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 2025-01-22.

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% 3 8% 5% 5% 5% 4 6% 7% 33%	105163
	Night	15% 11% 4 8% 5% 5% 4 4 5% 7% 33%	104203
Transparency	Day	48% 8% 13% 20% 11% 1	108769
	Night	48% 7% 13% 20% 11% 1	108685
Seeing	Day	18% 12% 28% 24% 13% 5%	14654
	Night	19% 10% 29% 28% 12% 1	24217

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	15% 17% 5% 10% 6% 4 4 4 4 6% 25%	2597
	11th to 20th	Day	12% 21% 7% 14% 8% 6% 5% 3 4 6% 15%	2903
	21st to end	Day	22% 19% 6% 12% 6% 6% 5% 3 4 3 14%	3133
February	1st to 10th	Day	17% 17% 5% 11% 7% 6% 5% 4 4 5% 19%	1710
	11th to 20th	Day	11% 16% 6% 14% 7% 5% 4 5% 4 5% 23%	1663
	21st to end	Day	18% 14% 4 11% 6% 5% 4 4 5% 5% 23%	1486
March	1st to 10th	Day	18% 12% 5% 9% 6% 4 4 4 4 5% 29%	3432
	11th to 20th	Day	11% 16% 5% 8% 6% 5% 5% 3 5% 6% 31%	3480
	21st to end	Day	17% 10% 3 8% 5% 4 4 3 5% 6% 35%	3815
April	1st to 10th	Day	18% 12% 4 8% 5% 5% 5% 3 5% 6% 31%	2235
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 8% 35%	2318
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 3 4 5% 34%	2470
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	4108
	11th to 20th	Day	14% 10% 4 9% 5% 5% 5% 3 5% 7% 32%	4322
	21st to end	Day	10% 6% 2 6% 4 4 4 4 5% 8% 46%	4837
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 4 6% 8% 40%	2660
	11th to 20th	Day	11% 5% 3 7% 5% 4 5% 4 8% 9% 40%	2660
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 8% 42%	2660
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 5% 7% 10% 44%	4440
	11th to 20th	Day	11% 5% 2 5% 4 5% 5% 4 7% 9% 44%	4411
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	4466
August	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 6% 8% 38%	4104
	11th to 20th	Day	12% 6% 3 7% 4 5% 6% 5% 8% 11% 34%	4044
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4349
September	1st to 10th	Day	13% 7% 3 5% 4 4 5% 4 6% 8% 41%	2280
	11th to 20th	Day	10% 6% 2 5% 5% 5% 4 3 5% 7% 49%	2157
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 3 7% 8% 35%	2057
October	1st to 10th	Day	18% 9% 2 7% 5% 4 5% 4 6% 7% 35%	1900
	11th to 20th	Day	17% 10% 3 9% 5% 5% 4 3 5% 6% 32%	1900
	21st to end	Day	23% 11% 3 9% 6% 5% 5% 4 5% 6% 25%	2090
November	1st to 10th	Day	12% 10% 3 9% 5% 4 4 4 6% 8% 35%	1730
	11th to 20th	Day	12% 13% 4 9% 7% 5% 5% 3 6% 7% 30%	1600
	21st to end	Day	18% 16% 3 9% 5% 4 4 5% 6% 6% 24%	1600
December	1st to 10th	Day	18% 16% 5% 10% 6% 6% 6% 4 5% 6% 17%	2600
	11th to 20th	Day	19% 16% 5% 8% 6% 4 5% 4 5% 4 25%	2986
	21st to end	Day	24% 17% 7% 12% 6% 5% 4 3 4 5% 13%	3960
January	1st to 10th	Night	16% 15% 5% 11% 6% 5% 5% 3 4 5% 24%	4569
	11th to 20th	Night	13% 17% 6% 11% 7% 6% 6% 4 5% 4 21%	4273
	21st to end	Night	23% 17% 5% 10% 5% 5% 4 3 4 5% 19%	4546
February	1st to 10th	Night	15% 16% 5% 10% 7% 5% 4 4 6% 6% 23%	2470
	11th to 20th	Night	14% 14% 5% 10% 6% 6% 4 4 5% 7% 27%	2403
	21st to end	Night	16% 13% 5% 11% 5% 5% 4 4 6% 7% 24%	1964
March	1st to 10th	Night	20% 9% 3 8% 6% 4 5% 4 4 5% 32%	3408
	11th to 20th	Night	15% 12% 3 7% 4 4 4 3 5% 7% 36%	3479
	21st to end	Night	15% 8% 3 7% 4 4 4 4 5% 5% 42%	3817
April	1st to 10th	Night	18% 11% 4 7% 5% 5% 3 4 5% 6% 32%	1944
	11th to 20th	Night	19% 7% 3 6% 4 5% 4 4 5% 6% 36%	1827
	21st to end	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	19% 10% 3 8% 4 3 3 3 5% 6% 36%	2833
	11th to 20th	Night	14% 8% 4 9% 5% 6% 5% 4 6% 7% 32%	2569
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 8% 49%	2738
June	1st to 10th	Night	11% 6% 2 7% 6% 5% 5% 5% 7% 8% 39%	1520
	11th to 20th	Night	9% 6% 2 6% 6% 6% 4 4 7% 8% 43%	1520
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 8% 49%	2520
	11th to 20th	Night	10% 4 2 6% 4 3 3 4 6% 9% 49%	2545
	21st to end	Night	8% 4 2 6% 4 4 4 4 7% 8% 49%	3087
August	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 45%	2825
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 5% 8% 44%	2785
	21st to end	Night	8% 4 2 6% 4 4 4 3 6% 10% 50%	3274
September	1st to 10th	Night	13% 6% 1 6% 4 4 5% 3 5% 7% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 3 4 3 3 5% 7% 54%	2019
	21st to end	Night	15% 7% 2 9% 5% 4 4 4 5% 7% 38%	2123
October	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 5% 6% 41%	2280
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 4 5% 7% 33%	2280
	21st to end	Night	23% 9% 3 7% 5% 4 5% 4 5% 5% 30%	2508
November	1st to 10th	Night	15% 8% 3 6% 4 5% 4 4 5% 7% 39%	2563
	11th to 20th	Night	14% 12% 3 7% 6% 5% 5% 4 6% 8% 31%	2800
	21st to end	Night	18% 11% 3 8% 5% 5% 4 4 6% 7% 28%	2800
December	1st to 10th	Night	14% 16% 4 10% 6% 5% 5% 4 6% 8% 21%	4580
	11th to 20th	Night	15% 18% 5% 9% 5% 5% 4 3 5% 5% 25%	5208
	21st to end	Night	24% 16% 6% 11% 5% 5% 4 4 4 5% 16%	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	59% 9% 14% 11% 7% 1	2598
	11th to 20th	Day	71% 8% 9% 8% 3	2903
	21st to end	Day	73% 8% 9% 7% 3	3133
February	1st to 10th	Day	68% 6% 9% 12% 4 1	1710
	11th to 20th	Day	62% 6% 11% 15% 6% 1	1672
	21st to end	Day	62% 7% 11% 13% 7% 1	1486
March	1st to 10th	Day	55% 7% 12% 20% 5% 1	3435
	11th to 20th	Day	52% 8% 15% 17% 7% 1	3480
	21st to end	Day	48% 8% 10% 22% 11% 1	3815
April	1st to 10th	Day	54% 5% 11% 17% 12% 1	2235
	11th to 20th	Day	48% 6% 11% 19% 14% 2	2318
	21st to end	Day	51% 6% 11% 17% 12% 2	2470
May	1st to 10th	Day	47% 7% 12% 19% 12% 2	4120
	11th to 20th	Day	49% 9% 14% 19% 7% 1	4326
	21st to end	Day	34% 8% 13% 25% 17% 3	4837
June	1st to 10th	Day	40% 8% 12% 22% 15% 2	2660
	11th to 20th	Day	38% 9% 14% 24% 14% 1	2660
	21st to end	Day	37% 8% 14% 25% 15% 1	2660
July	1st to 10th	Day	33% 8% 13% 27% 17% 2	4370
	11th to 20th	Day	34% 8% 14% 30% 13% 1	4336
	21st to end	Day	35% 9% 15% 22% 18% 1	4324
August	1st to 10th	Day	42% 8% 15% 22% 13% 1	4091
	11th to 20th	Day	40% 10% 16% 24% 10% 1	4083
	21st to end	Day	34% 7% 12% 33% 13% 1	4349
September	1st to 10th	Day	38% 8% 15% 25% 12% 1	2280
	11th to 20th	Day	34% 6% 13% 29% 16% 1	2157
	21st to end	Day	44% 8% 15% 24% 9% 1	2057
October	1st to 10th	Day	36% 6% 17% 24% 16% 1	3160
	11th to 20th	Day	42% 6% 13% 24% 15%	3140
	21st to end	Day	49% 7% 15% 22% 7%	3428
November	1st to 10th	Day	46% 7% 14% 22% 9% 1	1730
	11th to 20th	Day	52% 8% 14% 16% 10% 1	1600
	21st to end	Day	58% 7% 16% 14% 5%	1600
December	1st to 10th	Day	66% 8% 15% 9% 2	2600
	11th to 20th	Day	61% 6% 12% 11% 9% 1	2986
	21st to end	Day	72% 8% 9% 8% 4	3960
January	1st to 10th	Night	59% 8% 15% 13% 5%	4569
	11th to 20th	Night	62% 10% 13% 12% 3	4273
	21st to end	Night	68% 7% 10% 12% 3	4546
February	1st to 10th	Night	61% 7% 11% 15% 5% 1	2470
	11th to 20th	Night	56% 7% 12% 17% 7%	2405
	21st to end	Night	59% 6% 13% 15% 7%	1964
March	1st to 10th	Night	53% 6% 13% 21% 6% 1	3418
	11th to 20th	Night	47% 7% 15% 18% 11% 2	3480
	21st to end	Night	42% 6% 9% 24% 17% 2	3817
April	1st to 10th	Night	52% 6% 9% 19% 12% 1	1944
	11th to 20th	Night	49% 6% 8% 19% 15% 2	1827
	21st to end	Night	46% 7% 10% 19% 15% 3	1710
May	1st to 10th	Night	48% 6% 10% 19% 13% 4	2840
	11th to 20th	Night	45% 10% 15% 19% 8% 2	2570
	21st to end	Night	32% 7% 11% 25% 22% 2	2739
June	1st to 10th	Night	39% 8% 12% 22% 17% 1	1520
	11th to 20th	Night	37% 6% 12% 25% 18% 2	1520
	21st to end	Night	37% 6% 13% 25% 17% 2	1520
July	1st to 10th	Night	33% 6% 12% 23% 23% 4	2480
	11th to 20th	Night	31% 7% 14% 31% 16% 1	2497
	21st to end	Night	31% 8% 16% 24% 19% 2	2988
August	1st to 10th	Night	36% 7% 13% 26% 15% 3	2814
	11th to 20th	Night	37% 8% 17% 27% 11% 1	2812
	21st to end	Night	29% 7% 14% 34% 15% 1	3274
September	1st to 10th	Night	37% 6% 13% 25% 16% 2	1900
	11th to 20th	Night	30% 5% 13% 33% 19% 1	2019
	21st to end	Night	45% 6% 12% 24% 13% 1	2123
October	1st to 10th	Night	34% 5% 10% 26% 24% 1	3800
	11th to 20th	Night	44% 5% 13% 24% 13%	3779
	21st to end	Night	40% 5% 16% 27% 12% 1	4120
November	1st to 10th	Night	43% 6% 14% 24% 13% 1	2563
	11th to 20th	Night	50% 7% 15% 18% 10% 1	2800
	21st to end	Night	55% 8% 17% 16% 4	2800
December	1st to 10th	Night	59% 8% 17% 13% 3	4580
	11th to 20th	Night	60% 7% 10% 13% 10% 1	5208
	21st to end	Night	68% 9% 10% 9% 4	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	22% 22% 31% 16% 8% 2	200
	11th to 20th	Day	26% 23% 33% 13% 4 1	294
	21st to end	Day	23% 21% 31% 21% 4 1	300
February	1st to 10th	Day	24% 20% 41% 11% 4	270
	11th to 20th	Day	24% 23% 37% 14% 1	270
	21st to end	Day	31% 25% 28% 15% 1	246
March	1st to 10th	Day	23% 15% 34% 21% 7%	460
	11th to 20th	Day	22% 16% 38% 18% 6% 1	460
	21st to end	Day	28% 13% 31% 20% 7% 1	506
April	1st to 10th	Day	21% 15% 34% 21% 8% 1	460
	11th to 20th	Day	23% 20% 24% 20% 9% 4	452
	21st to end	Day	25% 10% 31% 23% 10% 2	460
May	1st to 10th	Day	22% 9% 29% 25% 10% 4	463
	11th to 20th	Day	17% 8% 23% 29% 19% 4	487
	21st to end	Day	12% 9% 24% 29% 18% 7%	529
June	1st to 10th	Day	12% 10% 29% 32% 14% 3	490
	11th to 20th	Day	12% 7% 28% 29% 20% 4	490
	21st to end	Day	16% 7% 29% 30% 14% 5%	490
July	1st to 10th	Day	9% 5% 17% 31% 26% 11%	485
	11th to 20th	Day	15% 6% 21% 28% 20% 10%	484
	21st to end	Day	9% 4 24% 36% 19% 8%	532
August	1st to 10th	Day	11% 3 21% 30% 21% 13%	486
	11th to 20th	Day	12% 5% 21% 30% 19% 13%	479
	21st to end	Day	12% 5% 22% 29% 20% 11%	539
September	1st to 10th	Day	14% 7% 23% 24% 20% 12%	490
	11th to 20th	Day	9% 6% 20% 23% 25% 17%	490
	21st to end	Day	15% 10% 28% 24% 16% 8%	490
October	1st to 10th	Day	19% 12% 27% 19% 18% 6%	490
	11th to 20th	Day	18% 17% 33% 20% 9% 3	490
	21st to end	Day	30% 14% 27% 20% 8%	539
November	1st to 10th	Day	17% 24% 31% 18% 7% 3	313
	11th to 20th	Day	21% 25% 29% 18% 7% 2	200
	21st to end	Day	25% 20% 39% 14% 3	200
December	1st to 10th	Day	27% 17% 34% 17% 5% 1	200
	11th to 20th	Day	27% 25% 32% 14% 3	200
	21st to end	Day	23% 21% 36% 16% 3	220
January	1st to 10th	Night	24% 16% 36% 17% 6% 1	800
	11th to 20th	Night	28% 18% 31% 19% 4	800
	21st to end	Night	19% 15% 38% 22% 6%	841
February	1st to 10th	Night	24% 19% 35% 20% 3	760
	11th to 20th	Night	21% 15% 35% 25% 4	743
	21st to end	Night	23% 15% 34% 25% 3	628
March	1st to 10th	Night	20% 11% 35% 27% 7%	760
	11th to 20th	Night	23% 13% 33% 23% 7% 1	760
	21st to end	Night	19% 11% 33% 27% 9% 1	836
April	1st to 10th	Night	22% 12% 32% 29% 5%	760
	11th to 20th	Night	21% 10% 34% 25% 9% 1	703
	21st to end	Night	19% 7% 31% 34% 9%	570
May	1st to 10th	Night	19% 5% 27% 30% 17% 1	570
	11th to 20th	Night	14% 4 25% 41% 14%	422
	21st to end	Night	14% 4 24% 35% 21% 2	411
June	1st to 10th	Night	12% 3 31% 37% 16% 1	380
	11th to 20th	Night	13% 3 28% 38% 18%	380
	21st to end	Night	12% 5% 24% 42% 17% 1	380
July	1st to 10th	Night	6% 3 20% 45% 25% 1	370
	11th to 20th	Night	12% 3 23% 40% 20% 3	380
	21st to end	Night	10% 3 18% 44% 23% 3	624
August	1st to 10th	Night	8% 4 13% 41% 29% 3	567
	11th to 20th	Night	9% 2 17% 34% 35% 3	564
	21st to end	Night	9% 3 20% 36% 29% 3	712
September	1st to 10th	Night	14% 6% 18% 32% 24% 6%	760
	11th to 20th	Night	13% 3 17% 30% 30% 6%	760
	21st to end	Night	16% 7% 22% 29% 21% 5%	760
October	1st to 10th	Night	17% 7% 27% 27% 19% 2	760
	11th to 20th	Night	21% 13% 28% 26% 10% 1	760
	21st to end	Night	26% 12% 29% 26% 6%	836
November	1st to 10th	Night	20% 12% 30% 26% 11% 2	780
	11th to 20th	Night	21% 15% 33% 22% 9% 2	800
	21st to end	Night	25% 13% 35% 20% 6% 1	800
December	1st to 10th	Night	27% 18% 32% 18% 6%	800
	11th to 20th	Night	24% 20% 33% 20% 5%	800
	21st to end	Night	30% 14% 31% 22% 3	880

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