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 2024-01-31.

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% 9% 4 7% 6% 3 5% 5% 6% 7% 33%	75101
	Night	15% 10% 4 7% 6% 3 5% 4 5% 7% 35%	75039
Transparency	Day	48% 7% 12% 20% 12% 1	74814
	Night	48% 7% 12% 20% 12% 1	74847
Seeing	Day	19% 13% 29% 22% 12% 5%	14478
	Night	20% 11% 29% 27% 12% 1	24423

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	16% 15% 7% 10% 8% 4 5% 5% 5% 5% 20%	1517
	11th to 20th	Day	18% 18% 8% 9% 8% 4 5% 5% 5% 5% 16%	1532
	21st to end	Day	17% 18% 9% 11% 7% 3 5% 4 4 6% 17%	1872
February	1st to 10th	Day	16% 16% 5% 10% 9% 4 6% 6% 4 5% 20%	1620
	11th to 20th	Day	15% 11% 5% 11% 9% 5% 6% 5% 5% 6% 23%	1728
	21st to end	Day	18% 13% 5% 8% 6% 4 5% 4 6% 6% 26%	1480
March	1st to 10th	Day	17% 12% 5% 9% 7% 4 4 5% 5% 7% 27%	1836
	11th to 20th	Day	15% 13% 5% 7% 5% 3 4 3 5% 6% 33%	1980
	21st to end	Day	21% 13% 5% 8% 7% 3 4 4 4 5% 26%	2178
April	1st to 10th	Day	15% 11% 4 8% 6% 4 5% 6% 4 6% 31%	2095
	11th to 20th	Day	18% 7% 4 7% 6% 3 5% 5% 5% 8% 33%	2258
	21st to end	Day	19% 9% 3 7% 4 3 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	16% 8% 3 8% 5% 3 5% 5% 6% 8% 32%	2340
	11th to 20th	Day	18% 9% 4 7% 6% 3 5% 4 6% 7% 33%	2301
	21st to end	Day	11% 5% 3 6% 6% 3 6% 7% 6% 8% 39%	2531
June	1st to 10th	Day	11% 5% 3 5% 5% 4 6% 6% 6% 8% 40%	2461
	11th to 20th	Day	10% 6% 3 5% 6% 4 5% 5% 6% 9% 42%	2520
	21st to end	Day	10% 5% 3 5% 6% 4 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	8% 5% 2 4 5% 3 5% 5% 6% 10% 45%	2520
	11th to 20th	Day	8% 3 3 4 5% 4 5% 6% 6% 11% 44%	2520
	21st to end	Day	7% 5% 2 6% 6% 4 6% 6% 7% 8% 42%	2557
August	1st to 10th	Day	7% 5% 1 5% 6% 4 6% 6% 7% 10% 44%	2327
	11th to 20th	Day	9% 5% 2 5% 6% 4 7% 6% 8% 10% 39%	2303
	21st to end	Day	10% 4 2 5% 5% 4 5% 6% 6% 11% 42%	2518
September	1st to 10th	Day	12% 6% 2 6% 5% 3 5% 5% 7% 8% 41%	2034
	11th to 20th	Day	9% 5% 3 6% 5% 3 5% 6% 7% 9% 44%	1980
	21st to end	Day	15% 6% 2 5% 4 3 5% 5% 6% 8% 42%	1980
October	1st to 10th	Day	11% 7% 3 7% 5% 3 5% 5% 6% 7% 42%	1947
	11th to 20th	Day	18% 9% 3 6% 5% 3 5% 4 6% 6% 35%	1800
	21st to end	Day	20% 8% 3 7% 6% 4 4 6% 6% 6% 30%	1841
November	1st to 10th	Day	11% 8% 3 6% 5% 3 5% 4 7% 7% 40%	1672
	11th to 20th	Day	17% 13% 4 7% 6% 3 4 4 4 5% 32%	1710
	21st to end	Day	19% 12% 5% 7% 6% 3 4 5% 6% 7% 28%	1710
December	1st to 10th	Day	15% 16% 8% 12% 7% 4 4 4 4 6% 21%	1710
	11th to 20th	Day	14% 15% 6% 10% 6% 4 5% 5% 5% 6% 23%	2080
	21st to end	Day	23% 21% 8% 12% 5% 3 4 3 4 5% 12%	2783
January	1st to 10th	Night	18% 14% 6% 8% 6% 4 5% 4 4 6% 25%	2648
	11th to 20th	Night	18% 15% 7% 11% 7% 3 5% 4 4 5% 20%	2644
	21st to end	Night	17% 16% 6% 9% 7% 4 5% 4 4 5% 21%	2711
February	1st to 10th	Night	17% 15% 6% 9% 8% 3 5% 4 4 5% 23%	2340
	11th to 20th	Night	15% 14% 5% 9% 6% 4 5% 4 5% 6% 27%	2116
	21st to end	Night	21% 12% 4 8% 6% 3 5% 4 4 5% 28%	1776
March	1st to 10th	Night	15% 10% 5% 8% 7% 4 4 4 5% 6% 34%	2124
	11th to 20th	Night	14% 9% 3 6% 5% 3 4 5% 5% 6% 39%	1980
	21st to end	Night	20% 9% 5% 7% 5% 3 4 4 4 7% 33%	2178
April	1st to 10th	Night	14% 10% 4 7% 5% 3 5% 4 5% 7% 37%	1881
	11th to 20th	Night	18% 6% 3 6% 6% 4 5% 4 5% 6% 37%	1650
	21st to end	Night	17% 9% 2 6% 5% 3 4 5% 5% 7% 37%	1620
May	1st to 10th	Night	19% 7% 3 6% 6% 4 6% 5% 5% 7% 31%	1620
	11th to 20th	Night	18% 7% 2 6% 5% 3 5% 3 5% 6% 39%	1592
	21st to end	Night	8% 6% 3 6% 5% 3 5% 6% 6% 9% 44%	1744
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 7% 8% 44%	1494
	11th to 20th	Night	10% 5% 3 6% 5% 2 4 5% 6% 9% 44%	1440
	21st to end	Night	10% 5% 2 4 4 2 4 6% 8% 11% 44%	1440
July	1st to 10th	Night	8% 3 2 4 4 3 4 4 6% 8% 52%	1440
	11th to 20th	Night	8% 4 2 5% 4 3 4 5% 5% 9% 51%	1440
	21st to end	Night	6% 4 1 4 5% 3 5% 6% 7% 10% 49%	1759
August	1st to 10th	Night	6% 4 2 4 5% 3 5% 5% 6% 10% 50%	1611
	11th to 20th	Night	8% 4 1 4 4 3 5% 6% 7% 9% 48%	1598
	21st to end	Night	8% 4 2 5% 5% 2 5% 4 5% 8% 50%	1835
September	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 5% 5% 8% 47%	1906
	11th to 20th	Night	9% 5% 2 5% 5% 3 3 3 5% 8% 52%	1980
	21st to end	Night	12% 5% 3 6% 5% 3 4 5% 5% 6% 46%	1980
October	1st to 10th	Night	13% 6% 3 5% 5% 4 4 4 5% 8% 44%	2011
	11th to 20th	Night	17% 9% 3 6% 5% 3 5% 4 5% 6% 38%	2160
	21st to end	Night	22% 9% 2 5% 5% 3 4 5% 4 6% 34%	2511
November	1st to 10th	Night	13% 8% 3 5% 5% 3 4 4 5% 6% 43%	2406
	11th to 20th	Night	15% 10% 4 7% 6% 4 4 5% 5% 7% 33%	2470
	21st to end	Night	20% 12% 4 8% 7% 3 4 4 4 6% 28%	2470
December	1st to 10th	Night	17% 14% 7% 10% 6% 4 4 4 4 6% 24%	2470
	11th to 20th	Night	17% 14% 5% 9% 7% 3 5% 4 5% 6% 26%	3110
	21st to end	Night	27% 19% 7% 9% 6% 3 4 4 4 5% 12%	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	66% 6% 11% 13% 4	1518
	11th to 20th	Day	70% 6% 13% 8% 3	1532
	21st to end	Day	69% 5% 8% 11% 6% 1	1872
February	1st to 10th	Day	66% 6% 10% 12% 6%	1620
	11th to 20th	Day	62% 6% 10% 14% 7%	1738
	21st to end	Day	58% 6% 13% 15% 6% 2	1480
March	1st to 10th	Day	57% 6% 13% 18% 6%	1836
	11th to 20th	Day	52% 5% 12% 19% 10% 2	1980
	21st to end	Day	61% 5% 9% 20% 6% 1	2178
April	1st to 10th	Day	53% 7% 11% 17% 13%	2095
	11th to 20th	Day	48% 7% 11% 20% 12% 2	2258
	21st to end	Day	49% 6% 12% 20% 10% 2	2340
May	1st to 10th	Day	49% 7% 12% 19% 11% 1	2340
	11th to 20th	Day	50% 7% 11% 17% 14% 2	2305
	21st to end	Day	40% 9% 12% 24% 12% 2	2531
June	1st to 10th	Day	39% 8% 11% 25% 13% 3	2461
	11th to 20th	Day	37% 8% 13% 22% 16% 3	2520
	21st to end	Day	39% 8% 14% 20% 15% 4	2520
July	1st to 10th	Day	35% 8% 12% 25% 18% 3	2451
	11th to 20th	Day	32% 9% 15% 25% 17% 3	2445
	21st to end	Day	36% 8% 13% 23% 17% 2	2415
August	1st to 10th	Day	33% 9% 15% 28% 15% 1	2314
	11th to 20th	Day	36% 10% 14% 24% 14% 1	2303
	21st to end	Day	34% 10% 12% 24% 18% 3	2518
September	1st to 10th	Day	39% 8% 14% 20% 16% 3	2034
	11th to 20th	Day	34% 7% 13% 29% 17% 1	1980
	21st to end	Day	38% 8% 14% 24% 16% 1	1980
October	1st to 10th	Day	40% 7% 11% 26% 15% 2	1947
	11th to 20th	Day	47% 5% 13% 21% 13% 1	1800
	21st to end	Day	52% 6% 13% 19% 10% 1	1841
November	1st to 10th	Day	41% 7% 12% 23% 16% 1	1672
	11th to 20th	Day	54% 5% 11% 18% 11% 1	1710
	21st to end	Day	55% 6% 14% 17% 7%	1710
December	1st to 10th	Day	66% 6% 11% 13% 4	1710
	11th to 20th	Day	60% 7% 12% 16% 4	2077
	21st to end	Day	76% 6% 8% 7% 3	2783
January	1st to 10th	Night	60% 6% 11% 16% 8%	2648
	11th to 20th	Night	66% 6% 13% 11% 5%	2644
	21st to end	Night	65% 5% 12% 13% 5% 1	2711
February	1st to 10th	Night	62% 7% 11% 15% 4 1	2340
	11th to 20th	Night	58% 5% 10% 18% 8% 1	2117
	21st to end	Night	57% 6% 11% 17% 8% 2	1776
March	1st to 10th	Night	53% 6% 10% 20% 10% 1	2124
	11th to 20th	Night	45% 7% 12% 18% 16% 2	1980
	21st to end	Night	51% 4 10% 24% 9% 2	2178
April	1st to 10th	Night	46% 7% 11% 18% 15% 2	1881
	11th to 20th	Night	46% 6% 10% 19% 17% 2	1650
	21st to end	Night	45% 7% 11% 20% 15% 2	1620
May	1st to 10th	Night	52% 7% 11% 17% 12% 2	1620
	11th to 20th	Night	45% 6% 9% 20% 17% 2	1593
	21st to end	Night	35% 9% 12% 27% 14% 2	1745
June	1st to 10th	Night	35% 9% 11% 26% 17% 2	1494
	11th to 20th	Night	35% 9% 12% 25% 16% 3	1440
	21st to end	Night	32% 9% 14% 25% 19% 1	1440
July	1st to 10th	Night	30% 9% 11% 27% 20% 3	1400
	11th to 20th	Night	29% 9% 14% 26% 20% 2	1392
	21st to end	Night	27% 9% 16% 24% 19% 4	1661
August	1st to 10th	Night	28% 9% 15% 30% 16% 2	1600
	11th to 20th	Night	30% 10% 14% 28% 16% 2	1598
	21st to end	Night	32% 7% 13% 23% 22% 3	1835
September	1st to 10th	Night	36% 7% 12% 24% 17% 4	1906
	11th to 20th	Night	31% 6% 11% 27% 23% 1	1980
	21st to end	Night	38% 7% 12% 24% 17% 2	1980
October	1st to 10th	Night	38% 6% 13% 22% 20% 1	2011
	11th to 20th	Night	47% 6% 11% 20% 15% 1	2160
	21st to end	Night	50% 6% 11% 22% 10% 1	2511
November	1st to 10th	Night	42% 5% 12% 24% 17% 1	2406
	11th to 20th	Night	48% 7% 13% 20% 10% 1	2470
	21st to end	Night	57% 6% 11% 19% 6%	2470
December	1st to 10th	Night	61% 6% 12% 15% 7%	2470
	11th to 20th	Night	57% 8% 14% 14% 7% 1	3112
	21st to end	Night	73% 6% 11% 7% 2	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% 21% 31% 18% 2 1	270
	11th to 20th	Day	30% 20% 37% 9% 4	270
	21st to end	Day	32% 18% 28% 19% 3	294
February	1st to 10th	Day	31% 20% 38% 9% 2	240
	11th to 20th	Day	30% 20% 37% 12% 1	240
	21st to end	Day	30% 27% 34% 7% 2	198
March	1st to 10th	Day	25% 24% 33% 13% 4	276
	11th to 20th	Day	25% 14% 36% 18% 6% 1	420
	21st to end	Day	34% 15% 30% 15% 6%	462
April	1st to 10th	Day	21% 17% 33% 20% 8% 1	420
	11th to 20th	Day	26% 20% 26% 17% 8% 3	412
	21st to end	Day	28% 15% 32% 15% 9% 2	420
May	1st to 10th	Day	24% 18% 27% 18% 10% 4	423
	11th to 20th	Day	21% 13% 34% 16% 12% 4	447
	21st to end	Day	12% 9% 29% 24% 18% 8%	485
June	1st to 10th	Day	11% 9% 32% 31% 13% 3	571
	11th to 20th	Day	13% 10% 31% 31% 11% 4	630
	21st to end	Day	12% 9% 31% 30% 14% 5%	630
July	1st to 10th	Day	9% 5% 25% 29% 20% 12%	621
	11th to 20th	Day	11% 7% 27% 28% 19% 8%	618
	21st to end	Day	7% 6% 29% 28% 20% 10%	498
August	1st to 10th	Day	9% 3 22% 29% 22% 14%	446
	11th to 20th	Day	13% 4 22% 29% 20% 11%	439
	21st to end	Day	11% 6% 23% 28% 23% 9%	495
September	1st to 10th	Day	16% 6% 21% 28% 20% 10%	450
	11th to 20th	Day	9% 6% 21% 26% 27% 11%	450
	21st to end	Day	19% 11% 24% 22% 17% 7%	450
October	1st to 10th	Day	15% 10% 27% 25% 16% 8%	450
	11th to 20th	Day	21% 20% 30% 20% 7% 1	450
	21st to end	Day	25% 22% 31% 17% 4 1	356
November	1st to 10th	Day	18% 19% 34% 19% 8% 3	270
	11th to 20th	Day	28% 23% 26% 18% 5%	270
	21st to end	Day	27% 24% 32% 16% 1	270
December	1st to 10th	Day	26% 27% 29% 11% 7%	270
	11th to 20th	Day	23% 27% 34% 13% 3	270
	21st to end	Day	30% 20% 35% 12% 3	297
January	1st to 10th	Night	25% 17% 33% 20% 5%	950
	11th to 20th	Night	28% 16% 34% 17% 5% 1	934
	21st to end	Night	23% 15% 35% 22% 4	835
February	1st to 10th	Night	28% 13% 34% 23% 3	720
	11th to 20th	Night	24% 14% 36% 23% 3	703
	21st to end	Night	29% 14% 37% 17% 3	592
March	1st to 10th	Night	21% 14% 31% 28% 6% 1	720
	11th to 20th	Night	20% 16% 34% 24% 6% 1	720
	21st to end	Night	24% 12% 31% 24% 8%	792
April	1st to 10th	Night	20% 15% 34% 26% 4 1	621
	11th to 20th	Night	21% 12% 33% 25% 8% 1	533
	21st to end	Night	21% 9% 34% 27% 9% 1	540
May	1st to 10th	Night	22% 8% 29% 29% 10% 1	540
	11th to 20th	Night	20% 6% 30% 29% 13% 1	531
	21st to end	Night	10% 4 28% 35% 20% 3	584
June	1st to 10th	Night	12% 4 29% 39% 16%	540
	11th to 20th	Night	13% 4 25% 39% 17% 1	540
	21st to end	Night	12% 4 23% 36% 22% 2	540
July	1st to 10th	Night	10% 3 19% 37% 28% 3	525
	11th to 20th	Night	9% 4 21% 38% 23% 4	522
	21st to end	Night	7% 2 19% 42% 27% 3	591
August	1st to 10th	Night	8% 2 17% 42% 28% 3	537
	11th to 20th	Night	12% 2 16% 34% 31% 5%	534
	21st to end	Night	9% 2 16% 37% 31% 4	594
September	1st to 10th	Night	12% 5% 20% 33% 26% 4	646
	11th to 20th	Night	10% 5% 19% 33% 30% 4	720
	21st to end	Night	15% 7% 22% 30% 23% 3	720
October	1st to 10th	Night	16% 8% 25% 30% 19% 3	720
	11th to 20th	Night	21% 15% 29% 23% 9% 2	720
	21st to end	Night	25% 15% 30% 24% 7%	792
November	1st to 10th	Night	17% 15% 31% 26% 10% 2	740
	11th to 20th	Night	23% 16% 32% 21% 7% 1	760
	21st to end	Night	27% 13% 33% 21% 6%	760
December	1st to 10th	Night	26% 15% 34% 20% 5%	760
	11th to 20th	Night	26% 14% 35% 21% 4	802
	21st to end	Night	31% 14% 33% 18% 3	1045

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