Joyce Observatory 2
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joyce Observatory 2 clear sky chart from 2005-11-06 to 2025-10-04.

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	6% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	126146
	Night	7% 5% 2 7% 5% 5% 5% 5% 7% 10% 42%	123251
Transparency	Day	22% 39% 13% 15% 8% 3	129891
	Night	24% 39% 11% 14% 9% 3	127576
Seeing	Day	7% 3 15% 26% 26% 21%	16840
	Night	8% 3 16% 34% 29% 10%	26879

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	8% 6% 2 5% 4 4 4 3 6% 9% 49%	4056
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 5% 8% 32%	4060
	21st to end	Day	11% 5% 2 6% 4 4 4 3 5% 9% 46%	4879
February	1st to 10th	Day	8% 4 2 5% 4 5% 3 4 7% 8% 50%	2000
	11th to 20th	Day	5% 3 2 4 3 3 4 4 7% 8% 59%	1948
	21st to end	Day	7% 3 1 4 3 3 4 4 6% 10% 55%	1750
March	1st to 10th	Day	6% 4 2 6% 4 5% 5% 4 6% 7% 51%	4935
	11th to 20th	Day	3 2 1 4 3 3 3 3 5% 8% 64%	4980
	21st to end	Day	6% 4 1 5% 4 4 4 3 6% 8% 55%	5465
April	1st to 10th	Day	5% 2 1 4 3 3 4 3 8% 10% 57%	2200
	11th to 20th	Day	6% 3 2 5% 3 4 4 4 7% 11% 52%	2176
	21st to end	Day	4 3 1 4 3 4 5% 4 8% 9% 56%	2363
May	1st to 10th	Day	1 1 1 3 2 3 4 4 6% 9% 66%	5716
	11th to 20th	Day	6% 3 1 5% 4 4 4 3 6% 10% 54%	5861
	21st to end	Day	3 3 1 4 4 4 5% 4 7% 11% 55%	6433
June	1st to 10th	Day	10% 8% 3 8% 6% 6% 6% 6% 8% 10% 29%	2600
	11th to 20th	Day	7% 5% 2 6% 6% 7% 7% 7% 9% 13% 31%	2600
	21st to end	Day	6% 6% 2 8% 6% 7% 7% 6% 9% 12% 29%	2600
July	1st to 10th	Day	4 5% 3 9% 6% 7% 8% 6% 10% 13% 28%	5900
	11th to 20th	Day	4 4 2 7% 6% 8% 9% 8% 11% 14% 27%	5900
	21st to end	Day	3 4 2 6% 6% 6% 7% 7% 11% 14% 34%	6424
August	1st to 10th	Day	5% 4 3 9% 8% 7% 8% 6% 9% 13% 28%	4203
	11th to 20th	Day	3 3 2 7% 5% 6% 7% 6% 9% 15% 37%	3753
	21st to end	Day	5% 4 2 7% 7% 8% 8% 7% 11% 14% 26%	3949
September	1st to 10th	Day	6% 5% 3 7% 8% 7% 9% 7% 10% 13% 26%	2200
	11th to 20th	Day	9% 6% 2 8% 6% 7% 8% 6% 9% 10% 29%	2200
	21st to end	Day	6% 5% 3 8% 7% 8% 7% 5% 9% 11% 32%	2200
October	1st to 10th	Day	8% 5% 2 8% 6% 6% 7% 5% 6% 9% 37%	2123
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 4 6% 10% 46%	1929
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 50%	2090
November	1st to 10th	Day	6% 4 2 4 4 4 4 3 7% 8% 53%	1957
	11th to 20th	Day	7% 5% 3 6% 5% 4 5% 4 6% 9% 48%	2000
	21st to end	Day	6% 5% 2 5% 5% 4 4 4 7% 9% 50%	1927
December	1st to 10th	Day	3 3 1 3 2 3 3 4 6% 7% 63%	2930
	11th to 20th	Day	8% 6% 2 5% 4 5% 4 4 7% 9% 47%	3373
	21st to end	Day	9% 6% 3 8% 6% 6% 6% 5% 7% 8% 36%	4466
January	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 8% 10% 45%	5889
	11th to 20th	Night	17% 9% 4 9% 5% 5% 5% 4 5% 7% 29%	5900
	21st to end	Night	13% 5% 2 7% 4 4 4 4 7% 8% 42%	6060
February	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 6% 8% 47%	2400
	11th to 20th	Night	8% 4 2 5% 3 4 4 3 6% 8% 53%	2338
	21st to end	Night	6% 4 1 3 4 4 4 3 6% 8% 55%	1877
March	1st to 10th	Night	5% 5% 2 6% 4 4 4 4 6% 9% 52%	4918
	11th to 20th	Night	5% 2 1 3 4 3 4 4 7% 11% 56%	4980
	21st to end	Night	7% 5% 2 6% 3 4 4 4 6% 7% 51%	5467
April	1st to 10th	Night	4 3 2 4 4 4 3 5% 7% 9% 56%	2200
	11th to 20th	Night	4 4 1 5% 4 5% 5% 4 6% 9% 54%	2173
	21st to end	Night	4 3 2 4 5% 5% 5% 4 6% 8% 55%	2035
May	1st to 10th	Night	3 3 2 6% 4 5% 5% 4 7% 9% 52%	4219
	11th to 20th	Night	6% 3 2 5% 4 4 4 3 6% 8% 56%	4032
	21st to end	Night	4 3 2 5% 4 4 4 4 6% 10% 55%	4421
June	1st to 10th	Night	10% 5% 3 7% 7% 6% 5% 4 8% 10% 35%	1800
	11th to 20th	Night	5% 7% 2 7% 7% 6% 6% 6% 8% 12% 36%	1800
	21st to end	Night	5% 5% 3 8% 6% 8% 6% 7% 9% 13% 30%	1800
July	1st to 10th	Night	4 6% 3 10% 8% 8% 8% 7% 10% 11% 24%	4058
	11th to 20th	Night	5% 7% 3 9% 8% 8% 9% 7% 11% 13% 22%	4057
	21st to end	Night	5% 6% 3 9% 6% 7% 8% 6% 10% 12% 27%	4427
August	1st to 10th	Night	6% 7% 3 9% 6% 6% 6% 6% 8% 12% 30%	2939
	11th to 20th	Night	4 3 2 7% 6% 6% 7% 6% 10% 15% 33%	3261
	21st to end	Night	7% 5% 2 8% 6% 7% 8% 7% 10% 13% 25%	3949
September	1st to 10th	Night	6% 6% 3 9% 6% 7% 7% 6% 9% 13% 27%	2200
	11th to 20th	Night	9% 7% 3 8% 6% 6% 6% 5% 9% 10% 31%	2200
	21st to end	Night	9% 7% 3 9% 6% 7% 7% 5% 9% 9% 30%	2200
October	1st to 10th	Night	8% 6% 2 8% 4 4 6% 6% 8% 9% 38%	2124
	11th to 20th	Night	7% 6% 2 5% 5% 5% 5% 4 7% 9% 45%	2248
	21st to end	Night	9% 4 2 6% 5% 4 4 4 7% 8% 47%	2508
November	1st to 10th	Night	7% 6% 2 5% 4 5% 5% 4 6% 8% 48%	2341
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 4 6% 8% 49%	2400
	21st to end	Night	7% 5% 2 5% 4 4 4 4 7% 10% 49%	2468
December	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 8% 63%	4250
	11th to 20th	Night	9% 6% 2 5% 3 4 3 4 5% 8% 51%	4822
	21st to end	Night	10% 8% 2 8% 5% 5% 6% 5% 7% 10% 33%	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	24% 27% 6% 16% 20% 7%	4057
	11th to 20th	Day	42% 27% 7% 13% 10% 1	4060
	21st to end	Day	27% 29% 4 24% 15% 1	4879
February	1st to 10th	Day	11% 62% 7% 10% 7% 3	2000
	11th to 20th	Day	8% 60% 3 12% 13% 5%	1958
	21st to end	Day	13% 62% 5% 9% 8% 3	1750
March	1st to 10th	Day	27% 30% 10% 13% 15% 4	4935
	11th to 20th	Day	13% 29% 11% 15% 16% 16%	4980
	21st to end	Day	22% 29% 7% 22% 14% 6%	5465
April	1st to 10th	Day	8% 62% 7% 13% 8% 3	2200
	11th to 20th	Day	9% 62% 6% 7% 9% 6%	2176
	21st to end	Day	10% 62% 4 10% 13% 1	2363
May	1st to 10th	Day	11% 30% 20% 26% 10% 2	5749
	11th to 20th	Day	18% 29% 14% 28% 10% 1	5865
	21st to end	Day	14% 30% 18% 28% 9%	6447
June	1st to 10th	Day	20% 62% 10% 7% 1	2600
	11th to 20th	Day	20% 64% 10% 6%	2600
	21st to end	Day	17% 63% 14% 6%	2600
July	1st to 10th	Day	34% 31% 29% 6%	5835
	11th to 20th	Day	32% 31% 29% 8%	5831
	21st to end	Day	26% 31% 25% 17% 1	6240
August	1st to 10th	Day	31% 39% 22% 6% 2	4190
	11th to 20th	Day	19% 40% 23% 14% 4	3789
	21st to end	Day	26% 40% 25% 7% 1	3949
September	1st to 10th	Day	20% 61% 14% 6%	2200
	11th to 20th	Day	18% 59% 13% 8% 2	2200
	21st to end	Day	18% 59% 10% 8% 4 1	2200
October	1st to 10th	Day	40% 38% 11% 7% 3	3513
	11th to 20th	Day	17% 38% 6% 18% 19% 2	3182
	21st to end	Day	17% 36% 4 21% 17% 4	3428
November	1st to 10th	Day	12% 56% 8% 14% 7% 3	1957
	11th to 20th	Day	18% 57% 5% 9% 7% 4	2000
	21st to end	Day	15% 57% 4 10% 8% 5%	1927
December	1st to 10th	Day	10% 35% 11% 20% 15% 8%	2930
	11th to 20th	Day	23% 32% 7% 15% 15% 8%	3370
	21st to end	Day	36% 28% 11% 17% 8%	4466
January	1st to 10th	Night	25% 27% 6% 17% 19% 5%	5889
	11th to 20th	Night	46% 27% 8% 10% 7% 1	5900
	21st to end	Night	30% 29% 4 21% 14% 3	6060
February	1st to 10th	Night	13% 62% 6% 11% 7% 2	2400
	11th to 20th	Night	10% 60% 3 10% 11% 6%	2339
	21st to end	Night	12% 62% 5% 8% 11% 3	1877
March	1st to 10th	Night	25% 31% 10% 15% 13% 6%	4918
	11th to 20th	Night	15% 29% 11% 16% 14% 15%	4980
	21st to end	Night	25% 30% 6% 20% 15% 4	5467
April	1st to 10th	Night	8% 61% 6% 15% 7% 2	2200
	11th to 20th	Night	8% 63% 6% 9% 10% 4	2173
	21st to end	Night	10% 62% 5% 12% 10% 1	2035
May	1st to 10th	Night	23% 31% 16% 20% 7% 2	4249
	11th to 20th	Night	18% 29% 13% 31% 9%	4033
	21st to end	Night	17% 30% 14% 27% 12% 1	4429
June	1st to 10th	Night	17% 62% 12% 9%	1800
	11th to 20th	Night	18% 65% 11% 6%	1800
	21st to end	Night	16% 64% 13% 7% 1	1800
July	1st to 10th	Night	40% 31% 24% 4	4015
	11th to 20th	Night	36% 34% 24% 5%	4006
	21st to end	Night	37% 30% 20% 12%	4297
August	1st to 10th	Night	30% 40% 20% 5% 5%	2928
	11th to 20th	Night	23% 40% 23% 12% 2	3291
	21st to end	Night	27% 43% 26% 4	3949
September	1st to 10th	Night	19% 62% 13% 6%	2200
	11th to 20th	Night	18% 59% 12% 9% 2	2200
	21st to end	Night	20% 60% 9% 8% 4	2200
October	1st to 10th	Night	42% 39% 10% 6% 3	3514
	11th to 20th	Night	21% 38% 7% 17% 16% 1	3734
	21st to end	Night	21% 36% 5% 20% 14% 3	4120
November	1st to 10th	Night	13% 57% 7% 13% 7% 2	2341
	11th to 20th	Night	16% 57% 5% 9% 9% 3	2400
	21st to end	Night	14% 57% 5% 10% 10% 4	2468
December	1st to 10th	Night	11% 35% 10% 23% 9% 10%	4250
	11th to 20th	Night	21% 33% 8% 15% 17% 7%	4824
	21st to end	Night	37% 29% 14% 12% 7% 1	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	8% 8% 40% 29% 13% 1	300
	11th to 20th	Day	10% 10% 35% 30% 10% 4	300
	21st to end	Day	14% 8% 43% 29% 5% 1	330
February	1st to 10th	Day	5% 7% 30% 33% 18% 7%	300
	11th to 20th	Day	4 6% 32% 38% 16% 4	300
	21st to end	Day	5% 5% 26% 39% 21% 3	349
March	1st to 10th	Day	4 6% 35% 36% 17% 1	500
	11th to 20th	Day	6% 6% 27% 35% 23% 4	500
	21st to end	Day	4 5% 29% 36% 23% 4	550
April	1st to 10th	Day	6% 4 29% 34% 23% 4	500
	11th to 20th	Day	7% 5% 24% 37% 22% 5%	492
	21st to end	Day	6% 3 17% 35% 33% 6%	500
May	1st to 10th	Day	4 2 16% 38% 27% 14%	503
	11th to 20th	Day	10% 4 16% 30% 23% 18%	527
	21st to end	Day	7% 2 8% 23% 34% 26%	573
June	1st to 10th	Day	12% 3 8% 20% 30% 27%	530
	11th to 20th	Day	8% 2 5% 19% 29% 37%	530
	21st to end	Day	5% 2 2 13% 29% 49%	530
July	1st to 10th	Day	6% 1 1 11% 25% 55%	525
	11th to 20th	Day	4 2 1 9% 31% 52%	524
	21st to end	Day	8% 2 1 14% 26% 49%	576
August	1st to 10th	Day	7% 2 2 13% 30% 47%	526
	11th to 20th	Day	4 2 1 9% 32% 53%	519
	21st to end	Day	9% 2 4 17% 33% 35%	583
September	1st to 10th	Day	7% 3 4 11% 35% 40%	530
	11th to 20th	Day	8% 3 4 18% 33% 34%	530
	21st to end	Day	7% 2 4 21% 34% 32%	530
October	1st to 10th	Day	11% 3 7% 33% 33% 13%	502
	11th to 20th	Day	7% 2 14% 33% 33% 11%	490
	21st to end	Day	10% 3 15% 34% 27% 11%	539
November	1st to 10th	Day	7% 3 18% 33% 29% 10%	495
	11th to 20th	Day	10% 4 20% 34% 26% 6%	500
	21st to end	Day	8% 3 23% 35% 26% 6%	427
December	1st to 10th	Day	6% 6% 30% 33% 17% 8%	300
	11th to 20th	Day	9% 5% 29% 32% 19% 6%	300
	21st to end	Day	9% 5% 26% 36% 21% 3	330
January	1st to 10th	Night	9% 6% 26% 41% 17% 1	800
	11th to 20th	Night	11% 5% 26% 42% 15% 1	800
	21st to end	Night	14% 6% 30% 37% 14%	880
February	1st to 10th	Night	11% 4 28% 38% 19% 1	800
	11th to 20th	Night	9% 5% 24% 43% 18% 1	783
	21st to end	Night	8% 4 21% 48% 19%	660
March	1st to 10th	Night	4 4 33% 44% 15%	800
	11th to 20th	Night	7% 4 24% 46% 18% 1	800
	21st to end	Night	5% 3 27% 47% 18% 1	880
April	1st to 10th	Night	6% 3 23% 47% 20% 1	800
	11th to 20th	Night	5% 4 26% 45% 18% 1	791
	21st to end	Night	5% 2 23% 46% 23% 1	800
May	1st to 10th	Night	3 2 14% 49% 29% 3	685
	11th to 20th	Night	7% 2 10% 45% 32% 4	591
	21st to end	Night	5% 3 6% 37% 41% 8%	650
June	1st to 10th	Night	11% 2 3 27% 44% 13%	600
	11th to 20th	Night	6% 2 5% 14% 50% 24%	600
	21st to end	Night	5% 2 2 13% 45% 34%	600
July	1st to 10th	Night	8% 2 2 5% 44% 39%	585
	11th to 20th	Night	6% 3 1 7% 43% 41%	582
	21st to end	Night	4 1 1 8% 48% 38%	657
August	1st to 10th	Night	8% 2 2 6% 49% 33%	597
	11th to 20th	Night	6% 2 1 7% 42% 41%	629
	21st to end	Night	11% 3 4 16% 40% 26%	880
September	1st to 10th	Night	9% 2 4 15% 46% 26%	800
	11th to 20th	Night	12% 2 3 20% 41% 21%	800
	21st to end	Night	12% 3 5% 25% 38% 17%	800
October	1st to 10th	Night	13% 3 10% 39% 29% 7%	773
	11th to 20th	Night	9% 3 14% 36% 33% 5%	760
	21st to end	Night	12% 2 16% 41% 26% 4	836
November	1st to 10th	Night	9% 2 20% 42% 24% 2	780
	11th to 20th	Night	8% 4 22% 42% 22% 3	800
	21st to end	Night	9% 4 19% 43% 24% 1	800
December	1st to 10th	Night	6% 4 23% 40% 26% 2	800
	11th to 20th	Night	8% 5% 22% 40% 23% 2	800
	21st to end	Night	9% 4 23% 40% 23% 1	880

Page generated at 2025-10-01 01:39:08. Version ac07

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