Culver's Urban Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Culver's Urban Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 6% 2 6% 4 4 5% 4 6% 8% 41%	126906
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	124149
Transparency	Day	40% 6% 12% 20% 18% 5%	130693
	Night	38% 5% 11% 19% 20% 6%	128430
Seeing	Day	16% 10% 23% 23% 19% 9%	16343
	Night	16% 8% 23% 29% 20% 3	26372

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	18% 10% 3 5% 2 3 3 2 3 4 46%	4056
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 3 5% 52%	4060
	21st to end	Day	23% 9% 3 6% 4 3 3 3 3 5% 40%	4630
February	1st to 10th	Day	16% 9% 2 6% 4 5% 4 4 5% 6% 39%	2000
	11th to 20th	Day	23% 7% 2 5% 3 3 4 3 4 6% 40%	1948
	21st to end	Day	21% 7% 2 7% 4 3 4 3 4 5% 40%	1650
March	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 5% 34%	4615
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 50%	4980
	21st to end	Day	17% 5% 2 6% 5% 4 4 4 4 5% 44%	5465
April	1st to 10th	Day	12% 7% 2 5% 4 4 4 4 6% 7% 45%	2255
	11th to 20th	Day	11% 6% 2 7% 4 4 4 3 5% 7% 47%	2374
	21st to end	Day	11% 5% 1 5% 4 4 4 4 6% 8% 48%	2565
May	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 6% 9% 36%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 6% 5% 6% 9% 29%	5861
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 8% 39%	6433
June	1st to 10th	Day	10% 6% 3 7% 7% 5% 6% 5% 8% 10% 34%	2600
	11th to 20th	Day	5% 5% 2 5% 5% 5% 7% 6% 10% 11% 41%	2600
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 5% 9% 11% 38%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 9% 11% 35%	5900
	11th to 20th	Day	7% 4 2 6% 6% 6% 6% 6% 8% 12% 36%	5900
	21st to end	Day	11% 7% 3 8% 7% 7% 7% 6% 8% 12% 25%	6424
August	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 38%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	3921
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 47%	4312
September	1st to 10th	Day	11% 6% 3 7% 5% 4 5% 5% 7% 8% 40%	2206
	11th to 20th	Day	15% 7% 2 6% 4 5% 4 4 6% 8% 41%	2200
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	12% 5% 2 5% 3 4 3 3 4 5% 55%	2200
	11th to 20th	Day	11% 5% 1 4 3 2 3 3 5% 7% 57%	2086
	21st to end	Day	15% 5% 2 5% 3 3 3 2 4 5% 54%	2200
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 2 4 5% 55%	1867
	11th to 20th	Day	13% 5% 1 4 4 4 3 3 5% 5% 53%	1890
	21st to end	Day	14% 5% 3 6% 4 4 3 3 6% 6% 47%	1890
December	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 4 6% 45%	3020
	11th to 20th	Day	17% 7% 3 7% 4 5% 4 3 4 5% 41%	3436
	21st to end	Day	23% 6% 3 5% 3 3 4 3 5% 6% 39%	4466
January	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 5% 6% 47%	5889
	11th to 20th	Night	14% 8% 3 6% 3 3 4 3 4 5% 48%	5900
	21st to end	Night	20% 9% 2 6% 4 3 3 2 4 4 43%	6324
February	1st to 10th	Night	16% 7% 2 6% 4 4 3 3 5% 6% 44%	2400
	11th to 20th	Night	23% 7% 2 6% 4 3 3 3 4 4 41%	2338
	21st to end	Night	20% 7% 3 6% 4 3 3 4 5% 7% 40%	1980
March	1st to 10th	Night	21% 9% 3 6% 4 4 4 3 5% 6% 35%	5235
	11th to 20th	Night	11% 5% 2 4 4 3 4 3 5% 8% 52%	4980
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 47%	2160
	11th to 20th	Night	14% 5% 2 4 4 3 3 4 5% 6% 51%	1975
	21st to end	Night	9% 6% 2 6% 4 4 3 3 5% 8% 50%	1835
May	1st to 10th	Night	11% 7% 3 8% 5% 4 6% 5% 6% 9% 36%	4033
	11th to 20th	Night	20% 7% 3 6% 5% 5% 5% 4 6% 8% 30%	4032
	21st to end	Night	13% 6% 2 5% 4 4 4 5% 7% 9% 41%	4421
June	1st to 10th	Night	7% 5% 2 7% 6% 5% 6% 6% 8% 11% 35%	1800
	11th to 20th	Night	5% 5% 2 6% 5% 5% 5% 6% 8% 12% 43%	1800
	21st to end	Night	6% 4 2 5% 5% 4 6% 5% 8% 11% 44%	1800
July	1st to 10th	Night	9% 5% 3 9% 6% 6% 5% 5% 7% 9% 35%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 6% 9% 12% 35%	4057
	21st to end	Night	12% 8% 3 8% 6% 6% 6% 5% 7% 10% 29%	4427
August	1st to 10th	Night	8% 5% 3 7% 4 5% 5% 5% 7% 13% 38%	2915
	11th to 20th	Night	9% 6% 2 7% 4 4 4 4 5% 8% 46%	3124
	21st to end	Night	8% 5% 2 5% 4 4 4 4 6% 8% 51%	3586
September	1st to 10th	Night	9% 5% 3 7% 6% 5% 5% 4 7% 9% 40%	2171
	11th to 20th	Night	13% 6% 2 5% 4 5% 5% 4 6% 7% 43%	2200
	21st to end	Night	13% 4 2 6% 4 4 4 4 6% 7% 47%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 55%	2200
	11th to 20th	Night	10% 4 2 5% 3 3 3 2 4 5% 59%	2312
	21st to end	Night	13% 4 2 4 3 3 3 3 4 5% 56%	2640
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 6% 55%	2646
	11th to 20th	Night	14% 5% 2 4 3 3 2 2 4 6% 54%	2730
	21st to end	Night	15% 6% 2 5% 4 4 3 4 4 7% 47%	2730
December	1st to 10th	Night	17% 7% 2 6% 4 3 2 3 4 6% 48%	4380
	11th to 20th	Night	19% 7% 2 5% 3 3 4 3 4 5% 45%	4914
	21st to end	Night	22% 7% 3 6% 3 3 3 3 4 6% 40%	6490

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	42% 4 5% 20% 23% 7%	4057
	11th to 20th	Day	34% 3 6% 20% 28% 8%	4060
	21st to end	Day	48% 2 6% 16% 21% 7%	4630
February	1st to 10th	Day	45% 4 8% 19% 20% 5%	2000
	11th to 20th	Day	45% 4 7% 19% 19% 6%	1958
	21st to end	Day	45% 4 8% 14% 21% 8%	1650
March	1st to 10th	Day	53% 4 7% 11% 19% 8%	4615
	11th to 20th	Day	35% 4 7% 14% 24% 17%	4980
	21st to end	Day	42% 4 8% 22% 20% 4	5465
April	1st to 10th	Day	37% 5% 11% 22% 20% 5%	2255
	11th to 20th	Day	36% 5% 9% 18% 24% 7%	2374
	21st to end	Day	32% 6% 13% 27% 19% 3	2565
May	1st to 10th	Day	44% 6% 15% 20% 14% 1	5897
	11th to 20th	Day	50% 7% 15% 19% 8% 1	5865
	21st to end	Day	41% 6% 12% 22% 18% 1	6447
June	1st to 10th	Day	39% 11% 18% 21% 9% 2	2600
	11th to 20th	Day	30% 11% 21% 26% 11% 1	2600
	21st to end	Day	34% 11% 18% 22% 12% 3	2600
July	1st to 10th	Day	39% 8% 19% 26% 7% 1	5835
	11th to 20th	Day	39% 8% 29% 19% 4 1	5831
	21st to end	Day	50% 9% 18% 19% 4	6240
August	1st to 10th	Day	38% 7% 14% 32% 8%	4221
	11th to 20th	Day	35% 7% 18% 24% 14% 2	3959
	21st to end	Day	34% 6% 14% 24% 20% 3	4311
September	1st to 10th	Day	39% 9% 15% 16% 18% 4	2206
	11th to 20th	Day	40% 8% 12% 20% 20% 2	2200
	21st to end	Day	37% 5% 13% 17% 21% 7%	2200
October	1st to 10th	Day	27% 3 6% 13% 38% 13%	3590
	11th to 20th	Day	20% 2 6% 15% 29% 28%	3378
	21st to end	Day	29% 2 5% 22% 29% 12%	3538
November	1st to 10th	Day	33% 3 7% 16% 31% 10%	1867
	11th to 20th	Day	33% 3 7% 19% 27% 10%	1890
	21st to end	Day	37% 5% 7% 19% 26% 6%	1890
December	1st to 10th	Day	42% 3 5% 18% 26% 7%	3020
	11th to 20th	Day	47% 3 8% 16% 19% 6%	3433
	21st to end	Day	45% 3 9% 25% 16% 2	4466
January	1st to 10th	Night	37% 3 6% 19% 29% 6%	5889
	11th to 20th	Night	39% 3 5% 19% 26% 8%	5900
	21st to end	Night	45% 2 5% 15% 27% 6%	6324
February	1st to 10th	Night	41% 4 9% 19% 20% 8%	2400
	11th to 20th	Night	47% 3 6% 17% 20% 7%	2339
	21st to end	Night	44% 5% 7% 16% 19% 10%	1980
March	1st to 10th	Night	50% 3 8% 13% 19% 7%	5235
	11th to 20th	Night	30% 4 7% 14% 28% 16%	4980
	21st to end	Night	40% 4 7% 21% 20% 9%	5467
April	1st to 10th	Night	35% 5% 11% 19% 24% 6%	2160
	11th to 20th	Night	33% 5% 8% 21% 24% 10%	1975
	21st to end	Night	32% 6% 11% 26% 20% 5%	1835
May	1st to 10th	Night	43% 8% 16% 20% 12% 2	4060
	11th to 20th	Night	50% 6% 16% 19% 9% 1	4033
	21st to end	Night	36% 7% 14% 24% 16% 2	4429
June	1st to 10th	Night	37% 12% 18% 19% 12% 2	1800
	11th to 20th	Night	30% 10% 21% 25% 13% 1	1800
	21st to end	Night	31% 11% 19% 21% 14% 4	1800
July	1st to 10th	Night	44% 9% 17% 23% 6% 1	4015
	11th to 20th	Night	38% 11% 27% 17% 5% 1	4006
	21st to end	Night	50% 10% 20% 16% 4 1	4297
August	1st to 10th	Night	38% 7% 17% 30% 7%	2904
	11th to 20th	Night	37% 6% 17% 24% 15% 1	3152
	21st to end	Night	32% 6% 14% 22% 24% 2	3586
September	1st to 10th	Night	38% 9% 14% 18% 17% 4	2171
	11th to 20th	Night	38% 8% 12% 21% 19% 3	2200
	21st to end	Night	36% 7% 12% 17% 20% 8%	2200
October	1st to 10th	Night	27% 2 9% 12% 38% 11%	3590
	11th to 20th	Night	18% 2 5% 17% 33% 24%	3759
	21st to end	Night	25% 2 4 23% 29% 17%	4252
November	1st to 10th	Night	32% 4 7% 18% 27% 12%	2646
	11th to 20th	Night	32% 3 6% 18% 29% 11%	2730
	21st to end	Night	36% 4 7% 19% 25% 7%	2730
December	1st to 10th	Night	39% 3 5% 18% 28% 7%	4380
	11th to 20th	Night	41% 3 10% 16% 24% 6%	4916
	21st to end	Night	45% 3 7% 27% 16% 2	6490

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% 25% 36% 12% 1	300
	11th to 20th	Day	26% 21% 31% 16% 5% 1	300
	21st to end	Day	25% 24% 37% 11% 3 1	330
February	1st to 10th	Day	21% 20% 35% 21% 4	300
	11th to 20th	Day	37% 23% 26% 14% 1	300
	21st to end	Day	27% 22% 36% 11% 3	249
March	1st to 10th	Day	26% 18% 32% 17% 6% 1	357
	11th to 20th	Day	20% 20% 32% 21% 6% 1	500
	21st to end	Day	20% 17% 34% 21% 7% 1	550
April	1st to 10th	Day	19% 15% 32% 22% 10% 1	500
	11th to 20th	Day	14% 13% 36% 22% 14% 1	492
	21st to end	Day	17% 10% 24% 30% 16% 3	500
May	1st to 10th	Day	14% 12% 31% 24% 14% 5%	503
	11th to 20th	Day	15% 6% 24% 28% 17% 10%	527
	21st to end	Day	17% 6% 17% 27% 23% 11%	573
June	1st to 10th	Day	13% 2 18% 26% 27% 13%	530
	11th to 20th	Day	5% 4 12% 31% 24% 23%	530
	21st to end	Day	8% 2 15% 28% 29% 17%	530
July	1st to 10th	Day	10% 2 8% 24% 32% 25%	525
	11th to 20th	Day	9% 2 7% 21% 34% 27%	524
	21st to end	Day	8% 1 8% 22% 39% 22%	576
August	1st to 10th	Day	13% 4 13% 24% 25% 21%	526
	11th to 20th	Day	12% 3 9% 23% 36% 17%	519
	21st to end	Day	13% 2 10% 28% 25% 21%	583
September	1st to 10th	Day	10% 3 15% 29% 28% 14%	530
	11th to 20th	Day	15% 5% 13% 23% 32% 12%	530
	21st to end	Day	15% 4 17% 26% 27% 11%	530
October	1st to 10th	Day	15% 4 21% 32% 20% 8%	530
	11th to 20th	Day	12% 10% 24% 26% 24% 5%	530
	21st to end	Day	18% 8% 26% 27% 18% 3	583
November	1st to 10th	Day	22% 8% 28% 22% 17% 3	345
	11th to 20th	Day	17% 16% 36% 22% 8% 1	330
	21st to end	Day	14% 18% 34% 23% 9% 1	330
December	1st to 10th	Day	30% 21% 29% 13% 5% 1	330
	11th to 20th	Day	24% 17% 31% 22% 6% 1	321
	21st to end	Day	32% 15% 32% 16% 5% 1	330
January	1st to 10th	Night	25% 19% 32% 20% 4	800
	11th to 20th	Night	23% 15% 33% 23% 7%	800
	21st to end	Night	20% 17% 36% 24% 3	880
February	1st to 10th	Night	19% 14% 33% 29% 5%	800
	11th to 20th	Night	26% 16% 33% 23% 2	783
	21st to end	Night	23% 16% 36% 23% 2	660
March	1st to 10th	Night	20% 13% 34% 29% 5%	800
	11th to 20th	Night	18% 12% 35% 25% 9% 1	800
	21st to end	Night	18% 14% 29% 30% 8%	880
April	1st to 10th	Night	14% 14% 31% 29% 11%	760
	11th to 20th	Night	16% 8% 29% 33% 13% 1	593
	21st to end	Night	11% 8% 26% 36% 19% 1	600
May	1st to 10th	Night	11% 7% 25% 35% 22% 1	600
	11th to 20th	Night	15% 3 24% 31% 23% 4	591
	21st to end	Night	13% 2 15% 33% 31% 6%	650
June	1st to 10th	Night	9% 1 12% 40% 33% 6%	600
	11th to 20th	Night	6% 2 15% 30% 36% 12%	600
	21st to end	Night	6% 1 10% 35% 37% 10%	600
July	1st to 10th	Night	10% 3 6% 24% 45% 12%	585
	11th to 20th	Night	9% 1 7% 20% 50% 13%	582
	21st to end	Night	9% 2 8% 25% 47% 10%	657
August	1st to 10th	Night	11% 3 8% 28% 42% 9%	597
	11th to 20th	Night	10% 2 10% 28% 41% 9%	594
	21st to end	Night	14% 2 10% 26% 39% 9%	660
September	1st to 10th	Night	11% 2 13% 30% 36% 9%	771
	11th to 20th	Night	16% 2 12% 29% 38% 3	800
	21st to end	Night	15% 3 13% 36% 32% 2	800
October	1st to 10th	Night	13% 4 19% 35% 24% 6%	800
	11th to 20th	Night	14% 6% 25% 32% 21% 3	800
	21st to end	Night	15% 8% 22% 34% 18% 2	880
November	1st to 10th	Night	15% 7% 25% 32% 20% 1	820
	11th to 20th	Night	17% 10% 29% 30% 13% 1	840
	21st to end	Night	19% 12% 28% 28% 13%	840
December	1st to 10th	Night	22% 13% 32% 27% 7%	840
	11th to 20th	Night	24% 10% 31% 26% 9%	829
	21st to end	Night	27% 14% 26% 28% 5%	880

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