John J. McCarthy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John J. McCarthy Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	17% 7% 2 6% 4 4 4 4 6% 7% 38%	101289
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	97734
Transparency	Day	43% 6% 12% 22% 15% 2	104822
	Night	43% 5% 11% 23% 15% 2	102290
Seeing	Day	24% 17% 30% 19% 8% 2	14974
	Night	22% 15% 30% 24% 8% 1	24030

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	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2517
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 3 4 5% 35%	2520
	21st to end	Day	21% 12% 5% 9% 4 3 3 3 4 5% 31%	2903
February	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 5% 7% 35%	1738
	11th to 20th	Day	15% 7% 3 7% 5% 4 4 4 5% 7% 40%	1848
	21st to end	Day	20% 9% 3 7% 5% 4 3 3 5% 5% 36%	1570
March	1st to 10th	Day	27% 10% 3 6% 4 4 3 2 3 4 34%	3116
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 8% 41%	3277
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 37%	4166
April	1st to 10th	Day	18% 10% 2 7% 4 3 4 3 5% 7% 36%	2280
	11th to 20th	Day	18% 6% 2 6% 5% 3 5% 3 5% 7% 39%	2254
	21st to end	Day	20% 6% 2 6% 4 4 4 4 4 6% 40%	2356
May	1st to 10th	Day	27% 8% 3 6% 3 3 4 3 6% 7% 30%	4106
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 4 5% 6% 32%	4349
	21st to end	Day	14% 5% 2 5% 3 4 4 4 6% 8% 44%	4837
June	1st to 10th	Day	18% 5% 2 5% 4 5% 5% 5% 7% 9% 35%	2660
	11th to 20th	Day	16% 8% 2 5% 5% 4 5% 4 7% 9% 36%	2660
	21st to end	Day	13% 6% 2 6% 6% 4 5% 4 5% 10% 37%	2660
July	1st to 10th	Day	12% 6% 3 7% 5% 6% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	13% 5% 2 5% 5% 5% 6% 4 7% 9% 38%	4440
	21st to end	Day	15% 7% 3 7% 6% 5% 6% 4 6% 7% 33%	4814
August	1st to 10th	Day	20% 7% 2 7% 5% 4 5% 4 7% 8% 31%	4262
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 6% 8% 41%	3730
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 46%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 6% 6% 43%	2280
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 5% 6% 50%	2172
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 6% 40%	2090
October	1st to 10th	Day	23% 6% 2 5% 4 3 4 3 4 5% 40%	2023
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 6% 7% 45%	1900
	21st to end	Day	22% 5% 2 5% 4 4 4 4 5% 5% 40%	1891
November	1st to 10th	Day	13% 6% 3 5% 3 3 3 2 5% 6% 50%	1698
	11th to 20th	Day	18% 7% 2 4 3 3 4 3 5% 7% 44%	1710
	21st to end	Day	17% 7% 2 5% 4 4 3 3 6% 6% 43%	1644
December	1st to 10th	Day	20% 8% 3 7% 4 4 3 4 5% 6% 36%	1520
	11th to 20th	Day	22% 6% 2 5% 3 3 4 4 5% 6% 42%	1906
	21st to end	Day	20% 10% 3 8% 6% 5% 4 3 5% 7% 30%	2772
January	1st to 10th	Night	17% 6% 2 5% 3 4 4 3 5% 6% 45%	4429
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 5% 6% 39%	4440
	21st to end	Night	23% 12% 4 7% 4 3 3 3 4 5% 32%	4752
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 39%	2441
	11th to 20th	Night	15% 8% 2 6% 5% 4 4 3 6% 6% 40%	2218
	21st to end	Night	19% 8% 2 7% 4 3 3 3 5% 6% 40%	1884
March	1st to 10th	Night	24% 11% 2 6% 3 3 4 3 4 5% 34%	3724
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 4 5% 6% 44%	3667
	21st to end	Night	18% 10% 3 6% 4 3 3 3 4 6% 40%	3466
April	1st to 10th	Night	18% 8% 2 5% 5% 5% 5% 4 6% 6% 36%	1900
	11th to 20th	Night	18% 7% 2 6% 4 4 4 4 5% 6% 38%	1875
	21st to end	Night	19% 8% 2 7% 4 3 3 3 5% 5% 41%	1823
May	1st to 10th	Night	25% 9% 3 7% 5% 4 4 3 5% 7% 28%	2834
	11th to 20th	Night	22% 9% 3 7% 5% 4 3 4 4 6% 33%	2573
	21st to end	Night	16% 7% 2 5% 3 4 3 3 5% 8% 42%	2738
June	1st to 10th	Night	17% 6% 2 6% 5% 3 5% 4 4 7% 41%	1520
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 6% 9% 40%	1520
	21st to end	Night	14% 6% 2 5% 4 4 4 4 7% 9% 42%	1520
July	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 4 7% 9% 38%	2520
	11th to 20th	Night	15% 7% 3 5% 5% 4 4 4 7% 8% 38%	2520
	21st to end	Night	18% 8% 3 6% 4 5% 4 4 5% 7% 36%	2739
August	1st to 10th	Night	22% 7% 3 7% 5% 5% 5% 4 6% 7% 31%	2632
	11th to 20th	Night	15% 6% 3 7% 5% 4 4 4 5% 7% 41%	3097
	21st to end	Night	12% 5% 1 4 2 3 3 3 5% 7% 54%	3476
September	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 4 7% 46%	1900
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 4 7% 54%	2006
	21st to end	Night	18% 7% 2 4 3 3 5% 3 4 5% 44%	2090
October	1st to 10th	Night	21% 6% 3 5% 3 4 3 3 4 5% 42%	2155
	11th to 20th	Night	15% 5% 2 6% 4 4 3 3 5% 7% 47%	2280
	21st to end	Night	22% 6% 2 5% 3 4 4 3 5% 6% 39%	2705
November	1st to 10th	Night	13% 6% 2 6% 4 3 4 4 6% 6% 48%	2453
	11th to 20th	Night	19% 7% 2 5% 4 3 4 2 5% 6% 44%	2470
	21st to end	Night	14% 7% 1 6% 4 4 3 4 5% 7% 44%	2535
December	1st to 10th	Night	19% 8% 2 5% 4 3 4 3 5% 6% 39%	2660
	11th to 20th	Night	20% 7% 2 4 4 4 4 4 6% 6% 38%	3288
	21st to end	Night	22% 10% 3 7% 5% 4 3 3 5% 6% 33%	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	40% 4 9% 34% 11% 2	2518
	11th to 20th	Day	50% 5% 10% 24% 10% 1	2520
	21st to end	Day	56% 4 8% 20% 11% 2	2903
February	1st to 10th	Day	47% 5% 12% 21% 13% 2	1738
	11th to 20th	Day	42% 4 11% 23% 17% 3	1857
	21st to end	Day	49% 4 10% 22% 13% 3	1570
March	1st to 10th	Day	55% 3 10% 18% 11% 2	3119
	11th to 20th	Day	35% 7% 13% 31% 13% 1	3277
	21st to end	Day	47% 4 6% 17% 20% 6%	4166
April	1st to 10th	Day	47% 5% 10% 20% 15% 3	2280
	11th to 20th	Day	43% 6% 10% 21% 17% 3	2254
	21st to end	Day	44% 6% 10% 21% 16% 4	2356
May	1st to 10th	Day	53% 6% 13% 15% 12% 1	4120
	11th to 20th	Day	50% 8% 12% 19% 10% 1	4353
	21st to end	Day	36% 8% 11% 26% 18% 2	4837
June	1st to 10th	Day	43% 7% 13% 23% 13% 1	2660
	11th to 20th	Day	44% 7% 13% 24% 11% 1	2660
	21st to end	Day	41% 8% 13% 23% 13% 1	2660
July	1st to 10th	Day	43% 8% 12% 20% 15% 1	4370
	11th to 20th	Day	41% 6% 16% 23% 13% 1	4365
	21st to end	Day	49% 7% 12% 17% 14% 1	4661
August	1st to 10th	Day	50% 7% 14% 21% 8%	4248
	11th to 20th	Day	39% 6% 16% 29% 10%	3766
	21st to end	Day	35% 6% 12% 29% 17% 1	4180
September	1st to 10th	Day	40% 8% 9% 23% 19% 1	2280
	11th to 20th	Day	35% 6% 9% 24% 24% 2	2172
	21st to end	Day	44% 6% 9% 20% 17% 3	2090
October	1st to 10th	Day	39% 5% 9% 18% 25% 5%	3361
	11th to 20th	Day	28% 4 9% 25% 29% 5%	3140
	21st to end	Day	32% 3 15% 24% 23% 3	3091
November	1st to 10th	Day	36% 5% 12% 26% 19% 3	1698
	11th to 20th	Day	40% 4 11% 23% 19% 3	1710
	21st to end	Day	41% 4 13% 28% 12% 2	1644
December	1st to 10th	Day	48% 4 14% 18% 14% 1	1520
	11th to 20th	Day	43% 4 15% 20% 16% 2	1906
	21st to end	Day	53% 5% 10% 22% 9% 1	2772
January	1st to 10th	Night	40% 4 11% 31% 11% 4	4429
	11th to 20th	Night	44% 5% 12% 25% 11% 3	4440
	21st to end	Night	55% 4 8% 20% 11% 2	4752
February	1st to 10th	Night	46% 4 12% 23% 12% 2	2441
	11th to 20th	Night	44% 5% 10% 19% 18% 4	2220
	21st to end	Night	45% 4 9% 23% 14% 5%	1884
March	1st to 10th	Night	53% 4 10% 19% 12% 2	3734
	11th to 20th	Night	38% 6% 13% 24% 16% 2	3668
	21st to end	Night	46% 4 6% 14% 24% 6%	3466
April	1st to 10th	Night	46% 5% 10% 20% 16% 2	1900
	11th to 20th	Night	44% 6% 11% 19% 19% 1	1875
	21st to end	Night	45% 5% 8% 22% 16% 4	1823
May	1st to 10th	Night	55% 7% 11% 16% 10% 1	2840
	11th to 20th	Night	52% 9% 11% 16% 10% 1	2574
	21st to end	Night	40% 7% 14% 25% 13% 2	2739
June	1st to 10th	Night	45% 6% 9% 22% 17% 1	1520
	11th to 20th	Night	40% 7% 16% 26% 10% 1	1520
	21st to end	Night	36% 9% 14% 26% 14% 1	1520
July	1st to 10th	Night	42% 8% 16% 19% 14% 1	2480
	11th to 20th	Night	43% 7% 15% 22% 12% 1	2472
	21st to end	Night	47% 7% 13% 15% 16% 2	2651
August	1st to 10th	Night	53% 7% 13% 18% 9% 1	2622
	11th to 20th	Night	43% 6% 13% 29% 8% 1	3127
	21st to end	Night	30% 6% 12% 35% 16% 1	3476
September	1st to 10th	Night	40% 5% 9% 26% 18% 1	1900
	11th to 20th	Night	32% 6% 10% 24% 26% 2	2006
	21st to end	Night	42% 6% 8% 21% 21% 2	2090
October	1st to 10th	Night	40% 5% 11% 22% 20% 2	3597
	11th to 20th	Night	27% 5% 8% 28% 28% 4	3779
	21st to end	Night	33% 4 13% 29% 19% 2	4455
November	1st to 10th	Night	36% 6% 11% 22% 21% 4	2453
	11th to 20th	Night	41% 4 9% 24% 19% 4	2470
	21st to end	Night	40% 4 13% 29% 12% 1	2535
December	1st to 10th	Night	46% 5% 12% 22% 14% 1	2660
	11th to 20th	Night	45% 5% 13% 19% 16% 3	3288
	21st to end	Night	52% 4 11% 21% 11% 1	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	36% 22% 26% 14% 2	180
	11th to 20th	Day	34% 28% 25% 11% 1	180
	21st to end	Day	27% 27% 33% 9% 3 1	236
February	1st to 10th	Day	35% 29% 28% 7% 1	298
	11th to 20th	Day	31% 26% 33% 9% 1	456
	21st to end	Day	34% 22% 31% 10% 3	382
March	1st to 10th	Day	31% 19% 35% 12% 3	460
	11th to 20th	Day	28% 27% 31% 11% 2 1	460
	21st to end	Day	33% 22% 31% 9% 3 1	506
April	1st to 10th	Day	32% 26% 25% 11% 6% 1	460
	11th to 20th	Day	30% 24% 27% 13% 5% 1	452
	21st to end	Day	30% 19% 32% 13% 4 1	460
May	1st to 10th	Day	34% 20% 28% 11% 5% 2	463
	11th to 20th	Day	24% 19% 32% 19% 5% 1	487
	21st to end	Day	20% 12% 30% 26% 10% 2	529
June	1st to 10th	Day	22% 12% 31% 23% 10% 1	490
	11th to 20th	Day	18% 14% 33% 23% 10% 2	490
	21st to end	Day	17% 13% 32% 25% 11% 2	490
July	1st to 10th	Day	17% 8% 32% 26% 14% 3	485
	11th to 20th	Day	13% 6% 34% 30% 14% 2	484
	21st to end	Day	15% 11% 36% 27% 11% 2	532
August	1st to 10th	Day	15% 10% 29% 29% 12% 5%	486
	11th to 20th	Day	16% 8% 29% 29% 14% 5%	479
	21st to end	Day	13% 10% 30% 25% 15% 6%	539
September	1st to 10th	Day	19% 12% 26% 23% 15% 6%	490
	11th to 20th	Day	14% 11% 28% 26% 12% 9%	490
	21st to end	Day	22% 14% 29% 21% 9% 4	490
October	1st to 10th	Day	31% 10% 27% 14% 14% 3	490
	11th to 20th	Day	19% 21% 29% 21% 9% 2	490
	21st to end	Day	31% 13% 28% 19% 8% 1	434
November	1st to 10th	Day	22% 17% 27% 23% 9% 2	374
	11th to 20th	Day	31% 22% 26% 16% 4 1	370
	21st to end	Day	24% 23% 30% 18% 5%	304
December	1st to 10th	Day	31% 22% 31% 13% 4	180
	11th to 20th	Day	41% 22% 26% 10% 1	180
	21st to end	Day	31% 26% 25% 13% 6%	198
January	1st to 10th	Night	24% 19% 38% 16% 3	760
	11th to 20th	Night	27% 21% 31% 18% 2	760
	21st to end	Night	23% 23% 34% 17% 3	836
February	1st to 10th	Night	26% 20% 34% 18% 1	760
	11th to 20th	Night	25% 23% 37% 13% 1	743
	21st to end	Night	29% 23% 29% 17% 2	628
March	1st to 10th	Night	26% 22% 33% 17% 3	760
	11th to 20th	Night	23% 25% 33% 17% 2	760
	21st to end	Night	25% 22% 32% 17% 4	836
April	1st to 10th	Night	26% 21% 32% 18% 2	760
	11th to 20th	Night	23% 20% 33% 19% 4	751
	21st to end	Night	23% 22% 33% 18% 5% 1	683
May	1st to 10th	Night	27% 16% 31% 21% 5%	570
	11th to 20th	Night	22% 16% 36% 22% 4	426
	21st to end	Night	19% 8% 30% 33% 9% 1	411
June	1st to 10th	Night	19% 8% 32% 31% 11%	380
	11th to 20th	Night	18% 7% 29% 33% 12%	380
	21st to end	Night	16% 4 29% 41% 10%	380
July	1st to 10th	Night	16% 5% 21% 38% 21%	370
	11th to 20th	Night	15% 5% 23% 41% 15% 1	368
	21st to end	Night	13% 5% 27% 39% 16%	416
August	1st to 10th	Night	13% 4 26% 41% 15% 1	439
	11th to 20th	Night	15% 4 24% 37% 18% 2	750
	21st to end	Night	15% 7% 21% 36% 19% 2	836
September	1st to 10th	Night	17% 7% 29% 32% 13% 3	760
	11th to 20th	Night	15% 8% 24% 31% 18% 3	760
	21st to end	Night	22% 8% 26% 28% 14% 2	760
October	1st to 10th	Night	24% 11% 26% 26% 13% 1	760
	11th to 20th	Night	18% 15% 31% 27% 9% 1	760
	21st to end	Night	27% 14% 29% 23% 7%	836
November	1st to 10th	Night	18% 15% 31% 26% 8% 1	755
	11th to 20th	Night	24% 18% 31% 19% 7% 1	760
	21st to end	Night	23% 18% 31% 22% 5% 1	760
December	1st to 10th	Night	29% 18% 31% 18% 4 1	760
	11th to 20th	Night	26% 21% 30% 21% 3	760
	21st to end	Night	29% 18% 34% 17% 2	836

Page generated at 2024-11-01 22:47:36. Version ac07

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