East Fork Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the East Fork 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% 6% 2 6% 4 4 4 3 5% 7% 45%	105387
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	104097
Transparency	Day	38% 4 9% 22% 20% 6%	109035
	Night	34% 4 9% 21% 23% 9%	108576
Seeing	Day	15% 9% 24% 27% 20% 5%	15419
	Night	15% 9% 25% 31% 18% 2	25348

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	18% 11% 2 5% 3 3 2 2 2 5% 46%	2937
	11th to 20th	Day	18% 6% 2 4 2 2 3 3 4 5% 52%	2940
	21st to end	Day	24% 7% 2 5% 3 3 2 2 3 3 45%	3144
February	1st to 10th	Day	21% 9% 3 6% 5% 4 3 3 2 4 39%	1710
	11th to 20th	Day	21% 8% 3 7% 5% 3 3 3 4 4 41%	1794
	21st to end	Day	24% 8% 2 5% 3 3 4 2 5% 4 42%	1727
March	1st to 10th	Day	19% 11% 3 6% 3 3 3 3 4 6% 39%	3436
	11th to 20th	Day	19% 7% 3 6% 4 5% 4 3 6% 6% 37%	3480
	21st to end	Day	18% 8% 3 7% 4 4 4 3 5% 6% 39%	3814
April	1st to 10th	Day	14% 7% 3 6% 4 4 4 3 4 6% 45%	2147
	11th to 20th	Day	17% 7% 2 5% 4 4 4 3 5% 7% 42%	2254
	21st to end	Day	14% 5% 2 5% 5% 4 4 4 6% 7% 44%	2280
May	1st to 10th	Day	23% 8% 3 5% 4 4 3 3 5% 6% 35%	3911
	11th to 20th	Day	15% 5% 2 7% 4 4 4 3 5% 7% 44%	4082
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4477
June	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 5% 7% 10% 41%	2470
	11th to 20th	Day	6% 5% 1 6% 5% 5% 6% 5% 7% 9% 46%	2470
	21st to end	Day	5% 3 2 5% 6% 5% 5% 4 7% 10% 47%	2470
July	1st to 10th	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 10% 38%	4120
	11th to 20th	Day	6% 3 1 5% 4 4 5% 4 6% 10% 53%	4120
	21st to end	Day	12% 9% 3 7% 5% 5% 5% 4 6% 8% 34%	4465
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 7% 67%	4104
	11th to 20th	Day	5% 2 1 4 4 4 5% 4 7% 10% 54%	3892
	21st to end	Day	6% 4 2 7% 4 5% 6% 5% 9% 9% 42%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 5% 4 3 5% 7% 48%	2241
	11th to 20th	Day	12% 5% 2 7% 5% 6% 4 3 5% 8% 44%	2090
	21st to end	Day	8% 3 1 4 5% 4 4 3 5% 7% 56%	2090
October	1st to 10th	Day	7% 3 1 4 3 3 4 2 5% 8% 61%	1900
	11th to 20th	Day	14% 5% 1 4 2 3 3 3 5% 5% 55%	1900
	21st to end	Day	16% 5% 2 4 3 3 3 2 3 4 54%	2090
November	1st to 10th	Day	14% 6% 2 5% 4 4 3 2 4 6% 50%	1958
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 4 7% 54%	2000
	21st to end	Day	19% 6% 3 5% 3 3 3 2 3 5% 48%	1875
December	1st to 10th	Day	20% 6% 2 5% 3 3 2 2 3 4 51%	2940
	11th to 20th	Day	18% 9% 3 6% 4 3 3 3 4 5% 43%	3391
	21st to end	Day	18% 9% 3 6% 3 4 3 2 4 4 44%	4488
January	1st to 10th	Night	16% 9% 2 5% 3 3 2 2 3 5% 49%	4238
	11th to 20th	Night	18% 5% 1 4 3 2 2 2 3 5% 55%	4250
	21st to end	Night	25% 10% 2 4 3 2 2 2 3 3 45%	4546
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 4 5% 47%	2470
	11th to 20th	Night	21% 7% 2 5% 3 3 3 2 4 5% 45%	2286
	21st to end	Night	19% 7% 2 6% 4 2 3 3 4 5% 48%	1727
March	1st to 10th	Night	22% 9% 2 5% 4 3 4 3 4 5% 38%	3419
	11th to 20th	Night	19% 7% 2 6% 4 3 4 4 4 6% 41%	3480
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 45%	3817
April	1st to 10th	Night	12% 4 2 4 3 4 4 4 5% 7% 51%	2031
	11th to 20th	Night	13% 7% 2 5% 4 3 4 4 5% 7% 47%	1875
	21st to end	Night	11% 6% 2 5% 3 3 4 3 4 7% 52%	1900
May	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 5% 7% 37%	2985
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 5% 7% 47%	2823
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 8% 42%	3090
June	1st to 10th	Night	5% 3 2 5% 3 4 5% 4 6% 9% 56%	1710
	11th to 20th	Night	5% 4 1 4 3 4 3 3 5% 8% 61%	1710
	21st to end	Night	4 4 1 4 4 3 4 4 8% 10% 56%	1710
July	1st to 10th	Night	8% 3 2 5% 3 4 4 4 7% 11% 48%	2850
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 10% 59%	2850
	21st to end	Night	9% 7% 3 7% 5% 4 4 4 6% 8% 43%	3098
August	1st to 10th	Night	2 1 2 2 2 3 3 4 7% 74%	2836
	11th to 20th	Night	4 2 1 3 2 2 4 4 6% 11% 60%	2942
	21st to end	Night	4 3 1 4 3 4 5% 5% 8% 9% 53%	3476
September	1st to 10th	Night	8% 4 1 5% 3 4 4 3 5% 7% 55%	1933
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 8% 52%	2090
	21st to end	Night	7% 4 1 4 3 2 2 3 5% 7% 62%	2090
October	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 6% 66%	2261
	11th to 20th	Night	12% 4 1 3 2 3 3 2 4 4 61%	2280
	21st to end	Night	15% 5% 1 4 3 2 2 2 3 5% 58%	2508
November	1st to 10th	Night	13% 5% 1 5% 3 3 3 3 4 5% 55%	2340
	11th to 20th	Night	9% 4 2 6% 4 3 3 3 4 5% 57%	2400
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 51%	2520
December	1st to 10th	Night	17% 6% 3 5% 3 3 3 2 4 4 50%	4250
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 4 4 45%	4816
	21st to end	Night	16% 11% 2 5% 3 3 3 2 4 4 48%	6490

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	45% 2 5% 20% 24% 4	2938
	11th to 20th	Day	36% 3 7% 26% 24% 4	2940
	21st to end	Day	47% 2 4 16% 22% 10%	3144
February	1st to 10th	Day	50% 3 5% 16% 17% 8%	1710
	11th to 20th	Day	48% 3 4 18% 19% 7%	1803
	21st to end	Day	47% 3 5% 13% 22% 9%	1727
March	1st to 10th	Day	48% 5% 6% 14% 22% 6%	3436
	11th to 20th	Day	46% 5% 7% 22% 17% 4	3480
	21st to end	Day	46% 4 7% 19% 21% 2	3814
April	1st to 10th	Day	40% 4 8% 22% 18% 8%	2147
	11th to 20th	Day	42% 6% 7% 18% 19% 9%	2254
	21st to end	Day	39% 5% 10% 20% 22% 4	2280
May	1st to 10th	Day	49% 5% 8% 24% 10% 3	3946
	11th to 20th	Day	39% 6% 10% 22% 22% 1	4086
	21st to end	Day	48% 7% 13% 19% 10% 3	4489
June	1st to 10th	Day	35% 8% 15% 26% 16% 1	2470
	11th to 20th	Day	33% 6% 15% 24% 18% 3	2470
	21st to end	Day	30% 7% 13% 29% 18% 2	2470
July	1st to 10th	Day	41% 6% 20% 24% 8%	4056
	11th to 20th	Day	26% 5% 17% 33% 18% 1	4051
	21st to end	Day	47% 5% 13% 24% 10%	4325
August	1st to 10th	Day	17% 4 14% 31% 27% 6%	4090
	11th to 20th	Day	26% 5% 20% 36% 12% 1	3931
	21st to end	Day	35% 6% 16% 27% 15% 1	4179
September	1st to 10th	Day	35% 6% 11% 20% 18% 11%	2241
	11th to 20th	Day	39% 7% 10% 18% 19% 7%	2090
	21st to end	Day	28% 5% 9% 22% 25% 11%	2090
October	1st to 10th	Day	14% 3 7% 12% 25% 39%	3160
	11th to 20th	Day	21% 2 3 16% 33% 25%	3140
	21st to end	Day	31% 2 3 23% 32% 9%	3428
November	1st to 10th	Day	37% 4 7% 19% 24% 10%	1958
	11th to 20th	Day	32% 4 7% 18% 26% 12%	2000
	21st to end	Day	41% 4 6% 16% 25% 9%	1875
December	1st to 10th	Day	41% 2 5% 25% 23% 5%	2940
	11th to 20th	Day	47% 2 4 16% 22% 9%	3389
	21st to end	Day	45% 2 4 19% 25% 5%	4488
January	1st to 10th	Night	40% 3 4 15% 30% 7%	4238
	11th to 20th	Night	35% 3 6% 19% 28% 8%	4250
	21st to end	Night	46% 3 6% 15% 21% 9%	4546
February	1st to 10th	Night	42% 3 7% 19% 18% 11%	2470
	11th to 20th	Night	44% 3 5% 17% 22% 9%	2287
	21st to end	Night	41% 3 7% 16% 20% 13%	1727
March	1st to 10th	Night	48% 5% 6% 14% 17% 11%	3419
	11th to 20th	Night	43% 5% 8% 17% 19% 7%	3480
	21st to end	Night	40% 4 9% 20% 24% 4	3817
April	1st to 10th	Night	33% 5% 11% 21% 19% 11%	2031
	11th to 20th	Night	37% 5% 8% 19% 20% 11%	1875
	21st to end	Night	33% 5% 9% 24% 25% 4	1900
May	1st to 10th	Night	45% 7% 9% 21% 12% 6%	3015
	11th to 20th	Night	36% 6% 12% 21% 21% 3	2824
	21st to end	Night	38% 7% 15% 24% 14% 3	3099
June	1st to 10th	Night	24% 8% 15% 31% 20% 3	1710
	11th to 20th	Night	22% 6% 13% 31% 23% 5%	1710
	21st to end	Night	22% 6% 13% 29% 25% 4	1710
July	1st to 10th	Night	30% 5% 22% 33% 9% 1	2805
	11th to 20th	Night	21% 4 16% 40% 18% 2	2796
	21st to end	Night	39% 5% 14% 30% 11% 1	2999
August	1st to 10th	Night	11% 4 14% 30% 34% 7%	2826
	11th to 20th	Night	19% 5% 19% 38% 17% 3	2969
	21st to end	Night	25% 7% 16% 26% 24% 2	3476
September	1st to 10th	Night	29% 8% 12% 20% 21% 10%	1933
	11th to 20th	Night	31% 6% 10% 22% 22% 9%	2090
	21st to end	Night	23% 4 9% 22% 28% 13%	2090
October	1st to 10th	Night	14% 2 5% 15% 21% 43%	3768
	11th to 20th	Night	18% 2 3 14% 34% 28%	3779
	21st to end	Night	27% 2 4 20% 31% 17%	4120
November	1st to 10th	Night	32% 4 8% 16% 28% 12%	2340
	11th to 20th	Night	30% 3 7% 17% 28% 15%	2400
	21st to end	Night	37% 4 6% 18% 26% 10%	2520
December	1st to 10th	Night	40% 3 5% 20% 27% 6%	4250
	11th to 20th	Night	45% 2 7% 14% 22% 10%	4817
	21st to end	Night	42% 2 5% 18% 27% 6%	6490

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	27% 17% 35% 17% 4	300
	11th to 20th	Day	26% 20% 34% 17% 2	300
	21st to end	Day	25% 23% 36% 12% 4	300
February	1st to 10th	Day	28% 19% 39% 10% 3 1	270
	11th to 20th	Day	33% 22% 33% 11% 2	270
	21st to end	Day	39% 24% 31% 4 2	225
March	1st to 10th	Day	23% 22% 35% 16% 4	270
	11th to 20th	Day	24% 26% 37% 10% 3	270
	21st to end	Day	22% 24% 37% 14% 3	297
April	1st to 10th	Day	23% 16% 39% 17% 5%	327
	11th to 20th	Day	25% 16% 31% 18% 9%	452
	21st to end	Day	17% 15% 35% 23% 7% 2	460
May	1st to 10th	Day	19% 10% 38% 27% 6% 1	544
	11th to 20th	Day	15% 6% 31% 29% 16% 3	675
	21st to end	Day	12% 7% 28% 33% 16% 4	734
June	1st to 10th	Day	8% 3 17% 36% 30% 6%	680
	11th to 20th	Day	6% 3 16% 42% 27% 5%	680
	21st to end	Day	6% 2 16% 36% 30% 10%	680
July	1st to 10th	Day	8% 2 14% 33% 33% 10%	671
	11th to 20th	Day	4 1 8% 40% 37% 10%	668
	21st to end	Day	8% 2 9% 34% 35% 12%	739
August	1st to 10th	Day	4 1 8% 29% 47% 11%	676
	11th to 20th	Day	9% 1 8% 30% 43% 9%	581
	21st to end	Day	10% 1 6% 24% 41% 18%	539
September	1st to 10th	Day	10% 1 10% 27% 39% 13%	451
	11th to 20th	Day	6% 4 12% 29% 36% 12%	300
	21st to end	Day	11% 5% 13% 37% 27% 8%	300
October	1st to 10th	Day	5% 4 22% 43% 22% 4	300
	11th to 20th	Day	13% 7% 32% 32% 11% 4	300
	21st to end	Day	24% 12% 28% 24% 11% 2	330
November	1st to 10th	Day	21% 7% 32% 30% 9% 1	300
	11th to 20th	Day	16% 12% 39% 25% 7% 1	300
	21st to end	Day	22% 19% 31% 19% 8% 1	300
December	1st to 10th	Day	21% 16% 37% 17% 7% 2	300
	11th to 20th	Day	25% 20% 33% 16% 4 1	300
	21st to end	Day	22% 21% 38% 15% 3 1	330
January	1st to 10th	Night	22% 14% 31% 27% 6%	1000
	11th to 20th	Night	24% 15% 31% 25% 6%	1000
	21st to end	Night	22% 15% 33% 27% 3	1051
February	1st to 10th	Night	22% 15% 35% 23% 5%	950
	11th to 20th	Night	28% 14% 34% 21% 3	812
	21st to end	Night	23% 21% 33% 21% 2	471
March	1st to 10th	Night	23% 12% 32% 27% 5%	570
	11th to 20th	Night	20% 13% 33% 28% 4 1	570
	21st to end	Night	17% 12% 39% 26% 6%	627
April	1st to 10th	Night	13% 15% 39% 26% 7%	570
	11th to 20th	Night	16% 15% 34% 26% 10%	563
	21st to end	Night	14% 10% 36% 27% 11% 1	570
May	1st to 10th	Night	10% 9% 37% 33% 11% 1	570
	11th to 20th	Night	12% 5% 31% 34% 17% 2	561
	21st to end	Night	9% 5% 29% 36% 20% 2	617
June	1st to 10th	Night	5% 2 13% 40% 39% 2	570
	11th to 20th	Night	5% 2 16% 42% 32% 2	570
	21st to end	Night	5% 12% 40% 36% 7%	570
July	1st to 10th	Night	5% 1 9% 35% 44% 6%	555
	11th to 20th	Night	4 1 7% 37% 45% 6%	552
	21st to end	Night	5% 1 7% 37% 41% 9%	624
August	1st to 10th	Night	4 1 7% 38% 44% 7%	567
	11th to 20th	Night	7% 7% 41% 38% 6%	564
	21st to end	Night	6% 1 7% 36% 42% 8%	627
September	1st to 10th	Night	10% 1 10% 32% 41% 6%	570
	11th to 20th	Night	10% 1 11% 34% 40% 4	570
	21st to end	Night	8% 3 12% 38% 35% 4	570
October	1st to 10th	Night	7% 2 14% 43% 28% 5%	741
	11th to 20th	Night	12% 8% 24% 38% 18% 1	760
	21st to end	Night	17% 8% 26% 29% 18% 2	836
November	1st to 10th	Night	14% 6% 27% 38% 14% 1	780
	11th to 20th	Night	11% 11% 30% 38% 10% 1	800
	21st to end	Night	18% 14% 31% 26% 11% 1	920
December	1st to 10th	Night	19% 12% 30% 32% 7%	1000
	11th to 20th	Night	22% 12% 31% 29% 7%	1000
	21st to end	Night	23% 14% 30% 27% 6%	1100

Page generated at 2025-01-19 02:06:50. Version ac07

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