Otterholt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Otterholt clear sky chart from 2005-11-06 to 2024-07-02.

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	14% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	89652
	Night	13% 10% 4 8% 5% 4 4 4 5% 6% 36%	89586
Transparency	Day	47% 6% 13% 22% 10% 1	89406
	Night	47% 6% 12% 22% 12% 1	89437
Seeing	Day	17% 10% 29% 26% 14% 4	15533
	Night	17% 9% 31% 31% 11% 1	25480

By Month

Percentage Cloud Cover SE (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% 17% 7% 14% 6% 5% 5% 3 5% 5% 23%	2527
	11th to 20th	Day	7% 11% 6% 14% 8% 10% 10% 6% 6% 5% 16%	2530
	21st to end	Day	10% 19% 8% 15% 5% 4 4 4 5% 6% 21%	2984
February	1st to 10th	Day	8% 13% 4 10% 6% 5% 5% 5% 5% 7% 32%	1710
	11th to 20th	Day	9% 10% 5% 10% 7% 4 4 4 6% 7% 34%	1664
	21st to end	Day	12% 12% 5% 10% 6% 4 4 3 5% 6% 33%	1534
March	1st to 10th	Day	12% 9% 3 7% 4 4 5% 3 4 6% 43%	3211
	11th to 20th	Day	9% 10% 3 9% 5% 5% 4 4 5% 7% 38%	3480
	21st to end	Day	20% 12% 3 12% 6% 4 5% 3 4 6% 26%	3964
April	1st to 10th	Day	16% 12% 3 7% 5% 5% 5% 4 6% 7% 32%	2280
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 4 5% 6% 30%	2254
	21st to end	Day	21% 11% 3 8% 5% 4 4 4 5% 6% 30%	2403
May	1st to 10th	Day	18% 7% 2 6% 4 5% 4 4 4 6% 41%	4399
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 5% 7% 41%	4399
	21st to end	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 35%	4824
June	1st to 10th	Day	12% 6% 2 5% 4 5% 4 4 6% 9% 42%	2660
	11th to 20th	Day	12% 7% 2 8% 6% 5% 4 4 7% 8% 36%	2660
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 8% 10% 39%	2660
July	1st to 10th	Day	7% 4 1 4 4 4 6% 5% 7% 10% 48%	2535
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 7% 9% 46%	2520
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 8% 49%	2743
August	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	2413
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 7% 43%	2301
	21st to end	Day	12% 5% 2 5% 4 3 5% 4 6% 8% 47%	2406
September	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	2160
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 45%	2160
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 40%	1903
October	1st to 10th	Day	16% 7% 2 7% 4 4 5% 4 5% 7% 39%	1800
	11th to 20th	Day	17% 13% 3 6% 5% 4 4 4 5% 6% 32%	1800
	21st to end	Day	18% 11% 4 7% 4 4 4 3 4 5% 35%	1877
November	1st to 10th	Day	13% 12% 4 7% 5% 4 4 3 5% 6% 37%	1630
	11th to 20th	Day	14% 14% 6% 10% 7% 5% 4 4 5% 5% 26%	1520
	21st to end	Day	15% 11% 5% 7% 5% 4 3 4 6% 7% 34%	1520
December	1st to 10th	Day	11% 14% 6% 13% 8% 6% 6% 4 5% 5% 22%	1520
	11th to 20th	Day	12% 15% 7% 11% 8% 5% 5% 3 5% 5% 25%	1918
	21st to end	Day	23% 18% 5% 14% 5% 4 3 3 4 3 18%	2783
January	1st to 10th	Night	11% 15% 7% 14% 6% 4 4 3 5% 5% 28%	4428
	11th to 20th	Night	7% 12% 5% 14% 8% 8% 7% 7% 6% 5% 21%	4440
	21st to end	Night	11% 20% 9% 13% 5% 4 4 3 5% 5% 22%	4679
February	1st to 10th	Night	7% 11% 6% 11% 6% 6% 4 3 5% 6% 34%	2470
	11th to 20th	Night	8% 11% 3 9% 6% 5% 5% 3 6% 7% 37%	2403
	21st to end	Night	12% 12% 4 7% 5% 5% 5% 4 6% 7% 34%	1919
March	1st to 10th	Night	8% 7% 3 7% 4 4 4 4 5% 9% 45%	3673
	11th to 20th	Night	9% 7% 3 10% 6% 4 5% 4 7% 6% 40%	3480
	21st to end	Night	24% 11% 4 11% 5% 4 3 2 3 4 28%	3666
April	1st to 10th	Night	20% 9% 2 7% 4 5% 4 4 5% 7% 34%	1900
	11th to 20th	Night	23% 9% 3 5% 4 4 4 3 5% 5% 34%	1875
	21st to end	Night	19% 10% 3 7% 5% 4 4 3 6% 7% 35%	1775
May	1st to 10th	Night	16% 8% 2 6% 5% 4 4 4 6% 6% 38%	2506
	11th to 20th	Night	15% 5% 2 5% 4 3 3 2 5% 6% 51%	2506
	21st to end	Night	13% 7% 3 8% 5% 5% 4 4 6% 7% 40%	2743
June	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 8% 8% 45%	1520
	11th to 20th	Night	10% 6% 2 5% 5% 5% 4 4 6% 8% 45%	1520
	21st to end	Night	8% 5% 2 6% 4 4 5% 5% 7% 10% 46%	1520
July	1st to 10th	Night	6% 5% 1 4 3 3 3 4 6% 9% 55%	1453
	11th to 20th	Night	9% 4 2 4 4 4 4 4 7% 10% 48%	1440
	21st to end	Night	4 3 1 4 3 4 4 3 7% 9% 56%	1575
August	1st to 10th	Night	8% 5% 1 6% 4 5% 5% 4 6% 8% 50%	1523
	11th to 20th	Night	13% 7% 2 4 3 3 3 4 6% 8% 48%	1600
	21st to end	Night	11% 5% 2 6% 4 4 3 3 4 6% 52%	1948
September	1st to 10th	Night	10% 4 3 6% 4 4 4 4 5% 7% 51%	1800
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 51%	1800
	21st to end	Night	15% 6% 2 6% 3 4 5% 3 5% 7% 45%	2038
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 46%	2160
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 4 7% 38%	2160
	21st to end	Night	18% 10% 4 7% 5% 4 4 4 4 5% 36%	2479
November	1st to 10th	Night	14% 10% 3 7% 4 4 4 3 5% 8% 39%	2444
	11th to 20th	Night	14% 11% 3 10% 6% 4 4 3 5% 6% 34%	2660
	21st to end	Night	14% 10% 4 9% 5% 4 4 4 5% 6% 34%	2660
December	1st to 10th	Night	11% 13% 5% 12% 7% 5% 4 3 5% 7% 28%	2660
	11th to 20th	Night	12% 15% 5% 11% 7% 5% 5% 4 5% 5% 25%	3279
	21st to end	Night	24% 17% 5% 10% 5% 4 4 3 4 4 20%	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	63% 5% 11% 17% 4	2528
	11th to 20th	Day	66% 6% 9% 13% 6%	2530
	21st to end	Day	63% 5% 13% 14% 4	2984
February	1st to 10th	Day	51% 6% 13% 20% 9% 1	1710
	11th to 20th	Day	48% 6% 13% 23% 10% 1	1673
	21st to end	Day	51% 6% 12% 20% 9% 2	1534
March	1st to 10th	Day	41% 5% 16% 21% 14% 2	3211
	11th to 20th	Day	43% 5% 13% 27% 12% 1	3480
	21st to end	Day	61% 5% 9% 20% 5% 1	3964
April	1st to 10th	Day	50% 6% 11% 19% 12% 1	2280
	11th to 20th	Day	55% 6% 10% 17% 11% 1	2254
	21st to end	Day	54% 6% 14% 17% 9% 1	2403
May	1st to 10th	Day	42% 6% 11% 26% 12% 2	4440
	11th to 20th	Day	40% 7% 14% 24% 13% 1	4403
	21st to end	Day	48% 7% 11% 22% 11% 1	4837
June	1st to 10th	Day	36% 8% 15% 28% 12% 1	2660
	11th to 20th	Day	44% 8% 13% 21% 12% 1	2660
	21st to end	Day	37% 9% 18% 26% 10% 1	2660
July	1st to 10th	Day	29% 9% 17% 31% 13% 1	2466
	11th to 20th	Day	33% 7% 16% 25% 17% 2	2445
	21st to end	Day	30% 8% 18% 28% 15% 1	2591
August	1st to 10th	Day	37% 9% 16% 28% 9% 2	2398
	11th to 20th	Day	39% 8% 15% 27% 11%	2301
	21st to end	Day	33% 7% 15% 28% 15% 1	2406
September	1st to 10th	Day	36% 8% 14% 29% 13%	2160
	11th to 20th	Day	37% 8% 16% 31% 8% 1	2160
	21st to end	Day	43% 6% 13% 28% 9%	1903
October	1st to 10th	Day	43% 6% 13% 28% 9% 1	1800
	11th to 20th	Day	51% 6% 13% 20% 9% 1	1800
	21st to end	Day	51% 5% 14% 21% 9%	1877
November	1st to 10th	Day	47% 6% 11% 21% 12% 2	1630
	11th to 20th	Day	58% 6% 13% 16% 7% 1	1520
	21st to end	Day	49% 5% 16% 22% 7% 2	1520
December	1st to 10th	Day	63% 6% 13% 14% 4	1520
	11th to 20th	Day	62% 4 14% 14% 5% 1	1915
	21st to end	Day	72% 5% 7% 11% 6%	2783
January	1st to 10th	Night	58% 6% 11% 18% 7% 1	4428
	11th to 20th	Night	62% 7% 12% 13% 7%	4440
	21st to end	Night	64% 6% 8% 17% 5% 1	4679
February	1st to 10th	Night	50% 6% 11% 21% 10% 2	2470
	11th to 20th	Night	45% 6% 14% 25% 10% 1	2404
	21st to end	Night	49% 6% 12% 21% 11% 1	1919
March	1st to 10th	Night	37% 6% 15% 26% 14% 2	3673
	11th to 20th	Night	41% 6% 12% 26% 14% 1	3480
	21st to end	Night	61% 4 7% 18% 9% 1	3666
April	1st to 10th	Night	50% 5% 8% 18% 17% 1	1900
	11th to 20th	Night	50% 6% 8% 19% 14% 2	1875
	21st to end	Night	47% 6% 11% 21% 14% 1	1775
May	1st to 10th	Night	43% 7% 14% 17% 18% 1	2530
	11th to 20th	Night	35% 5% 8% 29% 20% 3	2507
	21st to end	Night	40% 7% 14% 22% 16% 1	2751
June	1st to 10th	Night	32% 10% 14% 27% 16% 1	1520
	11th to 20th	Night	35% 7% 14% 24% 19% 1	1520
	21st to end	Night	31% 10% 15% 29% 13% 2	1520
July	1st to 10th	Night	23% 7% 17% 32% 19% 2	1413
	11th to 20th	Night	29% 9% 14% 28% 18% 3	1392
	21st to end	Night	22% 7% 18% 31% 19% 3	1487
August	1st to 10th	Night	31% 7% 14% 31% 14% 4	1514
	11th to 20th	Night	34% 8% 15% 26% 16% 2	1600
	21st to end	Night	34% 6% 12% 29% 16% 3	1948
September	1st to 10th	Night	32% 7% 13% 33% 15% 1	1800
	11th to 20th	Night	31% 6% 16% 32% 15%	1800
	21st to end	Night	37% 6% 14% 31% 11% 1	2038
October	1st to 10th	Night	38% 6% 12% 30% 13% 1	2160
	11th to 20th	Night	45% 6% 14% 22% 12% 1	2160
	21st to end	Night	49% 6% 13% 20% 12% 1	2479
November	1st to 10th	Night	43% 5% 13% 24% 13% 1	2444
	11th to 20th	Night	51% 5% 15% 17% 12% 1	2660
	21st to end	Night	49% 6% 15% 22% 6% 1	2660
December	1st to 10th	Night	56% 6% 13% 20% 5%	2660
	11th to 20th	Night	59% 6% 12% 17% 5% 1	3281
	21st to end	Night	68% 4 8% 15% 5%	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	17% 11% 32% 26% 10% 3	270
	11th to 20th	Day	19% 9% 38% 26% 8%	270
	21st to end	Day	17% 8% 40% 24% 10% 1	297
February	1st to 10th	Day	16% 13% 45% 17% 8% 1	270
	11th to 20th	Day	15% 19% 39% 19% 7% 1	270
	21st to end	Day	14% 14% 45% 24% 2	225
March	1st to 10th	Day	19% 12% 39% 20% 10% 1	270
	11th to 20th	Day	20% 10% 37% 23% 8% 2	270
	21st to end	Day	23% 13% 31% 22% 7% 4	383
April	1st to 10th	Day	23% 19% 29% 22% 7%	460
	11th to 20th	Day	29% 12% 25% 25% 8% 2	452
	21st to end	Day	26% 14% 28% 21% 9% 2	578
May	1st to 10th	Day	20% 15% 29% 26% 8% 2	653
	11th to 20th	Day	20% 12% 27% 25% 12% 4	675
	21st to end	Day	19% 10% 26% 23% 18% 4	734
June	1st to 10th	Day	11% 9% 28% 30% 17% 4	680
	11th to 20th	Day	14% 9% 33% 29% 12% 3	680
	21st to end	Day	14% 6% 29% 29% 15% 6%	680
July	1st to 10th	Day	7% 4 21% 29% 27% 11%	627
	11th to 20th	Day	10% 5% 27% 31% 19% 8%	618
	21st to end	Day	8% 4 24% 33% 22% 8%	684
August	1st to 10th	Day	11% 4 22% 33% 23% 6%	626
	11th to 20th	Day	13% 4 19% 29% 27% 8%	615
	21st to end	Day	14% 6% 22% 28% 23% 5%	525
September	1st to 10th	Day	13% 7% 25% 30% 20% 5%	450
	11th to 20th	Day	12% 7% 27% 32% 20% 2	450
	21st to end	Day	22% 11% 26% 27% 12% 3	337
October	1st to 10th	Day	19% 10% 29% 21% 19% 3	270
	11th to 20th	Day	21% 19% 30% 23% 7%	270
	21st to end	Day	25% 14% 38% 16% 5% 1	297
November	1st to 10th	Day	20% 11% 36% 23% 9% 1	270
	11th to 20th	Day	21% 17% 34% 20% 6%	270
	21st to end	Day	21% 17% 35% 19% 7%	270
December	1st to 10th	Day	15% 14% 39% 21% 10% 1	270
	11th to 20th	Day	22% 13% 37% 21% 6%	270
	21st to end	Day	28% 10% 40% 19% 2 1	297
January	1st to 10th	Night	14% 11% 37% 29% 8%	950
	11th to 20th	Night	17% 10% 37% 28% 7% 1	950
	21st to end	Night	17% 10% 34% 31% 7%	1045
February	1st to 10th	Night	12% 12% 40% 29% 7% 1	950
	11th to 20th	Night	13% 14% 40% 26% 7% 1	929
	21st to end	Night	17% 12% 33% 31% 6%	663
March	1st to 10th	Night	16% 12% 37% 28% 7% 1	726
	11th to 20th	Night	16% 11% 36% 29% 7% 1	570
	21st to end	Night	21% 9% 32% 26% 9% 3	627
April	1st to 10th	Night	22% 11% 34% 27% 5%	570
	11th to 20th	Night	26% 9% 30% 28% 7% 1	563
	21st to end	Night	22% 8% 32% 29% 8% 1	570
May	1st to 10th	Night	19% 9% 32% 32% 6% 1	570
	11th to 20th	Night	18% 7% 31% 31% 11% 1	561
	21st to end	Night	15% 8% 23% 40% 15% 1	617
June	1st to 10th	Night	12% 4 31% 42% 11% 1	570
	11th to 20th	Night	13% 4 32% 41% 10%	570
	21st to end	Night	9% 3 24% 46% 16% 1	570
July	1st to 10th	Night	7% 4 21% 40% 27% 2	530
	11th to 20th	Night	11% 4 24% 40% 19% 2	522
	21st to end	Night	5% 3 20% 47% 22% 2	591
August	1st to 10th	Night	9% 4 14% 47% 25% 1	537
	11th to 20th	Night	15% 3 19% 33% 27% 3	534
	21st to end	Night	14% 3 25% 32% 24% 2	594
September	1st to 10th	Night	9% 3 28% 36% 20% 3	540
	11th to 20th	Night	11% 4 27% 37% 18% 2	540
	21st to end	Night	16% 10% 29% 30% 14% 1	666
October	1st to 10th	Night	18% 8% 27% 33% 14% 1	720
	11th to 20th	Night	20% 13% 31% 29% 8%	720
	21st to end	Night	23% 10% 30% 29% 8%	792
November	1st to 10th	Night	19% 9% 34% 29% 9% 1	778
	11th to 20th	Night	19% 12% 35% 26% 8%	950
	21st to end	Night	20% 14% 34% 26% 6%	950
December	1st to 10th	Night	18% 9% 37% 28% 8% 1	950
	11th to 20th	Night	21% 10% 36% 24% 8% 2	950
	21st to end	Night	27% 10% 33% 24% 6%	1045

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