 8% 3 7% 4 4 4 3 5% 7% 48%	8505
February	1st to 10th	Night	8% 11% 4 9% 6% 5% 5% 4 7% 8% 34%	2730
	11th to 20th	Night	9% 11% 3 6% 6% 4 5% 3 5% 8% 40%	2537
	21st to end	Night	5% 8% 3 6% 6% 5% 6% 4 7% 8% 41%	2076
March	1st to 10th	Night	9% 11% 3 7% 4 4 5% 3 5% 7% 41%	5391
	11th to 20th	Night	5% 8% 3 6% 4 4 5% 4 6% 7% 49%	5110
	21st to end	Night	6% 7% 2 6% 5% 5% 6% 4 7% 10% 43%	5610
April	1st to 10th	Night	6% 9% 3 7% 4 4 5% 4 5% 6% 47%	2095
	11th to 20th	Night	7% 9% 3 5% 4 3 4 4 5% 6% 48%	1811
	21st to end	Night	6% 6% 2 6% 4 4 4 4 6% 7% 52%	1800
May	1st to 10th	Night	9% 6% 2 4 3 3 3 4 5% 7% 54%	4100
	11th to 20th	Night	6% 3 2 5% 4 4 5% 4 7% 6% 54%	4074
	21st to end	Night	5% 4 1 4 3 3 4 3 5% 7% 60%	4430
June	1st to 10th	Night	5% 2 2 5% 4 4 4 5% 7% 9% 53%	1495
	11th to 20th	Night	5% 4 2 3 2 4 3 4 5% 5% 63%	1400
	21st to end	Night	3 2 3 2 3 3 2 5% 5% 71%	1533
July	1st to 10th	Night	2 1 1 3 2 3 4 3 4 4 74%	3640
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 5% 75%	3640
	21st to end	Night	1 1 1 3 3 3 4 4 5% 7% 67%	4325
August	1st to 10th	Night	2 3 1 3 2 3 4 3 5% 9% 63%	4033
	11th to 20th	Night	2 2 1 3 3 4 4 4 6% 7% 65%	4051
	21st to end	Night	3 3 1 4 3 3 4 3 4 5% 66%	4570
September	1st to 10th	Night	2 2 1 3 2 2 3 3 4 5% 72%	2115
	11th to 20th	Night	5% 3 1 4 3 3 4 4 5% 5% 65%	2200
	21st to end	Night	6% 3 1 4 2 2 3 2 4 4 68%	2200
October	1st to 10th	Night	4 3 1 3 2 2 2 2 4 5% 71%	3659
	11th to 20th	Night	7% 5% 2 6% 4 4 3 3 4 5% 58%	3898
	21st to end	Night	5% 6% 2 5% 3 5% 5% 4 6% 9% 51%	4527
November	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 49%	2665
	11th to 20th	Night	8% 6% 3 7% 5% 4 4 4 6% 8% 45%	2730
	21st to end	Night	6% 8% 3 7% 5% 5% 5% 4 6% 7% 44%	2835
December	1st to 10th	Night	6% 7% 2 7% 3 4 3 3 5% 7% 53%	4710
	11th to 20th	Night	10% 12% 4 9% 4 4 5% 4 6% 9% 32%	5596
	21st to end	Night	11% 11% 5% 10% 6% 5% 5% 4 6% 7% 30%	7247

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	53% 7% 20% 17% 2	4723
	11th to 20th	Day	39% 8% 19% 25% 8% 1	4740
	21st to end	Day	40% 8% 16% 25% 11% 1	5699
February	1st to 10th	Day	45% 9% 18% 21% 5% 1	1890
	11th to 20th	Day	45% 10% 17% 20% 8% 1	1977
	21st to end	Day	43% 8% 15% 23% 9% 2	1730
March	1st to 10th	Day	43% 7% 12% 27% 8% 3	4721
	11th to 20th	Day	34% 6% 12% 33% 15% 1	5110
	21st to end	Day	41% 10% 19% 23% 7%	5607
April	1st to 10th	Day	38% 6% 16% 21% 16% 3	2325
	11th to 20th	Day	33% 5% 11% 27% 19% 3	2538
	21st to end	Day	31% 6% 13% 28% 21% 2	2600
May	1st to 10th	Day	26% 6% 9% 24% 29% 6%	5880
	11th to 20th	Day	28% 5% 7% 26% 28% 6%	5844
	21st to end	Day	21% 6% 8% 28% 30% 7%	6461
June	1st to 10th	Day	24% 7% 12% 29% 24% 4	2895
	11th to 20th	Day	22% 4 9% 24% 28% 13%	3000
	21st to end	Day	12% 4 5% 20% 36% 24%	2855
July	1st to 10th	Day	10% 2 3 11% 24% 50%	6251
	11th to 20th	Day	12% 3 3 6% 25% 51%	6245
	21st to end	Day	15% 3 3 9% 33% 37%	6350
August	1st to 10th	Day	19% 4 4 15% 25% 33%	4208
	11th to 20th	Day	12% 3 3 15% 41% 27%	4197
	21st to end	Day	15% 3 3 17% 32% 31%	4613
September	1st to 10th	Day	15% 4 5% 17% 31% 28%	2284
	11th to 20th	Day	17% 5% 8% 26% 28% 16%	2200
	21st to end	Day	20% 4 6% 24% 34% 12%	2200
October	1st to 10th	Day	21% 3 7% 19% 32% 18%	3537
	11th to 20th	Day	23% 4 7% 34% 27% 5%	3250
	21st to end	Day	25% 5% 12% 36% 19% 4	3270
November	1st to 10th	Day	37% 6% 13% 27% 16% 2	1854
	11th to 20th	Day	36% 9% 19% 26% 8% 1	1890
	21st to end	Day	40% 7% 19% 24% 9% 1	1760
December	1st to 10th	Day	31% 7% 22% 33% 6% 1	2700
	11th to 20th	Day	49% 8% 19% 21% 2	2800
	21st to end	Day	52% 8% 14% 18% 8%	3980
January	1st to 10th	Night	53% 7% 17% 19% 4	8172
	11th to 20th	Night	39% 8% 20% 28% 5%	8220
	21st to end	Night	34% 7% 18% 29% 11% 2	8505
February	1st to 10th	Night	46% 9% 18% 21% 5% 1	2730
	11th to 20th	Night	41% 7% 19% 22% 11%	2538
	21st to end	Night	37% 10% 17% 27% 9% 1	2076
March	1st to 10th	Night	39% 7% 11% 26% 12% 4	5391
	11th to 20th	Night	32% 7% 13% 30% 17% 1	5110
	21st to end	Night	34% 8% 17% 27% 12% 1	5610
April	1st to 10th	Night	36% 6% 13% 22% 17% 5%	2095
	11th to 20th	Night	35% 6% 10% 27% 19% 2	1811
	21st to end	Night	30% 5% 11% 25% 27% 2	1800
May	1st to 10th	Night	27% 7% 9% 25% 25% 7%	4097
	11th to 20th	Night	26% 6% 8% 25% 27% 8%	4075
	21st to end	Night	22% 6% 8% 23% 32% 10%	4430
June	1st to 10th	Night	25% 7% 15% 24% 24% 5%	1495
	11th to 20th	Night	22% 5% 6% 21% 32% 15%	1400
	21st to end	Night	14% 4 5% 14% 31% 32%	1533
July	1st to 10th	Night	14% 3 3 8% 20% 52%	3600
	11th to 20th	Night	11% 2 2 5% 21% 59%	3592
	21st to end	Night	15% 3 3 8% 33% 37%	4201
August	1st to 10th	Night	18% 4 4 13% 22% 39%	2928
	11th to 20th	Night	16% 4 4 14% 33% 30%	2928
	21st to end	Night	14% 3 3 14% 30% 36%	3293
September	1st to 10th	Night	15% 4 5% 14% 29% 34%	2115
	11th to 20th	Night	20% 5% 5% 24% 27% 20%	2200
	21st to end	Night	21% 3 5% 20% 35% 16%	2200
October	1st to 10th	Night	17% 3 6% 16% 36% 21%	3659
	11th to 20th	Night	30% 4 5% 28% 27% 7%	3898
	21st to end	Night	29% 6% 13% 31% 19% 3	4527
November	1st to 10th	Night	33% 6% 14% 26% 18% 3	2665
	11th to 20th	Night	35% 7% 18% 27% 12% 1	2730
	21st to end	Night	36% 7% 18% 28% 9% 1	2835
December	1st to 10th	Night	30% 7% 21% 35% 7% 1	4710
	11th to 20th	Night	48% 8% 22% 21% 2	5597
	21st to end	Night	51% 8% 16% 17% 8%	7247

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	19% 9% 33% 26% 11% 2	220
	11th to 20th	Day	10% 9% 29% 32% 18% 3	220
	21st to end	Day	16% 6% 25% 32% 19% 2	336
February	1st to 10th	Day	18% 14% 29% 28% 9% 2	330
	11th to 20th	Day	15% 11% 35% 25% 12% 2	453
	21st to end	Day	14% 15% 28% 32% 10% 2	446
March	1st to 10th	Day	13% 12% 33% 29% 11% 1	540
	11th to 20th	Day	12% 12% 31% 30% 15% 1	540
	21st to end	Day	13% 15% 31% 25% 14% 2	594
April	1st to 10th	Day	12% 13% 31% 20% 18% 6%	505
	11th to 20th	Day	12% 16% 30% 26% 12% 4	492
	21st to end	Day	10% 13% 31% 24% 18% 4	500
May	1st to 10th	Day	7% 15% 25% 28% 17% 7%	503
	11th to 20th	Day	14% 12% 22% 26% 17% 9%	528
	21st to end	Day	10% 8% 20% 26% 23% 14%	573
June	1st to 10th	Day	7% 7% 22% 27% 26% 12%	625
	11th to 20th	Day	8% 10% 26% 30% 21% 6%	730
	21st to end	Day	3 4 23% 36% 24% 10%	585
July	1st to 10th	Day	2 4 19% 32% 31% 11%	526
	11th to 20th	Day	2 3 21% 37% 31% 6%	524
	21st to end	Day	3 3 12% 33% 34% 15%	575
August	1st to 10th	Day	5% 4 19% 30% 31% 11%	527
	11th to 20th	Day	3 3 16% 28% 34% 17%	517
	21st to end	Day	4 4 17% 24% 33% 18%	583
September	1st to 10th	Day	3 5% 15% 27% 32% 19%	530
	11th to 20th	Day	5% 5% 19% 27% 30% 15%	530
	21st to end	Day	6% 5% 20% 28% 28% 13%	530
October	1st to 10th	Day	12% 3 20% 28% 25% 13%	493
	11th to 20th	Day	9% 10% 25% 22% 24% 11%	330
	21st to end	Day	12% 9% 23% 35% 17% 4	363
November	1st to 10th	Day	14% 8% 26% 31% 20% 2	330
	11th to 20th	Day	9% 8% 26% 37% 18% 2	330
	21st to end	Day	14% 6% 28% 33% 15% 4	330
December	1st to 10th	Day	18% 7% 26% 31% 13% 5%	330
	11th to 20th	Day	20% 10% 24% 31% 13% 2	250
	21st to end	Day	22% 13% 24% 26% 12% 3	242
January	1st to 10th	Night	14% 8% 27% 37% 12% 1	840
	11th to 20th	Night	13% 8% 22% 39% 16% 2	840
	21st to end	Night	13% 5% 22% 43% 16% 1	924
February	1st to 10th	Night	14% 8% 32% 34% 11% 1	840
	11th to 20th	Night	14% 7% 31% 36% 11% 1	824
	21st to end	Night	8% 8% 34% 38% 11% 1	692
March	1st to 10th	Night	12% 7% 28% 41% 11% 1	840
	11th to 20th	Night	9% 8% 28% 40% 14% 1	840
	21st to end	Night	12% 8% 29% 39% 11% 1	924
April	1st to 10th	Night	10% 10% 24% 39% 16% 2	804
	11th to 20th	Night	10% 10% 33% 35% 11% 1	623
	21st to end	Night	8% 7% 32% 36% 16% 1	600
May	1st to 10th	Night	8% 8% 29% 36% 19% 1	600
	11th to 20th	Night	8% 6% 23% 36% 25% 2	591
	21st to end	Night	12% 6% 19% 35% 25% 4	632
June	1st to 10th	Night	6% 5% 20% 41% 26% 3	400
	11th to 20th	Night	8% 7% 24% 46% 14% 2	400
	21st to end	Night	4 1 19% 48% 25% 3	400
July	1st to 10th	Night	3 2 11% 57% 26% 2	390
	11th to 20th	Night	1 1 14% 58% 25% 2	388
	21st to end	Night	2 1 10% 53% 32% 2	588
August	1st to 10th	Night	4 3 14% 51% 27% 2	597
	11th to 20th	Night	2 2 14% 50% 30% 2	594
	21st to end	Night	4 3 15% 48% 27% 3	682
September	1st to 10th	Night	4 2 14% 45% 31% 5%	800
	11th to 20th	Night	6% 2 14% 40% 32% 6%	800
	21st to end	Night	8% 4 16% 37% 30% 6%	800
October	1st to 10th	Night	9% 5% 19% 38% 28% 1	800
	11th to 20th	Night	9% 6% 19% 39% 24% 4	800
	21st to end	Night	8% 6% 24% 40% 19% 2	880
November	1st to 10th	Night	12% 4 25% 40% 17% 2	820
	11th to 20th	Night	10% 7% 27% 41% 14% 1	840
	21st to end	Night	9% 7% 25% 39% 18% 2	945
December	1st to 10th	Night	13% 7% 27% 39% 12% 1	1050
	11th to 20th	Night	17% 7% 24% 40% 12% 1	1050
	21st to end	Night	19% 9% 31% 31% 9% 1	987

Page generated at 2026-03-30 00:18:44. Version ac07

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