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 2025-08-05.

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%	125692
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	122380
Transparency	Day	43% 5% 11% 22% 15% 3	129207
	Night	43% 5% 11% 22% 16% 3	126952
Seeing	Day	25% 17% 30% 18% 8% 2	15912
	Night	23% 15% 30% 24% 7% 1	24940

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% 2 7% 5% 4 5% 4 5% 6% 44%	3597
	11th to 20th	Day	14% 9% 3 8% 5% 4 5% 3 4 6% 39%	3570
	21st to end	Day	18% 10% 4 8% 4 3 4 3 5% 4 38%	4161
February	1st to 10th	Day	17% 7% 3 7% 5% 4 6% 4 5% 7% 35%	1830
	11th to 20th	Day	15% 7% 3 7% 5% 4 4 4 5% 7% 39%	1948
	21st to end	Day	20% 9% 3 7% 4 4 3 3 5% 5% 38%	1650
March	1st to 10th	Day	23% 8% 2 5% 4 4 3 3 4 5% 39%	4476
	11th to 20th	Day	14% 6% 3 7% 4 5% 4 4 6% 7% 41%	4693
	21st to end	Day	20% 8% 2 6% 5% 4 4 4 6% 6% 37%	5970
April	1st to 10th	Day	18% 10% 2 7% 4 4 4 3 5% 7% 35%	2400
	11th to 20th	Day	18% 6% 2 6% 4 3 5% 4 6% 7% 39%	2374
	21st to end	Day	20% 6% 2 6% 4 4 4 4 4 6% 40%	2480
May	1st to 10th	Day	31% 8% 2 5% 4 4 4 3 6% 7% 26%	5886
	11th to 20th	Day	21% 8% 3 6% 4 4 4 3 5% 6% 35%	6241
	21st to end	Day	21% 7% 2 6% 3 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 5% 2 5% 4 5% 5% 5% 7% 9% 35%	2800
	11th to 20th	Day	16% 8% 2 5% 5% 4 5% 4 7% 9% 35%	2800
	21st to end	Day	13% 7% 2 6% 6% 4 5% 4 5% 10% 38%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 34%	6360
	21st to end	Day	14% 6% 3 6% 4 4 5% 4 5% 7% 41%	6926
August	1st to 10th	Day	19% 7% 2 7% 5% 4 5% 4 7% 8% 31%	4318
	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% 2 5% 3 3 3 2 5% 6% 51%	1761
	11th to 20th	Day	18% 7% 2 4 3 3 4 3 5% 7% 46%	1800
	21st to end	Day	18% 7% 2 5% 4 4 3 3 6% 6% 43%	1730
December	1st to 10th	Day	17% 7% 3 6% 4 4 3 3 5% 7% 43%	2600
	11th to 20th	Day	22% 7% 2 5% 3 3 3 3 5% 6% 43%	2986
	21st to end	Day	19% 9% 3 7% 5% 4 3 2 4 6% 36%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 5% 4 6% 7% 44%	6349
	11th to 20th	Night	14% 8% 3 8% 4 4 4 4 5% 6% 42%	6315
	21st to end	Night	18% 9% 3 6% 5% 4 4 3 5% 6% 38%	6794
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 39%	2569
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 3 6% 6% 39%	2338
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 6% 40%	1980
March	1st to 10th	Night	20% 9% 2 5% 3 3 4 3 4 6% 40%	5364
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 3 4 6% 42%	5251
	21st to end	Night	19% 10% 3 6% 4 4 3 3 4 5% 39%	4962
April	1st to 10th	Night	18% 9% 2 6% 5% 5% 4 5% 6% 6% 35%	2000
	11th to 20th	Night	18% 7% 2 6% 4 5% 4 4 5% 6% 38%	1975
	21st to end	Night	19% 7% 2 7% 4 3 4 3 5% 5% 41%	1919
May	1st to 10th	Night	31% 9% 3 6% 4 3 3 3 4 7% 26%	4054
	11th to 20th	Night	21% 10% 2 7% 4 4 4 4 5% 6% 34%	3695
	21st to end	Night	22% 8% 2 6% 3 4 3 3 5% 7% 36%	3926
June	1st to 10th	Night	17% 6% 3 7% 5% 4 5% 4 4 6% 41%	1600
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 4 6% 9% 39%	1600
	21st to end	Night	14% 6% 2 5% 4 4 4 4 7% 9% 42%	1600
July	1st to 10th	Night	14% 5% 2 6% 5% 4 5% 4 7% 9% 39%	3600
	11th to 20th	Night	17% 9% 3 6% 5% 4 5% 4 7% 8% 34%	3600
	21st to end	Night	13% 7% 2 5% 4 4 4 4 5% 6% 46%	3926
August	1st to 10th	Night	22% 7% 3 7% 5% 5% 5% 4 6% 7% 31%	2664
	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	12% 6% 2 5% 4 3 4 4 6% 6% 48%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 4 2 5% 6% 45%	2600
	21st to end	Night	14% 7% 1 6% 4 4 3 4 5% 7% 44%	2669
December	1st to 10th	Night	17% 8% 2 4 3 3 3 2 4 5% 47%	4580
	11th to 20th	Night	25% 7% 2 5% 3 3 3 3 5% 4 39%	5208
	21st to end	Night	20% 9% 3 7% 4 3 3 3 4 5% 38%	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	39% 5% 7% 29% 16% 3	3598
	11th to 20th	Day	46% 4 10% 21% 12% 7%	3600
	21st to end	Day	48% 4 8% 22% 16% 1	4161
February	1st to 10th	Day	47% 5% 12% 20% 14% 2	1830
	11th to 20th	Day	43% 4 10% 23% 17% 3	1957
	21st to end	Day	48% 4 10% 22% 14% 3	1650
March	1st to 10th	Day	47% 4 10% 16% 13% 10%	4479
	11th to 20th	Day	39% 5% 14% 29% 11% 3	4693
	21st to end	Day	47% 4 8% 15% 20% 6%	5970
April	1st to 10th	Day	48% 5% 10% 19% 15% 3	2400
	11th to 20th	Day	43% 6% 10% 21% 18% 3	2374
	21st to end	Day	43% 6% 9% 21% 17% 3	2480
May	1st to 10th	Day	56% 6% 14% 13% 9% 1	5897
	11th to 20th	Day	49% 7% 10% 18% 14% 1	6245
	21st to end	Day	46% 7% 10% 23% 13% 1	6949
June	1st to 10th	Day	44% 7% 13% 22% 13% 1	2800
	11th to 20th	Day	45% 7% 13% 23% 11% 1	2800
	21st to end	Day	41% 8% 13% 23% 13% 1	2800
July	1st to 10th	Day	44% 8% 12% 20% 14% 1	6290
	11th to 20th	Day	43% 6% 18% 21% 11% 1	6285
	21st to end	Day	42% 6% 11% 22% 18% 1	6728
August	1st to 10th	Day	49% 7% 14% 21% 9%	4304
	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	35% 5% 12% 25% 20% 3	1761
	11th to 20th	Day	38% 4 10% 23% 21% 4	1800
	21st to end	Day	41% 4 13% 28% 12% 2	1730
December	1st to 10th	Day	41% 4 16% 28% 11% 1	2600
	11th to 20th	Day	43% 3 14% 20% 17% 3	2986
	21st to end	Day	49% 4 7% 21% 15% 4	3960
January	1st to 10th	Night	37% 5% 10% 27% 19% 3	6349
	11th to 20th	Night	42% 4 14% 21% 14% 6%	6360
	21st to end	Night	47% 3 10% 18% 17% 4	6794
February	1st to 10th	Night	46% 4 12% 23% 12% 2	2569
	11th to 20th	Night	44% 5% 10% 19% 18% 4	2340
	21st to end	Night	46% 4 8% 23% 15% 4	1980
March	1st to 10th	Night	45% 4 11% 18% 14% 7%	5374
	11th to 20th	Night	43% 5% 14% 23% 12% 4	5252
	21st to end	Night	48% 4 6% 14% 23% 5%	4962
April	1st to 10th	Night	47% 5% 10% 20% 15% 3	2000
	11th to 20th	Night	45% 6% 11% 19% 19% 1	1975
	21st to end	Night	45% 5% 8% 22% 17% 4	1919
May	1st to 10th	Night	59% 5% 13% 12% 10% 1	4060
	11th to 20th	Night	50% 8% 11% 16% 12% 3	3696
	21st to end	Night	48% 6% 13% 22% 10% 1	3927
June	1st to 10th	Night	45% 5% 9% 22% 17% 2	1600
	11th to 20th	Night	41% 7% 15% 25% 10% 1	1600
	21st to end	Night	37% 9% 14% 25% 15% 1	1600
July	1st to 10th	Night	43% 7% 14% 21% 14% 1	3560
	11th to 20th	Night	48% 6% 15% 19% 10% 1	3552
	21st to end	Night	38% 6% 13% 24% 18% 1	3809
August	1st to 10th	Night	52% 7% 13% 17% 9% 1	2654
	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	2537
	11th to 20th	Night	40% 4 9% 23% 20% 4	2600
	21st to end	Night	40% 4 13% 29% 12% 1	2669
December	1st to 10th	Night	40% 4 12% 32% 12% 1	4580
	11th to 20th	Night	47% 4 12% 18% 16% 2	5208
	21st to end	Night	49% 3 9% 18% 17% 4	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	37% 24% 25% 13% 2	200
	11th to 20th	Day	34% 29% 25% 11% 1	200
	21st to end	Day	28% 29% 30% 8% 3 1	263
February	1st to 10th	Day	35% 30% 28% 7% 1	330
	11th to 20th	Day	33% 25% 31% 9% 1	496
	21st to end	Day	34% 22% 32% 10% 2	414
March	1st to 10th	Day	32% 20% 34% 11% 3	500
	11th to 20th	Day	29% 26% 32% 11% 2 1	500
	21st to end	Day	33% 23% 30% 9% 3 1	550
April	1st to 10th	Day	35% 25% 24% 10% 5% 1	500
	11th to 20th	Day	32% 23% 26% 13% 5% 1	492
	21st to end	Day	31% 19% 33% 13% 4 1	500
May	1st to 10th	Day	35% 20% 28% 11% 5% 2	503
	11th to 20th	Day	25% 19% 31% 19% 5% 1	527
	21st to end	Day	23% 12% 30% 24% 10% 2	573
June	1st to 10th	Day	22% 12% 32% 23% 10% 1	530
	11th to 20th	Day	20% 13% 33% 22% 9% 2	530
	21st to end	Day	17% 13% 32% 26% 11% 2	530
July	1st to 10th	Day	17% 8% 32% 27% 13% 3	525
	11th to 20th	Day	13% 7% 34% 29% 15% 2	524
	21st to end	Day	15% 10% 36% 27% 11% 2	576
August	1st to 10th	Day	15% 10% 29% 29% 11% 5%	502
	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% 18% 26% 23% 9% 2	395
	11th to 20th	Day	30% 23% 27% 16% 4 1	400
	21st to end	Day	27% 23% 28% 17% 5%	330
December	1st to 10th	Day	32% 22% 29% 13% 5%	200
	11th to 20th	Day	41% 22% 26% 10% 2	200
	21st to end	Day	30% 25% 27% 12% 6%	220
January	1st to 10th	Night	25% 20% 37% 15% 3	800
	11th to 20th	Night	28% 21% 30% 19% 2	800
	21st to end	Night	23% 23% 36% 16% 3	880
February	1st to 10th	Night	26% 20% 35% 18% 1	800
	11th to 20th	Night	27% 23% 36% 13% 1	783
	21st to end	Night	28% 23% 30% 17% 2	660
March	1st to 10th	Night	26% 22% 33% 16% 3	800
	11th to 20th	Night	23% 24% 33% 18% 3	800
	21st to end	Night	25% 22% 33% 16% 4	880
April	1st to 10th	Night	27% 21% 32% 17% 2	800
	11th to 20th	Night	24% 20% 33% 19% 4	791
	21st to end	Night	23% 22% 32% 18% 4 1	719
May	1st to 10th	Night	29% 16% 31% 20% 5%	600
	11th to 20th	Night	21% 16% 36% 23% 4	449
	21st to end	Night	20% 8% 30% 32% 9% 1	433
June	1st to 10th	Night	19% 8% 31% 31% 11%	400
	11th to 20th	Night	19% 7% 30% 33% 12%	400
	21st to end	Night	16% 4 29% 41% 11%	400
July	1st to 10th	Night	16% 4 20% 39% 21%	390
	11th to 20th	Night	15% 5% 23% 40% 15% 1	388
	21st to end	Night	12% 5% 27% 39% 16%	438
August	1st to 10th	Night	13% 4 25% 41% 15% 1	447
	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% 32% 26% 8% 1	780
	11th to 20th	Night	24% 19% 31% 19% 7% 1	800
	21st to end	Night	23% 18% 31% 22% 5% 1	800
December	1st to 10th	Night	29% 18% 31% 18% 5% 1	800
	11th to 20th	Night	27% 20% 30% 21% 3	800
	21st to end	Night	29% 18% 33% 18% 2	880

Page generated at 2025-08-02 01:32:28. Version ac07

Copyright © 2025 Attilla Danko.