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 2026-06-16.

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% 29%	138076
	Night	15% 13% 5% 11% 6% 5% 4 4 5% 6% 27%	133930
Transparency	Day	54% 5% 11% 20% 10% 1	141647
	Night	57% 4 11% 18% 9% 1	138447
Seeing	Day	24% 18% 29% 18% 8% 3	16594
	Night	23% 17% 28% 22% 8% 1	25936

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	14% 21% 8% 18% 10% 4 4 3 3 4 13%	4661
	11th to 20th	Day	10% 19% 9% 20% 10% 6% 5% 3 4 4 9%	4845
	21st to end	Day	13% 15% 7% 15% 12% 10% 7% 3 4 3 10%	5780
February	1st to 10th	Day	16% 17% 7% 14% 9% 5% 5% 4 4 4 15%	1890
	11th to 20th	Day	12% 19% 8% 18% 8% 5% 4 3 3 4 15%	1843
	21st to end	Day	18% 17% 5% 14% 7% 4 4 3 4 5% 18%	1692
March	1st to 10th	Day	16% 16% 6% 12% 6% 4 3 2 4 4 26%	4892
	11th to 20th	Day	11% 17% 5% 9% 6% 4 5% 3 5% 6% 30%	5090
	21st to end	Day	14% 14% 5% 13% 6% 5% 4 3 4 5% 26%	5697
April	1st to 10th	Day	17% 15% 4 10% 4 4 4 3 4 6% 27%	2520
	11th to 20th	Day	19% 12% 3 7% 6% 4 4 3 5% 5% 32%	2494
	21st to end	Day	21% 12% 3 7% 4 4 4 3 4 6% 31%	2646
May	1st to 10th	Day	23% 9% 3 7% 5% 5% 5% 4 5% 6% 27%	7877
	11th to 20th	Day	15% 10% 3 8% 5% 5% 5% 3 5% 7% 32%	8221
	21st to end	Day	19% 7% 3 7% 4 5% 4 4 6% 7% 34%	9060
June	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 6% 8% 36%	2940
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 9% 34%	2870
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 7% 38%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 5% 7% 11% 36%	6360
	21st to end	Day	9% 5% 3 7% 5% 5% 5% 4 6% 8% 42%	6730
August	1st to 10th	Day	17% 7% 2 7% 4 4 4 4 6% 8% 37%	4234
	11th to 20th	Day	14% 7% 3 8% 6% 6% 6% 5% 6% 10% 29%	4174
	21st to end	Day	14% 7% 2 7% 4 4 5% 4 6% 9% 39%	4314
September	1st to 10th	Day	14% 7% 2 6% 3 5% 5% 4 6% 8% 40%	2400
	11th to 20th	Day	14% 5% 2 6% 4 4 4 4 6% 8% 44%	2400
	21st to end	Day	16% 7% 3 8% 5% 6% 5% 5% 6% 7% 32%	2094
October	1st to 10th	Day	19% 8% 2 6% 4 3 4 4 6% 6% 37%	2000
	11th to 20th	Day	19% 10% 3 9% 6% 5% 4 3 5% 6% 29%	2000
	21st to end	Day	24% 14% 4 7% 5% 4 5% 3 5% 5% 25%	2190
November	1st to 10th	Day	14% 15% 4 8% 5% 5% 4 4 4 5% 31%	1848
	11th to 20th	Day	12% 19% 5% 10% 6% 4 4 3 5% 6% 25%	1680
	21st to end	Day	17% 18% 5% 10% 6% 4 4 4 5% 6% 20%	1680
December	1st to 10th	Day	17% 20% 7% 14% 7% 5% 4 4 5% 5% 12%	2680
	11th to 20th	Day	17% 20% 7% 14% 5% 4 4 2 4 4 19%	3066
	21st to end	Day	19% 20% 7% 15% 8% 4 4 3 4 5% 12%	4048
January	1st to 10th	Night	13% 20% 9% 22% 8% 5% 4 3 3 3 11%	8234
	11th to 20th	Night	9% 20% 10% 19% 10% 6% 5% 4 5% 4 8%	8038
	21st to end	Night	12% 16% 6% 13% 10% 10% 6% 4 5% 5% 13%	8401
February	1st to 10th	Night	15% 16% 6% 15% 7% 5% 4 3 5% 5% 18%	2730
	11th to 20th	Night	12% 15% 7% 15% 9% 6% 6% 4 5% 5% 17%	2663
	21st to end	Night	14% 15% 6% 16% 8% 5% 4 3 4 5% 20%	2113
March	1st to 10th	Night	15% 11% 6% 12% 8% 4 4 3 5% 5% 26%	5166
	11th to 20th	Night	14% 16% 5% 9% 5% 3 3 3 5% 7% 31%	5089
	21st to end	Night	16% 13% 5% 12% 5% 5% 5% 3 5% 5% 26%	5477
April	1st to 10th	Night	18% 14% 4 7% 5% 5% 4 4 5% 7% 27%	2100
	11th to 20th	Night	17% 12% 4 8% 4 4 4 3 5% 6% 33%	2075
	21st to end	Night	17% 9% 3 6% 5% 3 3 4 7% 7% 35%	1991
May	1st to 10th	Night	25% 10% 3 8% 5% 5% 5% 3 5% 6% 25%	5019
	11th to 20th	Night	15% 8% 3 7% 4 5% 5% 4 7% 7% 36%	4639
	21st to end	Night	18% 9% 2 7% 4 4 4 3 6% 7% 35%	5114
June	1st to 10th	Night	14% 7% 3 7% 5% 4 5% 4 6% 8% 37%	1680
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 5% 7% 8% 41%	1640
	21st to end	Night	10% 8% 2 7% 5% 5% 4 5% 6% 8% 40%	1600
July	1st to 10th	Night	10% 6% 3 7% 4 5% 4 4 6% 11% 39%	3600
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	3600
	21st to end	Night	9% 6% 2 6% 4 3 4 4 5% 8% 48%	4105
August	1st to 10th	Night	14% 7% 3 7% 4 4 4 4 6% 8% 40%	2915
	11th to 20th	Night	15% 6% 2 6% 4 5% 5% 4 6% 7% 39%	2875
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	3550
September	1st to 10th	Night	14% 5% 2 5% 4 3 5% 3 5% 7% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 7% 48%	2000
	21st to end	Night	16% 8% 2 6% 4 4 5% 4 7% 7% 37%	2301
October	1st to 10th	Night	18% 7% 3 7% 4 4 4 3 5% 7% 37%	2400
	11th to 20th	Night	20% 9% 3 6% 5% 5% 5% 4 6% 7% 32%	2400
	21st to end	Night	25% 11% 3 7% 5% 3 4 3 5% 6% 28%	2650
November	1st to 10th	Night	16% 11% 4 8% 5% 4 5% 4 4 6% 34%	2667
	11th to 20th	Night	15% 13% 5% 9% 6% 4 5% 4 6% 7% 26%	2940
	21st to end	Night	17% 17% 5% 9% 6% 4 5% 5% 5% 7% 22%	2940
December	1st to 10th	Night	12% 19% 7% 17% 8% 6% 5% 4 5% 5% 12%	4720
	11th to 20th	Night	17% 21% 7% 12% 5% 4 4 3 4 4 19%	5348
	21st to end	Night	22% 19% 7% 11% 8% 4 4 2 4 5% 13%	7150

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 9% 9% 3	4662
	11th to 20th	Day	78% 3 6% 7% 4 1	4876
	21st to end	Day	78% 3 8% 9% 2	5780
February	1st to 10th	Day	73% 4 10% 11% 3	1890
	11th to 20th	Day	74% 4 6% 13% 3	1852
	21st to end	Day	68% 4 8% 13% 6% 1	1692
March	1st to 10th	Day	62% 3 9% 17% 8%	4895
	11th to 20th	Day	55% 4 17% 14% 9% 1	5090
	21st to end	Day	60% 5% 5% 20% 9% 1	5697
April	1st to 10th	Day	58% 4 9% 18% 10% 1	2520
	11th to 20th	Day	53% 4 9% 19% 13% 1	2494
	21st to end	Day	55% 4 8% 18% 13% 2	2646
May	1st to 10th	Day	56% 5% 9% 19% 10% 1	7886
	11th to 20th	Day	50% 5% 9% 23% 11% 1	8225
	21st to end	Day	48% 5% 11% 23% 12% 2	9061
June	1st to 10th	Day	46% 5% 10% 20% 18% 1	2940
	11th to 20th	Day	43% 7% 13% 23% 13% 1	2870
	21st to end	Day	42% 7% 12% 23% 15% 1	2800
July	1st to 10th	Day	39% 6% 11% 28% 15% 1	6290
	11th to 20th	Day	39% 7% 14% 24% 16% 1	6285
	21st to end	Day	38% 6% 13% 28% 15%	6536
August	1st to 10th	Day	44% 5% 13% 22% 15% 1	4221
	11th to 20th	Day	47% 7% 14% 25% 7%	4213
	21st to end	Day	42% 5% 9% 34% 9% 1	4314
September	1st to 10th	Day	41% 7% 10% 29% 12% 1	2400
	11th to 20th	Day	37% 6% 11% 29% 16% 1	2400
	21st to end	Day	49% 5% 11% 22% 11% 1	2094
October	1st to 10th	Day	41% 6% 13% 25% 14% 1	3260
	11th to 20th	Day	48% 4 15% 20% 13%	3240
	21st to end	Day	49% 5% 13% 24% 9% 1	3516
November	1st to 10th	Day	55% 5% 10% 19% 9% 2	1848
	11th to 20th	Day	60% 5% 12% 16% 6% 1	1680
	21st to end	Day	65% 4 13% 15% 3	1680
December	1st to 10th	Day	74% 4 12% 8% 1	2680
	11th to 20th	Day	71% 3 12% 8% 7%	3066
	21st to end	Day	76% 4 10% 8% 2	4048
January	1st to 10th	Night	79% 3 7% 8% 3	8234
	11th to 20th	Night	77% 4 7% 7% 5%	8082
	21st to end	Night	72% 3 8% 11% 6%	8401
February	1st to 10th	Night	68% 4 10% 13% 6%	2730
	11th to 20th	Night	67% 4 10% 14% 5%	2665
	21st to end	Night	66% 3 10% 13% 8%	2113
March	1st to 10th	Night	60% 3 12% 18% 7%	5176
	11th to 20th	Night	53% 4 15% 19% 7% 3	5090
	21st to end	Night	58% 5% 7% 19% 10% 1	5477
April	1st to 10th	Night	57% 5% 9% 19% 9% 1	2100
	11th to 20th	Night	52% 5% 11% 18% 14% 1	2075
	21st to end	Night	47% 4 11% 23% 13% 2	1991
May	1st to 10th	Night	59% 5% 8% 16% 10% 2	5026
	11th to 20th	Night	45% 7% 13% 22% 12% 2	4640
	21st to end	Night	47% 5% 12% 20% 14% 1	5115
June	1st to 10th	Night	44% 6% 10% 24% 16% 1	1680
	11th to 20th	Night	38% 6% 14% 24% 16% 2	1640
	21st to end	Night	40% 6% 13% 25% 15% 1	1600
July	1st to 10th	Night	39% 5% 14% 26% 14% 2	3560
	11th to 20th	Night	36% 6% 13% 27% 16% 1	3552
	21st to end	Night	34% 5% 12% 28% 19% 1	3985
August	1st to 10th	Night	41% 5% 13% 27% 13% 1	2904
	11th to 20th	Night	42% 7% 13% 28% 9%	2902
	21st to end	Night	33% 6% 12% 36% 12% 1	3550
September	1st to 10th	Night	36% 6% 13% 30% 13% 2	2000
	11th to 20th	Night	36% 4 12% 31% 16% 1	2000
	21st to end	Night	44% 6% 13% 24% 13% 1	2301
October	1st to 10th	Night	45% 4 9% 27% 14% 1	3920
	11th to 20th	Night	46% 4 14% 21% 14% 1	3899
	21st to end	Night	47% 4 12% 26% 10% 1	4274
November	1st to 10th	Night	51% 6% 10% 23% 9% 2	2667
	11th to 20th	Night	56% 5% 13% 17% 8% 1	2940
	21st to end	Night	61% 5% 14% 16% 4	2940
December	1st to 10th	Night	72% 5% 12% 9% 1	4720
	11th to 20th	Night	70% 4 7% 13% 7% 1	5348
	21st to end	Night	75% 5% 7% 10% 4	7150

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	32% 31% 25% 7% 5%	220
	11th to 20th	Day	31% 34% 26% 7% 1	256
	21st to end	Day	27% 29% 31% 10% 2	363
February	1st to 10th	Day	33% 31% 28% 7% 2	330
	11th to 20th	Day	33% 31% 33% 2 1	330
	21st to end	Day	37% 27% 28% 8%	408
March	1st to 10th	Day	26% 28% 31% 12% 2 1	540
	11th to 20th	Day	35% 23% 28% 11% 3 1	540
	21st to end	Day	32% 25% 26% 12% 4 1	594
April	1st to 10th	Day	34% 21% 28% 12% 4 1	540
	11th to 20th	Day	33% 25% 23% 12% 5% 2	532
	21st to end	Day	30% 19% 31% 14% 5% 1	540
May	1st to 10th	Day	29% 18% 30% 15% 5% 3	543
	11th to 20th	Day	20% 16% 34% 21% 7% 2	567
	21st to end	Day	21% 15% 32% 20% 9% 4	617
June	1st to 10th	Day	20% 16% 33% 23% 8% 1	570
	11th to 20th	Day	16% 15% 32% 26% 9% 2	550
	21st to end	Day	16% 16% 36% 22% 8% 2	530
July	1st to 10th	Day	12% 7% 33% 30% 14% 4	525
	11th to 20th	Day	12% 9% 31% 29% 15% 4	524
	21st to end	Day	13% 9% 36% 28% 11% 3	576
August	1st to 10th	Day	13% 7% 27% 31% 16% 6%	526
	11th to 20th	Day	16% 8% 26% 29% 14% 7%	519
	21st to end	Day	15% 10% 31% 23% 14% 7%	583
September	1st to 10th	Day	17% 13% 24% 24% 15% 8%	530
	11th to 20th	Day	13% 8% 25% 20% 22% 12%	530
	21st to end	Day	19% 14% 28% 21% 14% 4	530
October	1st to 10th	Day	23% 12% 27% 19% 14% 4	530
	11th to 20th	Day	28% 19% 29% 17% 6% 2	530
	21st to end	Day	32% 18% 30% 15% 4	577
November	1st to 10th	Day	27% 20% 29% 16% 8%	422
	11th to 20th	Day	29% 27% 28% 12% 4	220
	21st to end	Day	35% 26% 31% 7% 1	220
December	1st to 10th	Day	34% 22% 30% 10% 3	220
	11th to 20th	Day	35% 29% 24% 11% 1	220
	21st to end	Day	35% 26% 26% 10% 2	242
January	1st to 10th	Night	26% 31% 29% 12% 3	840
	11th to 20th	Night	28% 27% 29% 14% 2	840
	21st to end	Night	21% 29% 32% 16% 3	924
February	1st to 10th	Night	28% 25% 31% 15% 1	840
	11th to 20th	Night	26% 29% 29% 14% 2	823
	21st to end	Night	28% 27% 30% 14% 1	692
March	1st to 10th	Night	26% 21% 33% 17% 3	840
	11th to 20th	Night	30% 21% 28% 17% 3	840
	21st to end	Night	28% 17% 33% 16% 6% 1	924
April	1st to 10th	Night	27% 20% 32% 18% 3	840
	11th to 20th	Night	26% 18% 30% 20% 4 1	831
	21st to end	Night	24% 13% 30% 26% 7%	731
May	1st to 10th	Night	24% 11% 26% 29% 9%	546
	11th to 20th	Night	16% 9% 33% 35% 7%	414
	21st to end	Night	14% 9% 34% 27% 15%	455
June	1st to 10th	Night	17% 9% 30% 30% 14% 1	420
	11th to 20th	Night	12% 11% 26% 38% 12%	410
	21st to end	Night	12% 8% 33% 38% 9%	400
July	1st to 10th	Night	9% 5% 24% 43% 17% 2	390
	11th to 20th	Night	11% 4 28% 38% 18% 1	388
	21st to end	Night	13% 4 28% 37% 16% 3	516
August	1st to 10th	Night	11% 5% 25% 38% 19% 2	597
	11th to 20th	Night	13% 6% 21% 34% 23% 4	594
	21st to end	Night	12% 6% 26% 33% 21% 2	857
September	1st to 10th	Night	16% 9% 25% 28% 20% 3	800
	11th to 20th	Night	16% 6% 18% 30% 25% 6%	800
	21st to end	Night	18% 10% 25% 27% 18% 3	800
October	1st to 10th	Night	23% 11% 27% 21% 17% 2	800
	11th to 20th	Night	24% 16% 30% 22% 7% 1	800
	21st to end	Night	33% 14% 31% 19% 3	880
November	1st to 10th	Night	27% 16% 29% 21% 6% 1	820
	11th to 20th	Night	25% 21% 30% 16% 6% 1	840
	21st to end	Night	30% 23% 27% 15% 5%	840
December	1st to 10th	Night	32% 23% 27% 14% 4	840
	11th to 20th	Night	30% 25% 28% 14% 3	840
	21st to end	Night	34% 23% 29% 12% 2	924

Page generated at 2026-06-13 02:31:42. Version ac07

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