Ritter Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ritter Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	137264
	Night	14% 11% 4 9% 5% 4 4 3 5% 7% 34%	134868
Transparency	Day	48% 7% 12% 21% 12% 1	140863
	Night	48% 6% 11% 21% 13% 1	139342
Seeing	Day	20% 14% 29% 21% 11% 4	17743
	Night	20% 12% 29% 27% 12% 1	27575

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	14% 17% 7% 17% 6% 5% 4 4 5% 5% 16%	4691
	11th to 20th	Day	12% 16% 6% 14% 9% 6% 7% 4 4 4 17%	4748
	21st to end	Day	17% 16% 7% 13% 7% 5% 6% 4 5% 4 15%	5780
February	1st to 10th	Day	14% 16% 4 11% 8% 5% 6% 4 5% 5% 21%	1890
	11th to 20th	Day	14% 12% 5% 13% 7% 6% 5% 3 5% 5% 24%	2025
	21st to end	Day	16% 13% 5% 9% 5% 5% 5% 3 6% 6% 27%	1730
March	1st to 10th	Day	17% 13% 6% 10% 5% 4 4 3 4 5% 28%	4674
	11th to 20th	Day	12% 12% 4 8% 4 4 4 3 6% 6% 37%	5090
	21st to end	Day	17% 14% 4 10% 6% 5% 5% 4 5% 5% 25%	5586
April	1st to 10th	Day	16% 12% 3 8% 6% 5% 6% 5% 5% 6% 29%	2445
	11th to 20th	Day	17% 8% 4 7% 6% 4 4 3 6% 7% 33%	2639
	21st to end	Day	18% 10% 3 7% 4 4 4 4 5% 8% 34%	2730
May	1st to 10th	Day	19% 9% 3 8% 4 5% 5% 4 6% 7% 32%	7616
	11th to 20th	Day	15% 8% 3 7% 5% 5% 6% 4 6% 8% 34%	7626
	21st to end	Day	15% 6% 3 6% 4 4 4 4 5% 7% 41%	8391
June	1st to 10th	Day	11% 6% 2 6% 4 5% 6% 5% 7% 8% 40%	2872
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 3 6% 9% 42%	2871
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 41%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 42%	6360
	21st to end	Day	8% 6% 3 6% 5% 5% 5% 4 7% 10% 41%	6434
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	4234
	11th to 20th	Day	9% 6% 2 6% 5% 6% 7% 5% 8% 10% 37%	4174
	21st to end	Day	8% 5% 2 7% 4 5% 5% 5% 7% 11% 41%	4560
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	2260
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 8% 48%	2200
	21st to end	Day	16% 7% 2 6% 4 3 5% 4 6% 7% 40%	2200
October	1st to 10th	Day	11% 7% 2 7% 4 4 4 4 6% 7% 45%	2163
	11th to 20th	Day	17% 9% 3 7% 4 4 5% 3 6% 6% 37%	2000
	21st to end	Day	18% 9% 3 7% 4 5% 4 4 6% 6% 32%	2045
November	1st to 10th	Day	12% 9% 3 7% 5% 5% 4 4 6% 7% 40%	1852
	11th to 20th	Day	17% 14% 3 8% 6% 5% 3 4 4 5% 32%	1890
	21st to end	Day	18% 14% 5% 9% 5% 4 4 3 6% 7% 26%	1890
December	1st to 10th	Day	13% 14% 6% 13% 7% 5% 4 4 5% 6% 22%	3020
	11th to 20th	Day	19% 19% 6% 9% 6% 5% 4 4 5% 5% 18%	3348
	21st to end	Day	26% 21% 6% 12% 4 4 3 2 4 4 14%	4070
January	1st to 10th	Night	14% 16% 6% 15% 6% 5% 4 3 4 5% 21%	8232
	11th to 20th	Night	10% 17% 7% 15% 7% 5% 5% 4 5% 5% 19%	8223
	21st to end	Night	16% 14% 6% 10% 5% 5% 5% 4 5% 6% 24%	8401
February	1st to 10th	Night	15% 15% 5% 10% 7% 5% 5% 3 5% 6% 25%	2730
	11th to 20th	Night	14% 14% 4 10% 6% 5% 4 4 5% 6% 27%	2479
	21st to end	Night	19% 13% 4 9% 5% 4 5% 3 4 6% 28%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 4 4 2 4 6% 37%	5399
	11th to 20th	Night	11% 7% 3 7% 4 5% 4 4 5% 7% 43%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	16% 12% 3 7% 4 4 4 3 5% 6% 34%	2193
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 3 6% 6% 37%	1928
	21st to end	Night	16% 10% 2 6% 4 4 4 4 6% 6% 38%	1890
May	1st to 10th	Night	20% 10% 3 8% 6% 4 5% 3 5% 7% 30%	5237
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 40%	5234
	21st to end	Night	14% 6% 2 7% 4 4 4 4 6% 8% 42%	5761
June	1st to 10th	Night	10% 5% 2 5% 5% 5% 4 4 7% 7% 44%	1742
	11th to 20th	Night	10% 6% 3 7% 4 3 5% 4 6% 8% 44%	1640
	21st to end	Night	10% 5% 1 5% 4 4 4 5% 8% 11% 44%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 10% 47%	3618
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 5% 6% 9% 45%	3617
	21st to end	Night	8% 5% 2 6% 4 5% 4 4 6% 9% 46%	4415
August	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 9% 43%	2915
	11th to 20th	Night	9% 5% 1 5% 3 5% 4 4 6% 8% 49%	2875
	21st to end	Night	7% 5% 2 6% 4 4 5% 3 5% 9% 49%	3321
September	1st to 10th	Night	9% 6% 2 5% 5% 5% 4 4 5% 7% 50%	2118
	11th to 20th	Night	8% 5% 2 5% 4 4 3 3 4 7% 56%	2200
	21st to end	Night	14% 6% 3 7% 4 4 4 3 5% 6% 44%	2200
October	1st to 10th	Night	12% 6% 2 5% 5% 4 3 3 5% 7% 47%	2235
	11th to 20th	Night	16% 9% 2 7% 4 5% 4 3 5% 6% 39%	2400
	21st to end	Night	20% 9% 2 6% 5% 4 4 4 4 6% 36%	2791
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 3 5% 6% 44%	2666
	11th to 20th	Night	15% 11% 4 8% 5% 4 4 4 5% 7% 34%	2730
	21st to end	Night	19% 14% 4 9% 5% 4 4 3 5% 6% 26%	2730
December	1st to 10th	Night	12% 13% 7% 12% 6% 5% 4 3 5% 6% 26%	4380
	11th to 20th	Night	16% 18% 6% 12% 6% 4 4 3 4 5% 22%	5064
	21st to end	Night	27% 19% 6% 10% 4 4 3 3 4 5% 15%	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	68% 7% 10% 12% 3	4692
	11th to 20th	Day	70% 5% 8% 8% 9%	4748
	21st to end	Day	72% 5% 8% 10% 6%	5780
February	1st to 10th	Day	63% 7% 11% 13% 6%	1890
	11th to 20th	Day	61% 6% 10% 15% 8%	2035
	21st to end	Day	56% 6% 14% 16% 6% 2	1730
March	1st to 10th	Day	58% 6% 10% 18% 8%	4674
	11th to 20th	Day	45% 6% 16% 24% 9% 1	5090
	21st to end	Day	58% 6% 11% 16% 9% 1	5586
April	1st to 10th	Day	53% 7% 11% 17% 12%	2445
	11th to 20th	Day	48% 7% 12% 20% 11% 2	2639
	21st to end	Day	48% 6% 12% 22% 11% 2	2730
May	1st to 10th	Day	49% 7% 11% 20% 12% 1	7651
	11th to 20th	Day	45% 8% 12% 22% 12% 1	7630
	21st to end	Day	41% 7% 9% 19% 20% 5%	8405
June	1st to 10th	Day	38% 8% 11% 25% 15% 3	2872
	11th to 20th	Day	37% 8% 13% 23% 16% 3	2871
	21st to end	Day	38% 8% 14% 21% 16% 4	2800
July	1st to 10th	Day	36% 6% 11% 28% 16% 2	6291
	11th to 20th	Day	33% 8% 15% 27% 16% 1	6285
	21st to end	Day	36% 7% 13% 22% 20% 2	6250
August	1st to 10th	Day	35% 8% 15% 27% 15% 1	4221
	11th to 20th	Day	36% 9% 13% 30% 12% 1	4213
	21st to end	Day	34% 8% 13% 32% 12% 2	4559
September	1st to 10th	Day	37% 7% 14% 23% 17% 2	2260
	11th to 20th	Day	32% 7% 12% 30% 18% 1	2200
	21st to end	Day	41% 7% 13% 23% 16% 1	2200
October	1st to 10th	Day	28% 5% 12% 32% 21% 1	3527
	11th to 20th	Day	39% 4 10% 27% 19% 1	3240
	21st to end	Day	42% 5% 12% 24% 16% 1	3282
November	1st to 10th	Day	42% 6% 11% 24% 16% 1	1852
	11th to 20th	Day	54% 5% 10% 18% 11% 2	1890
	21st to end	Day	57% 6% 13% 18% 7%	1890
December	1st to 10th	Day	61% 7% 15% 13% 4	3020
	11th to 20th	Day	67% 5% 10% 14% 3	3345
	21st to end	Day	75% 5% 7% 8% 4	4070
January	1st to 10th	Night	64% 6% 9% 15% 5%	8233
	11th to 20th	Night	66% 5% 9% 11% 8%	8223
	21st to end	Night	60% 5% 13% 14% 8%	8401
February	1st to 10th	Night	60% 7% 11% 16% 6% 1	2730
	11th to 20th	Night	56% 6% 11% 19% 8% 1	2480
	21st to end	Night	56% 6% 11% 17% 8% 1	2076
March	1st to 10th	Night	49% 5% 11% 24% 10% 1	5399
	11th to 20th	Night	38% 7% 14% 26% 14% 1	5090
	21st to end	Night	48% 5% 11% 21% 14% 2	5588
April	1st to 10th	Night	49% 7% 11% 17% 14% 2	2193
	11th to 20th	Night	46% 6% 11% 19% 17% 2	1928
	21st to end	Night	44% 6% 11% 21% 15% 2	1890
May	1st to 10th	Night	54% 7% 8% 15% 13% 2	5264
	11th to 20th	Night	43% 7% 11% 25% 13% 1	5235
	21st to end	Night	39% 7% 11% 22% 18% 3	5771
June	1st to 10th	Night	35% 8% 10% 25% 20% 2	1742
	11th to 20th	Night	36% 8% 12% 26% 16% 3	1640
	21st to end	Night	32% 8% 15% 24% 20% 1	1600
July	1st to 10th	Night	31% 7% 12% 27% 18% 5%	3580
	11th to 20th	Night	34% 8% 15% 26% 16% 2	3572
	21st to end	Night	32% 8% 14% 22% 21% 4	4286
August	1st to 10th	Night	37% 7% 13% 25% 15% 2	2904
	11th to 20th	Night	31% 8% 12% 31% 18% 1	2902
	21st to end	Night	33% 7% 12% 30% 17% 2	3321
September	1st to 10th	Night	34% 7% 11% 26% 18% 3	2118
	11th to 20th	Night	29% 5% 10% 29% 25% 2	2200
	21st to end	Night	40% 7% 12% 22% 17% 2	2200
October	1st to 10th	Night	25% 4 9% 31% 30% 1	3651
	11th to 20th	Night	37% 4 10% 27% 21%	3899
	21st to end	Night	37% 4 9% 32% 16% 2	4504
November	1st to 10th	Night	41% 5% 11% 24% 17% 1	2666
	11th to 20th	Night	48% 7% 13% 21% 11% 2	2730
	21st to end	Night	58% 6% 11% 19% 6%	2730
December	1st to 10th	Night	58% 6% 12% 18% 6%	4380
	11th to 20th	Night	62% 6% 12% 13% 5%	5066
	21st to end	Night	72% 5% 10% 8% 5%	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	26% 20% 35% 16% 2 1	330
	11th to 20th	Day	31% 22% 35% 8% 4	330
	21st to end	Day	32% 19% 29% 17% 3	363
February	1st to 10th	Day	28% 20% 37% 12% 3	330
	11th to 20th	Day	28% 21% 34% 15% 2	330
	21st to end	Day	27% 29% 33% 10% 1	273
March	1st to 10th	Day	26% 26% 33% 12% 3	372
	11th to 20th	Day	29% 17% 33% 16% 5% 1	540
	21st to end	Day	32% 19% 30% 14% 5%	594
April	1st to 10th	Day	27% 18% 32% 16% 6% 1	540
	11th to 20th	Day	27% 21% 27% 16% 7% 2	532
	21st to end	Day	27% 15% 33% 15% 8% 1	540
May	1st to 10th	Day	25% 19% 29% 16% 8% 3	543
	11th to 20th	Day	20% 15% 34% 17% 11% 4	567
	21st to end	Day	14% 12% 29% 22% 16% 6%	617
June	1st to 10th	Day	13% 11% 32% 29% 12% 3	712
	11th to 20th	Day	13% 12% 33% 29% 10% 4	756
	21st to end	Day	12% 11% 31% 29% 13% 5%	730
July	1st to 10th	Day	10% 7% 26% 28% 19% 10%	721
	11th to 20th	Day	11% 8% 28% 28% 18% 8%	718
	21st to end	Day	8% 5% 27% 28% 22% 9%	586
August	1st to 10th	Day	10% 4 23% 30% 22% 12%	526
	11th to 20th	Day	13% 4 24% 30% 19% 10%	519
	21st to end	Day	10% 6% 23% 30% 22% 9%	583
September	1st to 10th	Day	15% 6% 22% 27% 20% 10%	530
	11th to 20th	Day	8% 5% 20% 28% 27% 11%	530
	21st to end	Day	21% 11% 24% 22% 16% 7%	530
October	1st to 10th	Day	14% 10% 25% 27% 16% 7%	530
	11th to 20th	Day	20% 19% 32% 20% 8% 2	530
	21st to end	Day	25% 22% 31% 16% 5% 1	428
November	1st to 10th	Day	21% 20% 33% 17% 7% 2	330
	11th to 20th	Day	31% 22% 26% 15% 5%	330
	21st to end	Day	31% 24% 30% 14% 2	330
December	1st to 10th	Day	27% 25% 29% 12% 6%	330
	11th to 20th	Day	27% 27% 31% 12% 3	330
	21st to end	Day	33% 20% 34% 10% 3	363
January	1st to 10th	Night	25% 18% 33% 20% 4	1050
	11th to 20th	Night	28% 17% 34% 16% 5% 1	1032
	21st to end	Night	23% 18% 34% 21% 3	924
February	1st to 10th	Night	26% 14% 33% 23% 3	840
	11th to 20th	Night	23% 15% 36% 23% 3	823
	21st to end	Night	28% 16% 37% 16% 2	692
March	1st to 10th	Night	22% 15% 31% 26% 5% 1	840
	11th to 20th	Night	21% 18% 34% 22% 5%	840
	21st to end	Night	24% 14% 30% 24% 7%	924
April	1st to 10th	Night	25% 15% 33% 23% 4 1	723
	11th to 20th	Night	21% 13% 34% 23% 8% 1	623
	21st to end	Night	21% 11% 34% 26% 8%	630
May	1st to 10th	Night	22% 10% 30% 28% 9% 1	630
	11th to 20th	Night	21% 7% 30% 28% 13% 1	621
	21st to end	Night	11% 6% 29% 34% 18% 2	683
June	1st to 10th	Night	12% 6% 28% 38% 16%	630
	11th to 20th	Night	13% 5% 26% 39% 16% 1	615
	21st to end	Night	12% 5% 24% 36% 21% 2	600
July	1st to 10th	Night	9% 4 22% 36% 26% 3	585
	11th to 20th	Night	9% 5% 23% 37% 23% 4	582
	21st to end	Night	7% 3 18% 43% 26% 3	657
August	1st to 10th	Night	8% 3 18% 43% 26% 3	597
	11th to 20th	Night	12% 2 18% 34% 30% 5%	594
	21st to end	Night	9% 3 17% 38% 30% 4	660
September	1st to 10th	Night	11% 6% 21% 32% 26% 4	718
	11th to 20th	Night	9% 4 18% 33% 31% 4	800
	21st to end	Night	16% 7% 22% 29% 22% 3	800
October	1st to 10th	Night	15% 8% 26% 30% 19% 2	800
	11th to 20th	Night	21% 15% 29% 25% 9% 2	800
	21st to end	Night	24% 15% 30% 24% 6%	880
November	1st to 10th	Night	17% 15% 31% 25% 9% 2	820
	11th to 20th	Night	23% 17% 31% 21% 7% 1	840
	21st to end	Night	28% 14% 32% 20% 5%	840
December	1st to 10th	Night	26% 16% 33% 20% 4	840
	11th to 20th	Night	26% 15% 34% 20% 4	887
	21st to end	Night	32% 15% 32% 18% 3	1155

Page generated at 2026-06-13 02:31:43. Version ac07

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