Weather and Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weather and Sky 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% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	132179
	Night	15% 11% 4 9% 5% 5% 4 4 5% 6% 33%	130634
Transparency	Day	49% 7% 12% 19% 11% 1	135774
	Night	49% 7% 12% 19% 12% 1	135125
Seeing	Day	19% 13% 28% 23% 12% 5%	16355
	Night	19% 11% 29% 27% 12% 1	25905

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% 16% 5% 12% 5% 4 5% 3 4 6% 25%	4661
	11th to 20th	Day	10% 22% 8% 17% 7% 7% 5% 3 4 4 12%	5219
	21st to end	Day	15% 15% 6% 16% 8% 8% 5% 3 4 4 17%	5780
February	1st to 10th	Day	16% 16% 5% 12% 7% 6% 6% 4 4 5% 20%	1890
	11th to 20th	Day	12% 16% 6% 14% 7% 4 4 4 4 6% 23%	1843
	21st to end	Day	17% 15% 4 11% 6% 5% 4 4 5% 5% 23%	1638
March	1st to 10th	Day	17% 11% 5% 11% 6% 4 4 3 4 5% 30%	5042
	11th to 20th	Day	11% 13% 5% 9% 6% 5% 5% 3 5% 7% 31%	5090
	21st to end	Day	18% 13% 4 8% 5% 4 4 3 4 5% 32%	5586
April	1st to 10th	Day	18% 12% 4 8% 5% 5% 5% 3 5% 6% 31%	2471
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 3 5% 8% 34%	2565
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 3 4 5% 35%	2730
May	1st to 10th	Day	18% 9% 3 8% 5% 4 4 4 5% 6% 34%	6018
	11th to 20th	Day	17% 10% 3 8% 5% 5% 4 3 5% 7% 32%	6295
	21st to end	Day	16% 7% 2 6% 4 5% 5% 4 5% 7% 38%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 40%	2800
	11th to 20th	Day	11% 5% 3 7% 5% 5% 5% 4 7% 9% 39%	2800
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	6317
	21st to end	Day	8% 4 2 6% 5% 4 5% 4 6% 9% 48%	6424
August	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 38%	4234
	11th to 20th	Day	12% 6% 3 7% 4 5% 6% 5% 7% 10% 34%	4174
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4486
September	1st to 10th	Day	13% 7% 3 5% 4 4 5% 4 6% 8% 42%	2400
	11th to 20th	Day	9% 6% 2 5% 5% 5% 4 3 5% 7% 49%	2270
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 3 8% 8% 35%	2165
October	1st to 10th	Day	18% 8% 2 6% 5% 4 5% 4 6% 7% 37%	2000
	11th to 20th	Day	17% 10% 3 9% 5% 5% 4 3 5% 7% 32%	2000
	21st to end	Day	22% 11% 3 9% 5% 5% 5% 4 5% 6% 25%	2200
November	1st to 10th	Day	13% 10% 3 9% 5% 4 5% 4 6% 8% 34%	1818
	11th to 20th	Day	12% 13% 4 8% 7% 5% 5% 3 6% 6% 30%	1680
	21st to end	Day	18% 16% 3 9% 5% 4 4 5% 6% 6% 24%	1680
December	1st to 10th	Day	18% 17% 5% 10% 6% 6% 6% 4 5% 6% 17%	2680
	11th to 20th	Day	18% 16% 5% 8% 6% 4 5% 4 5% 4 24%	3066
	21st to end	Day	24% 17% 7% 12% 6% 5% 4 3 4 5% 13%	4048
January	1st to 10th	Night	15% 15% 5% 11% 6% 5% 5% 3 5% 6% 25%	8234
	11th to 20th	Night	9% 17% 7% 14% 8% 6% 5% 4 5% 5% 21%	7662
	21st to end	Night	15% 14% 5% 14% 6% 5% 5% 3 5% 6% 23%	8401
February	1st to 10th	Night	15% 15% 5% 10% 7% 5% 4 4 6% 6% 25%	2730
	11th to 20th	Night	14% 13% 5% 10% 6% 6% 4 4 5% 7% 27%	2663
	21st to end	Night	15% 15% 5% 11% 5% 5% 4 4 5% 6% 24%	2164
March	1st to 10th	Night	16% 9% 4 8% 6% 5% 5% 4 5% 6% 32%	5018
	11th to 20th	Night	12% 10% 3 7% 5% 4 4 4 5% 8% 38%	5089
	21st to end	Night	18% 9% 2 7% 5% 4 4 3 4 5% 38%	5588
April	1st to 10th	Night	18% 10% 4 8% 5% 5% 4 4 5% 6% 31%	2148
	11th to 20th	Night	18% 8% 3 6% 4 5% 4 4 5% 6% 35%	2021
	21st to end	Night	15% 8% 3 7% 5% 4 5% 4 5% 7% 37%	1890
May	1st to 10th	Night	21% 10% 3 8% 5% 4 3 3 4 5% 32%	4143
	11th to 20th	Night	13% 7% 3 9% 5% 5% 5% 4 6% 7% 35%	3728
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 7% 43%	3926
June	1st to 10th	Night	10% 6% 2 7% 6% 5% 5% 5% 7% 8% 39%	1600
	11th to 20th	Night	9% 6% 2 7% 6% 6% 4 4 6% 8% 43%	1600
	21st to end	Night	11% 5% 2 5% 5% 5% 5% 4 6% 7% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 5% 4 4 4 6% 9% 48%	3600
	11th to 20th	Night	9% 4 2 6% 4 4 4 4 6% 8% 47%	3639
	21st to end	Night	9% 4 2 6% 4 4 4 4 6% 8% 51%	4428
August	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 46%	2915
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 5% 8% 44%	2875
	21st to end	Night	8% 4 2 6% 4 4 4 3 6% 10% 50%	3378
September	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 5% 7% 47%	2000
	11th to 20th	Night	10% 4 2 5% 3 4 3 3 4 7% 55%	2126
	21st to end	Night	15% 7% 2 9% 5% 4 4 4 4 7% 39%	2235
October	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 5% 6% 43%	2400
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 4 5% 6% 34%	2400
	21st to end	Night	22% 9% 3 7% 5% 4 5% 4 5% 5% 31%	2640
November	1st to 10th	Night	15% 8% 3 6% 4 5% 4 4 5% 7% 39%	2695
	11th to 20th	Night	14% 12% 3 7% 6% 5% 5% 4 6% 8% 31%	2940
	21st to end	Night	18% 12% 4 9% 6% 5% 4 4 6% 7% 27%	2940
December	1st to 10th	Night	14% 16% 5% 10% 6% 5% 5% 4 6% 8% 22%	4720
	11th to 20th	Night	15% 18% 5% 9% 5% 5% 4 3 5% 5% 26%	5348
	21st to end	Night	24% 15% 6% 11% 5% 5% 4 4 4 5% 16%	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	60% 7% 13% 11% 8% 1	4662
	11th to 20th	Day	75% 6% 8% 6% 4	5253
	21st to end	Day	70% 6% 11% 8% 5%	5780
February	1st to 10th	Day	67% 6% 9% 13% 4 1	1890
	11th to 20th	Day	62% 5% 12% 15% 5%	1852
	21st to end	Day	62% 6% 12% 13% 6% 1	1638
March	1st to 10th	Day	54% 6% 14% 16% 9%	5045
	11th to 20th	Day	50% 7% 17% 17% 7% 2	5090
	21st to end	Day	53% 6% 8% 21% 11% 1	5586
April	1st to 10th	Day	55% 5% 11% 17% 12% 1	2471
	11th to 20th	Day	49% 6% 11% 19% 13% 1	2565
	21st to end	Day	50% 6% 11% 18% 12% 2	2730
May	1st to 10th	Day	50% 6% 11% 17% 11% 5%	6027
	11th to 20th	Day	50% 8% 11% 18% 11% 1	6299
	21st to end	Day	43% 7% 12% 21% 14% 2	6949
June	1st to 10th	Day	40% 8% 12% 22% 16% 2	2800
	11th to 20th	Day	39% 9% 14% 24% 13% 1	2800
	21st to end	Day	37% 8% 14% 25% 15% 1	2800
July	1st to 10th	Day	34% 8% 13% 27% 17% 2	6290
	11th to 20th	Day	35% 8% 13% 28% 15% 1	6242
	21st to end	Day	32% 8% 14% 27% 19% 1	6240
August	1st to 10th	Day	41% 8% 15% 21% 13% 1	4221
	11th to 20th	Day	40% 9% 16% 24% 10% 1	4213
	21st to end	Day	34% 7% 12% 33% 12% 1	4486
September	1st to 10th	Day	38% 8% 15% 26% 12% 1	2400
	11th to 20th	Day	33% 6% 13% 31% 15% 1	2270
	21st to end	Day	44% 8% 15% 23% 9% 1	2165
October	1st to 10th	Day	36% 6% 16% 25% 16% 1	3260
	11th to 20th	Day	42% 6% 13% 24% 15%	3240
	21st to end	Day	49% 7% 14% 22% 8%	3538
November	1st to 10th	Day	47% 7% 14% 22% 8% 1	1818
	11th to 20th	Day	52% 8% 14% 15% 10% 1	1680
	21st to end	Day	58% 7% 15% 15% 5%	1680
December	1st to 10th	Day	66% 8% 15% 9% 2	2680
	11th to 20th	Day	61% 6% 12% 11% 9% 1	3066
	21st to end	Day	72% 8% 9% 8% 4	4048
January	1st to 10th	Night	58% 7% 14% 15% 6%	8234
	11th to 20th	Night	63% 7% 10% 12% 7% 1	7703
	21st to end	Night	62% 6% 9% 13% 10% 1	8401
February	1st to 10th	Night	60% 7% 11% 16% 5% 1	2730
	11th to 20th	Night	57% 7% 12% 17% 7%	2665
	21st to end	Night	60% 6% 12% 15% 7%	2164
March	1st to 10th	Night	51% 6% 13% 17% 11% 2	5028
	11th to 20th	Night	44% 6% 18% 20% 12% 1	5090
	21st to end	Night	46% 5% 9% 20% 17% 2	5588
April	1st to 10th	Night	53% 6% 9% 19% 11% 1	2148
	11th to 20th	Night	49% 6% 9% 19% 15% 2	2021
	21st to end	Night	46% 7% 10% 20% 14% 3	1890
May	1st to 10th	Night	54% 5% 9% 15% 12% 6%	4150
	11th to 20th	Night	45% 9% 13% 20% 11% 2	3729
	21st to end	Night	40% 7% 11% 24% 17% 2	3927
June	1st to 10th	Night	39% 8% 12% 22% 17% 2	1600
	11th to 20th	Night	37% 6% 13% 25% 17% 2	1600
	21st to end	Night	37% 6% 14% 24% 18% 2	1600
July	1st to 10th	Night	33% 7% 12% 23% 22% 3	3560
	11th to 20th	Night	32% 8% 13% 28% 17% 2	3591
	21st to end	Night	31% 6% 13% 24% 24% 2	4297
August	1st to 10th	Night	35% 7% 14% 26% 16% 3	2904
	11th to 20th	Night	37% 7% 16% 28% 11% 1	2902
	21st to end	Night	29% 7% 14% 34% 14% 1	3378
September	1st to 10th	Night	36% 6% 13% 27% 16% 2	2000
	11th to 20th	Night	29% 5% 13% 34% 18% 1	2126
	21st to end	Night	45% 6% 11% 23% 14% 1	2235
October	1st to 10th	Night	33% 5% 10% 27% 24% 1	3920
	11th to 20th	Night	44% 5% 13% 24% 14%	3899
	21st to end	Night	40% 5% 16% 27% 12% 1	4252
November	1st to 10th	Night	43% 5% 14% 24% 13% 1	2695
	11th to 20th	Night	50% 7% 15% 18% 10% 1	2940
	21st to end	Night	55% 8% 17% 16% 4	2940
December	1st to 10th	Night	59% 8% 16% 13% 3	4720
	11th to 20th	Night	59% 7% 10% 13% 10% 1	5348
	21st to end	Night	68% 9% 10% 9% 4	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	23% 22% 31% 15% 7% 1	220
	11th to 20th	Day	28% 25% 31% 12% 3 1	324
	21st to end	Day	25% 22% 30% 18% 3 1	363
February	1st to 10th	Day	26% 20% 38% 12% 4	330
	11th to 20th	Day	27% 23% 36% 13% 1	330
	21st to end	Day	32% 25% 28% 14% 1	296
March	1st to 10th	Day	24% 16% 34% 19% 6%	540
	11th to 20th	Day	26% 16% 35% 16% 6% 1	540
	21st to end	Day	30% 15% 30% 18% 6% 1	594
April	1st to 10th	Day	24% 16% 33% 19% 7% 1	540
	11th to 20th	Day	24% 20% 24% 20% 8% 3	532
	21st to end	Day	26% 10% 32% 22% 10% 1	540
May	1st to 10th	Day	22% 12% 31% 22% 9% 4	543
	11th to 20th	Day	18% 10% 24% 26% 18% 4	551
	21st to end	Day	14% 10% 25% 28% 17% 7%	573
June	1st to 10th	Day	13% 12% 30% 30% 13% 2	530
	11th to 20th	Day	12% 8% 29% 28% 19% 4	530
	21st to end	Day	15% 7% 30% 30% 15% 5%	530
July	1st to 10th	Day	10% 6% 18% 32% 25% 10%	525
	11th to 20th	Day	14% 8% 22% 28% 19% 9%	524
	21st to end	Day	9% 4 26% 35% 18% 8%	576
August	1st to 10th	Day	10% 3 21% 32% 21% 12%	526
	11th to 20th	Day	13% 5% 21% 30% 19% 13%	519
	21st to end	Day	12% 6% 23% 30% 19% 10%	583
September	1st to 10th	Day	14% 8% 22% 24% 20% 12%	530
	11th to 20th	Day	9% 6% 19% 24% 26% 17%	530
	21st to end	Day	15% 10% 28% 24% 16% 7%	530
October	1st to 10th	Day	18% 11% 28% 19% 18% 6%	530
	11th to 20th	Day	18% 17% 34% 20% 8% 3	530
	21st to end	Day	29% 15% 28% 20% 8%	583
November	1st to 10th	Day	19% 23% 31% 17% 7% 2	341
	11th to 20th	Day	21% 25% 28% 17% 7% 1	220
	21st to end	Day	27% 20% 38% 13% 3	220
December	1st to 10th	Day	28% 18% 32% 17% 4 1	220
	11th to 20th	Day	28% 25% 32% 13% 2	220
	21st to end	Day	24% 23% 34% 16% 3	242
January	1st to 10th	Night	24% 17% 37% 17% 5%	840
	11th to 20th	Night	28% 19% 31% 18% 4	840
	21st to end	Night	19% 15% 39% 21% 6%	924
February	1st to 10th	Night	23% 19% 35% 20% 3	840
	11th to 20th	Night	21% 16% 35% 25% 3	823
	21st to end	Night	24% 15% 34% 24% 2	692
March	1st to 10th	Night	21% 13% 35% 26% 6%	840
	11th to 20th	Night	24% 13% 33% 23% 7%	840
	21st to end	Night	21% 11% 33% 26% 8% 1	924
April	1st to 10th	Night	23% 12% 33% 27% 5%	840
	11th to 20th	Night	20% 11% 33% 25% 9% 1	777
	21st to end	Night	18% 8% 30% 35% 9%	630
May	1st to 10th	Night	19% 6% 28% 30% 16% 1	630
	11th to 20th	Night	14% 5% 26% 40% 14%	459
	21st to end	Night	15% 4 25% 34% 20% 2	433
June	1st to 10th	Night	12% 4 32% 36% 15% 1	400
	11th to 20th	Night	13% 4 29% 37% 18%	400
	21st to end	Night	12% 5% 24% 42% 17% 1	400
July	1st to 10th	Night	6% 3 20% 46% 23% 1	390
	11th to 20th	Night	11% 3 22% 41% 20% 2	401
	21st to end	Night	10% 3 18% 43% 23% 3	657
August	1st to 10th	Night	8% 4 13% 42% 30% 3	597
	11th to 20th	Night	10% 2 16% 35% 34% 4	594
	21st to end	Night	9% 3 20% 37% 29% 3	750
September	1st to 10th	Night	13% 7% 18% 32% 24% 5%	800
	11th to 20th	Night	12% 3 17% 31% 32% 6%	800
	21st to end	Night	16% 7% 23% 28% 21% 5%	800
October	1st to 10th	Night	17% 7% 27% 27% 20% 2	800
	11th to 20th	Night	22% 13% 28% 26% 10% 1	800
	21st to end	Night	26% 12% 30% 27% 6%	880
November	1st to 10th	Night	20% 12% 30% 25% 11% 1	820
	11th to 20th	Night	21% 15% 32% 22% 8% 1	840
	21st to end	Night	25% 13% 35% 20% 6%	840
December	1st to 10th	Night	26% 19% 31% 18% 6%	840
	11th to 20th	Night	23% 21% 33% 19% 4	840
	21st to end	Night	30% 14% 31% 21% 3	924

Page generated at 2026-05-14 01:58:56. Version ac07

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