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 2024-01-16.

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	7% 6% 3 5% 5% 3 4 4 5% 7% 52%	74623
	Night	7% 7% 3 6% 5% 3 4 5% 5% 7% 49%	75154
Transparency	Day	31% 6% 12% 22% 19% 10%	74334
	Night	33% 7% 13% 22% 16% 9%	74965
Seeing	Day	9% 8% 23% 29% 22% 9%	13674
	Night	10% 6% 23% 40% 19% 2	23534

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	11% 12% 5% 8% 7% 4 5% 5% 5% 7% 32%	1519
	11th to 20th	Day	10% 9% 5% 7% 5% 3 5% 5% 6% 7% 39%	1480
	21st to end	Day	10% 14% 5% 8% 5% 3 5% 6% 5% 7% 30%	1735
February	1st to 10th	Day	7% 10% 5% 7% 6% 4 5% 6% 5% 7% 38%	1620
	11th to 20th	Day	8% 10% 5% 9% 5% 3 5% 5% 6% 8% 37%	1679
	21st to end	Day	6% 13% 4 9% 6% 3 5% 4 5% 9% 35%	1480
March	1st to 10th	Day	7% 8% 4 7% 6% 4 5% 6% 5% 8% 39%	1836
	11th to 20th	Day	8% 9% 3 7% 6% 3 6% 5% 6% 8% 40%	1980
	21st to end	Day	10% 10% 5% 6% 6% 4 5% 5% 6% 8% 36%	2178
April	1st to 10th	Day	10% 8% 3 6% 6% 4 4 5% 7% 8% 41%	2081
	11th to 20th	Day	8% 8% 3 5% 5% 3 4 4 5% 7% 47%	2277
	21st to end	Day	7% 6% 2 6% 4 3 3 5% 6% 8% 50%	2340
May	1st to 10th	Day	7% 5% 2 5% 5% 4 4 5% 6% 7% 49%	2340
	11th to 20th	Day	8% 5% 2 4 6% 3 5% 5% 6% 9% 47%	2301
	21st to end	Day	9% 5% 2 5% 6% 3 5% 6% 6% 9% 42%	2541
June	1st to 10th	Day	7% 3 2 3 4 3 5% 5% 7% 9% 53%	2605
	11th to 20th	Day	6% 5% 1 3 4 1 4 3 5% 8% 61%	2700
	21st to end	Day	2 2 1 2 3 1 2 4 4 7% 73%	2571
July	1st to 10th	Day	2 1 1 2 3 2 3 4 6% 8% 69%	2520
	11th to 20th	Day	3 2 2 3 2 4 3 4 6% 71%	2520
	21st to end	Day	4 3 1 3 4 2 4 4 5% 5% 66%	2604
August	1st to 10th	Day	5% 3 1 3 4 2 4 4 6% 6% 61%	2328
	11th to 20th	Day	3 2 1 3 2 1 2 3 4 6% 75%	2299
	21st to end	Day	3 2 1 3 3 2 3 3 4 6% 70%	2552
September	1st to 10th	Day	4 2 1 2 2 2 2 3 4 7% 72%	2056
	11th to 20th	Day	4 3 1 2 3 2 3 4 4 6% 68%	1980
	21st to end	Day	7% 3 1 3 3 2 3 3 4 5% 66%	1980
October	1st to 10th	Day	9% 5% 2 5% 5% 2 4 4 5% 7% 53%	1947
	11th to 20th	Day	7% 4 2 4 3 2 3 4 5% 6% 58%	1800
	21st to end	Day	6% 4 2 6% 3 2 4 5% 4 8% 57%	1825
November	1st to 10th	Day	8% 9% 3 6% 6% 2 5% 4 5% 6% 46%	1674
	11th to 20th	Day	9% 6% 3 5% 5% 3 5% 4 6% 9% 45%	1710
	21st to end	Day	8% 9% 3 7% 6% 3 5% 5% 5% 7% 42%	1593
December	1st to 10th	Day	10% 10% 4 8% 6% 4 5% 4 4 7% 38%	1520
	11th to 20th	Day	10% 11% 5% 9% 6% 4 6% 6% 7% 8% 28%	1740
	21st to end	Day	9% 11% 5% 8% 7% 2 5% 5% 5% 7% 36%	2712
January	1st to 10th	Night	10% 12% 4 9% 6% 3 4 5% 5% 9% 34%	2646
	11th to 20th	Night	8% 11% 6% 9% 5% 3 5% 4 6% 8% 35%	2602
	21st to end	Night	10% 12% 5% 10% 6% 3 4 4 4 6% 36%	2618
February	1st to 10th	Night	7% 9% 4 9% 7% 4 6% 5% 7% 8% 35%	2340
	11th to 20th	Night	9% 9% 3 5% 6% 4 4 5% 5% 8% 43%	2166
	21st to end	Night	6% 8% 3 6% 7% 4 7% 5% 7% 8% 38%	1776
March	1st to 10th	Night	8% 8% 3 5% 6% 4 6% 6% 6% 8% 40%	2122
	11th to 20th	Night	7% 7% 3 5% 5% 3 5% 5% 6% 8% 46%	1980
	21st to end	Night	10% 9% 3 6% 5% 4 6% 6% 6% 7% 40%	2178
April	1st to 10th	Night	7% 8% 4 6% 5% 3 5% 5% 5% 6% 46%	1876
	11th to 20th	Night	8% 9% 4 5% 5% 2 4 5% 5% 6% 47%	1629
	21st to end	Night	7% 5% 3 4 4 3 3 4 6% 7% 54%	1620
May	1st to 10th	Night	7% 6% 2 5% 5% 3 4 5% 5% 6% 51%	1620
	11th to 20th	Night	6% 4 2 5% 6% 3 6% 6% 8% 8% 45%	1594
	21st to end	Night	11% 6% 2 5% 5% 3 5% 5% 5% 8% 45%	1730
June	1st to 10th	Night	5% 2 2 5% 5% 3 5% 5% 6% 9% 53%	1346
	11th to 20th	Night	5% 4 2 3 3 3 4 4 5% 5% 63%	1260
	21st to end	Night	3 2 2 3 2 2 3 5% 6% 71%	1379
July	1st to 10th	Night	2 2 1 2 3 2 5% 5% 6% 8% 65%	1440
	11th to 20th	Night	1 2 1 2 3 2 3 3 4 6% 73%	1440
	21st to end	Night	2 2 1 3 4 3 4 5% 5% 7% 65%	1710
August	1st to 10th	Night	4 4 1 4 3 3 4 4 5% 10% 59%	1611
	11th to 20th	Night	3 2 1 2 3 2 4 4 5% 8% 66%	1602
	21st to end	Night	3 3 1 4 3 2 3 3 4 5% 68%	1801
September	1st to 10th	Night	2 3 1 2 3 2 3 3 3 5% 73%	1903
	11th to 20th	Night	5% 3 1 3 3 2 4 4 5% 5% 65%	1980
	21st to end	Night	7% 3 1 3 3 2 3 3 4 4 68%	1980
October	1st to 10th	Night	8% 4 2 4 3 2 3 4 5% 6% 59%	2011
	11th to 20th	Night	7% 4 2 4 4 3 3 3 4 6% 61%	2160
	21st to end	Night	5% 5% 2 4 4 3 5% 5% 6% 8% 54%	2528
November	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 48%	2405
	11th to 20th	Night	9% 5% 3 6% 5% 3 4 5% 6% 8% 46%	2470
	21st to end	Night	7% 7% 4 6% 5% 3 5% 5% 6% 7% 45%	2565
December	1st to 10th	Night	9% 10% 4 7% 6% 3 5% 5% 6% 8% 37%	2660
	11th to 20th	Night	11% 11% 5% 9% 6% 3 6% 6% 6% 8% 29%	3448
	21st to end	Night	10% 11% 6% 9% 6% 3 5% 5% 5% 7% 34%	4958

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	51% 9% 20% 15% 3 1	1519
	11th to 20th	Day	44% 11% 19% 19% 6% 1	1480
	21st to end	Day	52% 9% 20% 16% 3 1	1735
February	1st to 10th	Day	44% 9% 20% 22% 5% 1	1620
	11th to 20th	Day	44% 10% 17% 21% 8% 1	1689
	21st to end	Day	45% 8% 16% 21% 7% 2	1480
March	1st to 10th	Day	41% 9% 15% 25% 9% 1	1836
	11th to 20th	Day	40% 8% 15% 26% 10% 1	1980
	21st to end	Day	44% 8% 19% 19% 9% 1	2178
April	1st to 10th	Day	39% 6% 15% 20% 16% 3	2081
	11th to 20th	Day	35% 5% 12% 26% 18% 3	2280
	21st to end	Day	31% 5% 13% 28% 21% 2	2340
May	1st to 10th	Day	30% 7% 8% 27% 23% 4	2340
	11th to 20th	Day	33% 6% 12% 28% 17% 4	2304
	21st to end	Day	36% 8% 13% 27% 14% 2	2541
June	1st to 10th	Day	25% 8% 13% 29% 21% 3	2605
	11th to 20th	Day	23% 5% 10% 25% 26% 13%	2700
	21st to end	Day	12% 4 5% 21% 35% 23%	2571
July	1st to 10th	Day	13% 4 4 18% 35% 27%	2451
	11th to 20th	Day	13% 3 4 11% 29% 39%	2445
	21st to end	Day	18% 4 5% 12% 24% 37%	2459
August	1st to 10th	Day	21% 5% 6% 17% 23% 27%	2313
	11th to 20th	Day	12% 3 4 19% 35% 26%	2299
	21st to end	Day	16% 4 4 19% 34% 24%	2552
September	1st to 10th	Day	14% 4 5% 17% 31% 29%	2056
	11th to 20th	Day	17% 5% 9% 25% 27% 17%	1980
	21st to end	Day	21% 4 7% 25% 34% 10%	1980
October	1st to 10th	Day	31% 5% 12% 26% 20% 6%	1947
	11th to 20th	Day	26% 6% 12% 33% 20% 4	1800
	21st to end	Day	27% 6% 17% 31% 18% 2	1825
November	1st to 10th	Day	38% 7% 13% 25% 15% 2	1674
	11th to 20th	Day	35% 9% 20% 27% 8% 1	1710
	21st to end	Day	41% 7% 19% 23% 10% 1	1593
December	1st to 10th	Day	46% 11% 18% 18% 6% 1	1520
	11th to 20th	Day	50% 10% 21% 17% 2	1739
	21st to end	Day	44% 10% 16% 20% 10%	2712
January	1st to 10th	Night	48% 9% 20% 17% 4 1	2646
	11th to 20th	Night	45% 10% 16% 22% 5% 1	2602
	21st to end	Night	49% 7% 19% 19% 5% 1	2618
February	1st to 10th	Night	45% 9% 19% 22% 4 1	2340
	11th to 20th	Night	39% 8% 19% 23% 11%	2167
	21st to end	Night	40% 11% 16% 25% 8% 1	1776
March	1st to 10th	Night	39% 9% 16% 26% 8% 2	2122
	11th to 20th	Night	33% 7% 16% 27% 15% 1	1980
	21st to end	Night	42% 8% 16% 25% 9% 1	2178
April	1st to 10th	Night	39% 6% 12% 21% 16% 6%	1876
	11th to 20th	Night	37% 7% 10% 27% 17% 2	1629
	21st to end	Night	29% 5% 11% 26% 27% 1	1620
May	1st to 10th	Night	30% 7% 10% 23% 25% 5%	1620
	11th to 20th	Night	32% 7% 12% 24% 20% 5%	1595
	21st to end	Night	36% 7% 13% 27% 15% 3	1730
June	1st to 10th	Night	26% 7% 16% 23% 24% 5%	1346
	11th to 20th	Night	22% 5% 7% 21% 32% 13%	1260
	21st to end	Night	14% 4 5% 14% 32% 31%	1379
July	1st to 10th	Night	16% 5% 4 13% 31% 32%	1400
	11th to 20th	Night	12% 3 3 10% 23% 48%	1392
	21st to end	Night	17% 4 3 12% 23% 41%	1615
August	1st to 10th	Night	21% 4 6% 18% 20% 31%	1602
	11th to 20th	Night	15% 4 4 14% 31% 33%	1602
	21st to end	Night	17% 4 4 14% 30% 30%	1801
September	1st to 10th	Night	14% 4 5% 14% 28% 36%	1903
	11th to 20th	Night	20% 5% 5% 22% 27% 22%	1980
	21st to end	Night	21% 3 5% 20% 35% 15%	1980
October	1st to 10th	Night	26% 4 10% 24% 26% 9%	2011
	11th to 20th	Night	26% 5% 8% 32% 25% 5%	2160
	21st to end	Night	26% 7% 15% 30% 19% 3	2528
November	1st to 10th	Night	34% 6% 14% 25% 17% 3	2405
	11th to 20th	Night	34% 7% 18% 27% 13% 1	2470
	21st to end	Night	36% 7% 18% 28% 10% 1	2565
December	1st to 10th	Night	44% 9% 21% 18% 7% 2	2660
	11th to 20th	Night	49% 9% 19% 20% 3	3449
	21st to end	Night	47% 9% 17% 18% 9%	4958

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	16% 10% 33% 26% 12% 2	180
	11th to 20th	Day	11% 11% 31% 29% 15% 4	170
	21st to end	Day	18% 7% 27% 27% 18% 2	244
February	1st to 10th	Day	15% 15% 32% 28% 9% 1	240
	11th to 20th	Day	13% 9% 36% 26% 13% 3	345
	21st to end	Day	14% 14% 29% 32% 10% 1	346
March	1st to 10th	Day	11% 11% 34% 32% 11% 2	420
	11th to 20th	Day	11% 9% 32% 32% 16%	420
	21st to end	Day	14% 15% 32% 24% 14% 2	462
April	1st to 10th	Day	14% 13% 30% 20% 17% 6%	420
	11th to 20th	Day	12% 17% 31% 25% 11% 4	412
	21st to end	Day	11% 13% 31% 24% 17% 4	420
May	1st to 10th	Day	6% 15% 25% 29% 17% 7%	423
	11th to 20th	Day	14% 9% 21% 26% 19% 11%	448
	21st to end	Day	10% 8% 19% 25% 23% 15%	485
June	1st to 10th	Day	8% 7% 21% 25% 26% 13%	535
	11th to 20th	Day	8% 10% 25% 30% 21% 6%	630
	21st to end	Day	3 3 22% 35% 26% 12%	501
July	1st to 10th	Day	2 3 18% 32% 33% 13%	446
	11th to 20th	Day	2 4 20% 38% 31% 6%	444
	21st to end	Day	2 3 11% 32% 36% 16%	487
August	1st to 10th	Day	6% 4 17% 30% 32% 12%	447
	11th to 20th	Day	3 3 14% 26% 35% 19%	437
	21st to end	Day	3 4 16% 24% 35% 18%	495
September	1st to 10th	Day	4 5% 16% 26% 30% 20%	450
	11th to 20th	Day	5% 4 17% 27% 31% 16%	450
	21st to end	Day	7% 5% 19% 27% 29% 12%	450
October	1st to 10th	Day	13% 3 19% 27% 25% 12%	417
	11th to 20th	Day	8% 12% 21% 22% 25% 11%	270
	21st to end	Day	12% 9% 23% 35% 18% 4	297
November	1st to 10th	Day	15% 7% 27% 30% 19% 1	270
	11th to 20th	Day	10% 7% 23% 38% 19% 2	270
	21st to end	Day	15% 5% 26% 34% 16% 4	270
December	1st to 10th	Day	20% 5% 26% 32% 13% 3	270
	11th to 20th	Day	20% 9% 22% 31% 15% 2	205
	21st to end	Day	18% 11% 25% 29% 14% 4	198
January	1st to 10th	Night	14% 8% 26% 37% 13% 1	760
	11th to 20th	Night	14% 8% 22% 37% 16% 2	744
	21st to end	Night	14% 5% 22% 40% 18% 1	792
February	1st to 10th	Night	13% 8% 32% 35% 12% 1	720
	11th to 20th	Night	13% 6% 30% 38% 12% 1	704
	21st to end	Night	9% 8% 32% 36% 13% 2	592
March	1st to 10th	Night	11% 6% 28% 41% 12% 1	720
	11th to 20th	Night	9% 6% 26% 42% 15% 1	720
	21st to end	Night	13% 8% 29% 38% 11% 1	792
April	1st to 10th	Night	11% 10% 23% 38% 16% 2	720
	11th to 20th	Night	11% 9% 33% 35% 12% 1	561
	21st to end	Night	9% 7% 31% 35% 17% 1	540
May	1st to 10th	Night	8% 7% 29% 36% 18% 1	540
	11th to 20th	Night	8% 5% 21% 35% 27% 2	531
	21st to end	Night	13% 6% 19% 32% 26% 5%	568
June	1st to 10th	Night	6% 5% 20% 39% 26% 4	360
	11th to 20th	Night	8% 7% 25% 44% 14% 2	360
	21st to end	Night	4 1 19% 47% 26% 3	360
July	1st to 10th	Night	3 1 9% 57% 28% 2	350
	11th to 20th	Night	1 14% 59% 24% 1	348
	21st to end	Night	2 1 9% 51% 34% 3	528
August	1st to 10th	Night	5% 2 12% 51% 28% 2	537
	11th to 20th	Night	2 1 13% 51% 31% 2	534
	21st to end	Night	4 2 15% 48% 27% 3	613
September	1st to 10th	Night	4 2 14% 44% 31% 5%	720
	11th to 20th	Night	7% 1 13% 39% 33% 6%	720
	21st to end	Night	8% 4 16% 36% 30% 6%	720
October	1st to 10th	Night	9% 5% 19% 38% 28% 1	720
	11th to 20th	Night	9% 5% 18% 40% 24% 4	720
	21st to end	Night	8% 6% 22% 41% 20% 3	792
November	1st to 10th	Night	13% 4 25% 39% 18% 2	740
	11th to 20th	Night	11% 7% 25% 41% 15% 1	760
	21st to end	Night	9% 6% 24% 38% 19% 2	855
December	1st to 10th	Night	13% 7% 27% 39% 12% 1	950
	11th to 20th	Night	17% 7% 22% 40% 13% 1	950
	21st to end	Night	19% 8% 30% 32% 10% 1	893

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