Anderson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anderson Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	109168
	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 32%	109064
Transparency	Day	52% 5% 11% 19% 12% 1	112842
	Night	51% 5% 10% 20% 12% 2	113529
Seeing	Day	18% 11% 28% 23% 14% 6%	15445
	Night	19% 10% 28% 29% 13% 2	25271

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	13% 18% 9% 22% 8% 6% 3 3 4 5% 9%	4056
	11th to 20th	Day	12% 21% 6% 15% 8% 6% 7% 4 4 3 14%	4060
	21st to end	Day	20% 16% 9% 15% 8% 7% 4 3 3 3 13%	4466
February	1st to 10th	Day	17% 18% 5% 11% 8% 5% 4 4 4 4 21%	1755
	11th to 20th	Day	13% 13% 7% 14% 8% 5% 4 4 4 5% 22%	1790
	21st to end	Day	16% 15% 4 10% 6% 5% 4 3 4 5% 27%	1570
March	1st to 10th	Day	21% 15% 7% 10% 5% 4 4 3 4 6% 22%	3272
	11th to 20th	Day	12% 14% 5% 9% 5% 5% 5% 3 4 5% 32%	3480
	21st to end	Day	18% 13% 6% 11% 5% 4 4 3 4 5% 27%	3815
April	1st to 10th	Day	13% 13% 4 9% 6% 5% 4 4 5% 7% 30%	2133
	11th to 20th	Day	16% 11% 3 7% 5% 4 4 4 5% 7% 34%	2421
	21st to end	Day	20% 9% 3 8% 4 4 5% 3 5% 7% 33%	2470
May	1st to 10th	Day	18% 10% 3 9% 5% 5% 4 4 5% 6% 30%	4082
	11th to 20th	Day	20% 8% 2 7% 5% 4 5% 3 6% 7% 34%	4081
	21st to end	Day	11% 5% 3 7% 5% 5% 5% 5% 7% 8% 38%	4476
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 4 4 7% 8% 37%	2470
	11th to 20th	Day	12% 6% 2 7% 5% 6% 5% 5% 6% 9% 37%	2575
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 10% 38%	2660
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 8% 42%	4130
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 4 7% 9% 39%	4466
August	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	4104
	11th to 20th	Day	9% 6% 3 6% 4 5% 6% 5% 8% 11% 37%	4044
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 6% 10% 41%	4514
September	1st to 10th	Day	13% 5% 2 5% 5% 4 4 4 5% 7% 46%	2095
	11th to 20th	Day	10% 6% 2 5% 5% 4 5% 4 5% 8% 45%	2090
	21st to end	Day	19% 8% 3 6% 4 4 4 3 5% 7% 39%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 4 6% 7% 42%	2018
	11th to 20th	Day	15% 8% 3 6% 5% 4 5% 5% 5% 7% 37%	1900
	21st to end	Day	19% 12% 3 7% 5% 5% 4 4 5% 7% 29%	2024
November	1st to 10th	Day	10% 10% 4 7% 5% 4 4 4 5% 7% 41%	1762
	11th to 20th	Day	17% 15% 5% 7% 5% 4 4 4 4 6% 31%	1800
	21st to end	Day	17% 17% 5% 9% 5% 4 5% 2 5% 6% 25%	1789
December	1st to 10th	Day	15% 14% 7% 17% 7% 5% 4 3 4 6% 20%	2600
	11th to 20th	Day	19% 18% 9% 12% 5% 4 3 3 4 5% 18%	3204
	21st to end	Day	22% 21% 9% 12% 6% 5% 4 3 3 3 13%	4466
January	1st to 10th	Night	14% 18% 8% 20% 8% 4 4 3 3 4 14%	5889
	11th to 20th	Night	10% 18% 9% 18% 9% 8% 6% 5% 4 3 11%	5900
	21st to end	Night	19% 16% 7% 14% 6% 4 3 3 4 4 20%	6490
February	1st to 10th	Night	18% 14% 6% 12% 7% 5% 4 3 4 5% 21%	2536
	11th to 20th	Night	14% 15% 5% 10% 6% 6% 5% 3 5% 5% 25%	2274
	21st to end	Night	18% 15% 5% 10% 7% 4 4 3 4 4 25%	1884
March	1st to 10th	Night	18% 15% 5% 8% 4 4 3 3 3 4 33%	3580
	11th to 20th	Night	13% 9% 3 6% 5% 4 5% 3 6% 6% 39%	3480
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 4 6% 35%	3817
April	1st to 10th	Night	15% 13% 3 6% 5% 5% 4 4 5% 6% 33%	2063
	11th to 20th	Night	17% 7% 4 7% 5% 4 4 4 6% 6% 36%	1706
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 36%	1710
May	1st to 10th	Night	22% 9% 4 8% 5% 5% 4 4 5% 7% 27%	2813
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 8% 38%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 7% 10% 46%	3079
June	1st to 10th	Night	10% 7% 3 5% 5% 4 4 4 6% 7% 44%	1710
	11th to 20th	Night	7% 6% 2 7% 4 5% 4 5% 7% 9% 43%	1596
	21st to end	Night	9% 4 2 6% 5% 5% 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	9% 4 2 6% 5% 5% 4 4 6% 8% 46%	2530
	11th to 20th	Night	8% 4 1 5% 4 4 4 4 7% 8% 51%	2826
	21st to end	Night	6% 6% 2 7% 5% 5% 5% 5% 7% 9% 45%	3087
August	1st to 10th	Night	10% 7% 4 7% 5% 5% 4 4 6% 9% 38%	2825
	11th to 20th	Night	11% 8% 3 7% 4 4 4 3 5% 7% 44%	2785
	21st to end	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 49%	3124
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 4 7% 51%	2064
	11th to 20th	Night	10% 5% 2 4 4 3 3 3 5% 7% 55%	2090
	21st to end	Night	14% 8% 3 5% 4 3 4 4 5% 6% 44%	2090
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2161
	11th to 20th	Night	17% 8% 3 7% 4 4 4 2 5% 6% 39%	2280
	21st to end	Night	22% 8% 2 7% 4 4 3 3 5% 6% 35%	2570
November	1st to 10th	Night	13% 8% 2 7% 5% 3 4 3 6% 7% 42%	2536
	11th to 20th	Night	15% 13% 4 9% 5% 4 4 3 5% 6% 33%	2600
	21st to end	Night	18% 15% 5% 8% 5% 4 4 4 5% 6% 27%	2602
December	1st to 10th	Night	13% 16% 6% 12% 7% 5% 4 4 5% 5% 22%	4580
	11th to 20th	Night	18% 17% 7% 12% 5% 4 4 3 5% 5% 20%	4965
	21st to end	Night	20% 20% 7% 12% 5% 5% 4 3 5% 5% 14%	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	79% 4 7% 8% 3	4057
	11th to 20th	Day	76% 3 8% 6% 7%	4060
	21st to end	Day	79% 3 3 9% 6%	4466
February	1st to 10th	Day	67% 4 9% 13% 6% 1	1755
	11th to 20th	Day	64% 5% 7% 15% 8% 1	1800
	21st to end	Day	61% 4 12% 15% 7% 2	1570
March	1st to 10th	Day	63% 5% 9% 18% 5%	3272
	11th to 20th	Day	53% 5% 12% 21% 8% 1	3480
	21st to end	Day	59% 5% 11% 17% 8%	3815
April	1st to 10th	Day	54% 5% 11% 18% 11% 1	2133
	11th to 20th	Day	50% 6% 11% 18% 13% 2	2421
	21st to end	Day	52% 4 12% 20% 11% 2	2470
May	1st to 10th	Day	53% 6% 11% 21% 8% 1	4120
	11th to 20th	Day	49% 6% 12% 25% 8% 1	4085
	21st to end	Day	40% 8% 12% 20% 15% 5%	4489
June	1st to 10th	Day	43% 6% 11% 25% 13% 1	2470
	11th to 20th	Day	43% 7% 12% 22% 15% 1	2575
	21st to end	Day	39% 8% 14% 20% 15% 3	2660
July	1st to 10th	Day	42% 6% 11% 22% 17% 2	4371
	11th to 20th	Day	34% 9% 13% 24% 19% 2	4060
	21st to end	Day	40% 7% 14% 17% 20% 3	4324
August	1st to 10th	Day	41% 8% 14% 23% 14% 1	4091
	11th to 20th	Day	38% 8% 11% 30% 12% 1	4083
	21st to end	Day	37% 7% 14% 27% 12% 3	4513
September	1st to 10th	Day	38% 6% 11% 22% 18% 4	2095
	11th to 20th	Day	37% 6% 11% 26% 19% 2	2090
	21st to end	Day	46% 4 10% 20% 19% 1	2090
October	1st to 10th	Day	29% 5% 12% 32% 21% 1	3356
	11th to 20th	Day	43% 5% 8% 22% 20% 2	3140
	21st to end	Day	43% 3 12% 23% 18% 1	3313
November	1st to 10th	Day	43% 5% 11% 24% 15% 2	1762
	11th to 20th	Day	56% 4 9% 20% 10% 2	1800
	21st to end	Day	61% 4 11% 16% 7% 1	1789
December	1st to 10th	Day	67% 4 13% 13% 3	2600
	11th to 20th	Day	69% 3 9% 14% 4	3201
	21st to end	Day	78% 4 5% 8% 4	4466
January	1st to 10th	Night	75% 4 6% 12% 3	5889
	11th to 20th	Night	79% 3 6% 7% 4	5900
	21st to end	Night	69% 3 10% 11% 6% 1	6490
February	1st to 10th	Night	66% 4 10% 13% 7%	2536
	11th to 20th	Night	60% 5% 9% 17% 8% 1	2275
	21st to end	Night	64% 4 8% 15% 8% 2	1884
March	1st to 10th	Night	56% 3 10% 21% 9%	3580
	11th to 20th	Night	44% 6% 14% 23% 11% 2	3480
	21st to end	Night	50% 5% 10% 25% 10% 1	3817
April	1st to 10th	Night	51% 5% 12% 16% 14% 2	2063
	11th to 20th	Night	47% 5% 11% 17% 17% 3	1706
	21st to end	Night	48% 5% 10% 21% 14% 2	1710
May	1st to 10th	Night	56% 7% 8% 18% 9% 2	2840
	11th to 20th	Night	45% 7% 11% 23% 13% 2	2813
	21st to end	Night	31% 8% 14% 28% 17% 3	3087
June	1st to 10th	Night	39% 5% 11% 24% 17% 3	1710
	11th to 20th	Night	34% 7% 15% 24% 16% 3	1596
	21st to end	Night	36% 7% 13% 25% 15% 3	1520
July	1st to 10th	Night	35% 7% 11% 26% 16% 6%	2490
	11th to 20th	Night	30% 6% 14% 31% 18% 1	2773
	21st to end	Night	34% 8% 15% 20% 21% 3	2988
August	1st to 10th	Night	42% 7% 12% 23% 15% 1	2814
	11th to 20th	Night	40% 5% 10% 28% 16% 1	2812
	21st to end	Night	31% 6% 14% 30% 15% 3	3124
September	1st to 10th	Night	34% 5% 12% 26% 17% 5%	2064
	11th to 20th	Night	28% 6% 11% 26% 26% 3	2090
	21st to end	Night	40% 6% 10% 22% 20% 2	2090
October	1st to 10th	Night	28% 4 7% 31% 29% 2	3603
	11th to 20th	Night	38% 4 10% 23% 21% 3	3779
	21st to end	Night	38% 3 9% 28% 18% 3	4231
November	1st to 10th	Night	42% 4 11% 23% 17% 2	2536
	11th to 20th	Night	52% 4 9% 21% 12% 3	2600
	21st to end	Night	58% 4 12% 17% 8% 1	2602
December	1st to 10th	Night	63% 5% 10% 17% 5%	4580
	11th to 20th	Night	66% 4 13% 13% 4	4967
	21st to end	Night	72% 5% 9% 10% 3	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	22% 21% 36% 15% 5% 1	300
	11th to 20th	Day	21% 25% 41% 8% 5%	300
	21st to end	Day	25% 21% 33% 17% 3 1	330
February	1st to 10th	Day	30% 18% 36% 13% 2 1	285
	11th to 20th	Day	24% 21% 33% 18% 3	270
	21st to end	Day	24% 23% 40% 11% 1	225
March	1st to 10th	Day	24% 15% 36% 18% 6%	357
	11th to 20th	Day	23% 15% 36% 20% 5% 1	460
	21st to end	Day	30% 13% 31% 21% 4 1	506
April	1st to 10th	Day	20% 16% 35% 19% 8% 2	460
	11th to 20th	Day	21% 18% 29% 17% 11% 4	452
	21st to end	Day	24% 12% 33% 21% 7% 2	460
May	1st to 10th	Day	24% 11% 31% 23% 8% 3	463
	11th to 20th	Day	23% 6% 30% 22% 14% 6%	487
	21st to end	Day	11% 7% 24% 28% 21% 8%	529
June	1st to 10th	Day	17% 6% 24% 33% 16% 5%	490
	11th to 20th	Day	13% 7% 25% 30% 20% 5%	595
	21st to end	Day	12% 7% 30% 29% 16% 5%	680
July	1st to 10th	Day	12% 6% 19% 28% 22% 14%	671
	11th to 20th	Day	10% 4 22% 29% 23% 12%	493
	21st to end	Day	10% 4 20% 29% 26% 12%	532
August	1st to 10th	Day	10% 5% 16% 27% 27% 15%	486
	11th to 20th	Day	11% 4 20% 30% 24% 11%	479
	21st to end	Day	10% 4 15% 29% 27% 15%	539
September	1st to 10th	Day	14% 3 20% 26% 24% 14%	490
	11th to 20th	Day	10% 3 12% 25% 33% 16%	490
	21st to end	Day	20% 10% 19% 24% 17% 9%	490
October	1st to 10th	Day	12% 5% 28% 26% 20% 9%	490
	11th to 20th	Day	15% 13% 33% 26% 10% 3	490
	21st to end	Day	25% 16% 31% 21% 5% 1	473
November	1st to 10th	Day	15% 12% 37% 23% 11% 3	300
	11th to 20th	Day	23% 19% 33% 15% 9%	300
	21st to end	Day	29% 15% 37% 16% 3	294
December	1st to 10th	Day	22% 16% 42% 15% 5% 2	200
	11th to 20th	Day	20% 20% 38% 18% 2	249
	21st to end	Day	28% 19% 32% 16% 4	330
January	1st to 10th	Night	24% 18% 36% 19% 4	800
	11th to 20th	Night	26% 18% 35% 17% 3 1	800
	21st to end	Night	24% 16% 35% 20% 4	880
February	1st to 10th	Night	25% 15% 35% 22% 3	781
	11th to 20th	Night	23% 14% 36% 23% 3	743
	21st to end	Night	27% 14% 37% 19% 3	628
March	1st to 10th	Night	23% 13% 34% 24% 6%	760
	11th to 20th	Night	18% 14% 36% 25% 6% 1	760
	21st to end	Night	24% 11% 32% 26% 6% 1	836
April	1st to 10th	Night	20% 14% 34% 27% 5% 1	733
	11th to 20th	Night	20% 11% 35% 24% 9% 2	563
	21st to end	Night	21% 8% 36% 27% 8% 1	570
May	1st to 10th	Night	20% 8% 31% 30% 10% 1	570
	11th to 20th	Night	22% 4 25% 32% 14% 2	561
	21st to end	Night	10% 3 22% 40% 22% 3	617
June	1st to 10th	Night	11% 5% 24% 41% 16% 2	570
	11th to 20th	Night	9% 4 21% 45% 18% 2	570
	21st to end	Night	11% 3 23% 42% 19% 2	570
July	1st to 10th	Night	9% 3 14% 41% 30% 4	555
	11th to 20th	Night	10% 2 17% 43% 24% 5%	552
	21st to end	Night	9% 2 14% 42% 30% 3	624
August	1st to 10th	Night	9% 2 13% 45% 27% 5%	567
	11th to 20th	Night	12% 2 14% 37% 31% 5%	564
	21st to end	Night	7% 3 13% 38% 33% 5%	627
September	1st to 10th	Night	13% 3 16% 34% 30% 4	734
	11th to 20th	Night	11% 2 13% 35% 33% 6%	760
	21st to end	Night	18% 6% 21% 31% 20% 4	760
October	1st to 10th	Night	14% 5% 24% 34% 19% 3	760
	11th to 20th	Night	21% 10% 29% 28% 10% 1	760
	21st to end	Night	25% 11% 30% 26% 7%	836
November	1st to 10th	Night	18% 11% 31% 28% 12% 1	780
	11th to 20th	Night	22% 16% 32% 22% 7% 1	800
	21st to end	Night	26% 14% 32% 24% 5%	800
December	1st to 10th	Night	25% 16% 34% 21% 4	800
	11th to 20th	Night	26% 16% 35% 19% 4	800
	21st to end	Night	26% 17% 33% 21% 3	880

Page generated at 2025-02-03 03:05:55. Version ac07

Copyright © 2025 Attilla Danko.