The Marian and Max Farash Center for Observational Astronomy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Marian and Max Farash Center for Observational Astronomy clear sky chart from 2005-11-06 to 2025-02-21.

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	15% 12% 4 10% 6% 5% 5% 4 5% 7% 28%	108773
	Night	15% 13% 5% 11% 6% 5% 4 4 5% 6% 27%	109787
Transparency	Day	54% 5% 11% 20% 9% 1	112391
	Night	56% 5% 11% 18% 9% 1	114333
Seeing	Day	23% 18% 30% 18% 8% 3	14858
	Night	23% 17% 29% 22% 9% 1	24308

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	11% 22% 8% 20% 9% 4 4 3 3 3 12%	3597
	11th to 20th	Day	12% 21% 10% 20% 8% 5% 4 3 4 4 9%	3723
	21st to end	Day	15% 17% 8% 15% 9% 8% 6% 3 5% 4 12%	4466
February	1st to 10th	Day	16% 18% 7% 14% 9% 5% 4 4 4 4 15%	1800
	11th to 20th	Day	12% 18% 8% 19% 8% 5% 4 3 3 4 15%	1753
	21st to end	Day	19% 16% 6% 14% 7% 4 4 3 4 5% 18%	1535
March	1st to 10th	Day	18% 16% 6% 10% 5% 4 3 2 4 5% 28%	3328
	11th to 20th	Day	10% 20% 4 10% 6% 4 5% 3 5% 6% 28%	3480
	21st to end	Day	14% 14% 4 13% 5% 4 4 3 5% 6% 27%	3896
April	1st to 10th	Day	17% 16% 4 10% 4 4 4 3 4 7% 28%	2280
	11th to 20th	Day	19% 11% 3 6% 5% 4 4 3 5% 5% 33%	2254
	21st to end	Day	21% 13% 3 7% 4 3 4 3 4 5% 31%	2394
May	1st to 10th	Day	20% 10% 3 7% 4 4 4 4 5% 6% 32%	4233
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 4 6% 8% 29%	4398
	21st to end	Day	12% 6% 2 6% 5% 5% 4 4 6% 8% 42%	4837
June	1st to 10th	Day	17% 8% 3 7% 5% 4 5% 4 6% 8% 34%	2660
	11th to 20th	Day	14% 5% 2 7% 5% 5% 6% 5% 8% 9% 35%	2660
	21st to end	Day	14% 5% 2 7% 5% 5% 5% 5% 7% 8% 38%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 8% 11% 38%	4440
	11th to 20th	Day	8% 5% 3 6% 5% 5% 6% 5% 8% 12% 36%	4440
	21st to end	Day	8% 6% 3 8% 5% 6% 6% 5% 7% 9% 36%	4674
August	1st to 10th	Day	17% 7% 2 7% 5% 4 4 4 6% 7% 36%	4104
	11th to 20th	Day	13% 7% 3 8% 6% 6% 6% 5% 7% 10% 29%	4044
	21st to end	Day	14% 7% 2 7% 4 4 5% 4 5% 9% 38%	4182
September	1st to 10th	Day	15% 8% 2 6% 3 5% 4 4 6% 8% 39%	2280
	11th to 20th	Day	14% 5% 1 6% 4 4 4 4 6% 8% 43%	2280
	21st to end	Day	17% 7% 3 8% 5% 6% 5% 5% 6% 7% 32%	1990
October	1st to 10th	Day	20% 9% 2 6% 4 3 4 4 6% 6% 34%	1900
	11th to 20th	Day	20% 10% 3 9% 6% 5% 4 3 5% 6% 29%	1900
	21st to end	Day	24% 14% 4 7% 5% 4 5% 3 4 5% 24%	2081
November	1st to 10th	Day	14% 15% 3 8% 5% 5% 4 4 5% 5% 32%	1758
	11th to 20th	Day	12% 19% 5% 10% 5% 4 5% 3 5% 6% 25%	1600
	21st to end	Day	18% 18% 5% 10% 6% 4 4 4 5% 6% 20%	1600
December	1st to 10th	Day	17% 20% 8% 14% 7% 5% 4 4 5% 5% 12%	2600
	11th to 20th	Day	18% 20% 7% 14% 5% 4 4 2 3 4 18%	2986
	21st to end	Day	19% 20% 7% 15% 8% 4 4 3 4 5% 12%	3960
January	1st to 10th	Night	14% 19% 8% 22% 8% 5% 4 3 3 3 11%	6349
	11th to 20th	Night	9% 20% 10% 20% 9% 6% 5% 4 5% 4 8%	6160
	21st to end	Night	14% 19% 7% 14% 8% 7% 5% 4 4 5% 14%	6490
February	1st to 10th	Night	15% 17% 6% 15% 7% 5% 4 3 5% 5% 18%	2600
	11th to 20th	Night	12% 15% 7% 15% 9% 6% 6% 4 5% 5% 17%	2533
	21st to end	Night	15% 14% 6% 16% 8% 5% 4 3 4 5% 21%	1919
March	1st to 10th	Night	19% 13% 4 9% 6% 4 4 3 4 5% 30%	3510
	11th to 20th	Night	15% 20% 6% 10% 5% 4 4 3 5% 6% 23%	3479
	21st to end	Night	16% 11% 4 10% 5% 4 4 3 5% 6% 32%	3736
April	1st to 10th	Night	18% 14% 4 7% 5% 4 4 4 5% 7% 28%	1900
	11th to 20th	Night	17% 11% 4 8% 4 4 4 3 5% 6% 33%	1875
	21st to end	Night	18% 9% 3 6% 5% 3 3 4 7% 7% 34%	1801
May	1st to 10th	Night	22% 10% 3 8% 4 5% 5% 3 5% 6% 30%	2705
	11th to 20th	Night	18% 7% 3 6% 4 5% 4 3 6% 8% 36%	2495
	21st to end	Night	11% 6% 2 6% 4 4 4 3 7% 8% 44%	2738
June	1st to 10th	Night	14% 7% 3 7% 5% 4 6% 4 6% 8% 36%	1520
	11th to 20th	Night	11% 6% 3 6% 4 4 4 5% 7% 8% 42%	1520
	21st to end	Night	10% 8% 2 6% 5% 5% 4 5% 6% 8% 40%	1520
July	1st to 10th	Night	10% 6% 3 8% 5% 5% 4 3 6% 9% 41%	2520
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 6% 9% 45%	2520
	21st to end	Night	10% 6% 3 7% 4 4 4 4 6% 8% 44%	2862
August	1st to 10th	Night	14% 7% 3 7% 4 4 4 4 6% 8% 40%	2825
	11th to 20th	Night	15% 6% 2 6% 5% 5% 5% 4 6% 7% 38%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	3441
September	1st to 10th	Night	15% 5% 2 5% 4 3 5% 4 5% 7% 45%	1900
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	1900
	21st to end	Night	16% 8% 2 6% 4 4 5% 4 6% 7% 37%	2186
October	1st to 10th	Night	19% 7% 3 8% 4 4 4 3 6% 6% 35%	2280
	11th to 20th	Night	20% 9% 3 6% 4 5% 5% 4 6% 7% 32%	2280
	21st to end	Night	26% 11% 3 7% 5% 3 4 3 4 6% 27%	2517
November	1st to 10th	Night	15% 11% 3 8% 5% 4 4 4 4 6% 34%	2537
	11th to 20th	Night	15% 12% 5% 9% 6% 4 5% 4 6% 7% 27%	2800
	21st to end	Night	17% 16% 5% 9% 5% 4 4 5% 5% 7% 22%	2800
December	1st to 10th	Night	13% 19% 7% 16% 8% 6% 5% 4 5% 5% 12%	4580
	11th to 20th	Night	18% 21% 7% 12% 5% 4 4 3 4 4 19%	5208
	21st to end	Night	22% 19% 7% 11% 8% 4 4 2 4 5% 13%	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	77% 3 7% 10% 3	3598
	11th to 20th	Day	78% 3 7% 7% 3 1	3754
	21st to end	Day	76% 3 10% 9% 2	4466
February	1st to 10th	Day	73% 4 10% 10% 4	1800
	11th to 20th	Day	73% 4 6% 13% 3	1762
	21st to end	Day	68% 4 9% 13% 6% 1	1535
March	1st to 10th	Day	60% 4 8% 21% 7%	3331
	11th to 20th	Day	57% 4 14% 15% 9% 1	3480
	21st to end	Day	56% 6% 7% 23% 8% 1	3896
April	1st to 10th	Day	57% 4 9% 18% 10% 1	2280
	11th to 20th	Day	52% 4 9% 19% 14% 1	2254
	21st to end	Day	55% 4 7% 17% 14% 2	2394
May	1st to 10th	Day	52% 4 10% 20% 13% 2	4245
	11th to 20th	Day	52% 6% 12% 21% 7% 1	4402
	21st to end	Day	39% 6% 12% 25% 15% 3	4837
June	1st to 10th	Day	47% 5% 11% 20% 17% 1	2660
	11th to 20th	Day	42% 7% 13% 24% 13% 1	2660
	21st to end	Day	42% 7% 12% 23% 14% 1	2660
July	1st to 10th	Day	38% 6% 12% 28% 14% 1	4370
	11th to 20th	Day	37% 7% 15% 26% 15%	4365
	21st to end	Day	42% 7% 15% 23% 12% 1	4525
August	1st to 10th	Day	45% 5% 13% 22% 14% 1	4091
	11th to 20th	Day	47% 7% 14% 25% 6%	4083
	21st to end	Day	42% 5% 9% 33% 9% 1	4182
September	1st to 10th	Day	41% 8% 10% 28% 12% 1	2280
	11th to 20th	Day	38% 6% 12% 27% 16% 1	2280
	21st to end	Day	50% 5% 11% 22% 11% 1	1990
October	1st to 10th	Day	42% 6% 13% 25% 14%	3160
	11th to 20th	Day	48% 4 15% 21% 13%	3140
	21st to end	Day	49% 5% 13% 25% 8% 1	3407
November	1st to 10th	Day	54% 6% 10% 19% 10% 2	1758
	11th to 20th	Day	59% 5% 12% 16% 7% 1	1600
	21st to end	Day	65% 4 13% 15% 3	1600
December	1st to 10th	Day	74% 4 13% 8% 1	2600
	11th to 20th	Day	71% 3 12% 8% 7%	2986
	21st to end	Day	76% 4 10% 8% 2	3960
January	1st to 10th	Night	78% 3 8% 8% 3	6349
	11th to 20th	Night	77% 4 8% 7% 3	6204
	21st to end	Night	73% 3 8% 9% 7%	6490
February	1st to 10th	Night	68% 4 10% 13% 6%	2600
	11th to 20th	Night	67% 5% 10% 13% 5% 1	2535
	21st to end	Night	65% 3 10% 14% 7%	1919
March	1st to 10th	Night	58% 3 11% 22% 6%	3520
	11th to 20th	Night	61% 5% 11% 16% 6% 1	3480
	21st to end	Night	52% 6% 7% 23% 12% 1	3736
April	1st to 10th	Night	55% 5% 10% 20% 9% 1	1900
	11th to 20th	Night	51% 4 10% 18% 15% 1	1875
	21st to end	Night	47% 4 11% 21% 14% 3	1801
May	1st to 10th	Night	54% 5% 11% 17% 10% 3	2712
	11th to 20th	Night	45% 7% 16% 22% 8% 1	2496
	21st to end	Night	36% 6% 14% 22% 20% 2	2739
June	1st to 10th	Night	44% 6% 11% 23% 16% 1	1520
	11th to 20th	Night	38% 6% 13% 24% 17% 2	1520
	21st to end	Night	40% 6% 12% 25% 15% 1	1520
July	1st to 10th	Night	40% 5% 13% 24% 15% 2	2480
	11th to 20th	Night	35% 6% 14% 29% 15% 1	2472
	21st to end	Night	37% 6% 14% 26% 15% 1	2771
August	1st to 10th	Night	41% 5% 13% 28% 12% 1	2814
	11th to 20th	Night	43% 7% 14% 28% 8%	2812
	21st to end	Night	33% 6% 12% 36% 13% 1	3441
September	1st to 10th	Night	37% 6% 13% 28% 13% 2	1900
	11th to 20th	Night	37% 4 12% 29% 17% 1	1900
	21st to end	Night	44% 6% 13% 24% 13% 1	2186
October	1st to 10th	Night	46% 5% 9% 27% 13% 1	3800
	11th to 20th	Night	46% 4 14% 21% 14% 1	3779
	21st to end	Night	47% 4 12% 26% 9% 1	4141
November	1st to 10th	Night	50% 6% 10% 22% 10% 2	2537
	11th to 20th	Night	55% 5% 13% 17% 9% 2	2800
	21st to end	Night	60% 5% 15% 16% 4	2800
December	1st to 10th	Night	73% 5% 13% 9% 1	4580
	11th to 20th	Night	70% 4 7% 12% 7% 1	5208
	21st to end	Night	74% 5% 7% 10% 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	29% 34% 25% 8% 5% 1	200
	11th to 20th	Day	30% 35% 27% 7% 1	233
	21st to end	Day	24% 29% 34% 10% 3	330
February	1st to 10th	Day	33% 32% 27% 6% 1	300
	11th to 20th	Day	32% 32% 33% 2 1	300
	21st to end	Day	37% 24% 29% 9% 1	347
March	1st to 10th	Day	23% 28% 32% 13% 2 1	460
	11th to 20th	Day	30% 25% 30% 11% 3 1	460
	21st to end	Day	31% 24% 26% 13% 5% 1	506
April	1st to 10th	Day	33% 20% 28% 13% 4 1	460
	11th to 20th	Day	31% 24% 23% 13% 6% 2	452
	21st to end	Day	30% 18% 31% 13% 6% 1	460
May	1st to 10th	Day	27% 16% 32% 16% 6% 3	463
	11th to 20th	Day	19% 14% 35% 23% 7% 2	487
	21st to end	Day	17% 15% 31% 22% 10% 4	529
June	1st to 10th	Day	20% 16% 33% 22% 8% 2	490
	11th to 20th	Day	16% 13% 31% 27% 10% 2	490
	21st to end	Day	16% 16% 36% 21% 9% 2	490
July	1st to 10th	Day	12% 7% 32% 30% 15% 4	485
	11th to 20th	Day	13% 9% 31% 29% 14% 4	484
	21st to end	Day	12% 10% 36% 29% 11% 2	532
August	1st to 10th	Day	14% 8% 28% 30% 15% 6%	486
	11th to 20th	Day	16% 8% 27% 28% 14% 7%	479
	21st to end	Day	15% 9% 31% 24% 14% 7%	539
September	1st to 10th	Day	17% 13% 24% 24% 13% 8%	490
	11th to 20th	Day	13% 8% 25% 19% 22% 12%	490
	21st to end	Day	19% 15% 28% 19% 15% 4	490
October	1st to 10th	Day	25% 13% 28% 18% 14% 3	490
	11th to 20th	Day	27% 19% 28% 18% 6% 2	490
	21st to end	Day	32% 18% 30% 15% 5%	534
November	1st to 10th	Day	26% 18% 30% 17% 8% 1	392
	11th to 20th	Day	28% 28% 28% 13% 4	200
	21st to end	Day	34% 26% 32% 8% 1	200
December	1st to 10th	Day	34% 22% 29% 11% 4 1	200
	11th to 20th	Day	35% 28% 24% 13% 2	200
	21st to end	Day	32% 28% 27% 11% 2	220
January	1st to 10th	Night	26% 30% 29% 12% 3	800
	11th to 20th	Night	27% 26% 30% 15% 3	800
	21st to end	Night	20% 29% 32% 16% 3	880
February	1st to 10th	Night	29% 25% 31% 14% 1	800
	11th to 20th	Night	26% 29% 29% 13% 2	783
	21st to end	Night	27% 28% 30% 14% 2	629
March	1st to 10th	Night	25% 21% 34% 17% 3	760
	11th to 20th	Night	28% 22% 28% 18% 4	760
	21st to end	Night	27% 16% 33% 17% 6% 1	836
April	1st to 10th	Night	26% 20% 33% 19% 3	760
	11th to 20th	Night	25% 19% 31% 20% 5% 1	751
	21st to end	Night	24% 13% 29% 26% 8%	661
May	1st to 10th	Night	23% 10% 25% 31% 10%	494
	11th to 20th	Night	16% 10% 32% 35% 7% 1	374
	21st to end	Night	12% 9% 34% 29% 17%	411
June	1st to 10th	Night	17% 9% 31% 28% 14% 1	380
	11th to 20th	Night	13% 10% 26% 38% 13%	380
	21st to end	Night	12% 8% 32% 38% 9%	380
July	1st to 10th	Night	9% 5% 24% 43% 17% 2	370
	11th to 20th	Night	11% 4 29% 37% 19% 1	368
	21st to end	Night	13% 4 28% 36% 16% 3	490
August	1st to 10th	Night	11% 5% 26% 37% 19% 2	567
	11th to 20th	Night	13% 6% 22% 34% 22% 4	564
	21st to end	Night	12% 6% 26% 33% 21% 2	814
September	1st to 10th	Night	17% 9% 25% 28% 20% 3	760
	11th to 20th	Night	17% 6% 19% 29% 24% 6%	760
	21st to end	Night	18% 11% 25% 26% 18% 3	760
October	1st to 10th	Night	23% 11% 27% 21% 16% 2	760
	11th to 20th	Night	25% 16% 29% 22% 7% 1	760
	21st to end	Night	34% 14% 31% 18% 3	836
November	1st to 10th	Night	26% 16% 29% 22% 7% 1	780
	11th to 20th	Night	25% 21% 31% 16% 7% 2	800
	21st to end	Night	29% 23% 28% 15% 5%	800
December	1st to 10th	Night	33% 23% 27% 14% 4	800
	11th to 20th	Night	30% 24% 28% 15% 3	800
	21st to end	Night	33% 22% 30% 12% 3	880

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