Joes Universe
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joes Universe clear sky chart from 2005-11-06 to 2025-09-19.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	126175
	Night	18% 8% 2 6% 4 3 4 3 5% 6% 41%	122878
Transparency	Day	42% 7% 12% 21% 15% 3	129704
	Night	42% 5% 12% 22% 16% 3	127436
Seeing	Day	20% 16% 32% 21% 9% 2	16136
	Night	21% 14% 32% 25% 8% 1	25133

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	12% 7% 3 7% 4 4 4 3 5% 5% 45%	3597
	11th to 20th	Day	15% 8% 2 7% 4 4 4 4 4 5% 42%	3570
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 5% 39%	4112
February	1st to 10th	Day	17% 8% 2 6% 5% 5% 5% 3 5% 6% 37%	1861
	11th to 20th	Day	15% 7% 3 6% 3 4 4 4 7% 7% 40%	1948
	21st to end	Day	21% 8% 3 6% 4 3 3 4 5% 6% 38%	1650
March	1st to 10th	Day	23% 9% 2 5% 3 3 3 2 4 5% 40%	4476
	11th to 20th	Day	15% 7% 3 5% 4 4 4 4 6% 7% 42%	4684
	21st to end	Day	21% 8% 2 6% 4 4 4 4 5% 6% 37%	5970
April	1st to 10th	Day	20% 9% 2 6% 5% 4 4 3 6% 7% 37%	2400
	11th to 20th	Day	20% 6% 2 5% 4 4 4 3 5% 7% 41%	2374
	21st to end	Day	21% 5% 2 5% 4 4 4 3 6% 5% 41%	2500
May	1st to 10th	Day	31% 8% 2 5% 4 3 4 3 5% 8% 27%	5886
	11th to 20th	Day	22% 8% 3 6% 4 4 4 3 5% 7% 34%	6151
	21st to end	Day	22% 6% 2 5% 4 4 4 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 5% 2 6% 5% 4 5% 5% 6% 8% 35%	2800
	11th to 20th	Day	17% 7% 2 6% 4 4 4 5% 6% 9% 36%	2800
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 6% 9% 38%	2800
July	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 4 6% 9% 34%	6360
	11th to 20th	Day	14% 6% 2 6% 4 5% 6% 5% 7% 11% 34%	6360
	21st to end	Day	17% 6% 2 5% 4 4 4 4 5% 7% 41%	6926
August	1st to 10th	Day	21% 6% 2 7% 5% 5% 5% 4 6% 8% 32%	4405
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 6% 8% 42%	3852
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 6% 8% 46%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	13% 4 2 5% 3 3 4 4 4 6% 50%	2245
	21st to end	Day	19% 6% 2 6% 4 3 3 3 5% 6% 41%	2090
October	1st to 10th	Day	22% 6% 2 5% 4 4 3 4 4 6% 41%	2052
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 5% 8% 47%	1892
	21st to end	Day	20% 6% 2 6% 3 4 4 4 4 6% 41%	1881
November	1st to 10th	Day	14% 6% 2 5% 4 4 3 2 5% 6% 49%	1761
	11th to 20th	Day	19% 5% 2 5% 2 3 3 3 5% 6% 48%	1800
	21st to end	Day	18% 7% 2 5% 4 3 4 3 6% 7% 43%	1765
December	1st to 10th	Day	16% 6% 2 5% 4 4 3 4 4 7% 45%	2600
	11th to 20th	Day	21% 8% 2 4 3 3 2 2 4 5% 46%	2986
	21st to end	Day	19% 10% 3 7% 4 4 2 3 4 6% 39%	3960
January	1st to 10th	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	15% 8% 3 7% 4 4 3 3 4 5% 45%	6315
	21st to end	Night	18% 8% 3 6% 4 4 5% 3 4 5% 39%	6840
February	1st to 10th	Night	17% 10% 3 5% 4 3 4 3 4 6% 40%	2537
	11th to 20th	Night	17% 8% 2 5% 4 4 4 2 5% 7% 41%	2338
	21st to end	Night	19% 7% 3 7% 4 4 4 3 4 6% 40%	1980
March	1st to 10th	Night	21% 10% 2 5% 3 3 3 2 4 5% 41%	5364
	11th to 20th	Night	16% 10% 3 6% 4 3 3 3 4 5% 41%	5273
	21st to end	Night	22% 9% 2 6% 3 3 3 3 4 5% 39%	4962
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 5% 7% 36%	2000
	11th to 20th	Night	20% 6% 2 5% 4 3 4 3 6% 6% 41%	1975
	21st to end	Night	20% 7% 2 6% 5% 3 3 3 4 5% 42%	1899
May	1st to 10th	Night	29% 11% 3 7% 4 3 3 3 4 6% 27%	4054
	11th to 20th	Night	22% 10% 3 6% 4 3 4 3 5% 6% 34%	3771
	21st to end	Night	23% 8% 3 6% 4 3 4 3 5% 6% 35%	3926
June	1st to 10th	Night	18% 6% 3 6% 5% 5% 4 4 5% 7% 39%	1600
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 6% 9% 40%	1600
	21st to end	Night	13% 7% 2 6% 4 4 4 4 5% 9% 42%	1600
July	1st to 10th	Night	18% 7% 2 6% 4 5% 4 4 6% 8% 37%	3600
	11th to 20th	Night	19% 8% 3 7% 4 4 4 4 6% 7% 33%	3600
	21st to end	Night	16% 6% 2 6% 4 4 4 4 5% 6% 44%	3926
August	1st to 10th	Night	23% 7% 2 7% 5% 5% 5% 4 6% 6% 31%	2711
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 4 5% 6% 42%	3192
	21st to end	Night	11% 4 1 4 4 3 3 3 5% 8% 55%	3586
September	1st to 10th	Night	14% 7% 2 6% 4 4 5% 3 4 7% 45%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	2110
	21st to end	Night	18% 6% 3 6% 3 4 4 3 4 5% 44%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 5% 6% 41%	2124
	11th to 20th	Night	15% 4 2 5% 3 3 4 3 4 6% 49%	2287
	21st to end	Night	22% 7% 2 5% 3 3 3 3 5% 5% 42%	2717
November	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 5% 6% 50%	2537
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 4 5% 47%	2600
	21st to end	Night	16% 6% 2 4 5% 3 3 4 4 6% 46%	2631
December	1st to 10th	Night	17% 7% 3 5% 3 3 3 3 3 4 48%	4580
	11th to 20th	Night	26% 7% 2 4 3 3 4 3 3 4 42%	5208
	21st to end	Night	20% 9% 3 7% 4 3 3 2 4 5% 41%	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	37% 6% 8% 28% 17% 3	3598
	11th to 20th	Day	42% 5% 11% 21% 13% 8%	3600
	21st to end	Day	45% 8% 8% 22% 15% 2	4112
February	1st to 10th	Day	45% 5% 14% 20% 14% 2	1861
	11th to 20th	Day	40% 6% 12% 23% 17% 3	1957
	21st to end	Day	46% 6% 12% 20% 14% 3	1650
March	1st to 10th	Day	45% 5% 11% 16% 12% 10%	4479
	11th to 20th	Day	37% 7% 16% 27% 11% 2	4684
	21st to end	Day	46% 5% 9% 15% 20% 6%	5970
April	1st to 10th	Day	46% 5% 10% 20% 15% 4	2400
	11th to 20th	Day	42% 6% 11% 20% 18% 3	2374
	21st to end	Day	44% 6% 10% 20% 17% 4	2500
May	1st to 10th	Day	54% 8% 15% 13% 9% 1	5897
	11th to 20th	Day	48% 7% 12% 18% 13% 1	6155
	21st to end	Day	45% 8% 11% 22% 13% 1	6949
June	1st to 10th	Day	43% 8% 12% 24% 12% 1	2800
	11th to 20th	Day	42% 8% 14% 22% 12% 1	2800
	21st to end	Day	40% 9% 14% 23% 13% 1	2800
July	1st to 10th	Day	45% 9% 11% 20% 14% 1	6290
	11th to 20th	Day	41% 9% 18% 21% 10% 1	6285
	21st to end	Day	41% 7% 11% 22% 18% 1	6728
August	1st to 10th	Day	47% 8% 15% 20% 9% 1	4391
	11th to 20th	Day	37% 7% 16% 28% 11% 1	3888
	21st to end	Day	32% 7% 15% 27% 17% 2	4312
September	1st to 10th	Day	39% 7% 11% 22% 19% 2	2400
	11th to 20th	Day	34% 6% 11% 23% 24% 2	2245
	21st to end	Day	44% 6% 10% 20% 18% 3	2090
October	1st to 10th	Day	36% 7% 10% 18% 23% 6%	3416
	11th to 20th	Day	27% 5% 10% 24% 30% 4	3120
	21st to end	Day	31% 5% 14% 24% 23% 3	3081
November	1st to 10th	Day	36% 5% 12% 23% 21% 3	1761
	11th to 20th	Day	37% 4 11% 22% 22% 4	1800
	21st to end	Day	41% 4 14% 27% 12% 3	1765
December	1st to 10th	Day	39% 5% 16% 27% 12% 1	2600
	11th to 20th	Day	41% 3 15% 19% 18% 3	2986
	21st to end	Day	47% 5% 8% 20% 15% 4	3960
January	1st to 10th	Night	36% 5% 10% 27% 20% 3	6349
	11th to 20th	Night	41% 4 14% 20% 15% 6%	6360
	21st to end	Night	47% 4 10% 19% 16% 5%	6840
February	1st to 10th	Night	46% 5% 12% 23% 12% 2	2537
	11th to 20th	Night	43% 4 11% 20% 18% 4	2340
	21st to end	Night	46% 4 8% 22% 15% 5%	1980
March	1st to 10th	Night	45% 5% 11% 18% 14% 7%	5374
	11th to 20th	Night	43% 6% 15% 21% 11% 4	5274
	21st to end	Night	47% 4 7% 13% 24% 5%	4962
April	1st to 10th	Night	46% 6% 11% 19% 14% 4	2000
	11th to 20th	Night	44% 5% 11% 19% 20% 1	1975
	21st to end	Night	44% 5% 9% 22% 16% 4	1899
May	1st to 10th	Night	58% 7% 13% 11% 10% 1	4060
	11th to 20th	Night	49% 9% 12% 15% 12% 3	3772
	21st to end	Night	50% 6% 13% 21% 9% 1	3927
June	1st to 10th	Night	45% 7% 11% 19% 18% 1	1600
	11th to 20th	Night	41% 8% 16% 24% 11% 1	1600
	21st to end	Night	39% 8% 13% 24% 15% 1	1600
July	1st to 10th	Night	45% 8% 13% 20% 12% 1	3560
	11th to 20th	Night	48% 8% 15% 19% 9% 1	3552
	21st to end	Night	39% 7% 13% 23% 17% 1	3809
August	1st to 10th	Night	52% 9% 13% 17% 9% 1	2701
	11th to 20th	Night	42% 7% 13% 29% 8% 1	3222
	21st to end	Night	27% 6% 14% 34% 18% 1	3586
September	1st to 10th	Night	39% 6% 10% 26% 18% 1	2000
	11th to 20th	Night	33% 6% 10% 23% 25% 2	2110
	21st to end	Night	43% 7% 8% 20% 20% 2	2090
October	1st to 10th	Night	38% 6% 12% 22% 19% 2	3540
	11th to 20th	Night	25% 5% 9% 27% 29% 4	3798
	21st to end	Night	32% 4 14% 28% 20% 3	4467
November	1st to 10th	Night	35% 5% 12% 22% 21% 4	2537
	11th to 20th	Night	40% 3 10% 23% 20% 4	2600
	21st to end	Night	39% 4 13% 29% 13% 2	2631
December	1st to 10th	Night	40% 4 12% 30% 13% 1	4580
	11th to 20th	Night	47% 4 11% 18% 17% 2	5208
	21st to end	Night	46% 4 10% 19% 16% 5%	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	27% 29% 26% 17% 2	200
	11th to 20th	Day	20% 36% 30% 14% 1 1	200
	21st to end	Day	21% 30% 33% 10% 5% 1	251
February	1st to 10th	Day	27% 25% 35% 12% 1	361
	11th to 20th	Day	24% 27% 35% 12% 1	496
	21st to end	Day	26% 26% 32% 13% 3	414
March	1st to 10th	Day	23% 24% 36% 14% 3	500
	11th to 20th	Day	20% 27% 35% 14% 3 1	500
	21st to end	Day	26% 23% 36% 11% 3 1	550
April	1st to 10th	Day	26% 26% 30% 12% 6% 1	500
	11th to 20th	Day	27% 22% 32% 14% 5% 1	492
	21st to end	Day	27% 18% 34% 15% 5% 1	500
May	1st to 10th	Day	29% 19% 32% 13% 5% 1	503
	11th to 20th	Day	21% 17% 36% 20% 6% 1	527
	21st to end	Day	20% 11% 30% 27% 10% 2	573
June	1st to 10th	Day	18% 12% 31% 25% 11% 2	530
	11th to 20th	Day	17% 12% 32% 27% 11% 1	530
	21st to end	Day	14% 9% 36% 27% 12% 2	530
July	1st to 10th	Day	16% 6% 30% 29% 16% 3	525
	11th to 20th	Day	13% 6% 29% 31% 18% 3	524
	21st to end	Day	14% 7% 33% 35% 10% 2	576
August	1st to 10th	Day	14% 7% 27% 35% 12% 5%	526
	11th to 20th	Day	15% 5% 25% 34% 16% 5%	519
	21st to end	Day	10% 11% 32% 26% 15% 6%	583
September	1st to 10th	Day	16% 10% 29% 23% 18% 4	530
	11th to 20th	Day	15% 7% 29% 28% 12% 8%	522
	21st to end	Day	21% 11% 31% 23% 10% 3	490
October	1st to 10th	Day	25% 10% 26% 21% 13% 4	490
	11th to 20th	Day	15% 20% 30% 24% 10% 2	485
	21st to end	Day	24% 16% 31% 20% 7% 1	429
November	1st to 10th	Day	16% 16% 31% 23% 11% 2	395
	11th to 20th	Day	22% 24% 29% 20% 5%	400
	21st to end	Day	22% 22% 30% 20% 5%	365
December	1st to 10th	Day	26% 19% 34% 18% 5%	200
	11th to 20th	Day	31% 26% 28% 14% 3	200
	21st to end	Day	20% 22% 35% 17% 5% 1	220
January	1st to 10th	Night	22% 22% 36% 18% 3	800
	11th to 20th	Night	25% 20% 33% 20% 3	800
	21st to end	Night	21% 19% 39% 18% 2	880
February	1st to 10th	Night	22% 19% 36% 21% 2	800
	11th to 20th	Night	25% 21% 37% 15% 2	783
	21st to end	Night	23% 21% 36% 17% 3	660
March	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	21% 17% 38% 20% 3 1	800
	21st to end	Night	22% 19% 38% 17% 4	880
April	1st to 10th	Night	24% 18% 35% 20% 3	800
	11th to 20th	Night	23% 16% 36% 22% 4	791
	21st to end	Night	21% 16% 36% 21% 5% 1	699
May	1st to 10th	Night	27% 11% 34% 22% 6%	600
	11th to 20th	Night	23% 12% 37% 23% 5%	486
	21st to end	Night	22% 6% 24% 37% 9% 1	433
June	1st to 10th	Night	19% 7% 30% 34% 10% 1	400
	11th to 20th	Night	18% 7% 29% 36% 12%	400
	21st to end	Night	17% 6% 25% 43% 10%	400
July	1st to 10th	Night	15% 4 23% 40% 17% 1	390
	11th to 20th	Night	15% 3 20% 44% 16% 2	388
	21st to end	Night	14% 4 23% 44% 13% 1	438
August	1st to 10th	Night	12% 3 28% 40% 16% 1	471
	11th to 20th	Night	15% 4 21% 38% 19% 2	785
	21st to end	Night	14% 6% 24% 36% 18% 2	880
September	1st to 10th	Night	17% 6% 27% 33% 16% 2	800
	11th to 20th	Night	17% 6% 23% 32% 18% 4	793
	21st to end	Night	21% 7% 27% 30% 15% 1	760
October	1st to 10th	Night	22% 10% 29% 25% 13% 2	760
	11th to 20th	Night	17% 13% 32% 28% 10% 1	760
	21st to end	Night	26% 12% 34% 21% 7%	836
November	1st to 10th	Night	16% 13% 34% 26% 10% 1	780
	11th to 20th	Night	22% 16% 35% 19% 6% 1	800
	21st to end	Night	21% 18% 32% 23% 5% 1	800
December	1st to 10th	Night	26% 16% 32% 20% 5% 1	800
	11th to 20th	Night	25% 19% 33% 20% 3	800
	21st to end	Night	25% 18% 34% 20% 2	880

Page generated at 2025-09-16 01:49:11. Version ac07

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