Rockridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rockridge Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	74703
	Night	12% 8% 4 7% 6% 3 4 4 5% 7% 40%	75038
Transparency	Day	46% 5% 13% 24% 11% 1	74407
	Night	45% 5% 13% 24% 13% 2	74854
Seeing	Day	17% 8% 30% 28% 14% 4	14816
	Night	16% 7% 32% 33% 11% 1	24834

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	8% 11% 8% 9% 7% 5% 7% 6% 5% 6% 29%	1517
	11th to 20th	Day	9% 15% 7% 12% 9% 4 6% 4 5% 6% 22%	1485
	21st to end	Day	9% 14% 6% 12% 9% 4 7% 6% 5% 8% 20%	1782
February	1st to 10th	Day	9% 11% 5% 9% 7% 5% 5% 6% 5% 7% 30%	1620
	11th to 20th	Day	10% 11% 5% 9% 8% 4 5% 7% 5% 6% 32%	1574
	21st to end	Day	13% 11% 4 9% 6% 4 5% 5% 5% 7% 30%	1383
March	1st to 10th	Day	14% 14% 6% 7% 6% 3 4 4 5% 5% 32%	1948
	11th to 20th	Day	14% 11% 4 7% 5% 4 5% 4 5% 7% 34%	1980
	21st to end	Day	20% 11% 5% 6% 6% 3 5% 4 4 7% 29%	2191
April	1st to 10th	Day	17% 9% 4 8% 5% 3 4 5% 5% 7% 34%	2160
	11th to 20th	Day	20% 12% 4 6% 4 3 5% 4 5% 6% 33%	2144
	21st to end	Day	21% 12% 4 6% 6% 3 4 5% 4 6% 30%	2340
May	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 5% 6% 7% 34%	2416
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 4 5% 8% 38%	2479
	21st to end	Day	15% 7% 3 6% 6% 4 5% 5% 5% 7% 37%	2725
June	1st to 10th	Day	11% 5% 2 4 5% 2 5% 5% 6% 9% 46%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 3 5% 5% 5% 9% 40%	2520
	21st to end	Day	9% 5% 2 5% 6% 4 6% 6% 7% 9% 41%	2520
July	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 8% 11% 48%	2520
	11th to 20th	Day	8% 4 2 4 4 3 4 5% 7% 8% 53%	2520
	21st to end	Day	6% 3 1 5% 4 3 6% 6% 7% 9% 49%	2686
August	1st to 10th	Day	8% 5% 3 6% 6% 4 6% 6% 7% 9% 41%	2328
	11th to 20th	Day	13% 4 2 5% 5% 3 5% 4 6% 8% 45%	2301
	21st to end	Day	8% 4 2 5% 4 3 3 4 6% 7% 53%	2466
September	1st to 10th	Day	13% 5% 2 5% 5% 3 4 5% 5% 6% 47%	2160
	11th to 20th	Day	13% 5% 2 4 4 3 3 5% 6% 7% 49%	2122
	21st to end	Day	15% 7% 2 5% 5% 4 4 4 6% 7% 42%	1900
October	1st to 10th	Day	16% 7% 2 6% 5% 3 3 4 5% 5% 43%	1800
	11th to 20th	Day	14% 8% 3 5% 6% 3 4 4 5% 7% 41%	1800
	21st to end	Day	18% 11% 4 5% 5% 2 4 4 5% 6% 37%	1976
November	1st to 10th	Day	11% 9% 4 8% 6% 3 4 4 5% 7% 40%	1559
	11th to 20th	Day	11% 13% 7% 10% 7% 3 4 4 5% 6% 30%	1520
	21st to end	Day	13% 10% 3 7% 6% 3 4 4 5% 7% 38%	1520
December	1st to 10th	Day	11% 12% 5% 8% 11% 5% 5% 5% 4 5% 28%	1520
	11th to 20th	Day	11% 12% 6% 10% 7% 4 5% 5% 5% 6% 29%	1918
	21st to end	Day	20% 13% 7% 17% 7% 3 3 3 4 5% 19%	2783
January	1st to 10th	Night	8% 9% 6% 10% 9% 5% 5% 5% 5% 6% 32%	2648
	11th to 20th	Night	10% 12% 6% 10% 7% 5% 5% 5% 6% 7% 27%	2501
	21st to end	Night	12% 13% 5% 12% 8% 3 5% 5% 5% 6% 25%	2574
February	1st to 10th	Night	7% 11% 6% 10% 8% 4 6% 4 4 7% 34%	2340
	11th to 20th	Night	8% 9% 4 8% 7% 5% 5% 4 5% 8% 39%	2273
	21st to end	Night	13% 10% 4 7% 7% 4 5% 4 4 6% 36%	1872
March	1st to 10th	Night	13% 9% 3 7% 7% 3 5% 4 4 7% 37%	2028
	11th to 20th	Night	13% 10% 4 7% 6% 3 5% 4 4 6% 38%	1980
	21st to end	Night	18% 10% 4 5% 6% 3 3 5% 5% 6% 34%	2165
April	1st to 10th	Night	14% 11% 3 6% 5% 3 5% 4 5% 7% 37%	1800
	11th to 20th	Night	20% 9% 4 9% 5% 2 4 4 4 6% 33%	1762
	21st to end	Night	18% 8% 5% 6% 6% 3 4 4 4 7% 36%	1620
May	1st to 10th	Night	17% 7% 3 5% 5% 3 4 5% 5% 7% 39%	1544
	11th to 20th	Night	16% 5% 2 5% 3 4 3 4 6% 8% 44%	1416
	21st to end	Night	13% 5% 2 7% 6% 3 5% 5% 4 8% 43%	1551
June	1st to 10th	Night	10% 5% 2 3 4 2 4 5% 6% 9% 49%	1440
	11th to 20th	Night	9% 5% 2 4 6% 4 5% 5% 6% 8% 47%	1440
	21st to end	Night	7% 3 2 4 4 3 4 5% 5% 9% 55%	1440
July	1st to 10th	Night	5% 3 1 3 3 1 4 4 7% 11% 56%	1440
	11th to 20th	Night	4 3 2 4 4 2 5% 5% 5% 9% 58%	1440
	21st to end	Night	6% 2 1 3 3 2 3 4 5% 8% 61%	1629
August	1st to 10th	Night	6% 4 2 4 5% 3 6% 6% 6% 8% 50%	1611
	11th to 20th	Night	9% 6% 3 3 3 3 3 4 5% 8% 54%	1600
	21st to end	Night	7% 5% 2 4 4 2 3 5% 5% 5% 57%	1890
September	1st to 10th	Night	10% 4 1 4 4 2 4 4 5% 8% 54%	1800
	11th to 20th	Night	10% 5% 2 3 4 2 3 4 4 8% 54%	1836
	21st to end	Night	12% 6% 2 4 4 3 4 4 4 5% 51%	2041
October	1st to 10th	Night	14% 6% 2 5% 4 2 3 4 4 6% 50%	2160
	11th to 20th	Night	15% 9% 3 5% 5% 3 4 4 5% 6% 42%	2160
	21st to end	Night	18% 11% 3 5% 5% 2 3 4 4 6% 38%	2380
November	1st to 10th	Night	13% 8% 3 5% 4 3 4 5% 5% 6% 43%	2514
	11th to 20th	Night	12% 11% 6% 9% 6% 3 5% 4 4 5% 35%	2660
	21st to end	Night	14% 8% 4 7% 7% 3 5% 3 4 6% 38%	2660
December	1st to 10th	Night	10% 11% 5% 10% 7% 4 5% 5% 5% 6% 33%	2660
	11th to 20th	Night	11% 11% 5% 9% 9% 4 5% 5% 5% 6% 28%	3279
	21st to end	Night	22% 12% 6% 12% 7% 3 4 4 4 5% 20%	4884

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	55% 5% 14% 19% 6%	1518
	11th to 20th	Day	64% 3 10% 14% 8%	1485
	21st to end	Day	62% 3 14% 14% 7% 1	1782
February	1st to 10th	Day	54% 4 15% 18% 9% 1	1620
	11th to 20th	Day	52% 6% 11% 22% 8% 1	1583
	21st to end	Day	54% 5% 11% 18% 11%	1383
March	1st to 10th	Day	55% 4 14% 20% 7% 1	1948
	11th to 20th	Day	51% 4 13% 23% 8% 1	1980
	21st to end	Day	56% 5% 10% 21% 7% 1	2191
April	1st to 10th	Day	50% 5% 11% 22% 11% 1	2160
	11th to 20th	Day	54% 4 10% 19% 11% 2	2144
	21st to end	Day	55% 4 12% 19% 8% 1	2340
May	1st to 10th	Day	49% 5% 12% 23% 9% 1	2416
	11th to 20th	Day	45% 5% 12% 25% 12% 1	2483
	21st to end	Day	46% 6% 11% 23% 13% 2	2725
June	1st to 10th	Day	34% 6% 14% 30% 14% 2	2520
	11th to 20th	Day	41% 6% 13% 24% 15% 1	2520
	21st to end	Day	36% 7% 17% 27% 11% 2	2520
July	1st to 10th	Day	30% 7% 17% 32% 14% 1	2451
	11th to 20th	Day	29% 5% 15% 28% 20% 3	2445
	21st to end	Day	29% 7% 16% 31% 16% 2	2537
August	1st to 10th	Day	37% 7% 13% 30% 10% 3	2314
	11th to 20th	Day	37% 6% 14% 32% 11% 1	2301
	21st to end	Day	30% 5% 15% 31% 19% 1	2466
September	1st to 10th	Day	37% 5% 13% 30% 15%	2160
	11th to 20th	Day	34% 5% 17% 33% 10%	2122
	21st to end	Day	41% 5% 12% 32% 10% 1	1900
October	1st to 10th	Day	43% 4 10% 29% 13% 1	1800
	11th to 20th	Day	43% 4 12% 25% 14% 2	1800
	21st to end	Day	49% 3 14% 23% 10% 1	1976
November	1st to 10th	Day	45% 4 13% 20% 14% 3	1559
	11th to 20th	Day	54% 4 12% 18% 10% 1	1520
	21st to end	Day	47% 5% 17% 23% 7% 1	1520
December	1st to 10th	Day	58% 6% 15% 14% 7%	1520
	11th to 20th	Day	55% 5% 19% 16% 5%	1915
	21st to end	Day	69% 5% 9% 12% 5%	2783
January	1st to 10th	Night	52% 4 12% 22% 10%	2648
	11th to 20th	Night	57% 5% 14% 17% 7% 1	2501
	21st to end	Night	60% 4 12% 16% 8% 1	2574
February	1st to 10th	Night	51% 3 13% 22% 10% 1	2340
	11th to 20th	Night	45% 5% 14% 25% 10% 2	2274
	21st to end	Night	51% 3 9% 22% 13% 2	1872
March	1st to 10th	Night	48% 5% 15% 23% 9% 1	2028
	11th to 20th	Night	48% 3 11% 23% 14% 1	1980
	21st to end	Night	51% 4 11% 24% 8% 1	2165
April	1st to 10th	Night	47% 4 10% 24% 14% 1	1800
	11th to 20th	Night	53% 3 8% 18% 15% 3	1762
	21st to end	Night	49% 4 9% 22% 14% 2	1620
May	1st to 10th	Night	44% 5% 13% 22% 14% 1	1544
	11th to 20th	Night	38% 5% 15% 24% 15% 3	1417
	21st to end	Night	40% 5% 13% 23% 17% 2	1552
June	1st to 10th	Night	31% 6% 13% 31% 18% 2	1440
	11th to 20th	Night	36% 4 15% 23% 19% 3	1440
	21st to end	Night	27% 6% 16% 32% 16% 3	1440
July	1st to 10th	Night	21% 5% 16% 36% 20% 3	1400
	11th to 20th	Night	24% 5% 14% 34% 19% 4	1392
	21st to end	Night	21% 4 16% 37% 17% 5%	1538
August	1st to 10th	Night	31% 6% 14% 30% 15% 4	1601
	11th to 20th	Night	30% 4 16% 30% 19% 1	1600
	21st to end	Night	28% 5% 13% 30% 20% 4	1890
September	1st to 10th	Night	28% 5% 12% 34% 20% 1	1800
	11th to 20th	Night	29% 5% 15% 33% 17% 1	1836
	21st to end	Night	35% 5% 12% 31% 17% 1	2041
October	1st to 10th	Night	37% 4 9% 32% 16% 3	2160
	11th to 20th	Night	43% 5% 13% 23% 15% 2	2160
	21st to end	Night	48% 4 11% 23% 13% 1	2380
November	1st to 10th	Night	41% 5% 12% 24% 16% 2	2514
	11th to 20th	Night	52% 4 11% 20% 12% 1	2660
	21st to end	Night	48% 4 16% 24% 8% 1	2660
December	1st to 10th	Night	52% 4 15% 21% 7% 1	2660
	11th to 20th	Night	55% 6% 13% 19% 6% 1	3281
	21st to end	Night	66% 5% 9% 15% 4	4884

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	14% 10% 37% 28% 9% 2	270
	11th to 20th	Day	18% 9% 41% 25% 6%	243
	21st to end	Day	19% 10% 36% 23% 8% 3	264
February	1st to 10th	Day	15% 16% 47% 18% 4	240
	11th to 20th	Day	18% 15% 38% 21% 7% 1	240
	21st to end	Day	22% 13% 36% 26% 3	198
March	1st to 10th	Day	22% 13% 40% 18% 8%	240
	11th to 20th	Day	23% 10% 37% 21% 9% 1	240
	21st to end	Day	30% 11% 26% 23% 8% 2	277
April	1st to 10th	Day	24% 11% 37% 21% 7%	420
	11th to 20th	Day	29% 13% 23% 23% 10% 1	422
	21st to end	Day	28% 9% 29% 22% 10% 2	600
May	1st to 10th	Day	21% 10% 33% 26% 9% 1	603
	11th to 20th	Day	21% 9% 28% 25% 13% 3	625
	21st to end	Day	18% 7% 24% 33% 15% 3	679
June	1st to 10th	Day	11% 6% 26% 31% 20% 6%	630
	11th to 20th	Day	14% 4 34% 32% 13% 3	630
	21st to end	Day	12% 5% 29% 30% 18% 5%	630
July	1st to 10th	Day	10% 3 20% 32% 25% 10%	621
	11th to 20th	Day	9% 5% 25% 31% 22% 7%	618
	21st to end	Day	7% 3 24% 38% 21% 6%	684
August	1st to 10th	Day	12% 1 20% 34% 26% 7%	626
	11th to 20th	Day	15% 3 18% 33% 24% 7%	615
	21st to end	Day	11% 3 25% 36% 19% 7%	585
September	1st to 10th	Day	16% 6% 26% 30% 17% 4	450
	11th to 20th	Day	16% 4 27% 30% 19% 4	412
	21st to end	Day	19% 9% 32% 22% 12% 5%	270
October	1st to 10th	Day	18% 6% 29% 26% 15% 6%	270
	11th to 20th	Day	17% 17% 35% 23% 8% 1	270
	21st to end	Day	26% 12% 34% 19% 8% 1	297
November	1st to 10th	Day	18% 12% 38% 20% 11% 1	270
	11th to 20th	Day	16% 15% 44% 21% 4	270
	21st to end	Day	21% 11% 44% 19% 5%	270
December	1st to 10th	Day	16% 10% 41% 26% 6% 1	270
	11th to 20th	Day	19% 13% 38% 24% 6%	270
	21st to end	Day	27% 9% 43% 16% 4	297
January	1st to 10th	Night	13% 9% 39% 30% 7% 1	950
	11th to 20th	Night	17% 9% 36% 31% 7% 1	908
	21st to end	Night	19% 8% 36% 31% 6% 1	990
February	1st to 10th	Night	13% 10% 40% 29% 7% 1	900
	11th to 20th	Night	14% 11% 41% 27% 7% 1	879
	21st to end	Night	18% 10% 36% 28% 8% 1	688
March	1st to 10th	Night	18% 9% 36% 27% 9% 1	588
	11th to 20th	Night	19% 7% 39% 28% 6% 1	540
	21st to end	Night	22% 9% 29% 27% 12% 2	594
April	1st to 10th	Night	20% 9% 36% 28% 7%	540
	11th to 20th	Night	24% 9% 29% 30% 8%	533
	21st to end	Night	22% 7% 32% 28% 10% 1	540
May	1st to 10th	Night	21% 6% 32% 33% 8% 1	540
	11th to 20th	Night	18% 7% 28% 34% 12% 1	531
	21st to end	Night	16% 5% 26% 38% 13% 1	584
June	1st to 10th	Night	13% 3 26% 41% 17%	540
	11th to 20th	Night	12% 3 30% 46% 10%	540
	21st to end	Night	8% 2 22% 49% 17% 1	540
July	1st to 10th	Night	6% 2 20% 43% 27% 2	525
	11th to 20th	Night	5% 3 22% 49% 18% 3	522
	21st to end	Night	6% 1 18% 54% 19% 2	591
August	1st to 10th	Night	8% 1 16% 47% 24% 2	537
	11th to 20th	Night	13% 2 15% 40% 26% 3	534
	21st to end	Night	9% 2 24% 38% 26% 2	594
September	1st to 10th	Night	13% 2 22% 41% 19% 4	540
	11th to 20th	Night	13% 4 29% 35% 16% 3	540
	21st to end	Night	15% 7% 31% 32% 13% 1	601
October	1st to 10th	Night	18% 6% 29% 30% 16% 1	720
	11th to 20th	Night	20% 9% 35% 29% 7% 1	720
	21st to end	Night	26% 9% 26% 31% 8%	792
November	1st to 10th	Night	17% 10% 34% 30% 8% 1	848
	11th to 20th	Night	18% 10% 38% 27% 6% 1	950
	21st to end	Night	17% 11% 39% 25% 8%	950
December	1st to 10th	Night	17% 8% 38% 30% 7% 1	950
	11th to 20th	Night	19% 7% 39% 27% 7% 1	950
	21st to end	Night	25% 10% 32% 27% 5%	1045

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