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 2024-01-31.

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	16% 11% 4 8% 6% 4 5% 5% 5% 7% 29%	74855
	Night	16% 12% 5% 8% 7% 3 5% 5% 5% 6% 29%	75271
Transparency	Day	55% 5% 11% 19% 10% 1	74565
	Night	55% 5% 11% 19% 9% 1	75096
Seeing	Day	23% 18% 30% 19% 8% 3	13514
	Night	23% 17% 29% 22% 9% 1	22963

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% 18% 9% 14% 11% 4 6% 5% 4 4 14%	1517
	11th to 20th	Day	15% 20% 8% 15% 10% 4 4 4 3 4 12%	1579
	21st to end	Day	11% 17% 9% 14% 12% 6% 5% 5% 5% 4 11%	1872
February	1st to 10th	Day	17% 16% 8% 13% 9% 4 5% 4 4 5% 14%	1620
	11th to 20th	Day	12% 17% 9% 16% 9% 5% 5% 4 3 5% 15%	1573
	21st to end	Day	20% 15% 5% 14% 8% 3 4 5% 4 5% 17%	1447
March	1st to 10th	Day	14% 14% 9% 13% 7% 5% 5% 4 5% 4 22%	1911
	11th to 20th	Day	15% 17% 6% 8% 7% 3 4 3 5% 5% 27%	1980
	21st to end	Day	18% 14% 5% 9% 6% 3 4 3 5% 5% 28%	2218
April	1st to 10th	Day	16% 14% 5% 9% 5% 3 4 4 5% 7% 28%	2160
	11th to 20th	Day	19% 10% 3 5% 5% 3 4 4 5% 6% 35%	2134
	21st to end	Day	22% 12% 4 6% 6% 3 4 4 4 5% 31%	2268
May	1st to 10th	Day	17% 11% 4 7% 6% 3 4 5% 5% 7% 32%	2409
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 5% 5% 7% 36%	2478
	21st to end	Day	14% 5% 2 5% 6% 4 5% 4 6% 9% 40%	2725
June	1st to 10th	Day	17% 6% 3 6% 6% 3 5% 5% 6% 8% 36%	2520
	11th to 20th	Day	15% 5% 2 6% 6% 4 5% 6% 7% 9% 36%	2520
	21st to end	Day	14% 5% 2 6% 6% 4 5% 5% 6% 8% 39%	2520
July	1st to 10th	Day	10% 6% 2 5% 5% 3 6% 5% 7% 8% 42%	2520
	11th to 20th	Day	11% 5% 3 5% 6% 5% 6% 6% 7% 10% 35%	2520
	21st to end	Day	11% 5% 3 7% 6% 4 6% 7% 7% 9% 34%	2671
August	1st to 10th	Day	11% 6% 2 6% 6% 4 5% 7% 8% 9% 38%	2327
	11th to 20th	Day	11% 7% 3 6% 7% 4 7% 7% 7% 9% 31%	2303
	21st to end	Day	15% 6% 2 6% 5% 4 6% 5% 6% 10% 35%	2378
September	1st to 10th	Day	15% 7% 3 5% 4 4 5% 5% 6% 9% 38%	2160
	11th to 20th	Day	15% 5% 2 5% 4 3 4 5% 6% 8% 42%	2160
	21st to end	Day	16% 6% 3 7% 6% 4 6% 6% 6% 7% 34%	1886
October	1st to 10th	Day	21% 9% 3 6% 5% 2 4 5% 6% 6% 34%	1800
	11th to 20th	Day	20% 10% 4 8% 8% 4 5% 4 5% 7% 28%	1800
	21st to end	Day	26% 14% 4 6% 6% 3 5% 4 4 5% 23%	1972
November	1st to 10th	Day	14% 14% 4 7% 6% 3 5% 5% 4 5% 32%	1669
	11th to 20th	Day	13% 18% 6% 9% 6% 3 5% 4 5% 6% 25%	1520
	21st to end	Day	18% 17% 6% 9% 7% 3 4 5% 4 6% 21%	1520
December	1st to 10th	Day	18% 20% 9% 12% 6% 3 4 4 4 5% 16%	1520
	11th to 20th	Day	17% 18% 8% 13% 8% 3 4 4 4 5% 16%	1906
	21st to end	Day	24% 23% 8% 12% 6% 3 3 4 3 3 11%	2772
January	1st to 10th	Night	14% 19% 7% 13% 10% 5% 6% 4 4 4 15%	2649
	11th to 20th	Night	16% 18% 8% 13% 11% 5% 5% 4 4 4 13%	2599
	21st to end	Night	12% 17% 8% 13% 11% 5% 6% 5% 5% 5% 14%	2711
February	1st to 10th	Night	16% 15% 7% 14% 8% 4 5% 4 5% 4 18%	2340
	11th to 20th	Night	13% 13% 8% 13% 10% 5% 6% 6% 4 5% 18%	2273
	21st to end	Night	15% 13% 7% 14% 9% 4 4 4 4 5% 21%	1808
March	1st to 10th	Night	14% 16% 6% 10% 8% 3 4 5% 5% 5% 23%	2034
	11th to 20th	Night	16% 14% 6% 9% 6% 3 4 4 4 6% 28%	1979
	21st to end	Night	18% 13% 4 8% 6% 3 4 4 4 6% 30%	2138
April	1st to 10th	Night	18% 13% 5% 6% 5% 4 4 5% 5% 7% 29%	1800
	11th to 20th	Night	17% 10% 4 7% 5% 3 5% 4 5% 6% 34%	1775
	21st to end	Night	18% 9% 4 5% 6% 3 3 4 6% 7% 34%	1707
May	1st to 10th	Night	18% 9% 3 6% 4 4 6% 5% 5% 8% 33%	1548
	11th to 20th	Night	13% 5% 3 5% 4 3 5% 4 7% 9% 40%	1415
	21st to end	Night	10% 5% 2 5% 6% 3 5% 4 6% 8% 46%	1550
June	1st to 10th	Night	14% 6% 3 6% 6% 3 5% 5% 6% 8% 37%	1440
	11th to 20th	Night	12% 5% 3 5% 5% 2 5% 6% 6% 8% 43%	1440
	21st to end	Night	10% 6% 3 5% 6% 4 4 6% 6% 9% 41%	1440
July	1st to 10th	Night	7% 5% 2 5% 6% 4 5% 5% 6% 8% 47%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 5% 6% 8% 47%	1440
	21st to end	Night	11% 5% 2 5% 5% 3 4 5% 7% 10% 44%	1644
August	1st to 10th	Night	9% 5% 2 6% 6% 3 4 5% 5% 8% 47%	1611
	11th to 20th	Night	11% 5% 3 5% 6% 4 6% 5% 6% 7% 42%	1598
	21st to end	Night	10% 5% 2 5% 5% 3 5% 5% 6% 9% 45%	1959
September	1st to 10th	Night	15% 5% 2 5% 5% 3 5% 5% 5% 7% 44%	1800
	11th to 20th	Night	14% 6% 2 5% 4 2 4 4 5% 7% 46%	1800
	21st to end	Night	15% 7% 2 5% 4 2 5% 5% 7% 8% 39%	2070
October	1st to 10th	Night	19% 7% 3 6% 6% 3 4 4 5% 6% 35%	2160
	11th to 20th	Night	20% 9% 3 5% 5% 4 5% 5% 5% 8% 31%	2160
	21st to end	Night	28% 11% 3 6% 5% 3 4 4 4 6% 26%	2384
November	1st to 10th	Night	16% 11% 4 7% 6% 3 5% 5% 4 6% 35%	2407
	11th to 20th	Night	15% 11% 5% 7% 8% 3 5% 5% 6% 7% 27%	2660
	21st to end	Night	17% 15% 5% 8% 6% 3 4 6% 5% 7% 23%	2660
December	1st to 10th	Night	17% 19% 7% 9% 7% 4 5% 5% 5% 5% 17%	2660
	11th to 20th	Night	17% 17% 7% 12% 7% 3 4 4 5% 4 19%	3288
	21st to end	Night	27% 19% 7% 10% 6% 3 4 4 4 5% 10%	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	73% 4 8% 11% 3	1518
	11th to 20th	Day	77% 4 7% 9% 2	1579
	21st to end	Day	77% 3 9% 8% 2	1872
February	1st to 10th	Day	73% 4 10% 9% 3	1620
	11th to 20th	Day	73% 4 6% 13% 3 1	1582
	21st to end	Day	70% 4 8% 12% 6% 1	1447
March	1st to 10th	Day	66% 5% 10% 15% 4 1	1914
	11th to 20th	Day	60% 4 8% 16% 10% 1	1980
	21st to end	Day	59% 3 7% 18% 11% 2	2218
April	1st to 10th	Day	56% 4 10% 18% 11% 1	2160
	11th to 20th	Day	50% 4 9% 20% 14% 2	2134
	21st to end	Day	56% 4 7% 18% 14% 2	2268
May	1st to 10th	Day	52% 4 9% 21% 12% 2	2409
	11th to 20th	Day	46% 6% 10% 23% 13% 3	2482
	21st to end	Day	41% 6% 14% 23% 14% 1	2725
June	1st to 10th	Day	47% 5% 11% 20% 17% 1	2520
	11th to 20th	Day	42% 7% 12% 25% 13% 2	2520
	21st to end	Day	42% 6% 13% 23% 15% 1	2520
July	1st to 10th	Day	39% 7% 11% 27% 14% 2	2450
	11th to 20th	Day	41% 7% 15% 24% 13%	2445
	21st to end	Day	44% 7% 14% 21% 13% 1	2522
August	1st to 10th	Day	39% 6% 16% 25% 13% 1	2314
	11th to 20th	Day	45% 7% 14% 24% 9% 1	2303
	21st to end	Day	44% 6% 11% 26% 11% 2	2378
September	1st to 10th	Day	42% 8% 11% 27% 11% 1	2160
	11th to 20th	Day	39% 6% 12% 26% 16% 1	2160
	21st to end	Day	48% 5% 11% 23% 12% 1	1886
October	1st to 10th	Day	50% 6% 13% 18% 13% 1	1800
	11th to 20th	Day	57% 4 14% 18% 7% 1	1800
	21st to end	Day	64% 4 10% 15% 6% 1	1972
November	1st to 10th	Day	54% 6% 10% 18% 10% 2	1669
	11th to 20th	Day	59% 5% 13% 15% 7% 1	1520
	21st to end	Day	65% 4 13% 15% 3	1520
December	1st to 10th	Day	72% 3 11% 12% 2	1520
	11th to 20th	Day	73% 3 13% 7% 4	1906
	21st to end	Day	80% 3 7% 8% 1	2772
January	1st to 10th	Night	73% 3 8% 11% 5%	2649
	11th to 20th	Night	75% 5% 10% 8% 3	2599
	21st to end	Night	72% 3 8% 11% 4 1	2711
February	1st to 10th	Night	69% 4 9% 12% 6%	2340
	11th to 20th	Night	67% 4 9% 14% 5% 1	2275
	21st to end	Night	66% 3 10% 14% 7%	1808
March	1st to 10th	Night	63% 4 10% 17% 6%	2044
	11th to 20th	Night	57% 4 8% 21% 8% 1	1980
	21st to end	Night	56% 4 7% 18% 13% 2	2138
April	1st to 10th	Night	55% 5% 10% 20% 10% 1	1800
	11th to 20th	Night	51% 4 10% 18% 15% 1	1775
	21st to end	Night	49% 4 11% 21% 13% 2	1707
May	1st to 10th	Night	50% 5% 10% 19% 14% 2	1548
	11th to 20th	Night	39% 7% 13% 25% 14% 2	1416
	21st to end	Night	36% 6% 17% 22% 17% 2	1551
June	1st to 10th	Night	44% 6% 11% 23% 16% 1	1440
	11th to 20th	Night	38% 6% 13% 24% 17% 2	1440
	21st to end	Night	39% 6% 12% 26% 15% 1	1440
July	1st to 10th	Night	34% 6% 13% 26% 18% 3	1400
	11th to 20th	Night	35% 6% 12% 30% 16% 2	1392
	21st to end	Night	35% 6% 15% 27% 14% 2	1553
August	1st to 10th	Night	35% 6% 15% 31% 14% 1	1600
	11th to 20th	Night	40% 6% 15% 30% 10% 1	1598
	21st to end	Night	34% 6% 14% 31% 13% 2	1959
September	1st to 10th	Night	37% 6% 14% 28% 12% 2	1800
	11th to 20th	Night	38% 4 11% 29% 17% 1	1800
	21st to end	Night	41% 6% 13% 25% 13% 1	2070
October	1st to 10th	Night	49% 5% 12% 20% 13% 1	2160
	11th to 20th	Night	50% 5% 16% 19% 9% 1	2160
	21st to end	Night	61% 4 11% 15% 8% 2	2384
November	1st to 10th	Night	50% 6% 10% 21% 10% 3	2407
	11th to 20th	Night	55% 5% 13% 17% 8% 2	2660
	21st to end	Night	60% 5% 15% 16% 4	2660
December	1st to 10th	Night	68% 5% 13% 12% 2	2660
	11th to 20th	Night	68% 5% 10% 14% 4	3288
	21st to end	Night	77% 4 9% 9% 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	28% 32% 26% 8% 6% 1	180
	11th to 20th	Day	29% 35% 27% 7% 1	209
	21st to end	Day	23% 29% 33% 12% 3	294
February	1st to 10th	Day	34% 34% 28% 4	240
	11th to 20th	Day	30% 35% 33% 3	240
	21st to end	Day	37% 24% 29% 9%	313
March	1st to 10th	Day	23% 30% 32% 13% 2	420
	11th to 20th	Day	29% 25% 30% 11% 3 1	420
	21st to end	Day	32% 23% 26% 13% 5% 1	462
April	1st to 10th	Day	32% 21% 28% 14% 4 1	420
	11th to 20th	Day	30% 24% 23% 14% 7% 3	412
	21st to end	Day	30% 19% 31% 13% 6% 1	420
May	1st to 10th	Day	27% 17% 31% 16% 6% 3	423
	11th to 20th	Day	19% 15% 36% 22% 6% 2	447
	21st to end	Day	18% 15% 30% 22% 11% 5%	485
June	1st to 10th	Day	19% 16% 35% 22% 7% 1	450
	11th to 20th	Day	16% 14% 31% 27% 10% 2	450
	21st to end	Day	16% 14% 36% 22% 9% 3	450
July	1st to 10th	Day	12% 7% 31% 30% 16% 4	445
	11th to 20th	Day	13% 8% 32% 29% 14% 5%	444
	21st to end	Day	13% 10% 36% 30% 9% 2	488
August	1st to 10th	Day	12% 7% 27% 31% 17% 6%	446
	11th to 20th	Day	16% 7% 27% 28% 14% 7%	439
	21st to end	Day	15% 9% 32% 24% 14% 7%	495
September	1st to 10th	Day	17% 12% 26% 24% 12% 9%	450
	11th to 20th	Day	15% 8% 27% 20% 21% 10%	450
	21st to end	Day	18% 15% 29% 20% 15% 4	450
October	1st to 10th	Day	26% 12% 28% 17% 14% 3	450
	11th to 20th	Day	27% 20% 28% 18% 5% 2	450
	21st to end	Day	34% 17% 28% 15% 4	491
November	1st to 10th	Day	25% 19% 30% 17% 8% 1	363
	11th to 20th	Day	30% 27% 27% 12% 4	180
	21st to end	Day	32% 24% 34% 9% 1	180
December	1st to 10th	Day	34% 22% 28% 11% 4 1	180
	11th to 20th	Day	33% 29% 23% 14% 1	180
	21st to end	Day	34% 28% 28% 9% 1	198
January	1st to 10th	Night	26% 29% 30% 12% 3	760
	11th to 20th	Night	26% 26% 30% 15% 3	760
	21st to end	Night	20% 28% 32% 17% 3	835
February	1st to 10th	Night	29% 26% 32% 12% 1	720
	11th to 20th	Night	25% 29% 30% 13% 2	703
	21st to end	Night	27% 28% 30% 14% 2	592
March	1st to 10th	Night	25% 21% 34% 17% 3	720
	11th to 20th	Night	28% 22% 28% 18% 4	720
	21st to end	Night	27% 15% 34% 17% 7% 1	792
April	1st to 10th	Night	25% 20% 33% 18% 3	720
	11th to 20th	Night	24% 19% 30% 21% 5% 2	711
	21st to end	Night	24% 13% 28% 27% 7%	627
May	1st to 10th	Night	23% 10% 25% 31% 11%	468
	11th to 20th	Night	15% 10% 32% 35% 7% 1	354
	21st to end	Night	12% 9% 34% 28% 17% 1	389
June	1st to 10th	Night	18% 9% 30% 29% 14% 1	360
	11th to 20th	Night	13% 10% 26% 37% 14%	360
	21st to end	Night	11% 8% 32% 40% 9%	360
July	1st to 10th	Night	8% 5% 24% 43% 18% 2	350
	11th to 20th	Night	11% 4 28% 36% 19% 1	348
	21st to end	Night	13% 4 29% 36% 15% 3	464
August	1st to 10th	Night	11% 4 26% 37% 20% 2	537
	11th to 20th	Night	12% 6% 21% 35% 22% 4	534
	21st to end	Night	12% 6% 27% 33% 21% 2	771
September	1st to 10th	Night	16% 8% 25% 28% 20% 3	720
	11th to 20th	Night	18% 6% 20% 29% 22% 5%	720
	21st to end	Night	17% 11% 26% 26% 18% 3	720
October	1st to 10th	Night	24% 11% 27% 21% 16% 2	720
	11th to 20th	Night	25% 16% 30% 22% 7%	720
	21st to end	Night	35% 14% 30% 18% 3	792
November	1st to 10th	Night	26% 15% 29% 22% 7% 1	740
	11th to 20th	Night	24% 21% 31% 16% 7% 2	760
	21st to end	Night	27% 23% 28% 16% 6%	760
December	1st to 10th	Night	33% 23% 27% 13% 4	760
	11th to 20th	Night	29% 24% 29% 15% 3	760
	21st to end	Night	33% 23% 29% 12% 2	836

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