Observatory on the Ridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory on the Ridge clear sky chart from 2005-11-06 to 2024-01-27.

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	16% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	74778
	Night	18% 11% 4 7% 6% 3 5% 5% 5% 6% 30%	75257
Transparency	Day	35% 38% 8% 12% 6% 1	74494
	Night	38% 38% 8% 10% 6% 1	75075
Seeing	Day	20% 15% 28% 22% 11% 3	13429
	Night	23% 14% 27% 24% 11% 1	22996

By Month

Percentage Cloud Cover NE (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	17% 19% 7% 11% 10% 5% 6% 4 4 5% 13%	1517
	11th to 20th	Day	19% 18% 7% 13% 8% 4 5% 4 4 5% 13%	1677
	21st to end	Day	16% 17% 8% 11% 11% 6% 5% 5% 5% 4 11%	1836
February	1st to 10th	Day	19% 17% 6% 13% 9% 4 5% 5% 4 5% 13%	1620
	11th to 20th	Day	15% 17% 7% 13% 9% 3 5% 5% 4 4 17%	1573
	21st to end	Day	22% 13% 5% 12% 9% 3 5% 5% 4 5% 18%	1399
March	1st to 10th	Day	17% 13% 6% 10% 7% 4 5% 4 4 6% 24%	1977
	11th to 20th	Day	14% 15% 5% 8% 7% 3 4 4 4 7% 29%	1980
	21st to end	Day	20% 11% 4 8% 5% 4 5% 4 5% 5% 28%	2178
April	1st to 10th	Day	18% 13% 5% 6% 5% 3 4 5% 4 5% 31%	2136
	11th to 20th	Day	19% 9% 3 6% 5% 3 4 4 5% 6% 36%	2157
	21st to end	Day	20% 10% 4 6% 6% 3 4 5% 4 5% 34%	2340
May	1st to 10th	Day	17% 9% 3 6% 6% 4 5% 5% 5% 8% 33%	2340
	11th to 20th	Day	15% 7% 4 6% 5% 4 4 5% 6% 6% 38%	2450
	21st to end	Day	11% 6% 3 5% 5% 3 4 5% 6% 9% 44%	2725
June	1st to 10th	Day	14% 7% 3 5% 5% 3 4 5% 6% 7% 39%	2520
	11th to 20th	Day	13% 4 2 5% 6% 4 5% 5% 6% 10% 40%	2520
	21st to end	Day	14% 6% 3 4 4 3 4 5% 6% 8% 45%	2520
July	1st to 10th	Day	8% 4 2 4 4 2 5% 6% 7% 10% 48%	2520
	11th to 20th	Day	12% 6% 2 6% 4 3 5% 5% 7% 10% 39%	2520
	21st to end	Day	9% 4 2 7% 5% 4 7% 6% 7% 10% 38%	2555
August	1st to 10th	Day	9% 5% 2 5% 5% 3 5% 6% 7% 10% 43%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 6% 7% 10% 39%	2303
	21st to end	Day	12% 5% 2 4 5% 3 4 4 7% 10% 44%	2448
September	1st to 10th	Day	15% 6% 2 5% 5% 3 4 5% 6% 7% 41%	2160
	11th to 20th	Day	12% 4 3 5% 5% 3 5% 5% 6% 8% 46%	2076
	21st to end	Day	15% 7% 3 7% 6% 5% 5% 5% 6% 8% 34%	1929
October	1st to 10th	Day	20% 8% 3 6% 6% 3 4 5% 5% 6% 34%	1800
	11th to 20th	Day	20% 9% 4 8% 7% 2 5% 5% 5% 7% 28%	1800
	21st to end	Day	26% 13% 5% 8% 7% 3 4 4 4 5% 21%	1980
November	1st to 10th	Day	15% 10% 4 7% 7% 4 5% 5% 5% 6% 33%	1657
	11th to 20th	Day	16% 12% 5% 8% 6% 3 5% 4 5% 7% 29%	1520
	21st to end	Day	22% 15% 3 8% 6% 3 5% 5% 5% 7% 21%	1520
December	1st to 10th	Day	20% 18% 7% 9% 8% 4 4 5% 5% 6% 14%	1520
	11th to 20th	Day	21% 19% 6% 11% 7% 3 5% 5% 4 5% 15%	1906
	21st to end	Day	27% 23% 6% 10% 6% 3 3 3 3 3 12%	2772
January	1st to 10th	Night	20% 18% 6% 10% 8% 4 5% 4 4 5% 16%	2649
	11th to 20th	Night	20% 20% 6% 11% 8% 3 5% 5% 5% 5% 13%	2499
	21st to end	Night	17% 17% 6% 11% 9% 4 5% 4 4 5% 17%	2664
February	1st to 10th	Night	20% 17% 8% 12% 7% 3 4 4 4 4 17%	2340
	11th to 20th	Night	18% 15% 5% 10% 8% 4 6% 5% 4 5% 20%	2273
	21st to end	Night	21% 15% 5% 11% 8% 4 4 4 5% 6% 19%	1853
March	1st to 10th	Night	16% 12% 4 9% 7% 3 5% 6% 5% 6% 28%	1970
	11th to 20th	Night	19% 12% 4 7% 6% 3 4 4 3 6% 32%	1979
	21st to end	Night	18% 11% 3 8% 6% 3 4 4 5% 6% 32%	2178
April	1st to 10th	Night	22% 10% 4 7% 5% 3 3 4 4 5% 31%	1823
	11th to 20th	Night	18% 9% 3 6% 6% 4 4 4 5% 6% 36%	1767
	21st to end	Night	18% 9% 3 6% 5% 2 5% 5% 6% 6% 35%	1620
May	1st to 10th	Night	17% 9% 3 7% 6% 3 5% 6% 5% 7% 34%	1620
	11th to 20th	Night	13% 5% 3 6% 6% 4 5% 5% 7% 8% 38%	1440
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	1550
June	1st to 10th	Night	13% 6% 3 6% 6% 5% 5% 7% 7% 7% 36%	1440
	11th to 20th	Night	11% 5% 2 5% 6% 4 6% 5% 5% 8% 43%	1440
	21st to end	Night	13% 6% 2 4 6% 3 6% 5% 5% 8% 41%	1440
July	1st to 10th	Night	6% 5% 2 4 7% 2 4 5% 5% 8% 52%	1440
	11th to 20th	Night	11% 5% 3 5% 5% 3 4 5% 7% 8% 45%	1440
	21st to end	Night	10% 4 2 5% 6% 4 4 5% 6% 9% 44%	1759
August	1st to 10th	Night	8% 6% 2 5% 5% 3 5% 5% 7% 9% 45%	1611
	11th to 20th	Night	10% 6% 2 5% 5% 3 5% 6% 6% 8% 44%	1598
	21st to end	Night	9% 5% 2 5% 6% 3 5% 5% 6% 9% 45%	1890
September	1st to 10th	Night	14% 7% 2 5% 5% 2 4 5% 6% 7% 44%	1800
	11th to 20th	Night	13% 4 2 5% 3 2 4 4 5% 6% 53%	1880
	21st to end	Night	16% 6% 3 5% 6% 3 5% 6% 7% 6% 37%	2030
October	1st to 10th	Night	19% 6% 2 6% 5% 3 4 4 5% 7% 38%	2160
	11th to 20th	Night	22% 9% 3 7% 6% 3 5% 5% 4 7% 29%	2160
	21st to end	Night	26% 9% 3 6% 6% 4 5% 5% 6% 6% 25%	2376
November	1st to 10th	Night	18% 9% 3 6% 4 3 4 4 5% 6% 38%	2416
	11th to 20th	Night	16% 11% 5% 7% 6% 4 5% 4 5% 7% 29%	2660
	21st to end	Night	20% 12% 4 9% 6% 4 5% 5% 5% 7% 22%	2660
December	1st to 10th	Night	22% 18% 6% 8% 6% 3 5% 5% 5% 6% 16%	2660
	11th to 20th	Night	18% 20% 6% 10% 7% 3 4 5% 4 5% 18%	3288
	21st to end	Night	31% 16% 6% 9% 6% 4 4 4 5% 5% 11%	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	51% 40% 5% 3 1	1518
	11th to 20th	Day	50% 41% 5% 3	1677
	21st to end	Day	53% 37% 6% 3 1	1836
February	1st to 10th	Day	50% 38% 6% 6% 1	1620
	11th to 20th	Day	48% 38% 6% 6% 2	1582
	21st to end	Day	49% 37% 6% 6% 2	1399
March	1st to 10th	Day	41% 37% 9% 11% 3	1980
	11th to 20th	Day	37% 38% 8% 10% 6% 1	1980
	21st to end	Day	37% 38% 6% 13% 5% 1	2178
April	1st to 10th	Day	34% 37% 8% 13% 8% 1	2136
	11th to 20th	Day	32% 36% 8% 13% 10% 1	2157
	21st to end	Day	34% 37% 6% 13% 9% 1	2340
May	1st to 10th	Day	33% 37% 7% 12% 9% 2	2340
	11th to 20th	Day	28% 40% 6% 14% 10% 2	2454
	21st to end	Day	24% 39% 10% 16% 9% 2	2725
June	1st to 10th	Day	27% 38% 11% 13% 10% 1	2520
	11th to 20th	Day	26% 39% 10% 17% 8% 1	2520
	21st to end	Day	26% 39% 10% 17% 8%	2520
July	1st to 10th	Day	22% 40% 8% 18% 10% 1	2450
	11th to 20th	Day	26% 40% 11% 14% 9%	2445
	21st to end	Day	25% 41% 12% 15% 7%	2412
August	1st to 10th	Day	24% 41% 10% 17% 7%	2314
	11th to 20th	Day	23% 39% 12% 16% 9% 1	2303
	21st to end	Day	25% 38% 10% 17% 9% 1	2448
September	1st to 10th	Day	29% 38% 8% 16% 8% 1	2160
	11th to 20th	Day	25% 39% 10% 19% 7%	2076
	21st to end	Day	34% 38% 9% 13% 5% 1	1929
October	1st to 10th	Day	34% 38% 9% 10% 8%	1800
	11th to 20th	Day	40% 38% 10% 10% 2	1800
	21st to end	Day	43% 37% 8% 7% 3 1	1980
November	1st to 10th	Day	30% 42% 7% 13% 7% 1	1657
	11th to 20th	Day	35% 42% 10% 8% 4 2	1520
	21st to end	Day	43% 41% 6% 7% 2	1520
December	1st to 10th	Day	47% 43% 5% 5%	1520
	11th to 20th	Day	51% 35% 8% 4 2	1906
	21st to end	Day	63% 27% 5% 4	2772
January	1st to 10th	Night	50% 40% 5% 4 1	2649
	11th to 20th	Night	51% 41% 5% 2 1	2499
	21st to end	Night	50% 37% 7% 5% 1	2664
February	1st to 10th	Night	49% 36% 7% 6% 1	2340
	11th to 20th	Night	45% 38% 8% 6% 3	2275
	21st to end	Night	47% 37% 6% 7% 3	1853
March	1st to 10th	Night	40% 38% 8% 10% 2 1	1980
	11th to 20th	Night	38% 37% 7% 11% 6% 1	1980
	21st to end	Night	37% 37% 5% 12% 7% 2	2178
April	1st to 10th	Night	36% 37% 6% 12% 8% 1	1823
	11th to 20th	Night	34% 36% 7% 12% 10% 1	1767
	21st to end	Night	35% 38% 6% 11% 9% 2	1620
May	1st to 10th	Night	32% 37% 5% 14% 10% 2	1620
	11th to 20th	Night	27% 37% 7% 15% 11% 2	1441
	21st to end	Night	24% 39% 9% 13% 13% 2	1551
June	1st to 10th	Night	30% 39% 7% 14% 9% 1	1440
	11th to 20th	Night	25% 38% 10% 14% 12% 1	1440
	21st to end	Night	29% 39% 9% 15% 8%	1440
July	1st to 10th	Night	22% 39% 8% 17% 12% 2	1400
	11th to 20th	Night	24% 39% 10% 16% 9% 1	1392
	21st to end	Night	22% 40% 11% 16% 10% 1	1661
August	1st to 10th	Night	26% 39% 10% 17% 7% 1	1600
	11th to 20th	Night	24% 38% 11% 17% 10%	1598
	21st to end	Night	26% 38% 8% 17% 9% 2	1890
September	1st to 10th	Night	28% 38% 8% 16% 9% 1	1800
	11th to 20th	Night	23% 37% 10% 19% 10% 1	1880
	21st to end	Night	32% 39% 9% 12% 8% 1	2030
October	1st to 10th	Night	33% 37% 8% 14% 8% 1	2160
	11th to 20th	Night	39% 38% 8% 10% 4	2160
	21st to end	Night	42% 38% 7% 8% 5% 1	2376
November	1st to 10th	Night	29% 41% 6% 16% 6% 2	2416
	11th to 20th	Night	36% 41% 9% 8% 5% 1	2660
	21st to end	Night	40% 42% 9% 7% 2	2660
December	1st to 10th	Night	48% 42% 6% 4 1	2660
	11th to 20th	Night	49% 36% 9% 5% 1	3288
	21st to end	Night	62% 28% 6% 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	27% 27% 28% 11% 6% 2	180
	11th to 20th	Day	27% 29% 28% 11% 3 1	255
	21st to end	Day	25% 27% 27% 17% 4	282
February	1st to 10th	Day	30% 27% 33% 8% 1	240
	11th to 20th	Day	26% 29% 35% 10%	240
	21st to end	Day	30% 24% 34% 11% 1	242
March	1st to 10th	Day	23% 21% 35% 17% 4 1	420
	11th to 20th	Day	24% 18% 35% 18% 5% 1	420
	21st to end	Day	29% 13% 32% 19% 7% 1	462
April	1st to 10th	Day	22% 22% 31% 18% 7% 1	420
	11th to 20th	Day	25% 20% 25% 18% 9% 3	412
	21st to end	Day	24% 14% 33% 21% 7% 2	420
May	1st to 10th	Day	26% 14% 27% 20% 9% 3	423
	11th to 20th	Day	17% 12% 28% 28% 13% 3	447
	21st to end	Day	15% 7% 29% 28% 15% 6%	485
June	1st to 10th	Day	17% 14% 28% 30% 10% 1	450
	11th to 20th	Day	13% 14% 24% 31% 15% 3	450
	21st to end	Day	16% 10% 31% 30% 10% 3	450
July	1st to 10th	Day	8% 8% 22% 33% 23% 5%	445
	11th to 20th	Day	13% 10% 27% 28% 18% 5%	444
	21st to end	Day	10% 9% 30% 31% 16% 5%	488
August	1st to 10th	Day	10% 6% 22% 31% 22% 8%	446
	11th to 20th	Day	13% 6% 23% 30% 21% 8%	439
	21st to end	Day	12% 7% 26% 30% 17% 7%	495
September	1st to 10th	Day	14% 9% 24% 28% 18% 7%	450
	11th to 20th	Day	11% 6% 21% 28% 24% 10%	450
	21st to end	Day	16% 14% 26% 21% 16% 6%	450
October	1st to 10th	Day	21% 13% 25% 17% 17% 6%	450
	11th to 20th	Day	25% 19% 30% 19% 5% 2	450
	21st to end	Day	34% 21% 22% 17% 5%	495
November	1st to 10th	Day	24% 18% 31% 18% 7% 2	311
	11th to 20th	Day	32% 27% 23% 14% 3 2	180
	21st to end	Day	33% 26% 29% 11% 1	180
December	1st to 10th	Day	32% 26% 28% 9% 5%	180
	11th to 20th	Day	32% 25% 26% 16% 2	180
	21st to end	Day	31% 32% 25% 11% 2	198
January	1st to 10th	Night	31% 24% 27% 13% 5%	760
	11th to 20th	Night	32% 23% 27% 14% 3	760
	21st to end	Night	25% 17% 35% 17% 5%	820
February	1st to 10th	Night	29% 24% 30% 14% 2	720
	11th to 20th	Night	26% 21% 32% 18% 3	703
	21st to end	Night	29% 19% 33% 18% 2	592
March	1st to 10th	Night	23% 16% 34% 23% 5%	720
	11th to 20th	Night	26% 16% 31% 21% 6% 1	720
	21st to end	Night	24% 11% 34% 21% 9% 1	792
April	1st to 10th	Night	26% 15% 30% 25% 4	720
	11th to 20th	Night	23% 14% 29% 23% 9% 1	703
	21st to end	Night	24% 8% 27% 30% 9% 1	540
May	1st to 10th	Night	21% 8% 23% 33% 13% 2	540
	11th to 20th	Night	14% 9% 26% 42% 9% 1	379
	21st to end	Night	14% 5% 28% 30% 22% 2	389
June	1st to 10th	Night	14% 4 31% 36% 15% 1	360
	11th to 20th	Night	14% 8% 25% 37% 16% 1	360
	21st to end	Night	13% 8% 26% 38% 14%	360
July	1st to 10th	Night	8% 4 21% 42% 21% 3	350
	11th to 20th	Night	12% 4 27% 36% 20% 2	348
	21st to end	Night	12% 4 24% 38% 21% 2	579
August	1st to 10th	Night	12% 4 17% 39% 25% 3	537
	11th to 20th	Night	10% 6% 18% 33% 30% 3	534
	21st to end	Night	13% 5% 22% 34% 23% 3	702
September	1st to 10th	Night	16% 7% 19% 33% 21% 4	720
	11th to 20th	Night	15% 6% 20% 28% 27% 5%	720
	21st to end	Night	19% 10% 22% 29% 18% 4	720
October	1st to 10th	Night	21% 8% 28% 23% 18% 2	720
	11th to 20th	Night	26% 19% 25% 23% 7% 1	720
	21st to end	Night	32% 16% 28% 19% 4	792
November	1st to 10th	Night	24% 15% 30% 23% 8% 1	740
	11th to 20th	Night	24% 22% 30% 16% 8% 1	760
	21st to end	Night	27% 22% 30% 16% 5%	760
December	1st to 10th	Night	36% 24% 24% 13% 4	760
	11th to 20th	Night	31% 25% 27% 15% 3	760
	21st to end	Night	35% 19% 28% 16% 2	836

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