Stillwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stillwell clear sky chart from 2005-11-06 to 2024-01-12.

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% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	74460
	Night	16% 10% 4 7% 6% 3 4 4 5% 7% 35%	75262
Transparency	Day	53% 3 10% 21% 12% 1	74181
	Night	51% 3 10% 22% 13% 2	75060
Seeing	Day	22% 14% 28% 20% 12% 4	13590
	Night	22% 11% 27% 26% 13% 2	23935

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	19% 16% 7% 9% 8% 4 4 4 4 5% 20%	1706
	11th to 20th	Day	21% 20% 7% 10% 8% 3 4 2 4 5% 18%	1635
	21st to end	Day	17% 20% 7% 11% 9% 4 3 5% 4 4 16%	1782
February	1st to 10th	Day	21% 16% 6% 10% 6% 4 4 3 3 4 23%	1620
	11th to 20th	Day	17% 12% 7% 13% 7% 3 5% 3 4 5% 23%	1671
	21st to end	Day	17% 16% 6% 9% 5% 3 3 3 3 5% 30%	1480
March	1st to 10th	Day	18% 12% 6% 7% 5% 3 4 5% 4 6% 29%	1912
	11th to 20th	Day	18% 13% 5% 7% 6% 3 4 4 4 6% 29%	1980
	21st to end	Day	24% 11% 5% 6% 5% 3 4 3 4 6% 28%	2178
April	1st to 10th	Day	15% 14% 3 7% 6% 3 4 4 4 7% 33%	1980
	11th to 20th	Day	18% 10% 3 7% 6% 2 5% 4 5% 6% 34%	2304
	21st to end	Day	20% 9% 3 7% 7% 3 5% 5% 5% 5% 31%	2340
May	1st to 10th	Day	18% 9% 3 7% 7% 4 6% 5% 5% 7% 29%	2340
	11th to 20th	Day	22% 8% 3 7% 6% 3 5% 4 5% 7% 31%	2301
	21st to end	Day	10% 6% 2 6% 7% 4 6% 7% 8% 10% 35%	2531
June	1st to 10th	Day	12% 6% 3 7% 6% 4 6% 6% 7% 7% 36%	2340
	11th to 20th	Day	12% 7% 3 7% 6% 5% 6% 6% 6% 8% 34%	2340
	21st to end	Day	12% 5% 2 4 6% 4 7% 6% 9% 10% 36%	2340
July	1st to 10th	Day	12% 7% 3 6% 6% 4 6% 5% 6% 9% 37%	2340
	11th to 20th	Day	11% 5% 3 5% 6% 3 6% 5% 6% 9% 40%	2340
	21st to end	Day	10% 5% 3 5% 7% 4 6% 6% 7% 10% 37%	2547
August	1st to 10th	Day	10% 5% 2 5% 6% 4 6% 7% 7% 11% 37%	2327
	11th to 20th	Day	11% 5% 3 6% 6% 4 7% 6% 7% 9% 35%	2303
	21st to end	Day	9% 5% 2 5% 6% 4 6% 6% 6% 10% 40%	2549
September	1st to 10th	Day	14% 5% 2 5% 5% 3 4 5% 5% 8% 43%	2012
	11th to 20th	Day	10% 5% 2 5% 5% 3 7% 6% 7% 8% 42%	1980
	21st to end	Day	19% 6% 3 5% 5% 3 5% 3 4 6% 40%	1980
October	1st to 10th	Day	13% 7% 3 7% 6% 3 5% 4 5% 7% 40%	1890
	11th to 20th	Day	16% 9% 3 6% 5% 3 5% 4 6% 6% 38%	1800
	21st to end	Day	22% 11% 3 6% 5% 4 3 4 5% 5% 33%	1964
November	1st to 10th	Day	11% 8% 3 7% 5% 2 4 5% 5% 6% 44%	1672
	11th to 20th	Day	18% 12% 4 6% 6% 3 4 4 5% 5% 33%	1710
	21st to end	Day	19% 14% 4 8% 5% 3 4 5% 4 6% 28%	1644
December	1st to 10th	Day	20% 18% 8% 9% 6% 3 4 4 3 5% 20%	1520
	11th to 20th	Day	19% 16% 6% 10% 6% 2 4 3 5% 6% 22%	1978
	21st to end	Day	25% 24% 6% 9% 6% 3 3 3 4 4 12%	3124
January	1st to 10th	Night	19% 16% 8% 8% 6% 4 5% 4 3 4 23%	2459
	11th to 20th	Night	20% 17% 6% 10% 8% 3 4 4 4 4 20%	2365
	21st to end	Night	18% 18% 6% 10% 8% 3 4 4 4 5% 21%	2574
February	1st to 10th	Night	21% 15% 5% 9% 7% 3 4 4 4 5% 23%	2340
	11th to 20th	Night	19% 13% 5% 10% 7% 4 5% 3 4 6% 24%	2171
	21st to end	Night	21% 14% 7% 8% 7% 3 4 4 4 4 24%	1776
March	1st to 10th	Night	17% 11% 5% 8% 6% 3 4 4 5% 6% 32%	2048
	11th to 20th	Night	17% 10% 4 6% 6% 3 3 4 5% 7% 35%	1980
	21st to end	Night	20% 10% 4 7% 6% 3 3 4 4 6% 33%	2178
April	1st to 10th	Night	17% 10% 3 6% 5% 3 4 4 4 7% 36%	1978
	11th to 20th	Night	16% 8% 3 6% 5% 3 5% 4 7% 6% 37%	1620
	21st to end	Night	16% 9% 2 6% 5% 4 4 4 5% 7% 38%	1620
May	1st to 10th	Night	18% 8% 3 6% 7% 3 5% 5% 6% 7% 31%	1620
	11th to 20th	Night	18% 7% 4 6% 5% 3 5% 3 4 6% 39%	1592
	21st to end	Night	8% 5% 3 5% 6% 3 6% 5% 7% 10% 42%	1744
June	1st to 10th	Night	9% 5% 2 6% 6% 3 5% 5% 5% 8% 45%	1620
	11th to 20th	Night	9% 6% 2 5% 5% 4 4 5% 6% 12% 42%	1620
	21st to end	Night	11% 4 2 4 6% 3 4 6% 6% 10% 44%	1620
July	1st to 10th	Night	8% 4 2 6% 5% 3 5% 5% 7% 9% 46%	1620
	11th to 20th	Night	8% 3 2 5% 5% 3 4 6% 6% 9% 48%	1620
	21st to end	Night	8% 5% 3 6% 5% 4 6% 6% 7% 9% 43%	1771
August	1st to 10th	Night	6% 4 2 5% 6% 4 5% 7% 8% 9% 44%	1611
	11th to 20th	Night	8% 6% 2 5% 4 3 5% 5% 6% 9% 46%	1598
	21st to end	Night	9% 4 2 4 4 3 5% 5% 6% 9% 49%	1788
September	1st to 10th	Night	14% 4 1 4 4 2 4 4 4 7% 51%	1945
	11th to 20th	Night	10% 3 2 4 3 2 3 4 5% 8% 55%	1980
	21st to end	Night	16% 6% 3 5% 4 3 3 4 5% 6% 46%	1980
October	1st to 10th	Night	12% 5% 2 5% 4 3 5% 4 6% 8% 46%	2070
	11th to 20th	Night	16% 8% 3 7% 6% 3 3 4 4 6% 39%	2160
	21st to end	Night	23% 9% 3 5% 4 3 4 4 4 6% 34%	2389
November	1st to 10th	Night	15% 7% 3 5% 4 3 4 3 5% 6% 45%	2406
	11th to 20th	Night	18% 10% 5% 6% 5% 3 4 3 5% 7% 34%	2470
	21st to end	Night	19% 12% 6% 7% 5% 3 5% 4 5% 5% 30%	2530
December	1st to 10th	Night	23% 16% 6% 9% 5% 3 3 5% 4 5% 22%	2660
	11th to 20th	Night	18% 16% 7% 8% 5% 3 4 3 4 6% 25%	3207
	21st to end	Night	25% 20% 7% 8% 7% 3 4 3 4 5% 15%	4532

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	68% 3 8% 15% 6%	1707
	11th to 20th	Day	72% 2 11% 11% 4	1635
	21st to end	Day	72% 2 7% 13% 4 1	1782
February	1st to 10th	Day	67% 2 8% 15% 7% 1	1620
	11th to 20th	Day	65% 3 7% 16% 8% 1	1680
	21st to end	Day	61% 1 9% 19% 8% 2	1480
March	1st to 10th	Day	57% 2 10% 23% 7%	1912
	11th to 20th	Day	58% 2 9% 19% 11% 2	1980
	21st to end	Day	60% 2 7% 21% 10% 1	2178
April	1st to 10th	Day	53% 2 11% 19% 12% 2	1980
	11th to 20th	Day	52% 2 11% 17% 15% 2	2304
	21st to end	Day	55% 3 8% 20% 12% 2	2340
May	1st to 10th	Day	55% 2 11% 20% 10% 2	2340
	11th to 20th	Day	54% 3 11% 18% 13% 1	2305
	21st to end	Day	43% 5% 13% 25% 13% 1	2531
June	1st to 10th	Day	45% 4 11% 25% 14% 1	2340
	11th to 20th	Day	47% 4 12% 22% 13% 1	2340
	21st to end	Day	41% 4 14% 23% 14% 3	2340
July	1st to 10th	Day	45% 4 11% 25% 15% 1	2275
	11th to 20th	Day	41% 4 11% 26% 16% 2	2271
	21st to end	Day	41% 4 13% 26% 14% 2	2405
August	1st to 10th	Day	41% 5% 13% 27% 14% 1	2314
	11th to 20th	Day	44% 5% 12% 26% 12% 1	2303
	21st to end	Day	40% 3 12% 25% 16% 5%	2549
September	1st to 10th	Day	39% 4 12% 23% 19% 3	2012
	11th to 20th	Day	39% 3 12% 27% 17% 2	1980
	21st to end	Day	47% 2 8% 23% 19% 1	1980
October	1st to 10th	Day	44% 4 9% 24% 19% 1	1890
	11th to 20th	Day	47% 2 11% 24% 15% 1	1800
	21st to end	Day	55% 2 11% 20% 10% 2	1964
November	1st to 10th	Day	42% 3 10% 27% 17% 2	1672
	11th to 20th	Day	55% 2 7% 22% 10% 3	1710
	21st to end	Day	59% 3 10% 19% 8% 1	1644
December	1st to 10th	Day	70% 1 9% 16% 4	1520
	11th to 20th	Day	66% 2 8% 19% 6%	1974
	21st to end	Day	78% 2 10% 7% 3	3124
January	1st to 10th	Night	67% 2 7% 17% 7%	2459
	11th to 20th	Night	69% 2 11% 14% 4	2365
	21st to end	Night	68% 2 8% 16% 6% 1	2574
February	1st to 10th	Night	66% 2 9% 16% 6% 1	2340
	11th to 20th	Night	64% 2 9% 17% 8% 1	2173
	21st to end	Night	65% 3 7% 14% 9% 2	1776
March	1st to 10th	Night	54% 2 9% 22% 12% 1	2048
	11th to 20th	Night	50% 3 10% 24% 10% 3	1980
	21st to end	Night	54% 2 7% 22% 13% 2	2178
April	1st to 10th	Night	49% 2 10% 21% 15% 2	1978
	11th to 20th	Night	47% 2 11% 18% 18% 3	1620
	21st to end	Night	48% 3 10% 21% 16% 2	1620
May	1st to 10th	Night	52% 4 11% 20% 11% 3	1620
	11th to 20th	Night	49% 3 11% 19% 15% 3	1593
	21st to end	Night	38% 4 16% 26% 15% 1	1745
June	1st to 10th	Night	38% 4 12% 25% 18% 3	1620
	11th to 20th	Night	37% 3 15% 25% 17% 2	1620
	21st to end	Night	37% 5% 12% 26% 18% 2	1620
July	1st to 10th	Night	36% 4 13% 23% 20% 4	1575
	11th to 20th	Night	32% 5% 13% 30% 18% 2	1566
	21st to end	Night	37% 4 13% 28% 15% 3	1672
August	1st to 10th	Night	33% 4 15% 28% 19% 1	1600
	11th to 20th	Night	36% 5% 14% 28% 16% 2	1598
	21st to end	Night	31% 4 13% 29% 19% 4	1788
September	1st to 10th	Night	35% 3 12% 26% 19% 6%	1945
	11th to 20th	Night	28% 3 12% 30% 25% 2	1980
	21st to end	Night	41% 3 10% 23% 23% 2	1980
October	1st to 10th	Night	37% 3 11% 26% 22% 2	2070
	11th to 20th	Night	48% 2 8% 25% 15% 1	2160
	21st to end	Night	53% 2 8% 23% 12% 2	2389
November	1st to 10th	Night	42% 2 11% 25% 17% 3	2406
	11th to 20th	Night	52% 3 8% 23% 13% 2	2470
	21st to end	Night	58% 2 11% 22% 6% 1	2530
December	1st to 10th	Night	66% 2 9% 17% 6%	2660
	11th to 20th	Night	62% 2 10% 20% 5%	3210
	21st to end	Night	75% 1 9% 12% 3	4532

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% 24% 30% 9% 5% 1	270
	11th to 20th	Day	35% 21% 36% 7% 1	245
	21st to end	Day	27% 31% 31% 9% 1	264
February	1st to 10th	Day	36% 26% 29% 8% 2	240
	11th to 20th	Day	32% 34% 29% 5%	240
	21st to end	Day	33% 33% 31% 3	198
March	1st to 10th	Day	28% 25% 31% 14% 3	352
	11th to 20th	Day	31% 19% 31% 12% 6% 1	420
	21st to end	Day	35% 17% 28% 17% 3	462
April	1st to 10th	Day	27% 19% 30% 16% 7% 2	420
	11th to 20th	Day	29% 17% 27% 16% 8% 4	412
	21st to end	Day	30% 17% 31% 14% 6% 2	420
May	1st to 10th	Day	29% 17% 27% 16% 6% 4	423
	11th to 20th	Day	25% 11% 32% 19% 9% 4	447
	21st to end	Day	12% 12% 27% 25% 17% 7%	485
June	1st to 10th	Day	15% 9% 33% 26% 13% 4	450
	11th to 20th	Day	18% 12% 23% 28% 14% 5%	450
	21st to end	Day	14% 9% 31% 26% 13% 6%	450
July	1st to 10th	Day	17% 4 19% 25% 23% 12%	445
	11th to 20th	Day	14% 4 25% 30% 18% 10%	444
	21st to end	Day	11% 5% 24% 30% 20% 9%	488
August	1st to 10th	Day	15% 2 19% 32% 21% 11%	446
	11th to 20th	Day	15% 5% 24% 29% 18% 9%	439
	21st to end	Day	9% 5% 22% 30% 24% 10%	495
September	1st to 10th	Day	14% 5% 22% 27% 23% 9%	450
	11th to 20th	Day	11% 4 18% 30% 26% 12%	450
	21st to end	Day	24% 8% 21% 25% 17% 5%	450
October	1st to 10th	Day	14% 9% 29% 26% 18% 5%	450
	11th to 20th	Day	21% 17% 32% 21% 8% 1	450
	21st to end	Day	31% 14% 31% 17% 6%	479
November	1st to 10th	Day	17% 20% 34% 17% 10% 2	270
	11th to 20th	Day	33% 15% 30% 16% 6% 1	270
	21st to end	Day	30% 21% 38% 10% 2	239
December	1st to 10th	Day	26% 27% 32% 10% 3 2	180
	11th to 20th	Day	26% 26% 35% 11% 3 1	200
	21st to end	Day	36% 20% 31% 13% 1	297
January	1st to 10th	Night	29% 19% 33% 16% 3	760
	11th to 20th	Night	30% 18% 32% 17% 3	728
	21st to end	Night	29% 20% 29% 19% 3	792
February	1st to 10th	Night	31% 17% 32% 18% 2	720
	11th to 20th	Night	30% 18% 33% 17% 2	703
	21st to end	Night	32% 19% 31% 16% 2	592
March	1st to 10th	Night	26% 19% 31% 21% 3	720
	11th to 20th	Night	25% 17% 29% 21% 7%	720
	21st to end	Night	28% 14% 31% 21% 6% 1	792
April	1st to 10th	Night	24% 14% 34% 22% 5% 1	720
	11th to 20th	Night	23% 10% 32% 23% 11% 1	556
	21st to end	Night	21% 11% 35% 25% 6% 1	540
May	1st to 10th	Night	24% 8% 29% 28% 11% 1	540
	11th to 20th	Night	22% 6% 25% 31% 15% 1	531
	21st to end	Night	10% 4 22% 37% 24% 2	584
June	1st to 10th	Night	11% 3 27% 39% 19% 1	540
	11th to 20th	Night	14% 5% 21% 38% 20% 2	540
	21st to end	Night	12% 3 21% 38% 24% 1	540
July	1st to 10th	Night	10% 2 16% 38% 28% 6%	525
	11th to 20th	Night	9% 2 19% 35% 30% 6%	522
	21st to end	Night	11% 2 17% 36% 31% 3	591
August	1st to 10th	Night	8% 1 13% 42% 32% 4	537
	11th to 20th	Night	11% 2 14% 40% 28% 5%	534
	21st to end	Night	10% 3 19% 30% 33% 7%	600
September	1st to 10th	Night	17% 3 17% 30% 28% 5%	720
	11th to 20th	Night	12% 3 14% 34% 32% 5%	720
	21st to end	Night	18% 6% 23% 29% 20% 4	720
October	1st to 10th	Night	15% 6% 23% 33% 19% 3	720
	11th to 20th	Night	21% 10% 32% 26% 12% 1	720
	21st to end	Night	29% 11% 28% 25% 7%	792
November	1st to 10th	Night	20% 9% 29% 28% 14% 1	740
	11th to 20th	Night	26% 14% 29% 21% 9% 1	760
	21st to end	Night	27% 14% 29% 23% 6% 1	760
December	1st to 10th	Night	35% 13% 32% 17% 4	760
	11th to 20th	Night	27% 17% 31% 21% 4	760
	21st to end	Night	33% 14% 30% 20% 3	836

Page generated at 2024-01-09 16:24:21. Version ac07

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