East-View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the East-View Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	13% 9% 3 7% 5% 5% 5% 4 6% 7% 37%	102150
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 36%	97020
Transparency	Day	46% 7% 14% 23% 10% 1	105655
	Night	47% 6% 13% 22% 10% 1	101602
Seeing	Day	16% 6% 24% 33% 17% 4	16120
	Night	17% 6% 26% 34% 15% 2	25989

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	9% 16% 8% 11% 5% 5% 3 3 5% 5% 30%	2527
	11th to 20th	Day	6% 12% 4 13% 9% 10% 9% 5% 8% 6% 18%	2530
	21st to end	Day	8% 14% 7% 16% 8% 6% 6% 4 5% 6% 20%	2919
February	1st to 10th	Day	8% 13% 5% 9% 6% 5% 5% 4 6% 7% 33%	1710
	11th to 20th	Day	8% 11% 4 8% 6% 4 5% 4 6% 5% 40%	1664
	21st to end	Day	9% 12% 3 10% 6% 4 5% 3 5% 6% 37%	1496
March	1st to 10th	Day	11% 9% 3 8% 5% 5% 5% 4 5% 6% 39%	3214
	11th to 20th	Day	8% 11% 5% 9% 5% 4 4 4 5% 6% 39%	3480
	21st to end	Day	19% 10% 4 9% 4 4 4 3 5% 7% 31%	3959
April	1st to 10th	Day	15% 10% 3 8% 6% 5% 6% 4 5% 6% 32%	2280
	11th to 20th	Day	18% 9% 4 6% 5% 5% 5% 4 5% 7% 32%	2254
	21st to end	Day	21% 9% 3 6% 5% 3 4 4 5% 6% 34%	2487
May	1st to 10th	Day	20% 8% 2 6% 4 4 3 3 5% 5% 40%	4428
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 5% 7% 42%	4399
	21st to end	Day	19% 7% 3 7% 5% 4 4 4 5% 7% 33%	4837
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 6% 9% 43%	2660
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 6% 7% 40%	2744
	21st to end	Day	14% 5% 2 5% 5% 5% 6% 5% 7% 10% 37%	2850
July	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 10% 44%	4636
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 4 7% 11% 48%	4440
	21st to end	Day	7% 4 2 5% 4 3 4 4 7% 9% 50%	4814
August	1st to 10th	Day	11% 5% 2 7% 4 5% 6% 5% 6% 8% 42%	4311
	11th to 20th	Day	13% 4 2 5% 4 3 4 4 6% 8% 48%	4041
	21st to end	Day	15% 7% 3 6% 3 4 4 4 6% 8% 41%	4292
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 6% 9% 48%	2280
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 4 6% 7% 44%	2280
	21st to end	Day	17% 8% 3 5% 4 4 4 3 5% 6% 42%	1997
October	1st to 10th	Day	20% 6% 2 7% 4 4 4 4 6% 6% 37%	1900
	11th to 20th	Day	15% 12% 3 7% 5% 5% 5% 3 5% 7% 33%	1900
	21st to end	Day	19% 11% 3 8% 5% 4 5% 3 5% 7% 31%	1964
November	1st to 10th	Day	14% 11% 3 8% 6% 5% 5% 5% 6% 5% 33%	1600
	11th to 20th	Day	15% 15% 6% 10% 7% 5% 5% 4 5% 4 25%	1520
	21st to end	Day	15% 11% 4 9% 6% 5% 5% 5% 6% 7% 28%	1520
December	1st to 10th	Day	11% 14% 5% 10% 8% 7% 6% 5% 5% 6% 23%	1520
	11th to 20th	Day	13% 16% 6% 11% 6% 7% 6% 5% 5% 5% 21%	1914
	21st to end	Day	28% 18% 5% 10% 5% 3 3 3 3 3 20%	2783
January	1st to 10th	Night	12% 14% 6% 9% 5% 5% 4 3 4 6% 31%	4428
	11th to 20th	Night	7% 11% 5% 15% 10% 7% 6% 4 5% 6% 24%	4440
	21st to end	Night	9% 22% 8% 11% 5% 5% 4 3 4 5% 25%	4747
February	1st to 10th	Night	7% 11% 5% 11% 7% 5% 4 3 5% 6% 36%	2470
	11th to 20th	Night	7% 11% 4 9% 6% 5% 4 3 5% 5% 40%	2403
	21st to end	Night	12% 12% 4 7% 5% 4 4 3 6% 6% 37%	1956
March	1st to 10th	Night	7% 9% 2 8% 5% 5% 5% 3 5% 7% 45%	3644
	11th to 20th	Night	11% 9% 5% 12% 5% 4 4 4 4 5% 36%	3479
	21st to end	Night	22% 12% 5% 10% 5% 3 3 2 4 4 30%	3672
April	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 5% 6% 34%	1900
	11th to 20th	Night	19% 10% 3 6% 5% 5% 4 4 5% 6% 34%	1875
	21st to end	Night	22% 10% 3 6% 4 3 3 4 5% 5% 34%	1689
May	1st to 10th	Night	19% 9% 3 6% 4 4 3 3 5% 7% 38%	2523
	11th to 20th	Night	15% 6% 2 5% 5% 4 4 3 5% 6% 45%	2506
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 6% 6% 31%	2750
June	1st to 10th	Night	14% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	1520
	11th to 20th	Night	13% 7% 3 5% 4 5% 5% 4 6% 7% 41%	1429
	21st to end	Night	13% 7% 1 6% 5% 4 5% 5% 6% 8% 41%	1330
July	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 6% 10% 47%	2329
	11th to 20th	Night	5% 3 2 4 3 3 3 4 6% 9% 57%	2530
	21st to end	Night	8% 5% 2 4 3 3 3 4 5% 8% 54%	2750
August	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 7% 47%	2612
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 7% 48%	2798
	21st to end	Night	19% 7% 3 7% 3 4 3 3 5% 7% 40%	3366
September	1st to 10th	Night	10% 5% 2 4 3 4 4 4 4 7% 51%	1900
	11th to 20th	Night	12% 4 2 5% 3 3 4 3 5% 8% 51%	1900
	21st to end	Night	17% 6% 2 5% 3 3 3 4 6% 8% 42%	2159
October	1st to 10th	Night	16% 7% 2 6% 4 5% 3 3 6% 6% 42%	2280
	11th to 20th	Night	16% 9% 3 7% 5% 5% 5% 4 5% 7% 36%	2280
	21st to end	Night	21% 10% 3 8% 4 4 4 4 5% 6% 31%	2633
November	1st to 10th	Night	15% 10% 3 7% 5% 4 5% 4 5% 7% 35%	2575
	11th to 20th	Night	17% 11% 4 8% 5% 5% 4 4 5% 7% 31%	2660
	21st to end	Night	13% 10% 3 8% 5% 4 5% 4 5% 7% 35%	2660
December	1st to 10th	Night	11% 15% 6% 10% 7% 5% 5% 3 6% 7% 25%	2660
	11th to 20th	Night	11% 17% 5% 10% 6% 5% 5% 4 5% 7% 24%	3283
	21st to end	Night	28% 19% 4 7% 4 3 3 2 3 4 23%	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	59% 5% 12% 18% 6%	2528
	11th to 20th	Day	64% 8% 10% 15% 3	2530
	21st to end	Day	63% 8% 10% 13% 5%	2919
February	1st to 10th	Day	50% 6% 13% 20% 11% 1	1710
	11th to 20th	Day	45% 5% 15% 24% 10% 1	1673
	21st to end	Day	48% 7% 13% 19% 12% 1	1496
March	1st to 10th	Day	46% 6% 13% 23% 10% 1	3216
	11th to 20th	Day	45% 6% 16% 20% 12%	3480
	21st to end	Day	54% 7% 11% 22% 5% 1	3959
April	1st to 10th	Day	53% 5% 11% 18% 12% 2	2280
	11th to 20th	Day	53% 5% 13% 17% 10% 1	2254
	21st to end	Day	52% 6% 11% 21% 10% 1	2487
May	1st to 10th	Day	46% 5% 13% 21% 14% 1	4440
	11th to 20th	Day	41% 8% 14% 26% 9% 2	4403
	21st to end	Day	49% 8% 11% 19% 12% 1	4837
June	1st to 10th	Day	36% 10% 16% 25% 13% 1	2660
	11th to 20th	Day	42% 8% 15% 21% 14%	2744
	21st to end	Day	40% 9% 16% 25% 9% 1	2850
July	1st to 10th	Day	35% 8% 20% 23% 14% 1	4565
	11th to 20th	Day	28% 7% 19% 33% 12% 1	4365
	21st to end	Day	29% 7% 18% 31% 14% 1	4662
August	1st to 10th	Day	38% 8% 17% 26% 11% 1	4296
	11th to 20th	Day	34% 7% 19% 31% 9%	4080
	21st to end	Day	40% 9% 16% 24% 10% 1	4291
September	1st to 10th	Day	34% 7% 19% 30% 9%	2280
	11th to 20th	Day	39% 8% 18% 28% 7%	2280
	21st to end	Day	44% 6% 15% 28% 7%	1997
October	1st to 10th	Day	36% 5% 16% 31% 11% 1	3160
	11th to 20th	Day	45% 5% 11% 26% 13%	3140
	21st to end	Day	51% 5% 17% 21% 6%	3215
November	1st to 10th	Day	52% 7% 12% 19% 9% 1	1600
	11th to 20th	Day	63% 6% 12% 13% 6%	1520
	21st to end	Day	56% 7% 13% 18% 5% 1	1520
December	1st to 10th	Day	61% 9% 13% 14% 3	1520
	11th to 20th	Day	62% 8% 10% 14% 4 1	1915
	21st to end	Day	72% 6% 5% 10% 6%	2783
January	1st to 10th	Night	55% 6% 14% 19% 6%	4428
	11th to 20th	Night	61% 6% 14% 14% 4	4440
	21st to end	Night	63% 6% 10% 17% 4	4747
February	1st to 10th	Night	49% 5% 12% 21% 11% 1	2470
	11th to 20th	Night	46% 5% 13% 23% 12% 1	2404
	21st to end	Night	49% 5% 12% 23% 10% 1	1956
March	1st to 10th	Night	40% 6% 15% 21% 17% 2	3655
	11th to 20th	Night	50% 5% 14% 21% 10%	3480
	21st to end	Night	60% 4 8% 17% 10% 1	3672
April	1st to 10th	Night	51% 5% 9% 17% 16% 1	1900
	11th to 20th	Night	50% 5% 11% 19% 13% 1	1875
	21st to end	Night	51% 4 9% 23% 12% 1	1689
May	1st to 10th	Night	46% 6% 13% 24% 11% 1	2530
	11th to 20th	Night	37% 8% 10% 26% 16% 3	2507
	21st to end	Night	50% 9% 11% 18% 10% 1	2751
June	1st to 10th	Night	41% 8% 14% 24% 12% 1	1520
	11th to 20th	Night	40% 9% 12% 22% 16% 2	1429
	21st to end	Night	40% 8% 14% 26% 12% 1	1330
July	1st to 10th	Night	31% 9% 21% 21% 16% 1	2291
	11th to 20th	Night	23% 8% 14% 42% 12% 2	2482
	21st to end	Night	27% 7% 14% 34% 16% 2	2662
August	1st to 10th	Night	35% 7% 13% 27% 15% 2	2603
	11th to 20th	Night	36% 6% 16% 33% 9% 1	2825
	21st to end	Night	44% 6% 14% 24% 10% 1	3366
September	1st to 10th	Night	31% 8% 15% 34% 12% 1	1900
	11th to 20th	Night	32% 7% 19% 32% 9%	1900
	21st to end	Night	41% 7% 13% 29% 9%	2159
October	1st to 10th	Night	30% 4 10% 35% 19% 3	3800
	11th to 20th	Night	44% 5% 12% 24% 15%	3779
	21st to end	Night	53% 6% 12% 21% 8%	4332
November	1st to 10th	Night	49% 6% 14% 21% 8% 1	2575
	11th to 20th	Night	54% 7% 15% 16% 8%	2660
	21st to end	Night	49% 8% 18% 19% 6% 1	2660
December	1st to 10th	Night	58% 7% 13% 18% 4	2660
	11th to 20th	Night	59% 8% 12% 16% 5% 1	3281
	21st to end	Night	67% 6% 9% 13% 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	14% 4 30% 32% 19% 1	270
	11th to 20th	Day	17% 4 23% 39% 16% 1	270
	21st to end	Day	11% 5% 23% 42% 14% 4	297
February	1st to 10th	Day	13% 7% 33% 36% 9% 2	270
	11th to 20th	Day	12% 9% 41% 24% 11% 3	270
	21st to end	Day	14% 10% 32% 40% 5%	225
March	1st to 10th	Day	15% 7% 27% 36% 13% 1	270
	11th to 20th	Day	13% 6% 28% 41% 9% 4	270
	21st to end	Day	22% 8% 24% 31% 12% 3	378
April	1st to 10th	Day	19% 11% 26% 33% 10% 1	460
	11th to 20th	Day	23% 8% 20% 34% 13% 2	452
	21st to end	Day	23% 8% 24% 29% 14% 3	638
May	1st to 10th	Day	20% 8% 28% 32% 10% 2	653
	11th to 20th	Day	19% 6% 24% 31% 16% 4	675
	21st to end	Day	19% 6% 21% 33% 19% 2	734
June	1st to 10th	Day	10% 7% 26% 35% 18% 3	680
	11th to 20th	Day	16% 6% 24% 38% 15% 2	680
	21st to end	Day	15% 5% 21% 34% 20% 5%	680
July	1st to 10th	Day	9% 2 17% 37% 26% 8%	671
	11th to 20th	Day	10% 4 23% 31% 22% 11%	668
	21st to end	Day	7% 3 17% 42% 24% 8%	739
August	1st to 10th	Day	13% 3 17% 34% 28% 5%	676
	11th to 20th	Day	14% 4 14% 32% 30% 6%	665
	21st to end	Day	17% 5% 17% 30% 25% 7%	612
September	1st to 10th	Day	11% 4 19% 38% 24% 4	490
	11th to 20th	Day	15% 4 22% 31% 23% 6%	490
	21st to end	Day	20% 9% 23% 26% 17% 5%	348
October	1st to 10th	Day	20% 6% 24% 31% 14% 4	300
	11th to 20th	Day	17% 9% 30% 31% 10% 2	300
	21st to end	Day	25% 8% 34% 25% 8%	330
November	1st to 10th	Day	20% 10% 29% 29% 11% 1	282
	11th to 20th	Day	19% 10% 34% 28% 9% 1	270
	21st to end	Day	19% 11% 27% 34% 8% 1	270
December	1st to 10th	Day	16% 7% 34% 29% 12% 1	270
	11th to 20th	Day	20% 13% 31% 27% 8% 1	270
	21st to end	Day	27% 8% 28% 30% 6% 1	297
January	1st to 10th	Night	14% 7% 32% 36% 11% 1	950
	11th to 20th	Night	17% 5% 35% 33% 10% 1	950
	21st to end	Night	17% 6% 31% 34% 11% 1	1045
February	1st to 10th	Night	12% 6% 34% 37% 9% 1	950
	11th to 20th	Night	12% 5% 37% 33% 11% 1	929
	21st to end	Night	17% 7% 31% 36% 10%	700
March	1st to 10th	Night	15% 6% 27% 40% 10% 1	721
	11th to 20th	Night	18% 5% 31% 36% 10% 1	570
	21st to end	Night	22% 5% 24% 32% 15% 3	627
April	1st to 10th	Night	22% 5% 30% 33% 9% 1	570
	11th to 20th	Night	23% 6% 22% 37% 11% 1	563
	21st to end	Night	26% 5% 19% 35% 13% 3	570
May	1st to 10th	Night	18% 6% 27% 34% 13% 2	570
	11th to 20th	Night	19% 4 21% 37% 17% 2	561
	21st to end	Night	18% 5% 16% 38% 22% 2	617
June	1st to 10th	Night	16% 4 19% 39% 20% 1	570
	11th to 20th	Night	16% 4 16% 47% 15% 1	570
	21st to end	Night	15% 4 16% 40% 22% 3	570
July	1st to 10th	Night	9% 1 14% 38% 34% 4	555
	11th to 20th	Night	9% 1 21% 36% 27% 5%	552
	21st to end	Night	8% 2 16% 41% 29% 4	624
August	1st to 10th	Night	10% 3 15% 41% 29% 2	567
	11th to 20th	Night	14% 3 15% 34% 30% 4	564
	21st to end	Night	18% 4 15% 35% 25% 4	627
September	1st to 10th	Night	11% 4 19% 37% 26% 3	570
	11th to 20th	Night	15% 3 17% 38% 25% 2	570
	21st to end	Night	20% 5% 25% 29% 18% 3	691
October	1st to 10th	Night	19% 7% 25% 33% 16% 1	760
	11th to 20th	Night	20% 9% 31% 29% 9% 1	760
	21st to end	Night	25% 9% 28% 29% 9%	836
November	1st to 10th	Night	19% 9% 29% 32% 10%	865
	11th to 20th	Night	20% 12% 31% 26% 11% 1	950
	21st to end	Night	19% 9% 35% 30% 7%	950
December	1st to 10th	Night	18% 11% 32% 32% 7%	950
	11th to 20th	Night	21% 9% 34% 26% 9% 1	950
	21st to end	Night	28% 7% 32% 25% 7% 1	1045

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