Treesdale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treesdale Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	16% 10% 4 8% 5% 4 4 4 5% 7% 31%	101328
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	98152
Transparency	Day	49% 8% 12% 20% 10% 1	104976
	Night	50% 7% 11% 20% 11% 2	102643
Seeing	Day	22% 14% 27% 21% 11% 4	14754
	Night	21% 12% 27% 27% 12% 1	24452

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	19% 21% 10% 11% 4 4 3 2 4 5% 18%	2808
	11th to 20th	Day	13% 23% 9% 16% 8% 4 2 2 3 3 17%	2840
	21st to end	Day	26% 19% 8% 15% 6% 4 3 2 3 4 10%	3124
February	1st to 10th	Day	20% 18% 5% 11% 7% 5% 3 3 3 4 22%	1710
	11th to 20th	Day	16% 15% 6% 13% 6% 5% 5% 4 4 5% 21%	1663
	21st to end	Day	21% 15% 5% 10% 5% 4 3 3 4 6% 24%	1568
March	1st to 10th	Day	21% 11% 4 9% 5% 4 4 3 5% 6% 28%	3435
	11th to 20th	Day	13% 11% 5% 10% 6% 5% 4 3 4 6% 32%	3480
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 6% 6% 31%	3815
April	1st to 10th	Day	16% 12% 4 8% 5% 4 4 4 6% 6% 30%	2192
	11th to 20th	Day	18% 11% 3 7% 5% 5% 4 3 5% 6% 32%	2289
	21st to end	Day	22% 10% 3 7% 5% 4 4 4 4 7% 32%	2470
May	1st to 10th	Day	24% 9% 3 8% 5% 4 4 4 6% 6% 27%	4082
	11th to 20th	Day	19% 8% 3 9% 5% 4 4 4 6% 7% 30%	4081
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 38%	4751
June	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 5% 7% 9% 33%	2660
	11th to 20th	Day	14% 7% 3 7% 6% 4 5% 4 7% 9% 35%	2660
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 7% 9% 37%	2660
July	1st to 10th	Day	10% 7% 3 6% 6% 5% 6% 4 6% 9% 40%	4325
	11th to 20th	Day	10% 6% 2 6% 5% 5% 4 4 7% 10% 40%	4120
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 5% 6% 8% 36%	4466
August	1st to 10th	Day	14% 8% 3 8% 6% 6% 6% 4 7% 8% 30%	4104
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4044
	21st to end	Day	8% 5% 3 7% 5% 6% 6% 5% 7% 10% 40%	4318
September	1st to 10th	Day	16% 6% 2 5% 4 4 3 3 5% 8% 43%	2280
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 5% 9% 44%	2099
	21st to end	Day	19% 8% 3 7% 4 5% 5% 3 3 6% 36%	2086
October	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 39%	1900
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 4 5% 5% 34%	1900
	21st to end	Day	23% 10% 3 6% 4 3 4 3 6% 7% 32%	2090
November	1st to 10th	Day	11% 9% 3 7% 5% 4 5% 4 4 5% 42%	1872
	11th to 20th	Day	17% 14% 4 8% 5% 3 4 3 5% 5% 31%	1658
	21st to end	Day	18% 16% 6% 8% 7% 4 4 4 5% 5% 24%	1576
December	1st to 10th	Day	21% 20% 7% 11% 5% 3 4 3 3 5% 17%	1520
	11th to 20th	Day	20% 17% 8% 11% 5% 3 4 3 4 6% 20%	1910
	21st to end	Day	25% 24% 6% 10% 5% 4 3 3 4 4 13%	2772
January	1st to 10th	Night	16% 21% 8% 12% 5% 4 4 3 3 4 18%	4132
	11th to 20th	Night	17% 19% 7% 14% 7% 6% 4 3 4 4 15%	4120
	21st to end	Night	29% 16% 7% 12% 6% 4 3 2 3 3 15%	4532
February	1st to 10th	Night	21% 14% 5% 10% 7% 5% 4 3 4 4 24%	2470
	11th to 20th	Night	19% 13% 4 11% 5% 5% 5% 3 4 5% 25%	2403
	21st to end	Night	21% 12% 5% 11% 5% 4 4 4 5% 4 25%	1884
March	1st to 10th	Night	24% 12% 3 6% 5% 3 3 3 4 5% 32%	3418
	11th to 20th	Night	18% 13% 5% 9% 3 3 3 2 4 7% 34%	3480
	21st to end	Night	18% 10% 3 7% 5% 4 3 3 4 5% 39%	3817
April	1st to 10th	Night	18% 12% 3 7% 6% 3 4 2 5% 7% 33%	1986
	11th to 20th	Night	17% 8% 2 7% 5% 5% 4 4 5% 7% 36%	1855
	21st to end	Night	15% 10% 3 8% 4 4 3 4 5% 8% 36%	1710
May	1st to 10th	Night	26% 10% 3 7% 4 4 3 3 5% 7% 28%	2813
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 3 6% 7% 36%	2812
	21st to end	Night	11% 6% 2 6% 4 4 4 5% 8% 10% 41%	2799
June	1st to 10th	Night	10% 6% 3 5% 5% 3 4 4 7% 8% 44%	1520
	11th to 20th	Night	11% 5% 2 7% 5% 4 5% 5% 7% 8% 41%	1520
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	1520
July	1st to 10th	Night	10% 9% 3 8% 5% 4 4 4 6% 8% 41%	2630
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 3 6% 9% 43%	2840
	21st to end	Night	10% 7% 3 8% 5% 5% 4 4 6% 7% 40%	3087
August	1st to 10th	Night	12% 8% 4 8% 6% 5% 4 4 7% 7% 35%	2825
	11th to 20th	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 42%	2785
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	3306
September	1st to 10th	Night	13% 6% 2 5% 4 3 4 5% 5% 6% 48%	1900
	11th to 20th	Night	10% 4 2 4 4 4 4 3 5% 9% 51%	2078
	21st to end	Night	17% 8% 2 6% 4 4 4 4 5% 5% 40%	2092
October	1st to 10th	Night	14% 6% 3 5% 4 4 5% 4 4 6% 45%	2280
	11th to 20th	Night	18% 7% 2 7% 5% 4 4 4 4 6% 40%	2280
	21st to end	Night	23% 9% 2 6% 4 4 3 3 4 6% 36%	2508
November	1st to 10th	Night	13% 9% 2 6% 4 3 4 3 6% 6% 43%	2422
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 6% 6% 31%	2740
	21st to end	Night	20% 14% 4 8% 4 4 3 4 5% 5% 28%	2759
December	1st to 10th	Night	22% 16% 7% 10% 5% 5% 4 2 5% 5% 19%	2660
	11th to 20th	Night	19% 18% 7% 10% 5% 4 4 3 4 5% 23%	3285
	21st to end	Night	25% 19% 6% 10% 6% 5% 4 3 4 5% 13%	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	68% 7% 10% 10% 5%	2809
	11th to 20th	Day	72% 5% 8% 11% 3 1	2840
	21st to end	Day	77% 6% 8% 7% 2	3124
February	1st to 10th	Day	67% 4 8% 13% 6% 1	1710
	11th to 20th	Day	63% 6% 11% 13% 6%	1672
	21st to end	Day	61% 5% 12% 14% 7% 1	1568
March	1st to 10th	Day	54% 8% 11% 21% 6%	3435
	11th to 20th	Day	50% 9% 13% 20% 7% 1	3480
	21st to end	Day	50% 9% 11% 19% 11% 1	3815
April	1st to 10th	Day	52% 7% 12% 17% 11% 2	2192
	11th to 20th	Day	51% 7% 11% 16% 14% 2	2289
	21st to end	Day	50% 6% 12% 21% 9% 1	2470
May	1st to 10th	Day	53% 9% 11% 19% 7% 1	4120
	11th to 20th	Day	50% 9% 13% 19% 7% 1	4085
	21st to end	Day	37% 11% 15% 22% 11% 3	4765
June	1st to 10th	Day	44% 10% 14% 21% 10% 1	2660
	11th to 20th	Day	43% 9% 14% 21% 12% 1	2660
	21st to end	Day	41% 9% 13% 25% 11% 1	2660
July	1st to 10th	Day	40% 8% 12% 23% 14% 3	4258
	11th to 20th	Day	37% 7% 13% 27% 15%	4051
	21st to end	Day	43% 8% 12% 19% 16% 1	4324
August	1st to 10th	Day	49% 9% 14% 19% 9%	4091
	11th to 20th	Day	42% 8% 12% 27% 11% 1	4083
	21st to end	Day	36% 9% 14% 27% 13% 2	4318
September	1st to 10th	Day	39% 8% 10% 21% 19% 3	2280
	11th to 20th	Day	36% 8% 15% 25% 15% 1	2099
	21st to end	Day	49% 6% 11% 21% 13%	2086
October	1st to 10th	Day	32% 6% 12% 30% 19% 1	3160
	11th to 20th	Day	43% 6% 10% 22% 13% 5%	3140
	21st to end	Day	37% 5% 13% 27% 17% 1	3428
November	1st to 10th	Day	42% 6% 11% 21% 17% 2	1872
	11th to 20th	Day	54% 5% 11% 18% 10% 2	1658
	21st to end	Day	61% 5% 12% 15% 6% 1	1576
December	1st to 10th	Day	70% 5% 10% 12% 3	1520
	11th to 20th	Day	66% 5% 11% 14% 3	1906
	21st to end	Day	74% 7% 9% 8% 2	2772
January	1st to 10th	Night	68% 8% 9% 12% 3	4132
	11th to 20th	Night	70% 7% 10% 10% 2	4120
	21st to end	Night	74% 5% 6% 11% 3	4532
February	1st to 10th	Night	62% 6% 9% 16% 6%	2470
	11th to 20th	Night	61% 5% 11% 16% 7%	2405
	21st to end	Night	61% 6% 8% 14% 9% 1	1884
March	1st to 10th	Night	54% 6% 10% 23% 7%	3418
	11th to 20th	Night	50% 7% 13% 23% 6% 1	3480
	21st to end	Night	45% 7% 12% 22% 13% 1	3817
April	1st to 10th	Night	51% 6% 9% 18% 13% 2	1986
	11th to 20th	Night	45% 8% 12% 17% 15% 3	1855
	21st to end	Night	45% 7% 12% 22% 12% 3	1710
May	1st to 10th	Night	55% 6% 10% 17% 10% 2	2840
	11th to 20th	Night	45% 10% 13% 22% 10% 1	2813
	21st to end	Night	32% 12% 16% 23% 13% 4	2806
June	1st to 10th	Night	35% 10% 15% 23% 16% 2	1520
	11th to 20th	Night	37% 10% 14% 23% 15% 2	1520
	21st to end	Night	36% 10% 13% 23% 16% 2	1520
July	1st to 10th	Night	41% 8% 10% 22% 17% 2	2587
	11th to 20th	Night	35% 8% 15% 25% 16% 1	2786
	21st to end	Night	40% 9% 10% 16% 22% 2	2988
August	1st to 10th	Night	46% 10% 13% 21% 11% 1	2814
	11th to 20th	Night	40% 7% 10% 28% 14% 1	2812
	21st to end	Night	33% 10% 13% 29% 13% 2	3306
September	1st to 10th	Night	33% 7% 14% 22% 19% 5%	1900
	11th to 20th	Night	30% 7% 14% 26% 22% 1	2078
	21st to end	Night	44% 6% 11% 21% 16% 1	2092
October	1st to 10th	Night	36% 5% 9% 25% 23% 1	3800
	11th to 20th	Night	38% 6% 11% 23% 15% 8%	3779
	21st to end	Night	35% 5% 10% 28% 20% 2	4120
November	1st to 10th	Night	40% 7% 11% 22% 17% 3	2422
	11th to 20th	Night	53% 5% 11% 19% 10% 3	2740
	21st to end	Night	56% 6% 12% 20% 6% 1	2759
December	1st to 10th	Night	66% 6% 12% 12% 4	2660
	11th to 20th	Night	64% 5% 12% 15% 3	3288
	21st to end	Night	71% 8% 11% 9% 2	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	29% 26% 29% 11% 4 1	263
	11th to 20th	Day	32% 29% 29% 8% 3	270
	21st to end	Day	27% 29% 32% 7% 4	297
February	1st to 10th	Day	32% 23% 36% 8% 1	270
	11th to 20th	Day	34% 30% 27% 9% 1	270
	21st to end	Day	41% 21% 28% 9%	253
March	1st to 10th	Day	28% 21% 30% 17% 4	460
	11th to 20th	Day	32% 20% 31% 12% 4 1	460
	21st to end	Day	30% 21% 27% 16% 4 1	506
April	1st to 10th	Day	28% 22% 26% 16% 7% 1	460
	11th to 20th	Day	23% 21% 27% 17% 8% 4	452
	21st to end	Day	28% 17% 30% 17% 5% 2	460
May	1st to 10th	Day	29% 17% 25% 18% 7% 3	463
	11th to 20th	Day	22% 11% 32% 19% 11% 4	487
	21st to end	Day	14% 10% 26% 28% 17% 6%	529
June	1st to 10th	Day	15% 11% 29% 27% 13% 4	490
	11th to 20th	Day	17% 12% 26% 27% 15% 5%	490
	21st to end	Day	14% 11% 32% 27% 13% 3	490
July	1st to 10th	Day	12% 7% 21% 28% 22% 10%	485
	11th to 20th	Day	14% 6% 26% 28% 18% 8%	484
	21st to end	Day	12% 5% 27% 31% 18% 7%	532
August	1st to 10th	Day	11% 8% 23% 29% 20% 9%	486
	11th to 20th	Day	16% 5% 24% 30% 16% 10%	479
	21st to end	Day	12% 4 25% 27% 22% 11%	539
September	1st to 10th	Day	16% 8% 23% 27% 19% 8%	490
	11th to 20th	Day	12% 5% 20% 33% 21% 11%	490
	21st to end	Day	25% 12% 22% 23% 13% 5%	490
October	1st to 10th	Day	17% 10% 26% 25% 18% 5%	490
	11th to 20th	Day	22% 16% 29% 23% 8% 3	490
	21st to end	Day	29% 15% 29% 19% 7%	539
November	1st to 10th	Day	19% 18% 33% 18% 10% 2	410
	11th to 20th	Day	28% 20% 33% 13% 5% 2	228
	21st to end	Day	23% 24% 40% 10% 3	194
December	1st to 10th	Day	31% 27% 26% 11% 4 2	180
	11th to 20th	Day	32% 21% 33% 11% 3	180
	21st to end	Day	35% 18% 31% 13% 3	198
January	1st to 10th	Night	27% 21% 34% 15% 3	760
	11th to 20th	Night	31% 16% 32% 18% 3	760
	21st to end	Night	26% 19% 33% 19% 3	836
February	1st to 10th	Night	28% 16% 29% 25% 2	760
	11th to 20th	Night	29% 20% 34% 15% 1	743
	21st to end	Night	28% 16% 36% 18% 3	628
March	1st to 10th	Night	24% 18% 32% 22% 3	760
	11th to 20th	Night	26% 19% 27% 21% 6% 1	760
	21st to end	Night	24% 15% 34% 22% 5% 1	836
April	1st to 10th	Night	24% 16% 30% 24% 5%	760
	11th to 20th	Night	21% 13% 33% 23% 9% 1	731
	21st to end	Night	19% 10% 35% 31% 5%	570
May	1st to 10th	Night	21% 10% 27% 32% 9% 1	570
	11th to 20th	Night	21% 8% 27% 32% 12%	561
	21st to end	Night	11% 3 25% 35% 23% 3	453
June	1st to 10th	Night	11% 5% 26% 40% 18% 1	380
	11th to 20th	Night	11% 6% 23% 39% 20% 1	380
	21st to end	Night	11% 2 29% 36% 20% 1	380
July	1st to 10th	Night	9% 2 20% 42% 21% 5%	425
	11th to 20th	Night	9% 5% 19% 35% 28% 5%	552
	21st to end	Night	11% 3 14% 43% 27% 2	624
August	1st to 10th	Night	10% 2 15% 43% 27% 2	567
	11th to 20th	Night	12% 5% 16% 34% 29% 4	564
	21st to end	Night	12% 3 19% 34% 27% 5%	731
September	1st to 10th	Night	15% 4 20% 32% 25% 4	760
	11th to 20th	Night	11% 4 16% 36% 29% 4	760
	21st to end	Night	18% 9% 24% 25% 19% 4	760
October	1st to 10th	Night	17% 8% 23% 28% 22% 1	760
	11th to 20th	Night	21% 10% 31% 27% 10% 1	760
	21st to end	Night	27% 11% 26% 27% 8%	836
November	1st to 10th	Night	17% 13% 28% 30% 12%	780
	11th to 20th	Night	26% 15% 29% 24% 6% 1	800
	21st to end	Night	27% 16% 30% 21% 6%	789
December	1st to 10th	Night	31% 16% 28% 19% 4	760
	11th to 20th	Night	27% 19% 30% 21% 3	760
	21st to end	Night	28% 17% 32% 21% 2	836

Page generated at 2024-11-25 00:22:21. Version ac07

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