WhistleStop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WhistleStop Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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 6% 7% 35%	132281
	Night	16% 10% 3 9% 5% 5% 5% 4 5% 6% 32%	130493
Transparency	Day	47% 6% 12% 22% 12% 1	135849
	Night	49% 6% 12% 20% 12% 1	135011
Seeing	Day	19% 11% 28% 25% 12% 3	16348
	Night	20% 10% 29% 28% 12% 1	25738

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	16% 12% 5% 11% 5% 5% 5% 4 5% 6% 24%	4661
	11th to 20th	Day	9% 22% 8% 18% 10% 6% 5% 4 4 3 11%	4788
	21st to end	Day	13% 15% 7% 12% 8% 5% 5% 4 5% 5% 21%	5780
February	1st to 10th	Day	13% 15% 5% 13% 7% 6% 5% 4 6% 5% 22%	1890
	11th to 20th	Day	14% 10% 4 15% 7% 6% 5% 3 5% 6% 26%	1843
	21st to end	Day	19% 12% 4 10% 5% 5% 5% 4 5% 7% 24%	1648
March	1st to 10th	Day	19% 9% 4 9% 5% 5% 4 4 5% 5% 32%	4922
	11th to 20th	Day	10% 11% 4 10% 6% 5% 5% 4 6% 8% 32%	5090
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 6% 39%	5667
April	1st to 10th	Day	18% 10% 3 8% 5% 4 5% 3 5% 7% 32%	2520
	11th to 20th	Day	19% 8% 3 7% 5% 3 4 3 4 6% 37%	2494
	21st to end	Day	19% 8% 2 7% 5% 4 5% 4 6% 6% 34%	2714
May	1st to 10th	Day	19% 9% 4 9% 5% 4 4 3 5% 6% 31%	6224
	11th to 20th	Day	15% 9% 3 9% 5% 4 4 5% 6% 6% 33%	6402
	21st to end	Day	18% 6% 2 6% 5% 4 4 4 6% 8% 38%	6949
June	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 5% 6% 8% 36%	2800
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 43%	2800
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 6% 7% 42%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 6% 10% 46%	6360
	11th to 20th	Day	9% 4 2 5% 4 4 5% 4 6% 9% 49%	6360
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 7% 9% 49%	6773
August	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 43%	4234
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 7% 10% 41%	4174
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	4386
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 47%	2400
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 7% 7% 52%	2386
	21st to end	Day	14% 6% 2 6% 4 5% 4 4 5% 7% 42%	2095
October	1st to 10th	Day	16% 7% 2 7% 5% 4 4 4 4 7% 38%	2000
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 4 5% 7% 33%	2000
	21st to end	Day	23% 10% 3 8% 4 4 4 3 6% 6% 28%	2190
November	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 5% 8% 36%	1777
	11th to 20th	Day	14% 11% 3 8% 6% 6% 5% 5% 6% 7% 29%	1680
	21st to end	Day	20% 12% 4 9% 6% 5% 4 3 5% 7% 25%	1680
December	1st to 10th	Day	18% 16% 5% 9% 7% 5% 6% 4 5% 5% 18%	2680
	11th to 20th	Day	17% 16% 5% 10% 5% 4 4 3 6% 6% 25%	3066
	21st to end	Day	26% 13% 5% 11% 5% 5% 5% 4 4 4 18%	4048
January	1st to 10th	Night	15% 13% 5% 12% 7% 5% 5% 4 5% 6% 23%	8234
	11th to 20th	Night	13% 16% 6% 17% 8% 6% 6% 5% 6% 4 12%	8080
	21st to end	Night	13% 13% 5% 13% 7% 6% 6% 5% 6% 5% 21%	8401
February	1st to 10th	Night	15% 15% 5% 11% 6% 5% 5% 4 5% 6% 24%	2730
	11th to 20th	Night	14% 13% 6% 9% 6% 5% 5% 5% 5% 6% 26%	2663
	21st to end	Night	16% 11% 3 8% 7% 5% 5% 5% 5% 6% 29%	2157
March	1st to 10th	Night	17% 9% 3 8% 6% 6% 5% 4 5% 5% 31%	5120
	11th to 20th	Night	12% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	5089
	21st to end	Night	21% 7% 2 6% 3 3 3 3 4 5% 41%	5506
April	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 3 6% 7% 34%	2100
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 37%	2075
	21st to end	Night	18% 7% 2 7% 3 4 5% 4 6% 7% 36%	1922
May	1st to 10th	Night	26% 8% 3 7% 4 4 5% 3 5% 5% 29%	3923
	11th to 20th	Night	13% 7% 2 7% 5% 5% 6% 4 7% 6% 36%	3623
	21st to end	Night	18% 6% 3 8% 4 4 5% 4 5% 7% 36%	3926
June	1st to 10th	Night	12% 7% 2 7% 6% 4 5% 4 6% 7% 39%	1600
	11th to 20th	Night	9% 6% 3 6% 5% 5% 5% 5% 7% 9% 42%	1600
	21st to end	Night	12% 7% 2 7% 5% 4 4 4 6% 7% 43%	1600
July	1st to 10th	Night	8% 5% 3 7% 5% 4 5% 4 6% 10% 43%	3600
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	3600
	21st to end	Night	9% 6% 2 5% 3 3 4 3 6% 8% 51%	4075
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	2915
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 5% 7% 49%	2875
	21st to end	Night	9% 5% 1 5% 3 4 5% 4 6% 9% 49%	3478
September	1st to 10th	Night	11% 5% 2 6% 5% 3 4 4 5% 9% 47%	2000
	11th to 20th	Night	10% 4 1 4 3 3 4 3 6% 6% 53%	2010
	21st to end	Night	17% 6% 3 7% 4 4 4 2 3 6% 44%	2305
October	1st to 10th	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	2400
	11th to 20th	Night	19% 8% 3 7% 4 5% 5% 4 6% 7% 32%	2400
	21st to end	Night	21% 8% 3 7% 4 4 5% 4 6% 6% 31%	2650
November	1st to 10th	Night	17% 7% 3 6% 4 5% 5% 4 5% 6% 38%	2738
	11th to 20th	Night	17% 10% 3 7% 6% 5% 4 4 6% 8% 30%	2940
	21st to end	Night	22% 11% 3 7% 5% 6% 5% 4 5% 7% 25%	2940
December	1st to 10th	Night	17% 17% 4 9% 6% 6% 5% 4 6% 6% 21%	4720
	11th to 20th	Night	19% 15% 4 9% 5% 5% 5% 4 6% 5% 23%	5348
	21st to end	Night	26% 14% 4 10% 5% 5% 5% 4 5% 5% 17%	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	56% 8% 11% 17% 8% 1	4662
	11th to 20th	Day	73% 6% 8% 7% 6% 1	4818
	21st to end	Day	63% 5% 9% 14% 9% 1	5780
February	1st to 10th	Day	59% 8% 11% 16% 5% 1	1890
	11th to 20th	Day	58% 7% 10% 15% 9% 1	1852
	21st to end	Day	60% 6% 10% 16% 7%	1648
March	1st to 10th	Day	52% 5% 12% 16% 14% 1	4925
	11th to 20th	Day	49% 6% 20% 15% 9% 2	5090
	21st to end	Day	48% 4 7% 22% 17% 2	5667
April	1st to 10th	Day	52% 5% 9% 19% 12% 2	2520
	11th to 20th	Day	49% 4 9% 22% 15% 1	2494
	21st to end	Day	50% 6% 9% 19% 12% 3	2714
May	1st to 10th	Day	52% 6% 10% 18% 11% 3	6233
	11th to 20th	Day	48% 9% 11% 16% 16% 1	6406
	21st to end	Day	44% 6% 13% 23% 11% 2	6949
June	1st to 10th	Day	44% 8% 12% 21% 15% 1	2800
	11th to 20th	Day	37% 7% 15% 27% 13% 1	2800
	21st to end	Day	39% 8% 14% 23% 15% 1	2800
July	1st to 10th	Day	36% 5% 14% 31% 15% 1	6290
	11th to 20th	Day	31% 5% 14% 33% 16% 1	6285
	21st to end	Day	32% 6% 14% 34% 13%	6578
August	1st to 10th	Day	41% 7% 14% 24% 13% 1	4221
	11th to 20th	Day	38% 7% 15% 31% 10%	4213
	21st to end	Day	34% 7% 13% 36% 10% 1	4386
September	1st to 10th	Day	36% 7% 14% 31% 12% 1	2400
	11th to 20th	Day	31% 6% 15% 33% 16% 1	2386
	21st to end	Day	41% 8% 14% 27% 10%	2095
October	1st to 10th	Day	37% 6% 12% 30% 14% 1	3260
	11th to 20th	Day	40% 5% 16% 24% 14% 1	3240
	21st to end	Day	48% 6% 16% 21% 10%	3516
November	1st to 10th	Day	46% 7% 14% 21% 11% 1	1777
	11th to 20th	Day	50% 7% 17% 16% 9% 1	1680
	21st to end	Day	58% 7% 13% 16% 6%	1680
December	1st to 10th	Day	63% 7% 16% 11% 3	2680
	11th to 20th	Day	60% 7% 10% 15% 9%	3066
	21st to end	Day	66% 7% 9% 12% 7%	4048
January	1st to 10th	Night	59% 7% 11% 15% 8%	8234
	11th to 20th	Night	66% 8% 9% 10% 7%	8125
	21st to end	Night	58% 7% 8% 14% 13% 1	8401
February	1st to 10th	Night	58% 6% 13% 16% 6% 1	2730
	11th to 20th	Night	56% 7% 10% 17% 9%	2665
	21st to end	Night	52% 5% 13% 20% 8% 1	2157
March	1st to 10th	Night	52% 6% 9% 17% 13% 4	5130
	11th to 20th	Night	44% 6% 17% 20% 11% 2	5090
	21st to end	Night	45% 3 9% 22% 19% 2	5506
April	1st to 10th	Night	49% 5% 11% 20% 13% 2	2100
	11th to 20th	Night	47% 6% 9% 19% 17% 1	2075
	21st to end	Night	45% 7% 11% 19% 15% 3	1922
May	1st to 10th	Night	56% 5% 10% 13% 12% 4	3930
	11th to 20th	Night	43% 6% 15% 20% 14% 2	3624
	21st to end	Night	46% 5% 12% 21% 14% 2	3927
June	1st to 10th	Night	42% 7% 11% 23% 14% 3	1600
	11th to 20th	Night	36% 5% 14% 21% 21% 3	1600
	21st to end	Night	41% 6% 13% 23% 15% 2	1600
July	1st to 10th	Night	36% 5% 15% 22% 21% 1	3560
	11th to 20th	Night	33% 4 12% 33% 17% 1	3552
	21st to end	Night	31% 5% 13% 33% 17% 1	3955
August	1st to 10th	Night	35% 6% 13% 28% 16% 2	2904
	11th to 20th	Night	36% 5% 16% 29% 12% 2	2902
	21st to end	Night	30% 6% 13% 36% 14% 1	3478
September	1st to 10th	Night	34% 7% 15% 29% 14% 1	2000
	11th to 20th	Night	30% 6% 12% 34% 17% 1	2010
	21st to end	Night	43% 5% 10% 27% 14% 1	2305
October	1st to 10th	Night	35% 5% 9% 28% 23% 1	3920
	11th to 20th	Night	42% 5% 13% 30% 9%	3899
	21st to end	Night	42% 6% 15% 27% 10% 1	4274
November	1st to 10th	Night	45% 6% 13% 21% 14% 1	2738
	11th to 20th	Night	49% 7% 14% 19% 10% 1	2940
	21st to end	Night	55% 9% 15% 15% 5%	2940
December	1st to 10th	Night	59% 6% 14% 16% 4	4720
	11th to 20th	Night	58% 8% 11% 13% 10% 1	5348
	21st to end	Night	67% 6% 9% 11% 6%	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	22% 20% 34% 16% 6% 2	220
	11th to 20th	Day	27% 24% 29% 16% 4 1	245
	21st to end	Day	24% 22% 31% 18% 4 1	363
February	1st to 10th	Day	23% 24% 35% 13% 4	330
	11th to 20th	Day	24% 21% 38% 16% 2	330
	21st to end	Day	28% 21% 34% 15% 2	364
March	1st to 10th	Day	23% 17% 35% 19% 5% 1	540
	11th to 20th	Day	26% 17% 31% 21% 4 1	540
	21st to end	Day	29% 11% 35% 16% 8%	594
April	1st to 10th	Day	25% 15% 34% 19% 6% 1	540
	11th to 20th	Day	26% 14% 30% 19% 8% 3	532
	21st to end	Day	23% 12% 35% 21% 8% 1	540
May	1st to 10th	Day	22% 10% 30% 25% 9% 4	543
	11th to 20th	Day	17% 9% 26% 30% 14% 4	551
	21st to end	Day	14% 5% 27% 30% 19% 4	573
June	1st to 10th	Day	17% 9% 26% 29% 16% 2	530
	11th to 20th	Day	11% 7% 29% 32% 18% 4	530
	21st to end	Day	19% 7% 26% 29% 15% 4	530
July	1st to 10th	Day	9% 3 23% 34% 26% 5%	525
	11th to 20th	Day	14% 4 23% 32% 19% 8%	524
	21st to end	Day	9% 4 26% 40% 15% 5%	576
August	1st to 10th	Day	12% 3 21% 33% 22% 8%	526
	11th to 20th	Day	12% 4 20% 32% 25% 7%	519
	21st to end	Day	10% 6% 24% 34% 18% 7%	583
September	1st to 10th	Day	12% 6% 24% 29% 21% 8%	530
	11th to 20th	Day	9% 4 17% 31% 25% 13%	530
	21st to end	Day	16% 6% 28% 28% 18% 5%	530
October	1st to 10th	Day	19% 11% 27% 20% 18% 5%	530
	11th to 20th	Day	20% 13% 33% 24% 8% 2	530
	21st to end	Day	27% 15% 28% 23% 7%	577
November	1st to 10th	Day	24% 18% 26% 23% 9% 1	351
	11th to 20th	Day	21% 20% 35% 20% 4 1	220
	21st to end	Day	27% 17% 32% 17% 6%	220
December	1st to 10th	Day	25% 18% 31% 19% 7%	220
	11th to 20th	Day	28% 25% 31% 13% 2	220
	21st to end	Day	33% 19% 33% 13% 1 1	242
January	1st to 10th	Night	25% 17% 35% 18% 6%	840
	11th to 20th	Night	27% 16% 30% 22% 5%	840
	21st to end	Night	21% 15% 33% 25% 6% 1	924
February	1st to 10th	Night	22% 16% 40% 20% 3	840
	11th to 20th	Night	22% 15% 37% 22% 4	823
	21st to end	Night	23% 15% 37% 21% 4	692
March	1st to 10th	Night	21% 15% 33% 26% 4	840
	11th to 20th	Night	22% 14% 34% 24% 5% 1	840
	21st to end	Night	21% 9% 34% 27% 8% 1	924
April	1st to 10th	Night	22% 9% 35% 27% 6%	840
	11th to 20th	Night	21% 11% 32% 25% 10% 1	831
	21st to end	Night	20% 8% 29% 30% 12% 1	662
May	1st to 10th	Night	23% 4 24% 33% 14% 3	540
	11th to 20th	Night	15% 3 27% 39% 16%	406
	21st to end	Night	12% 4 26% 34% 22% 2	433
June	1st to 10th	Night	13% 3 28% 39% 16% 1	400
	11th to 20th	Night	10% 5% 23% 38% 22% 2	400
	21st to end	Night	13% 5% 21% 42% 19% 1	400
July	1st to 10th	Night	5% 2 22% 45% 23% 3	390
	11th to 20th	Night	11% 3 17% 40% 26% 3	388
	21st to end	Night	9% 2 21% 44% 21% 2	499
August	1st to 10th	Night	11% 2 14% 38% 32% 3	597
	11th to 20th	Night	9% 3 18% 35% 31% 5%	594
	21st to end	Night	11% 4 19% 37% 27% 2	811
September	1st to 10th	Night	13% 3 21% 33% 26% 4	800
	11th to 20th	Night	12% 3 15% 32% 30% 7%	800
	21st to end	Night	17% 6% 22% 30% 21% 5%	800
October	1st to 10th	Night	18% 6% 27% 26% 21% 3	800
	11th to 20th	Night	23% 10% 28% 29% 9% 1	800
	21st to end	Night	24% 11% 32% 27% 6%	880
November	1st to 10th	Night	23% 11% 25% 29% 11% 1	820
	11th to 20th	Night	21% 15% 32% 24% 6% 1	840
	21st to end	Night	25% 13% 32% 23% 7%	840
December	1st to 10th	Night	27% 17% 33% 19% 4	840
	11th to 20th	Night	25% 15% 38% 18% 4	840
	21st to end	Night	31% 14% 35% 17% 3	924

Page generated at 2026-05-14 01:59:00. Version ac07

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