Joey's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joey's Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	7% 5% 2 7% 6% 6% 6% 5% 7% 10% 38%	131380
	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 44%	130473
Transparency	Day	38% 9% 15% 19% 13% 5%	135088
	Night	36% 9% 16% 21% 13% 6%	134835
Seeing	Day	8% 5% 18% 26% 24% 19%	17506
	Night	9% 4 19% 34% 26% 7%	27645

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	7% 5% 2 5% 4 4 4 4 6% 8% 53%	5258
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 4 5% 7% 33%	5280
	21st to end	Day	13% 6% 2 5% 3 4 4 3 6% 8% 46%	5917
February	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 45%	2100
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 6% 7% 51%	2048
	21st to end	Day	8% 7% 2 5% 4 4 5% 4 5% 7% 48%	1872
March	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 5% 9% 44%	5045
	11th to 20th	Day	4 3 1 5% 3 3 4 3 5% 9% 60%	5090
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 6% 8% 47%	5586
April	1st to 10th	Day	6% 3 2 6% 6% 6% 5% 4 7% 8% 48%	2211
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	2229
	21st to end	Day	5% 3 2 6% 5% 4 5% 4 7% 9% 49%	2400
May	1st to 10th	Day	3 3 1 5% 6% 5% 6% 5% 8% 11% 47%	5669
	11th to 20th	Day	8% 5% 2 7% 5% 4 5% 4 7% 9% 44%	5861
	21st to end	Day	4 4 2 6% 5% 6% 6% 5% 8% 10% 44%	6433
June	1st to 10th	Day	13% 7% 3 9% 7% 7% 8% 6% 7% 9% 24%	2600
	11th to 20th	Day	6% 6% 3 11% 9% 8% 8% 6% 8% 9% 26%	2600
	21st to end	Day	8% 7% 4 10% 8% 8% 8% 7% 7% 8% 26%	2600
July	1st to 10th	Day	6% 6% 3 10% 8% 8% 9% 8% 10% 14% 18%	5900
	11th to 20th	Day	5% 5% 3 8% 7% 7% 8% 6% 10% 14% 28%	5900
	21st to end	Day	4 5% 2 8% 8% 8% 8% 6% 10% 13% 29%	6424
August	1st to 10th	Day	4 7% 3 10% 7% 8% 8% 6% 9% 12% 25%	4234
	11th to 20th	Day	4 5% 2 8% 7% 7% 8% 6% 10% 13% 31%	3988
	21st to end	Day	7% 6% 3 10% 9% 7% 8% 6% 9% 11% 22%	3949
September	1st to 10th	Day	8% 7% 3 11% 8% 9% 9% 6% 8% 10% 21%	2200
	11th to 20th	Day	9% 7% 4 11% 8% 10% 9% 5% 8% 8% 22%	2200
	21st to end	Day	9% 7% 4 10% 7% 7% 8% 7% 7% 10% 26%	2200
October	1st to 10th	Day	13% 5% 3 9% 7% 7% 6% 5% 7% 7% 30%	2160
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 4 6% 9% 46%	2000
	21st to end	Day	6% 4 2 6% 6% 4 6% 5% 6% 9% 47%	2200
November	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 7% 8% 45%	2057
	11th to 20th	Day	8% 6% 2 6% 5% 5% 4 4 6% 7% 46%	2100
	21st to end	Day	8% 5% 2 7% 6% 5% 5% 5% 6% 9% 43%	2021
December	1st to 10th	Day	5% 4 2 5% 4 4 4 3 5% 8% 56%	3020
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 5% 7% 9% 39%	3463
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 4 6% 7% 38%	4565
January	1st to 10th	Night	7% 5% 2 5% 4 3 3 3 5% 7% 56%	7635
	11th to 20th	Night	16% 9% 4 8% 4 4 5% 4 5% 7% 34%	7680
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 8% 46%	8247
February	1st to 10th	Night	9% 7% 2 5% 4 3 4 3 5% 7% 49%	2520
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 5% 7% 49%	2458
	21st to end	Night	7% 5% 1 6% 4 4 4 3 5% 8% 54%	1932
March	1st to 10th	Night	10% 7% 3 6% 4 4 4 4 5% 7% 46%	5028
	11th to 20th	Night	5% 3 2 4 3 3 4 3 7% 9% 57%	5090
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	5588
April	1st to 10th	Night	6% 5% 1 4 4 4 5% 4 6% 10% 52%	2212
	11th to 20th	Night	6% 5% 2 6% 4 4 4 3 6% 8% 52%	2117
	21st to end	Night	4 4 1 4 4 4 4 4 6% 7% 57%	2000
May	1st to 10th	Night	4 3 2 7% 5% 5% 5% 4 7% 9% 49%	4267
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 49%	4032
	21st to end	Night	4 4 2 5% 4 5% 5% 4 7% 10% 50%	4421
June	1st to 10th	Night	9% 8% 3 8% 5% 6% 6% 6% 7% 10% 33%	1800
	11th to 20th	Night	6% 8% 3 8% 6% 7% 6% 6% 7% 12% 32%	1800
	21st to end	Night	8% 7% 3 8% 6% 6% 6% 5% 8% 12% 31%	1800
July	1st to 10th	Night	7% 8% 3 11% 7% 7% 8% 6% 9% 12% 22%	4058
	11th to 20th	Night	4 5% 3 8% 7% 7% 7% 7% 10% 13% 30%	4057
	21st to end	Night	4 6% 3 10% 8% 6% 7% 6% 9% 12% 30%	4427
August	1st to 10th	Night	4 6% 3 9% 6% 6% 8% 5% 8% 11% 34%	2915
	11th to 20th	Night	2 5% 3 9% 6% 7% 6% 6% 8% 11% 37%	3020
	21st to end	Night	7% 6% 3 8% 6% 6% 7% 6% 10% 12% 30%	3949
September	1st to 10th	Night	8% 7% 3 9% 7% 6% 6% 5% 10% 10% 29%	2200
	11th to 20th	Night	11% 9% 3 10% 7% 6% 7% 4 8% 9% 28%	2200
	21st to end	Night	9% 7% 3 8% 6% 5% 6% 5% 7% 9% 34%	2200
October	1st to 10th	Night	10% 6% 3 7% 6% 5% 4 5% 7% 8% 40%	2240
	11th to 20th	Night	6% 5% 1 6% 5% 4 5% 5% 6% 8% 50%	2400
	21st to end	Night	7% 5% 2 7% 4 3 4 3 5% 7% 51%	2640
November	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 4 5% 8% 49%	2461
	11th to 20th	Night	8% 7% 2 5% 5% 5% 4 4 6% 7% 48%	2520
	21st to end	Night	8% 6% 2 6% 4 4 5% 3 7% 9% 45%	2594
December	1st to 10th	Night	5% 3 1 4 3 3 4 4 6% 8% 58%	4380
	11th to 20th	Night	9% 7% 2 7% 4 4 4 4 6% 7% 47%	4952
	21st to end	Night	11% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	6633

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	29% 5% 10% 22% 24% 10%	5259
	11th to 20th	Day	51% 4 7% 15% 14% 9%	5280
	21st to end	Day	36% 3 9% 27% 15% 10%	5917
February	1st to 10th	Day	36% 6% 10% 15% 22% 11%	2100
	11th to 20th	Day	31% 6% 10% 21% 23% 10%	2058
	21st to end	Day	35% 5% 10% 17% 23% 10%	1872
March	1st to 10th	Day	36% 7% 12% 18% 20% 8%	5045
	11th to 20th	Day	22% 5% 9% 24% 22% 18%	5090
	21st to end	Day	33% 6% 10% 26% 17% 8%	5586
April	1st to 10th	Day	32% 7% 13% 23% 17% 8%	2211
	11th to 20th	Day	35% 9% 10% 18% 20% 8%	2229
	21st to end	Day	29% 7% 11% 22% 23% 8%	2400
May	1st to 10th	Day	26% 10% 20% 27% 14% 4	5701
	11th to 20th	Day	33% 9% 16% 28% 13% 1	5865
	21st to end	Day	30% 9% 16% 31% 13% 1	6447
June	1st to 10th	Day	52% 12% 16% 15% 5%	2600
	11th to 20th	Day	49% 13% 20% 16% 3	2600
	21st to end	Day	52% 13% 20% 12% 3 1	2600
July	1st to 10th	Day	49% 17% 25% 8% 1	5835
	11th to 20th	Day	41% 14% 33% 11%	5831
	21st to end	Day	40% 16% 23% 20% 1	6240
August	1st to 10th	Day	45% 15% 27% 10% 2	4221
	11th to 20th	Day	39% 16% 23% 15% 7%	4027
	21st to end	Day	50% 15% 22% 11% 2	3949
September	1st to 10th	Day	53% 16% 18% 9% 4	2200
	11th to 20th	Day	52% 14% 18% 12% 3	2200
	21st to end	Day	48% 12% 16% 13% 9% 2	2200
October	1st to 10th	Day	57% 10% 14% 9% 6% 4	3524
	11th to 20th	Day	27% 6% 10% 22% 29% 5%	3240
	21st to end	Day	28% 5% 8% 22% 30% 7%	3538
November	1st to 10th	Day	34% 7% 12% 18% 21% 9%	2057
	11th to 20th	Day	36% 7% 9% 20% 21% 8%	2100
	21st to end	Day	36% 7% 10% 21% 19% 8%	2021
December	1st to 10th	Day	28% 5% 14% 28% 16% 9%	3020
	11th to 20th	Day	39% 6% 12% 16% 19% 7%	3460
	21st to end	Day	43% 7% 11% 20% 15% 4	4565
January	1st to 10th	Night	27% 4 10% 26% 22% 10%	7636
	11th to 20th	Night	49% 5% 9% 16% 13% 8%	7680
	21st to end	Night	35% 6% 12% 23% 14% 10%	8247
February	1st to 10th	Night	34% 6% 12% 17% 20% 12%	2520
	11th to 20th	Night	33% 6% 11% 21% 20% 9%	2459
	21st to end	Night	28% 5% 13% 20% 25% 8%	1932
March	1st to 10th	Night	36% 7% 11% 21% 17% 7%	5028
	11th to 20th	Night	21% 7% 16% 23% 16% 17%	5090
	21st to end	Night	31% 6% 12% 26% 21% 5%	5588
April	1st to 10th	Night	26% 7% 18% 26% 16% 8%	2212
	11th to 20th	Night	29% 8% 13% 22% 21% 7%	2117
	21st to end	Night	24% 7% 16% 24% 23% 7%	2000
May	1st to 10th	Night	27% 10% 18% 29% 13% 4	4297
	11th to 20th	Night	31% 9% 19% 29% 11% 1	4033
	21st to end	Night	27% 10% 20% 31% 13%	4429
June	1st to 10th	Night	43% 13% 20% 20% 5%	1800
	11th to 20th	Night	42% 14% 21% 21% 2	1800
	21st to end	Night	43% 13% 24% 15% 5%	1800
July	1st to 10th	Night	49% 15% 25% 10% 1	4015
	11th to 20th	Night	38% 18% 34% 9%	4006
	21st to end	Night	40% 15% 24% 21% 1	4297
August	1st to 10th	Night	40% 15% 31% 12% 2	2904
	11th to 20th	Night	37% 13% 28% 16% 7%	3047
	21st to end	Night	41% 16% 29% 12% 1	3949
September	1st to 10th	Night	46% 19% 20% 12% 4	2200
	11th to 20th	Night	49% 15% 18% 16% 2	2200
	21st to end	Night	44% 11% 17% 18% 8% 2	2200
October	1st to 10th	Night	55% 10% 12% 13% 6% 3	3656
	11th to 20th	Night	31% 6% 10% 25% 24% 4	3899
	21st to end	Night	30% 5% 9% 20% 30% 6%	4252
November	1st to 10th	Night	32% 7% 13% 20% 19% 9%	2461
	11th to 20th	Night	34% 7% 11% 20% 20% 9%	2520
	21st to end	Night	33% 7% 10% 22% 20% 7%	2594
December	1st to 10th	Night	23% 5% 18% 30% 12% 12%	4380
	11th to 20th	Night	36% 7% 13% 20% 15% 9%	4954
	21st to end	Night	42% 7% 13% 21% 12% 5%	6633

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	12% 9% 38% 28% 11% 2	330
	11th to 20th	Day	6% 14% 37% 27% 12% 4	330
	21st to end	Day	13% 15% 40% 24% 8%	363
February	1st to 10th	Day	12% 12% 29% 25% 20% 3	330
	11th to 20th	Day	8% 8% 36% 31% 12% 5%	330
	21st to end	Day	10% 8% 28% 32% 20% 1	415
March	1st to 10th	Day	5% 10% 31% 36% 17% 1	540
	11th to 20th	Day	8% 8% 30% 29% 21% 3	540
	21st to end	Day	7% 8% 26% 31% 23% 5%	594
April	1st to 10th	Day	7% 9% 28% 34% 18% 5%	504
	11th to 20th	Day	8% 7% 26% 31% 20% 8%	492
	21st to end	Day	6% 3 20% 35% 23% 13%	500
May	1st to 10th	Day	5% 4 18% 32% 29% 12%	503
	11th to 20th	Day	11% 2 15% 29% 28% 16%	527
	21st to end	Day	6% 3 9% 24% 31% 27%	573
June	1st to 10th	Day	15% 3 10% 20% 29% 23%	530
	11th to 20th	Day	7% 2 9% 23% 24% 34%	530
	21st to end	Day	5% 2 5% 17% 28% 43%	530
July	1st to 10th	Day	6% 2 4 11% 27% 50%	525
	11th to 20th	Day	6% 1 4 10% 30% 48%	524
	21st to end	Day	5% 2 4 16% 25% 48%	576
August	1st to 10th	Day	5% 2 7% 14% 29% 42%	526
	11th to 20th	Day	5% 1 3 13% 26% 52%	519
	21st to end	Day	11% 2 7% 17% 26% 38%	583
September	1st to 10th	Day	8% 2 6% 15% 28% 41%	530
	11th to 20th	Day	10% 2 9% 23% 27% 29%	530
	21st to end	Day	10% 2 8% 20% 32% 28%	530
October	1st to 10th	Day	14% 4 16% 32% 24% 11%	530
	11th to 20th	Day	7% 5% 18% 29% 32% 8%	530
	21st to end	Day	6% 3 21% 37% 27% 6%	583
November	1st to 10th	Day	9% 6% 19% 32% 29% 5%	535
	11th to 20th	Day	11% 7% 19% 34% 23% 5%	540
	21st to end	Day	9% 6% 24% 36% 21% 5%	461
December	1st to 10th	Day	6% 9% 36% 30% 15% 4	330
	11th to 20th	Day	11% 5% 31% 35% 18% 2	330
	21st to end	Day	7% 9% 30% 33% 17% 4	363
January	1st to 10th	Night	11% 8% 31% 35% 14%	840
	11th to 20th	Night	11% 7% 29% 40% 12% 1	840
	21st to end	Night	14% 9% 34% 34% 9%	924
February	1st to 10th	Night	12% 8% 30% 37% 13% 1	840
	11th to 20th	Night	12% 6% 31% 39% 10% 1	823
	21st to end	Night	9% 7% 34% 40% 10%	692
March	1st to 10th	Night	5% 9% 34% 39% 13% 1	840
	11th to 20th	Night	8% 7% 28% 41% 15% 1	840
	21st to end	Night	7% 6% 29% 41% 16%	924
April	1st to 10th	Night	6% 7% 30% 42% 14% 1	805
	11th to 20th	Night	7% 6% 27% 43% 16% 1	791
	21st to end	Night	5% 5% 26% 43% 21% 1	800
May	1st to 10th	Night	5% 2 20% 48% 24% 2	707
	11th to 20th	Night	8% 3 13% 42% 30% 4	591
	21st to end	Night	7% 2 9% 37% 39% 7%	650
June	1st to 10th	Night	11% 3 7% 31% 42% 8%	600
	11th to 20th	Night	6% 2 6% 24% 40% 22%	600
	21st to end	Night	7% 3 4 19% 47% 20%	600
July	1st to 10th	Night	8% 2 3 13% 48% 25%	585
	11th to 20th	Night	6% 1 3 13% 45% 32%	582
	21st to end	Night	5% 2 3 13% 51% 27%	657
August	1st to 10th	Night	5% 1 4 14% 50% 27%	597
	11th to 20th	Night	3 2 4 13% 47% 32%	653
	21st to end	Night	12% 2 6% 20% 40% 20%	880
September	1st to 10th	Night	10% 2 6% 21% 41% 21%	800
	11th to 20th	Night	12% 3 10% 24% 39% 11%	800
	21st to end	Night	11% 2 7% 30% 40% 10%	800
October	1st to 10th	Night	12% 2 16% 37% 29% 5%	800
	11th to 20th	Night	7% 3 14% 38% 33% 4	800
	21st to end	Night	10% 2 18% 40% 28% 2	880
November	1st to 10th	Night	11% 4 20% 35% 27% 3	820
	11th to 20th	Night	8% 7% 20% 42% 21% 2	840
	21st to end	Night	9% 5% 27% 42% 17% 1	840
December	1st to 10th	Night	7% 5% 26% 40% 21% 2	840
	11th to 20th	Night	11% 6% 25% 39% 18% 1	840
	21st to end	Night	10% 6% 25% 39% 19% 1	924

Page generated at 2026-03-30 02:18:40. Version ac07

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