Clarkson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clarkson Observatory 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	18% 12% 4 9% 5% 5% 4 3 5% 6% 29%	108662
	Night	18% 15% 5% 10% 5% 4 4 3 4 5% 26%	109905
Transparency	Day	55% 6% 11% 19% 8% 1	112162
	Night	60% 6% 10% 16% 8% 1	114569
Seeing	Day	28% 17% 29% 18% 7% 2	14836
	Night	27% 16% 29% 21% 7% 1	24054

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% 23% 8% 17% 6% 5% 3 3 3 4 13%	3597
	11th to 20th	Day	11% 20% 9% 21% 8% 6% 4 2 3 4 12%	3570
	21st to end	Day	13% 20% 9% 14% 7% 8% 4 3 4 4 14%	4080
February	1st to 10th	Day	16% 19% 5% 15% 7% 5% 4 3 5% 4 18%	1800
	11th to 20th	Day	15% 17% 7% 14% 7% 5% 4 3 3 5% 20%	1814
	21st to end	Day	18% 20% 5% 10% 7% 5% 4 3 4 5% 19%	1570
March	1st to 10th	Day	22% 15% 5% 9% 5% 4 3 3 4 5% 25%	3116
	11th to 20th	Day	14% 22% 5% 10% 5% 5% 4 3 4 4 24%	3406
	21st to end	Day	16% 11% 4 10% 5% 4 3 3 4 5% 35%	4088
April	1st to 10th	Day	22% 15% 4 7% 4 4 4 3 4 4 30%	2280
	11th to 20th	Day	22% 11% 3 7% 4 4 4 2 4 4 35%	2254
	21st to end	Day	24% 11% 4 6% 4 4 4 3 5% 5% 31%	2362
May	1st to 10th	Day	21% 11% 3 8% 5% 5% 4 3 5% 6% 28%	4426
	11th to 20th	Day	17% 10% 4 9% 5% 6% 5% 4 5% 6% 30%	4398
	21st to end	Day	16% 7% 2 7% 4 5% 5% 4 6% 8% 35%	4837
June	1st to 10th	Day	25% 6% 2 7% 5% 5% 4 5% 6% 7% 28%	2660
	11th to 20th	Day	19% 6% 2 6% 6% 3 5% 5% 7% 8% 34%	2660
	21st to end	Day	22% 8% 2 6% 5% 5% 4 4 5% 7% 31%	2660
July	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	14% 6% 3 6% 5% 5% 5% 4 7% 9% 37%	4440
	21st to end	Day	13% 6% 2 6% 5% 4 5% 5% 7% 9% 39%	4814
August	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 4 7% 8% 33%	4401
	11th to 20th	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 36%	3897
	21st to end	Day	18% 6% 3 6% 4 4 4 4 6% 8% 38%	4180
September	1st to 10th	Day	17% 6% 2 6% 4 4 4 3 6% 7% 41%	2280
	11th to 20th	Day	15% 6% 2 5% 3 4 4 3 5% 6% 45%	2247
	21st to end	Day	19% 5% 2 6% 5% 5% 4 4 6% 6% 38%	2083
October	1st to 10th	Day	22% 8% 3 8% 4 4 4 4 5% 6% 31%	1900
	11th to 20th	Day	20% 13% 4 8% 5% 5% 5% 3 4 6% 27%	1900
	21st to end	Day	27% 14% 3 7% 4 4 4 2 5% 5% 25%	1910
November	1st to 10th	Day	16% 13% 3 9% 6% 5% 4 3 5% 6% 29%	1761
	11th to 20th	Day	15% 16% 4 9% 5% 5% 5% 4 5% 4 28%	1685
	21st to end	Day	20% 20% 5% 12% 5% 5% 4 4 4 5% 19%	1600
December	1st to 10th	Day	20% 22% 7% 17% 6% 4 3 3 3 3 10%	2600
	11th to 20th	Day	19% 19% 8% 12% 5% 3 4 3 4 4 20%	2986
	21st to end	Day	20% 20% 5% 12% 5% 4 4 3 3 4 20%	3960
January	1st to 10th	Night	13% 24% 9% 18% 6% 4 3 2 3 3 14%	6349
	11th to 20th	Night	11% 23% 10% 19% 8% 5% 5% 3 3 3 10%	6315
	21st to end	Night	16% 19% 8% 16% 6% 7% 5% 3 3 3 14%	6861
February	1st to 10th	Night	17% 20% 6% 14% 7% 4 3 3 4 4 17%	2600
	11th to 20th	Night	15% 16% 7% 12% 7% 5% 5% 3 4 4 21%	2470
	21st to end	Night	16% 17% 5% 13% 6% 3 3 3 5% 4 24%	1885
March	1st to 10th	Night	20% 14% 4 9% 6% 3 3 2 4 4 31%	3724
	11th to 20th	Night	19% 23% 6% 8% 4 3 3 3 4 5% 24%	3551
	21st to end	Night	16% 15% 4 9% 5% 4 3 2 3 4 35%	3544
April	1st to 10th	Night	21% 14% 4 9% 5% 3 4 2 4 5% 30%	1900
	11th to 20th	Night	22% 12% 3 7% 5% 3 4 3 4 5% 32%	1875
	21st to end	Night	23% 12% 3 7% 4 3 3 3 4 5% 33%	1824
May	1st to 10th	Night	24% 12% 3 6% 4 4 5% 4 5% 7% 26%	2524
	11th to 20th	Night	20% 11% 3 7% 4 4 4 3 5% 6% 32%	2495
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 9% 39%	2738
June	1st to 10th	Night	22% 7% 3 6% 6% 5% 5% 4 6% 7% 30%	1520
	11th to 20th	Night	17% 6% 3 6% 4 4 4 4 5% 9% 38%	1520
	21st to end	Night	20% 7% 3 6% 6% 4 6% 5% 5% 7% 30%	1520
July	1st to 10th	Night	18% 6% 3 7% 6% 5% 4 4 6% 6% 37%	2520
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 6% 8% 41%	2520
	21st to end	Night	14% 7% 3 7% 5% 4 5% 4 6% 7% 38%	2739
August	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 6% 36%	2525
	11th to 20th	Night	18% 8% 3 8% 4 4 4 3 5% 7% 36%	2899
	21st to end	Night	15% 6% 3 6% 4 4 4 4 6% 7% 40%	3476
September	1st to 10th	Night	16% 7% 2 6% 3 4 4 3 5% 6% 43%	1900
	11th to 20th	Night	15% 6% 2 4 3 4 3 3 4 7% 50%	1933
	21st to end	Night	17% 8% 3 6% 4 4 4 4 6% 7% 38%	2095
October	1st to 10th	Night	22% 8% 3 6% 4 4 4 3 5% 6% 33%	2280
	11th to 20th	Night	21% 10% 2 7% 5% 4 5% 3 5% 6% 32%	2280
	21st to end	Night	27% 13% 3 8% 5% 4 4 3 4 6% 24%	2688
November	1st to 10th	Night	20% 12% 4 7% 5% 4 5% 3 5% 5% 31%	2537
	11th to 20th	Night	18% 14% 4 8% 5% 5% 4 3 4 5% 29%	2714
	21st to end	Night	22% 17% 5% 10% 5% 4 4 3 4 5% 21%	2800
December	1st to 10th	Night	18% 28% 8% 14% 6% 4 3 3 3 4 10%	4580
	11th to 20th	Night	22% 20% 6% 12% 5% 4 3 2 2 3 20%	5208
	21st to end	Night	22% 19% 5% 13% 5% 5% 4 2 3 4 18%	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	76% 6% 7% 10% 2	3598
	11th to 20th	Day	76% 6% 7% 6% 5%	3600
	21st to end	Day	74% 5% 8% 7% 5%	4080
February	1st to 10th	Day	70% 5% 6% 14% 3 1	1800
	11th to 20th	Day	69% 4 8% 12% 6% 1	1823
	21st to end	Day	67% 6% 8% 12% 7% 1	1570
March	1st to 10th	Day	61% 6% 9% 17% 7% 1	3119
	11th to 20th	Day	63% 6% 11% 11% 8% 1	3406
	21st to end	Day	50% 5% 8% 19% 15% 2	4088
April	1st to 10th	Day	58% 4 7% 19% 10% 2	2280
	11th to 20th	Day	53% 5% 7% 19% 14% 2	2254
	21st to end	Day	55% 4 10% 15% 13% 3	2362
May	1st to 10th	Day	56% 6% 10% 19% 9% 1	4440
	11th to 20th	Day	52% 9% 11% 18% 9% 1	4402
	21st to end	Day	44% 8% 11% 24% 12% 1	4837
June	1st to 10th	Day	53% 8% 11% 17% 11% 1	2660
	11th to 20th	Day	46% 7% 12% 24% 10% 2	2660
	21st to end	Day	52% 8% 10% 20% 10%	2660
July	1st to 10th	Day	41% 7% 14% 28% 10% 1	4370
	11th to 20th	Day	41% 7% 16% 29% 7%	4365
	21st to end	Day	40% 8% 13% 31% 7%	4661
August	1st to 10th	Day	47% 6% 15% 23% 9%	4387
	11th to 20th	Day	45% 7% 17% 25% 6%	3936
	21st to end	Day	43% 6% 17% 24% 10%	4180
September	1st to 10th	Day	42% 6% 14% 26% 12% 1	2280
	11th to 20th	Day	38% 5% 12% 30% 14% 1	2247
	21st to end	Day	45% 7% 13% 25% 9%	2083
October	1st to 10th	Day	47% 5% 12% 26% 9%	3160
	11th to 20th	Day	52% 4 14% 23% 7%	3140
	21st to end	Day	49% 5% 17% 23% 5% 1	3122
November	1st to 10th	Day	56% 6% 11% 20% 6%	1761
	11th to 20th	Day	59% 7% 9% 15% 9% 2	1685
	21st to end	Day	69% 6% 11% 9% 5%	1600
December	1st to 10th	Day	80% 4 9% 6% 1	2600
	11th to 20th	Day	69% 6% 8% 12% 5%	2986
	21st to end	Day	68% 5% 7% 15% 5%	3960
January	1st to 10th	Night	77% 5% 8% 8% 2	6349
	11th to 20th	Night	79% 6% 6% 6% 3	6360
	21st to end	Night	76% 4 7% 7% 6% 1	6861
February	1st to 10th	Night	72% 4 8% 10% 5%	2600
	11th to 20th	Night	67% 5% 8% 15% 6%	2472
	21st to end	Night	64% 5% 8% 15% 8%	1885
March	1st to 10th	Night	57% 5% 8% 20% 9% 1	3734
	11th to 20th	Night	63% 6% 12% 12% 6% 1	3552
	21st to end	Night	54% 4 5% 21% 13% 3	3544
April	1st to 10th	Night	58% 5% 8% 18% 11% 1	1900
	11th to 20th	Night	55% 6% 8% 18% 12% 1	1875
	21st to end	Night	54% 4 9% 17% 13% 3	1824
May	1st to 10th	Night	56% 7% 12% 14% 10% 2	2530
	11th to 20th	Night	50% 8% 12% 19% 9% 1	2496
	21st to end	Night	42% 6% 13% 21% 17% 1	2739
June	1st to 10th	Night	51% 8% 13% 18% 10% 1	1520
	11th to 20th	Night	43% 7% 14% 20% 16% 1	1520
	21st to end	Night	51% 8% 12% 19% 10%	1520
July	1st to 10th	Night	46% 5% 12% 22% 14% 1	2480
	11th to 20th	Night	39% 7% 16% 30% 8%	2472
	21st to end	Night	43% 8% 13% 23% 12% 1	2651
August	1st to 10th	Night	48% 6% 14% 23% 9%	2515
	11th to 20th	Night	47% 7% 16% 22% 8% 1	2926
	21st to end	Night	41% 6% 15% 26% 11% 1	3476
September	1st to 10th	Night	42% 7% 12% 26% 13% 1	1900
	11th to 20th	Night	35% 6% 14% 31% 13%	1933
	21st to end	Night	44% 6% 11% 26% 12% 1	2095
October	1st to 10th	Night	48% 6% 12% 25% 9% 1	3800
	11th to 20th	Night	49% 5% 14% 21% 10%	3779
	21st to end	Night	54% 5% 16% 19% 6% 1	4426
November	1st to 10th	Night	55% 6% 13% 17% 8% 1	2537
	11th to 20th	Night	59% 4 12% 16% 7% 3	2714
	21st to end	Night	67% 5% 11% 11% 4	2800
December	1st to 10th	Night	80% 5% 8% 6% 1	4580
	11th to 20th	Night	71% 5% 6% 12% 5%	5208
	21st to end	Night	71% 5% 6% 10% 7% 1	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	32% 17% 34% 13% 5%	200
	11th to 20th	Day	31% 24% 32% 11% 3 1	200
	21st to end	Day	29% 23% 31% 16% 1	247
February	1st to 10th	Day	40% 29% 27% 4	300
	11th to 20th	Day	33% 30% 32% 5%	361
	21st to end	Day	37% 23% 30% 9% 1	382
March	1st to 10th	Day	33% 20% 31% 13% 3	460
	11th to 20th	Day	35% 23% 28% 12% 3 1	460
	21st to end	Day	37% 22% 23% 11% 6%	506
April	1st to 10th	Day	37% 19% 28% 12% 4	460
	11th to 20th	Day	35% 20% 23% 15% 4 2	452
	21st to end	Day	37% 19% 28% 10% 6% 1	460
May	1st to 10th	Day	31% 16% 29% 16% 6% 1	463
	11th to 20th	Day	23% 19% 30% 19% 8% 1	487
	21st to end	Day	22% 16% 29% 24% 9%	529
June	1st to 10th	Day	31% 16% 28% 18% 6% 1	490
	11th to 20th	Day	19% 17% 34% 21% 7% 3	490
	21st to end	Day	28% 15% 32% 15% 9% 1	490
July	1st to 10th	Day	17% 11% 33% 28% 11% 1	485
	11th to 20th	Day	15% 13% 33% 28% 8% 2	484
	21st to end	Day	18% 11% 33% 30% 8% 1	532
August	1st to 10th	Day	19% 12% 29% 29% 10% 2	486
	11th to 20th	Day	18% 10% 26% 30% 13% 4	479
	21st to end	Day	19% 12% 33% 22% 11% 4	539
September	1st to 10th	Day	19% 13% 29% 24% 10% 6%	490
	11th to 20th	Day	16% 11% 24% 24% 17% 8%	490
	21st to end	Day	20% 11% 30% 25% 11% 3	490
October	1st to 10th	Day	28% 17% 25% 18% 9% 4	490
	11th to 20th	Day	29% 19% 30% 14% 6% 1	490
	21st to end	Day	39% 14% 27% 17% 3	444
November	1st to 10th	Day	32% 18% 29% 16% 5% 1	395
	11th to 20th	Day	31% 24% 28% 10% 6% 1	285
	21st to end	Day	35% 23% 31% 9% 3	200
December	1st to 10th	Day	40% 13% 28% 18% 2	200
	11th to 20th	Day	33% 28% 31% 7% 1 1	200
	21st to end	Day	37% 20% 28% 12% 2 1	220
January	1st to 10th	Night	32% 22% 32% 12% 3	800
	11th to 20th	Night	32% 22% 27% 15% 3 1	800
	21st to end	Night	22% 23% 33% 18% 4	880
February	1st to 10th	Night	30% 23% 32% 14% 2	800
	11th to 20th	Night	28% 22% 36% 13% 2	783
	21st to end	Night	29% 19% 34% 15% 3	629
March	1st to 10th	Night	29% 19% 30% 19% 2	760
	11th to 20th	Night	32% 19% 30% 15% 3	760
	21st to end	Night	31% 17% 27% 19% 5% 1	836
April	1st to 10th	Night	32% 16% 30% 19% 3	760
	11th to 20th	Night	32% 18% 28% 16% 6%	751
	21st to end	Night	33% 14% 30% 17% 6%	684
May	1st to 10th	Night	29% 12% 33% 21% 5% 1	390
	11th to 20th	Night	22% 11% 35% 26% 5% 1	374
	21st to end	Night	18% 12% 34% 27% 8%	411
June	1st to 10th	Night	25% 10% 27% 31% 5% 1	380
	11th to 20th	Night	19% 10% 29% 29% 11% 1	380
	21st to end	Night	21% 9% 35% 26% 8% 1	380
July	1st to 10th	Night	12% 9% 32% 38% 8% 1	370
	11th to 20th	Night	13% 6% 33% 36% 13%	368
	21st to end	Night	16% 6% 34% 36% 7% 1	416
August	1st to 10th	Night	17% 7% 28% 34% 14%	384
	11th to 20th	Night	20% 6% 23% 33% 16% 1	626
	21st to end	Night	17% 8% 32% 30% 11% 2	836
September	1st to 10th	Night	18% 10% 29% 29% 12% 1	760
	11th to 20th	Night	17% 9% 24% 28% 18% 3	760
	21st to end	Night	20% 13% 25% 25% 15% 1	760
October	1st to 10th	Night	26% 13% 29% 20% 12%	760
	11th to 20th	Night	29% 16% 26% 22% 6% 1	760
	21st to end	Night	37% 14% 26% 19% 2	836
November	1st to 10th	Night	30% 17% 30% 17% 5% 1	780
	11th to 20th	Night	30% 19% 28% 15% 8% 1	800
	21st to end	Night	33% 22% 25% 14% 6%	800
December	1st to 10th	Night	38% 20% 25% 15% 2	800
	11th to 20th	Night	32% 23% 28% 13% 4 1	800
	21st to end	Night	36% 18% 28% 14% 3	880

Page generated at 2025-02-18 03:26:32. Version ac07

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