R2 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the R2 Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	14% 11% 4 9% 6% 5% 5% 4 5% 7% 31%	128108
	Night	13% 10% 4 10% 6% 5% 5% 4 5% 6% 31%	123220
Transparency	Day	51% 5% 12% 20% 11% 1	131700
	Night	51% 5% 12% 20% 11% 1	127716
Seeing	Day	21% 17% 31% 19% 9% 3	16249
	Night	20% 16% 30% 23% 9% 1	25280

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	13% 11% 6% 18% 8% 6% 6% 4 5% 4 18%	3605
	11th to 20th	Day	10% 17% 8% 18% 11% 7% 6% 4 5% 5% 11%	3934
	21st to end	Day	16% 18% 7% 16% 6% 5% 5% 4 5% 5% 14%	4466
February	1st to 10th	Day	13% 17% 5% 14% 9% 6% 6% 4 4 5% 17%	1800
	11th to 20th	Day	11% 14% 8% 15% 8% 6% 5% 4 5% 6% 19%	1753
	21st to end	Day	16% 17% 5% 11% 6% 5% 4 3 5% 5% 24%	1542
March	1st to 10th	Day	16% 12% 5% 11% 7% 4 3 3 4 5% 29%	4929
	11th to 20th	Day	9% 14% 5% 10% 6% 5% 5% 4 5% 7% 30%	4980
	21st to end	Day	16% 15% 5% 9% 6% 4 4 2 3 4 32%	5465
April	1st to 10th	Day	15% 15% 4 9% 5% 4 5% 3 5% 6% 29%	2347
	11th to 20th	Day	18% 11% 3 7% 4 4 3 4 5% 7% 34%	2482
	21st to end	Day	19% 10% 4 8% 4 4 3 3 6% 6% 33%	2600
May	1st to 10th	Day	17% 9% 3 7% 4 4 4 3 5% 7% 36%	5888
	11th to 20th	Day	15% 10% 4 8% 5% 5% 5% 4 5% 7% 34%	6262
	21st to end	Day	17% 7% 3 6% 5% 4 4 3 5% 7% 38%	6949
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 5% 6% 8% 36%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	2897
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 9% 38%	3000
July	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 9% 37%	6732
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 8% 47%	6438
August	1st to 10th	Day	13% 8% 3 7% 5% 4 6% 5% 7% 8% 36%	4234
	11th to 20th	Day	11% 7% 3 8% 6% 5% 6% 5% 8% 9% 33%	4174
	21st to end	Day	11% 5% 2 7% 4 5% 5% 4 6% 9% 42%	4525
September	1st to 10th	Day	13% 6% 2 8% 5% 5% 4 4 6% 7% 40%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 3 5% 8% 44%	2257
	21st to end	Day	15% 6% 3 8% 6% 6% 5% 4 6% 7% 34%	2175
October	1st to 10th	Day	19% 8% 3 8% 5% 4 4 4 5% 7% 34%	2000
	11th to 20th	Day	18% 10% 4 10% 6% 5% 5% 3 6% 5% 28%	2000
	21st to end	Day	24% 16% 4 9% 4 4 4 4 5% 6% 21%	2200
November	1st to 10th	Day	13% 13% 5% 9% 6% 4 5% 3 6% 6% 28%	1760
	11th to 20th	Day	11% 17% 4 10% 6% 6% 6% 3 6% 6% 23%	1680
	21st to end	Day	15% 18% 6% 13% 7% 5% 5% 4 5% 6% 17%	1680
December	1st to 10th	Day	17% 20% 7% 14% 7% 5% 5% 4 5% 6% 10%	2680
	11th to 20th	Day	17% 21% 6% 13% 5% 4 5% 3 4 5% 18%	3066
	21st to end	Day	22% 18% 7% 16% 7% 5% 4 4 4 4 9%	4048
January	1st to 10th	Night	13% 12% 6% 15% 11% 6% 5% 4 5% 6% 18%	6364
	11th to 20th	Night	10% 14% 7% 18% 12% 8% 6% 5% 4 5% 11%	5935
	21st to end	Night	13% 14% 6% 12% 8% 6% 5% 4 5% 6% 19%	6490
February	1st to 10th	Night	12% 15% 5% 11% 8% 5% 4 4 6% 7% 22%	2600
	11th to 20th	Night	10% 14% 6% 12% 7% 7% 6% 4 5% 5% 23%	2533
	21st to end	Night	11% 13% 4 12% 7% 5% 5% 5% 5% 7% 26%	2085
March	1st to 10th	Night	14% 10% 5% 10% 6% 6% 5% 4 5% 6% 27%	4909
	11th to 20th	Night	8% 10% 4 8% 5% 5% 6% 5% 7% 8% 34%	4979
	21st to end	Night	16% 9% 4 8% 5% 4 5% 4 5% 5% 35%	5467
April	1st to 10th	Night	17% 13% 3 9% 6% 4 4 3 5% 6% 30%	2051
	11th to 20th	Night	17% 9% 2 6% 5% 4 5% 4 5% 6% 37%	1883
	21st to end	Night	14% 11% 3 7% 5% 5% 4 4 5% 7% 35%	1800
May	1st to 10th	Night	18% 8% 2 7% 4 4 4 4 5% 7% 37%	4053
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 3 6% 7% 41%	3631
	21st to end	Night	17% 6% 2 5% 4 3 4 2 5% 7% 45%	3926
June	1st to 10th	Night	13% 5% 2 6% 4 6% 5% 3 6% 8% 43%	1600
	11th to 20th	Night	9% 4 2 7% 4 3 6% 5% 8% 9% 43%	1495
	21st to end	Night	12% 4 3 6% 4 3 4 4 7% 7% 47%	1400
July	1st to 10th	Night	6% 5% 3 7% 5% 4 4 4 5% 7% 48%	3238
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 5% 7% 52%	3600
	21st to end	Night	7% 4 2 5% 4 3 5% 4 6% 8% 51%	4397
August	1st to 10th	Night	10% 5% 3 6% 3 4 5% 3 5% 7% 49%	2915
	11th to 20th	Night	13% 5% 2 5% 3 4 3 4 5% 8% 49%	2875
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	3339
September	1st to 10th	Night	8% 5% 2 6% 4 4 4 5% 7% 7% 48%	2000
	11th to 20th	Night	10% 5% 1 4 3 3 3 4 6% 6% 56%	2140
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 4 6% 41%	2225
October	1st to 10th	Night	16% 6% 3 7% 4 5% 4 3 5% 7% 40%	2400
	11th to 20th	Night	18% 9% 3 7% 5% 5% 4 5% 5% 5% 34%	2400
	21st to end	Night	20% 13% 3 8% 5% 5% 5% 4 5% 6% 27%	2640
November	1st to 10th	Night	15% 11% 3 8% 5% 5% 5% 4 6% 6% 34%	2752
	11th to 20th	Night	14% 14% 4 8% 5% 5% 5% 5% 6% 7% 27%	2940
	21st to end	Night	15% 16% 5% 10% 6% 5% 6% 4 5% 6% 21%	2940
December	1st to 10th	Night	16% 15% 6% 14% 7% 6% 6% 5% 6% 6% 13%	4720
	11th to 20th	Night	12% 18% 7% 14% 7% 5% 5% 4 5% 5% 18%	5348
	21st to end	Night	21% 19% 8% 16% 6% 5% 4 3 4 4 11%	7150

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	64% 5% 13% 13% 4	3606
	11th to 20th	Day	74% 5% 10% 8% 3	3968
	21st to end	Day	71% 5% 10% 9% 4	4466
February	1st to 10th	Day	69% 4 7% 16% 4	1800
	11th to 20th	Day	66% 5% 8% 14% 6% 1	1762
	21st to end	Day	63% 4 8% 16% 7% 1	1542
March	1st to 10th	Day	57% 3 13% 14% 12%	4932
	11th to 20th	Day	54% 4 13% 20% 7% 2	4980
	21st to end	Day	56% 3 8% 19% 12% 2	5465
April	1st to 10th	Day	56% 4 9% 19% 10% 1	2347
	11th to 20th	Day	50% 4 8% 23% 14% 1	2482
	21st to end	Day	52% 4 11% 19% 12% 2	2600
May	1st to 10th	Day	48% 5% 12% 19% 12% 5%	5897
	11th to 20th	Day	50% 5% 10% 17% 17% 1	6266
	21st to end	Day	46% 4 12% 21% 15% 2	6949
June	1st to 10th	Day	44% 6% 12% 22% 16% 1	2800
	11th to 20th	Day	39% 5% 14% 25% 16% 1	2897
	21st to end	Day	42% 5% 13% 25% 14% 1	3000
July	1st to 10th	Day	39% 8% 15% 23% 14% 1	6660
	11th to 20th	Day	33% 6% 12% 32% 16% 1	6285
	21st to end	Day	33% 7% 14% 31% 15% 1	6253
August	1st to 10th	Day	43% 6% 14% 24% 11% 1	4221
	11th to 20th	Day	43% 7% 17% 24% 10%	4213
	21st to end	Day	37% 6% 15% 31% 10% 1	4525
September	1st to 10th	Day	42% 6% 12% 28% 10% 2	2400
	11th to 20th	Day	38% 4 12% 32% 13%	2257
	21st to end	Day	48% 5% 13% 25% 9%	2175
October	1st to 10th	Day	42% 6% 13% 26% 11% 1	3260
	11th to 20th	Day	43% 6% 15% 22% 14%	3240
	21st to end	Day	58% 6% 13% 17% 6%	3538
November	1st to 10th	Day	54% 6% 13% 20% 8%	1760
	11th to 20th	Day	60% 4 13% 15% 8% 1	1680
	21st to end	Day	69% 5% 12% 12% 2	1680
December	1st to 10th	Day	73% 6% 11% 7% 2	2680
	11th to 20th	Day	69% 6% 9% 9% 6%	3066
	21st to end	Day	77% 4 7% 7% 4	4048
January	1st to 10th	Night	64% 5% 12% 15% 4	6364
	11th to 20th	Night	73% 5% 10% 9% 3	5976
	21st to end	Night	64% 5% 11% 10% 8% 1	6490
February	1st to 10th	Night	61% 4 10% 18% 5%	2600
	11th to 20th	Night	61% 5% 9% 18% 7%	2535
	21st to end	Night	56% 4 13% 18% 8% 1	2085
March	1st to 10th	Night	56% 5% 11% 18% 9% 2	4919
	11th to 20th	Night	45% 5% 18% 22% 9% 2	4980
	21st to end	Night	50% 4 9% 20% 15% 2	5467
April	1st to 10th	Night	55% 4 9% 19% 12% 1	2051
	11th to 20th	Night	47% 4 9% 21% 16% 2	1883
	21st to end	Night	48% 4 11% 20% 16% 2	1800
May	1st to 10th	Night	48% 4 11% 15% 14% 6%	4060
	11th to 20th	Night	43% 5% 12% 24% 14% 2	3632
	21st to end	Night	40% 5% 13% 26% 15% 2	3927
June	1st to 10th	Night	39% 6% 10% 26% 17% 2	1600
	11th to 20th	Night	36% 6% 13% 25% 19% 1	1495
	21st to end	Night	36% 5% 14% 28% 16% 1	1400
July	1st to 10th	Night	33% 8% 14% 25% 18% 2	3202
	11th to 20th	Night	31% 6% 11% 32% 19% 1	3552
	21st to end	Night	30% 7% 12% 32% 18% 2	4267
August	1st to 10th	Night	35% 5% 13% 30% 16% 1	2904
	11th to 20th	Night	34% 5% 16% 31% 13% 1	2902
	21st to end	Night	32% 6% 15% 32% 13% 1	3339
September	1st to 10th	Night	33% 6% 15% 31% 13% 1	2000
	11th to 20th	Night	28% 4 12% 37% 18%	2140
	21st to end	Night	44% 4 13% 28% 11% 1	2225
October	1st to 10th	Night	34% 5% 10% 29% 20% 1	3920
	11th to 20th	Night	42% 6% 14% 24% 14%	3899
	21st to end	Night	45% 8% 15% 23% 9%	4252
November	1st to 10th	Night	49% 5% 12% 24% 9% 1	2752
	11th to 20th	Night	56% 5% 14% 17% 8% 1	2940
	21st to end	Night	62% 6% 15% 14% 2 1	2940
December	1st to 10th	Night	68% 8% 12% 10% 2	4720
	11th to 20th	Night	66% 6% 10% 11% 6%	5348
	21st to end	Night	76% 4 9% 8% 3	7150

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	21% 27% 34% 14% 3 1	202
	11th to 20th	Day	24% 30% 29% 13% 3 1	280
	21st to end	Day	26% 29% 30% 12% 2	330
February	1st to 10th	Day	32% 30% 25% 9% 3	300
	11th to 20th	Day	26% 34% 34% 5% 1	300
	21st to end	Day	37% 29% 27% 7%	249
March	1st to 10th	Day	25% 24% 34% 12% 5%	497
	11th to 20th	Day	26% 22% 35% 13% 4	500
	21st to end	Day	32% 20% 30% 13% 4 1	550
April	1st to 10th	Day	27% 22% 33% 12% 5%	500
	11th to 20th	Day	35% 18% 25% 12% 8% 2	492
	21st to end	Day	28% 18% 34% 14% 5% 2	500
May	1st to 10th	Day	22% 16% 33% 19% 8% 2	503
	11th to 20th	Day	20% 17% 32% 19% 9% 3	527
	21st to end	Day	18% 16% 30% 23% 10% 4	573
June	1st to 10th	Day	18% 15% 36% 22% 7% 2	530
	11th to 20th	Day	15% 13% 33% 25% 11% 2	627
	21st to end	Day	17% 13% 39% 20% 8% 3	730
July	1st to 10th	Day	11% 10% 32% 30% 14% 4	687
	11th to 20th	Day	12% 8% 32% 28% 13% 6%	524
	21st to end	Day	10% 8% 38% 27% 12% 6%	576
August	1st to 10th	Day	10% 7% 29% 30% 15% 9%	526
	11th to 20th	Day	9% 9% 29% 27% 18% 8%	519
	21st to end	Day	15% 9% 30% 23% 16% 7%	583
September	1st to 10th	Day	14% 14% 26% 25% 14% 8%	530
	11th to 20th	Day	10% 8% 25% 24% 19% 14%	530
	21st to end	Day	16% 14% 27% 24% 14% 5%	530
October	1st to 10th	Day	24% 14% 25% 20% 14% 4	530
	11th to 20th	Day	22% 21% 34% 15% 6% 2	530
	21st to end	Day	32% 19% 29% 15% 4 1	583
November	1st to 10th	Day	27% 24% 30% 16% 3 1	289
	11th to 20th	Day	30% 21% 33% 11% 5%	220
	21st to end	Day	29% 25% 37% 9% 1	220
December	1st to 10th	Day	29% 26% 30% 14% 2	220
	11th to 20th	Day	30% 30% 27% 10% 1 1	220
	21st to end	Day	29% 27% 36% 5% 2 1	242
January	1st to 10th	Night	23% 26% 32% 14% 4	805
	11th to 20th	Night	26% 23% 31% 17% 4	800
	21st to end	Night	19% 20% 35% 20% 6%	880
February	1st to 10th	Night	23% 26% 34% 17% 2	800
	11th to 20th	Night	23% 24% 36% 14% 3	783
	21st to end	Night	22% 25% 33% 18% 2	660
March	1st to 10th	Night	22% 24% 33% 19% 3	800
	11th to 20th	Night	23% 22% 30% 19% 5% 1	800
	21st to end	Night	23% 19% 31% 20% 6%	880
April	1st to 10th	Night	26% 18% 34% 20% 2	800
	11th to 20th	Night	22% 18% 32% 18% 8% 1	699
	21st to end	Night	22% 13% 32% 25% 7%	600
May	1st to 10th	Night	19% 8% 30% 31% 11% 2	600
	11th to 20th	Night	17% 8% 32% 31% 11%	424
	21st to end	Night	12% 9% 35% 32% 12%	433
June	1st to 10th	Night	15% 7% 35% 33% 10%	400
	11th to 20th	Night	11% 10% 29% 37% 13%	400
	21st to end	Night	12% 8% 31% 34% 15% 1	400
July	1st to 10th	Night	6% 5% 28% 41% 18% 2	390
	11th to 20th	Night	11% 8% 26% 36% 19% 1	388
	21st to end	Night	8% 5% 29% 38% 18% 2	639
August	1st to 10th	Night	9% 3 24% 38% 25% 2	597
	11th to 20th	Night	7% 6% 21% 40% 24% 3	594
	21st to end	Night	13% 4 28% 31% 22% 2	724
September	1st to 10th	Night	11% 8% 28% 28% 20% 4	800
	11th to 20th	Night	12% 7% 23% 30% 23% 6%	800
	21st to end	Night	19% 10% 22% 29% 17% 3	800
October	1st to 10th	Night	19% 10% 30% 24% 15% 2	800
	11th to 20th	Night	24% 16% 31% 21% 8% 1	800
	21st to end	Night	27% 18% 30% 20% 5%	880
November	1st to 10th	Night	24% 18% 28% 22% 7% 1	820
	11th to 20th	Night	23% 24% 26% 21% 5% 1	840
	21st to end	Night	26% 21% 31% 18% 3	840
December	1st to 10th	Night	27% 23% 33% 13% 3	840
	11th to 20th	Night	22% 26% 34% 15% 3	840
	21st to end	Night	31% 20% 30% 16% 2	924

Page generated at 2025-12-30 01:14:08. Version ac07

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