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 2025-01-22.

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% 11% 4 8% 5% 4 4 4 5% 7% 31%	105155
	Night	17% 12% 4 9% 5% 5% 4 4 5% 6% 29%	104196
Transparency	Day	29% 46% 8% 11% 6%	108760
	Night	32% 45% 7% 10% 5%	108679
Seeing	Day	20% 15% 27% 22% 11% 4	14697
	Night	23% 14% 27% 24% 11% 1	24239

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% 24% 7% 11% 7% 5% 4 4 4 5% 13%	2597
	11th to 20th	Day	13% 23% 7% 23% 9% 5% 3 2 3 4 8%	2870
	21st to end	Day	23% 19% 8% 12% 7% 6% 4 3 4 3 11%	3133
February	1st to 10th	Day	17% 18% 5% 14% 8% 5% 5% 4 4 5% 13%	1710
	11th to 20th	Day	14% 18% 6% 14% 8% 5% 4 4 4 4 18%	1663
	21st to end	Day	21% 13% 4 13% 9% 4 4 4 4 5% 18%	1485
March	1st to 10th	Day	19% 13% 4 8% 5% 4 4 3 4 6% 29%	3432
	11th to 20th	Day	11% 18% 5% 9% 5% 4 5% 3 5% 6% 29%	3480
	21st to end	Day	18% 11% 4 10% 5% 4 4 4 5% 6% 30%	3815
April	1st to 10th	Day	18% 14% 4 7% 4 5% 3 4 4 5% 31%	2255
	11th to 20th	Day	19% 10% 2 6% 4 5% 4 3 5% 6% 35%	2279
	21st to end	Day	20% 11% 3 7% 5% 4 4 4 4 4 34%	2470
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 4 5% 7% 31%	4108
	11th to 20th	Day	13% 10% 4 9% 6% 6% 5% 5% 6% 7% 30%	4356
	21st to end	Day	11% 7% 2 6% 4 3 4 4 6% 8% 45%	4837
June	1st to 10th	Day	14% 8% 2 6% 5% 4 5% 4 7% 7% 38%	2660
	11th to 20th	Day	12% 4 2 7% 5% 5% 5% 4 7% 9% 40%	2660
	21st to end	Day	14% 6% 3 5% 4 4 4 4 6% 7% 43%	2660
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	4440
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 7% 9% 42%	4440
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 4 7% 9% 43%	4474
August	1st to 10th	Day	17% 6% 2 6% 3 4 5% 4 6% 8% 39%	4104
	11th to 20th	Day	12% 6% 3 8% 5% 5% 6% 4 8% 10% 35%	4044
	21st to end	Day	11% 5% 2 5% 4 4 4 4 7% 9% 46%	4294
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 46%	2191
	21st to end	Day	15% 8% 3 8% 5% 6% 5% 4 6% 8% 33%	2036
October	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 5% 6% 35%	1900
	11th to 20th	Day	20% 9% 4 9% 5% 3 5% 4 5% 6% 29%	1900
	21st to end	Day	24% 13% 5% 9% 6% 4 4 4 4 6% 22%	2090
November	1st to 10th	Day	15% 10% 4 8% 6% 5% 5% 4 5% 6% 32%	1746
	11th to 20th	Day	15% 14% 4 9% 6% 5% 5% 3 5% 7% 29%	1600
	21st to end	Day	22% 16% 3 10% 5% 4 4 4 6% 7% 20%	1600
December	1st to 10th	Day	20% 18% 5% 11% 7% 6% 5% 5% 5% 6% 12%	2600
	11th to 20th	Day	22% 18% 5% 11% 5% 4 4 3 4 5% 19%	2986
	21st to end	Day	24% 21% 5% 11% 7% 5% 4 3 4 4 12%	3960
January	1st to 10th	Night	20% 21% 6% 11% 6% 5% 4 3 4 4 16%	4569
	11th to 20th	Night	15% 20% 6% 17% 8% 7% 5% 3 5% 4 9%	4306
	21st to end	Night	25% 20% 6% 13% 6% 4 3 2 3 3 12%	4546
February	1st to 10th	Night	19% 18% 6% 13% 6% 4 4 3 4 5% 18%	2470
	11th to 20th	Night	17% 15% 5% 11% 6% 5% 6% 4 5% 6% 20%	2403
	21st to end	Night	20% 15% 4 13% 7% 5% 4 3 5% 5% 19%	1965
March	1st to 10th	Night	21% 13% 3 9% 5% 4 4 3 3 4 32%	3408
	11th to 20th	Night	19% 16% 3 8% 4 4 4 3 4 6% 28%	3479
	21st to end	Night	17% 9% 2 7% 4 3 4 3 5% 6% 40%	3817
April	1st to 10th	Night	22% 12% 3 8% 5% 4 4 3 4 5% 31%	1924
	11th to 20th	Night	18% 9% 3 7% 5% 5% 4 3 5% 6% 35%	1866
	21st to end	Night	18% 9% 3 7% 5% 4 6% 4 6% 6% 34%	1710
May	1st to 10th	Night	22% 10% 3 7% 5% 4 4 3 4 6% 31%	2833
	11th to 20th	Night	13% 7% 3 8% 5% 6% 5% 4 7% 9% 34%	2534
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 5% 9% 44%	2738
June	1st to 10th	Night	13% 7% 3 7% 5% 6% 5% 5% 7% 7% 36%	1520
	11th to 20th	Night	11% 6% 2 6% 6% 5% 5% 4 6% 8% 42%	1520
	21st to end	Night	14% 6% 3 5% 5% 5% 5% 4 5% 8% 39%	1520
July	1st to 10th	Night	8% 6% 2 7% 5% 4 5% 4 6% 9% 44%	2520
	11th to 20th	Night	12% 5% 3 6% 4 4 4 4 6% 9% 44%	2520
	21st to end	Night	10% 4 2 6% 5% 5% 5% 4 7% 8% 45%	3075
August	1st to 10th	Night	10% 8% 2 7% 4 4 5% 4 5% 9% 41%	2825
	11th to 20th	Night	13% 8% 3 6% 4 4 5% 4 6% 8% 37%	2785
	21st to end	Night	8% 5% 2 5% 5% 4 5% 4 7% 10% 45%	3316
September	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 6% 7% 45%	1900
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 6% 53%	1985
	21st to end	Night	17% 7% 2 6% 5% 4 6% 4 7% 6% 36%	2143
October	1st to 10th	Night	18% 6% 2 7% 4 5% 4 3 5% 6% 39%	2280
	11th to 20th	Night	22% 10% 2 8% 4 5% 5% 3 4 6% 30%	2280
	21st to end	Night	24% 9% 3 7% 5% 5% 4 4 6% 6% 27%	2508
November	1st to 10th	Night	18% 10% 3 6% 4 4 4 3 5% 6% 38%	2547
	11th to 20th	Night	16% 12% 4 8% 6% 6% 5% 3 5% 7% 29%	2800
	21st to end	Night	20% 13% 4 10% 5% 5% 5% 4 5% 7% 22%	2800
December	1st to 10th	Night	19% 21% 6% 11% 6% 4 5% 4 5% 6% 13%	4580
	11th to 20th	Night	20% 21% 5% 10% 5% 4 5% 4 5% 5% 17%	5208
	21st to end	Night	26% 18% 5% 10% 5% 5% 4 3 5% 5% 14%	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	49% 39% 6% 4 3	2598
	11th to 20th	Day	54% 38% 4 3 1	2870
	21st to end	Day	49% 42% 6% 2	3133
February	1st to 10th	Day	26% 67% 4 3	1710
	11th to 20th	Day	24% 65% 3 6% 1	1672
	21st to end	Day	19% 66% 6% 6% 3	1485
March	1st to 10th	Day	32% 42% 6% 14% 5%	3435
	11th to 20th	Day	33% 43% 13% 9% 3	3480
	21st to end	Day	33% 43% 6% 14% 4 1	3815
April	1st to 10th	Day	22% 66% 3 6% 2	2255
	11th to 20th	Day	19% 66% 3 7% 4 1	2279
	21st to end	Day	20% 66% 4 5% 4 1	2470
May	1st to 10th	Day	32% 42% 9% 11% 6% 1	4120
	11th to 20th	Day	33% 45% 9% 12% 1	4360
	21st to end	Day	23% 41% 7% 17% 10% 2	4837
June	1st to 10th	Day	16% 67% 4 7% 6% 1	2660
	11th to 20th	Day	14% 66% 6% 7% 6%	2660
	21st to end	Day	15% 64% 4 10% 7%	2660
July	1st to 10th	Day	25% 36% 8% 18% 12% 1	4370
	11th to 20th	Day	23% 37% 9% 20% 11%	4365
	21st to end	Day	22% 37% 12% 13% 14% 1	4331
August	1st to 10th	Day	29% 39% 9% 14% 9% 1	4091
	11th to 20th	Day	29% 40% 12% 15% 4	4083
	21st to end	Day	21% 39% 8% 24% 8%	4294
September	1st to 10th	Day	15% 62% 6% 11% 5%	2280
	11th to 20th	Day	12% 61% 5% 11% 9% 1	2191
	21st to end	Day	18% 61% 7% 10% 4	2036
October	1st to 10th	Day	22% 39% 9% 19% 11% 1	3160
	11th to 20th	Day	25% 38% 12% 14% 11%	3140
	21st to end	Day	30% 38% 11% 16% 5%	3428
November	1st to 10th	Day	28% 58% 7% 5% 2 1	1746
	11th to 20th	Day	25% 59% 6% 8% 3	1600
	21st to end	Day	29% 58% 7% 4 1	1600
December	1st to 10th	Day	48% 38% 9% 4	2600
	11th to 20th	Day	49% 33% 8% 8% 3	2986
	21st to end	Day	55% 29% 7% 7% 2	3960
January	1st to 10th	Night	47% 41% 7% 4 1	4569
	11th to 20th	Night	52% 39% 6% 3 1	4306
	21st to end	Night	52% 41% 4 2 1	4546
February	1st to 10th	Night	25% 67% 4 4	2470
	11th to 20th	Night	23% 65% 4 6% 2	2405
	21st to end	Night	23% 66% 4 5% 2	1965
March	1st to 10th	Night	34% 41% 5% 15% 4	3418
	11th to 20th	Night	37% 41% 10% 7% 3 1	3480
	21st to end	Night	25% 40% 8% 16% 10%	3817
April	1st to 10th	Night	23% 65% 3 6% 2	1924
	11th to 20th	Night	17% 66% 4 6% 5% 1	1866
	21st to end	Night	16% 65% 5% 8% 5% 1	1710
May	1st to 10th	Night	35% 43% 6% 10% 5% 2	2840
	11th to 20th	Night	30% 45% 12% 13% 1	2535
	21st to end	Night	23% 42% 5% 16% 13% 1	2739
June	1st to 10th	Night	16% 66% 3 7% 7% 1	1520
	11th to 20th	Night	15% 65% 4 9% 6% 1	1520
	21st to end	Night	16% 64% 4 8% 7% 1	1520
July	1st to 10th	Night	27% 37% 8% 15% 12% 2	2480
	11th to 20th	Night	25% 37% 10% 20% 8%	2472
	21st to end	Night	24% 38% 11% 14% 12% 1	2977
August	1st to 10th	Night	27% 37% 9% 18% 7% 1	2814
	11th to 20th	Night	33% 38% 11% 15% 3	2812
	21st to end	Night	21% 39% 10% 21% 9%	3316
September	1st to 10th	Night	13% 62% 7% 11% 7% 1	1900
	11th to 20th	Night	12% 61% 6% 11% 9% 1	1985
	21st to end	Night	19% 61% 5% 10% 5%	2143
October	1st to 10th	Night	25% 38% 5% 17% 15%	3800
	11th to 20th	Night	29% 38% 8% 15% 10%	3779
	21st to end	Night	23% 38% 13% 20% 7%	4120
November	1st to 10th	Night	26% 57% 5% 7% 5%	2547
	11th to 20th	Night	24% 58% 5% 9% 3	2800
	21st to end	Night	28% 59% 7% 5% 1	2800
December	1st to 10th	Night	48% 38% 9% 4 1	4580
	11th to 20th	Night	50% 33% 6% 7% 2	5208
	21st to end	Night	53% 29% 8% 8% 2	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	26% 26% 31% 11% 6% 2	200
	11th to 20th	Day	26% 29% 29% 12% 3 1	284
	21st to end	Day	23% 27% 29% 16% 4	300
February	1st to 10th	Day	29% 26% 31% 11% 2	270
	11th to 20th	Day	26% 29% 35% 10%	270
	21st to end	Day	29% 24% 35% 11% 1	272
March	1st to 10th	Day	23% 20% 35% 17% 3 2	460
	11th to 20th	Day	24% 20% 34% 17% 5% 1	460
	21st to end	Day	28% 14% 32% 19% 6% 1	506
April	1st to 10th	Day	23% 22% 31% 18% 6% 1	460
	11th to 20th	Day	26% 21% 25% 17% 9% 3	452
	21st to end	Day	25% 15% 31% 21% 7% 2	460
May	1st to 10th	Day	26% 13% 27% 21% 10% 3	463
	11th to 20th	Day	16% 11% 28% 27% 14% 3	487
	21st to end	Day	14% 8% 30% 28% 15% 5%	529
June	1st to 10th	Day	17% 16% 27% 29% 11% 1	490
	11th to 20th	Day	12% 13% 23% 31% 17% 3	490
	21st to end	Day	16% 10% 31% 29% 11% 3	490
July	1st to 10th	Day	9% 8% 21% 34% 23% 5%	485
	11th to 20th	Day	13% 9% 25% 29% 19% 5%	484
	21st to end	Day	9% 8% 28% 32% 17% 6%	532
August	1st to 10th	Day	12% 6% 22% 31% 21% 8%	486
	11th to 20th	Day	14% 6% 23% 29% 21% 8%	479
	21st to end	Day	11% 7% 25% 30% 18% 8%	539
September	1st to 10th	Day	14% 8% 24% 28% 18% 8%	490
	11th to 20th	Day	10% 6% 19% 26% 24% 15%	490
	21st to end	Day	16% 15% 26% 21% 16% 6%	490
October	1st to 10th	Day	20% 12% 26% 19% 17% 6%	490
	11th to 20th	Day	24% 18% 30% 19% 7% 2	490
	21st to end	Day	32% 20% 24% 17% 6%	539
November	1st to 10th	Day	23% 18% 32% 18% 8% 1	340
	11th to 20th	Day	29% 27% 25% 15% 4 2	200
	21st to end	Day	34% 26% 29% 11% 1	200
December	1st to 10th	Day	31% 26% 27% 12% 6%	200
	11th to 20th	Day	33% 23% 25% 17% 3	200
	21st to end	Day	30% 30% 24% 12% 3 1	220
January	1st to 10th	Night	30% 24% 29% 13% 5%	800
	11th to 20th	Night	32% 22% 29% 15% 3	800
	21st to end	Night	24% 18% 36% 18% 5%	841
February	1st to 10th	Night	28% 24% 29% 16% 3	760
	11th to 20th	Night	26% 21% 32% 18% 3	743
	21st to end	Night	28% 20% 32% 18% 2	628
March	1st to 10th	Night	23% 17% 33% 22% 5%	760
	11th to 20th	Night	26% 17% 30% 20% 6% 1	760
	21st to end	Night	24% 12% 34% 20% 9% 1	836
April	1st to 10th	Night	26% 15% 30% 24% 4	760
	11th to 20th	Night	24% 14% 29% 22% 9% 1	742
	21st to end	Night	23% 9% 27% 30% 9% 1	570
May	1st to 10th	Night	21% 9% 23% 32% 13% 2	570
	11th to 20th	Night	14% 9% 26% 41% 10% 1	400
	21st to end	Night	13% 6% 27% 31% 21% 2	411
June	1st to 10th	Night	14% 6% 30% 35% 15% 1	380
	11th to 20th	Night	14% 7% 24% 37% 17% 1	380
	21st to end	Night	14% 8% 25% 38% 14%	380
July	1st to 10th	Night	8% 4 22% 42% 21% 2	370
	11th to 20th	Night	12% 4 26% 36% 20% 2	368
	21st to end	Night	11% 3 24% 36% 23% 2	612
August	1st to 10th	Night	11% 4 17% 39% 25% 3	567
	11th to 20th	Night	10% 6% 18% 33% 30% 3	564
	21st to end	Night	12% 5% 21% 34% 25% 3	741
September	1st to 10th	Night	16% 7% 19% 33% 21% 4	760
	11th to 20th	Night	14% 5% 19% 27% 28% 7%	760
	21st to end	Night	20% 10% 21% 28% 17% 4	760
October	1st to 10th	Night	21% 8% 27% 23% 18% 2	760
	11th to 20th	Night	26% 18% 24% 23% 8% 2	760
	21st to end	Night	31% 16% 29% 20% 5%	836
November	1st to 10th	Night	23% 15% 30% 23% 8% 1	780
	11th to 20th	Night	24% 22% 29% 16% 7% 1	800
	21st to end	Night	28% 23% 30% 16% 5%	800
December	1st to 10th	Night	35% 24% 24% 13% 4	800
	11th to 20th	Night	32% 25% 26% 15% 3	800
	21st to end	Night	35% 18% 28% 16% 2	880

Page generated at 2025-01-19 03:06:47. Version ac07

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