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 2026-06-01.

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% 4 9% 6% 5% 5% 4 5% 7% 30%	132613
	Night	14% 11% 4 10% 5% 5% 4 3 5% 6% 32%	130510
Transparency	Day	52% 5% 10% 20% 12% 1	136243
	Night	51% 5% 10% 20% 12% 2	134950
Seeing	Day	18% 12% 28% 23% 14% 6%	17218
	Night	19% 10% 28% 28% 12% 2	26992

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	14% 17% 8% 20% 8% 6% 3 3 4 5% 11%	5258
	11th to 20th	Day	10% 19% 7% 16% 8% 7% 7% 4 4 3 15%	5280
	21st to end	Day	17% 15% 8% 14% 9% 7% 6% 4 3 4 13%	5780
February	1st to 10th	Day	15% 17% 6% 12% 8% 5% 4 4 4 5% 21%	1890
	11th to 20th	Day	13% 13% 7% 15% 7% 5% 5% 4 4 5% 22%	1984
	21st to end	Day	15% 14% 5% 11% 5% 5% 5% 4 4 5% 27%	1730
March	1st to 10th	Day	17% 12% 7% 14% 5% 4 4 3 4 5% 25%	4807
	11th to 20th	Day	11% 12% 4 10% 5% 4 5% 3 5% 6% 35%	5090
	21st to end	Day	17% 14% 5% 11% 5% 5% 5% 3 5% 6% 25%	5586
April	1st to 10th	Day	14% 13% 4 9% 6% 5% 4 4 5% 7% 29%	2357
	11th to 20th	Day	16% 11% 3 7% 5% 4 4 4 5% 7% 33%	2679
	21st to end	Day	19% 10% 3 7% 5% 4 5% 3 5% 7% 34%	2730
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 4 4 5% 6% 29%	5992
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 3 6% 7% 34%	5991
	21st to end	Day	17% 6% 2 7% 4 4 5% 5% 6% 8% 36%	6576
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 4 4 7% 8% 38%	2600
	11th to 20th	Day	12% 6% 3 7% 5% 6% 5% 5% 6% 9% 36%	2711
	21st to end	Day	11% 5% 2 6% 5% 4 6% 5% 7% 10% 39%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 39%	6360
	11th to 20th	Day	9% 5% 2 7% 6% 5% 6% 5% 8% 9% 37%	5910
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	6424
August	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	4234
	11th to 20th	Day	9% 6% 3 7% 4 5% 6% 5% 8% 11% 37%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 6% 10% 41%	4657
September	1st to 10th	Day	13% 5% 2 5% 5% 4 4 3 5% 7% 47%	2205
	11th to 20th	Day	10% 6% 2 5% 5% 3 5% 4 5% 8% 46%	2200
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Day	13% 7% 3 6% 4 4 4 4 6% 7% 44%	2124
	11th to 20th	Day	15% 8% 3 6% 5% 4 5% 5% 5% 7% 38%	2000
	21st to end	Day	19% 12% 3 7% 4 5% 4 4 5% 7% 30%	2130
November	1st to 10th	Day	10% 10% 4 8% 5% 4 4 4 5% 7% 40%	1852
	11th to 20th	Day	16% 15% 4 7% 5% 4 4 4 4 6% 31%	1890
	21st to end	Day	17% 17% 6% 10% 5% 4 4 3 5% 6% 24%	1878
December	1st to 10th	Day	15% 14% 7% 17% 7% 4 4 3 4 5% 20%	2680
	11th to 20th	Day	18% 18% 8% 12% 5% 4 3 3 4 5% 19%	3289
	21st to end	Day	22% 21% 9% 12% 6% 5% 4 3 3 3 13%	4565
January	1st to 10th	Night	13% 19% 8% 19% 8% 5% 4 3 3 3 14%	7635
	11th to 20th	Night	8% 16% 9% 19% 9% 8% 6% 4 4 4 13%	7680
	21st to end	Night	16% 15% 6% 12% 6% 5% 5% 3 5% 6% 20%	8401
February	1st to 10th	Night	17% 14% 6% 13% 7% 5% 4 4 4 5% 22%	2730
	11th to 20th	Night	15% 14% 5% 11% 6% 6% 5% 3 5% 6% 24%	2520
	21st to end	Night	17% 15% 5% 11% 6% 4 4 3 4 4 26%	2076
March	1st to 10th	Night	17% 14% 5% 9% 4 4 3 3 4 5% 34%	5265
	11th to 20th	Night	11% 8% 3 6% 5% 4 5% 4 7% 7% 40%	5090
	21st to end	Night	16% 10% 4 9% 5% 5% 4 3 4 6% 35%	5588
April	1st to 10th	Night	16% 13% 4 6% 5% 4 4 4 5% 6% 32%	2280
	11th to 20th	Night	16% 7% 4 7% 5% 4 5% 4 6% 7% 35%	1888
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 37%	1890
May	1st to 10th	Night	22% 10% 4 9% 5% 5% 4 4 5% 6% 26%	4123
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 4 6% 9% 37%	4122
	21st to end	Night	14% 7% 2 5% 4 4 4 4 6% 8% 41%	4520
June	1st to 10th	Night	10% 7% 3 6% 5% 4 4 4 6% 7% 45%	1800
	11th to 20th	Night	8% 6% 2 7% 4 5% 5% 5% 7% 9% 43%	1680
	21st to end	Night	9% 4 2 5% 5% 5% 5% 4 7% 10% 43%	1600
July	1st to 10th	Night	8% 4 2 7% 5% 5% 4 4 7% 9% 44%	3618
	11th to 20th	Night	9% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	4030
	21st to end	Night	7% 6% 2 7% 4 4 5% 4 6% 8% 46%	4427
August	1st to 10th	Night	10% 7% 3 7% 5% 4 4 4 6% 9% 39%	2915
	11th to 20th	Night	11% 7% 3 7% 4 4 4 3 5% 7% 44%	2875
	21st to end	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 49%	3223
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 4 7% 52%	2173
	11th to 20th	Night	9% 4 1 4 4 3 3 3 5% 7% 57%	2200
	21st to end	Night	14% 8% 3 6% 4 3 4 4 5% 6% 44%	2200
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2275
	11th to 20th	Night	16% 8% 3 7% 4 4 4 2 5% 6% 40%	2400
	21st to end	Night	22% 8% 2 7% 4 5% 3 3 4 6% 36%	2706
November	1st to 10th	Night	13% 8% 2 7% 5% 3 4 3 6% 7% 42%	2666
	11th to 20th	Night	14% 13% 4 9% 5% 4 4 3 5% 6% 33%	2730
	21st to end	Night	18% 15% 6% 8% 5% 4 4 4 4 6% 27%	2732
December	1st to 10th	Night	13% 16% 6% 12% 7% 5% 4 3 5% 5% 22%	4720
	11th to 20th	Night	18% 17% 7% 12% 5% 4 4 3 5% 5% 20%	5099
	21st to end	Night	20% 20% 7% 12% 5% 5% 4 3 5% 5% 14%	6633

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	76% 4 9% 8% 2	5259
	11th to 20th	Day	74% 3 7% 7% 9%	5280
	21st to end	Day	76% 3 5% 11% 5%	5780
February	1st to 10th	Day	67% 4 9% 14% 6%	1890
	11th to 20th	Day	64% 5% 7% 15% 8% 1	1994
	21st to end	Day	60% 4 12% 15% 7% 2	1730
March	1st to 10th	Day	61% 4 8% 20% 7%	4807
	11th to 20th	Day	49% 4 13% 24% 8% 1	5090
	21st to end	Day	60% 4 10% 17% 9%	5586
April	1st to 10th	Day	55% 5% 11% 17% 11% 1	2357
	11th to 20th	Day	50% 6% 12% 18% 13% 2	2679
	21st to end	Day	51% 4 12% 21% 11% 2	2730
May	1st to 10th	Day	55% 6% 10% 17% 11% 1	6027
	11th to 20th	Day	48% 6% 13% 22% 10% 1	5995
	21st to end	Day	44% 7% 10% 22% 13% 4	6590
June	1st to 10th	Day	43% 6% 11% 25% 14% 1	2600
	11th to 20th	Day	44% 7% 12% 21% 14% 1	2711
	21st to end	Day	38% 8% 15% 21% 16% 3	2800
July	1st to 10th	Day	39% 6% 10% 29% 16% 1	6291
	11th to 20th	Day	39% 9% 14% 22% 16% 1	5840
	21st to end	Day	39% 7% 12% 19% 20% 3	6240
August	1st to 10th	Day	40% 8% 14% 23% 14% 1	4221
	11th to 20th	Day	38% 8% 11% 30% 12% 1	4213
	21st to end	Day	36% 7% 14% 28% 12% 2	4656
September	1st to 10th	Day	37% 6% 10% 24% 19% 4	2205
	11th to 20th	Day	35% 6% 11% 28% 19% 2	2200
	21st to end	Day	46% 4 10% 20% 20% 1	2200
October	1st to 10th	Day	29% 5% 12% 32% 22% 1	3462
	11th to 20th	Day	42% 5% 8% 22% 20% 2	3240
	21st to end	Day	43% 4 11% 22% 19% 1	3419
November	1st to 10th	Day	44% 5% 11% 23% 15% 2	1852
	11th to 20th	Day	55% 4 9% 20% 10% 2	1890
	21st to end	Day	62% 4 11% 15% 7% 1	1878
December	1st to 10th	Day	67% 4 13% 13% 4	2680
	11th to 20th	Day	69% 3 9% 14% 5%	3286
	21st to end	Day	78% 4 5% 8% 4	4565
January	1st to 10th	Night	76% 3 6% 11% 3	7636
	11th to 20th	Night	75% 4 6% 8% 6% 1	7680
	21st to end	Night	65% 3 10% 14% 7% 1	8401
February	1st to 10th	Night	65% 4 9% 14% 7% 1	2730
	11th to 20th	Night	62% 5% 9% 16% 8% 1	2521
	21st to end	Night	63% 4 8% 15% 8% 2	2076
March	1st to 10th	Night	55% 3 9% 22% 10% 1	5265
	11th to 20th	Night	41% 5% 14% 28% 10% 2	5090
	21st to end	Night	50% 5% 9% 21% 14% 1	5588
April	1st to 10th	Night	53% 4 11% 17% 14% 2	2280
	11th to 20th	Night	47% 5% 11% 17% 17% 2	1888
	21st to end	Night	48% 5% 10% 21% 14% 2	1890
May	1st to 10th	Night	58% 7% 8% 14% 11% 2	4150
	11th to 20th	Night	44% 7% 11% 22% 15% 1	4123
	21st to end	Night	40% 6% 13% 26% 13% 2	4528
June	1st to 10th	Night	39% 5% 11% 24% 18% 3	1800
	11th to 20th	Night	36% 7% 15% 24% 16% 3	1680
	21st to end	Night	36% 8% 13% 25% 16% 3	1600
July	1st to 10th	Night	34% 7% 13% 26% 16% 4	3580
	11th to 20th	Night	35% 7% 15% 27% 15% 1	3980
	21st to end	Night	35% 7% 13% 21% 23% 2	4297
August	1st to 10th	Night	41% 7% 12% 24% 15% 1	2904
	11th to 20th	Night	39% 5% 10% 29% 16% 1	2902
	21st to end	Night	31% 6% 13% 31% 15% 3	3223
September	1st to 10th	Night	33% 5% 12% 28% 17% 5%	2173
	11th to 20th	Night	27% 6% 11% 28% 26% 3	2200
	21st to end	Night	40% 6% 10% 22% 21% 2	2200
October	1st to 10th	Night	28% 4 7% 30% 30% 2	3717
	11th to 20th	Night	38% 4 10% 24% 22% 3	3899
	21st to end	Night	38% 4 9% 28% 18% 3	4367
November	1st to 10th	Night	42% 4 11% 24% 17% 2	2666
	11th to 20th	Night	52% 4 9% 21% 12% 2	2730
	21st to end	Night	59% 4 12% 17% 7% 1	2732
December	1st to 10th	Night	63% 4 10% 16% 6% 1	4720
	11th to 20th	Night	66% 4 13% 13% 4	5101
	21st to end	Night	72% 5% 9% 11% 3	6633

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	23% 20% 37% 15% 4 1	330
	11th to 20th	Day	24% 25% 38% 8% 4	330
	21st to end	Day	25% 22% 32% 17% 3 1	363
February	1st to 10th	Day	26% 19% 37% 13% 3 2	330
	11th to 20th	Day	26% 21% 33% 17% 2	330
	21st to end	Day	21% 25% 42% 11% 1	273
March	1st to 10th	Day	23% 17% 38% 17% 5%	427
	11th to 20th	Day	25% 16% 35% 19% 5% 1	540
	21st to end	Day	29% 16% 31% 20% 4 1	594
April	1st to 10th	Day	22% 17% 35% 17% 7% 2	540
	11th to 20th	Day	21% 19% 30% 16% 11% 4	532
	21st to end	Day	24% 12% 32% 22% 8% 2	540
May	1st to 10th	Day	25% 12% 31% 21% 7% 3	543
	11th to 20th	Day	22% 8% 31% 23% 12% 5%	567
	21st to end	Day	13% 8% 25% 27% 19% 7%	617
June	1st to 10th	Day	17% 6% 24% 33% 15% 5%	530
	11th to 20th	Day	13% 7% 26% 30% 19% 5%	641
	21st to end	Day	11% 7% 30% 30% 17% 5%	730
July	1st to 10th	Day	12% 5% 19% 29% 22% 13%	721
	11th to 20th	Day	10% 5% 22% 29% 23% 12%	533
	21st to end	Day	11% 4 19% 29% 25% 13%	576
August	1st to 10th	Day	9% 5% 16% 27% 29% 14%	526
	11th to 20th	Day	11% 4 19% 29% 25% 11%	519
	21st to end	Day	10% 5% 16% 29% 27% 14%	583
September	1st to 10th	Day	13% 4 20% 25% 24% 15%	530
	11th to 20th	Day	9% 3 12% 25% 34% 17%	530
	21st to end	Day	20% 9% 19% 25% 18% 10%	530
October	1st to 10th	Day	12% 5% 27% 26% 21% 9%	530
	11th to 20th	Day	15% 12% 35% 26% 9% 3	530
	21st to end	Day	25% 18% 31% 20% 5% 1	513
November	1st to 10th	Day	16% 12% 36% 22% 11% 3	330
	11th to 20th	Day	23% 20% 35% 14% 8%	330
	21st to end	Day	30% 16% 35% 16% 2	323
December	1st to 10th	Day	23% 17% 41% 14% 4 1	220
	11th to 20th	Day	22% 21% 37% 17% 3	274
	21st to end	Day	30% 19% 32% 15% 3	363
January	1st to 10th	Night	24% 18% 36% 19% 3	840
	11th to 20th	Night	25% 20% 35% 17% 3 1	840
	21st to end	Night	23% 18% 35% 20% 4	924
February	1st to 10th	Night	24% 16% 35% 22% 3	840
	11th to 20th	Night	23% 15% 36% 23% 2	823
	21st to end	Night	26% 15% 37% 18% 3	692
March	1st to 10th	Night	22% 14% 35% 23% 6%	840
	11th to 20th	Night	20% 14% 37% 23% 5% 1	840
	21st to end	Night	23% 13% 33% 24% 6% 1	924
April	1st to 10th	Night	22% 14% 33% 26% 4 1	810
	11th to 20th	Night	19% 13% 35% 23% 8% 1	623
	21st to end	Night	21% 8% 35% 27% 8% 1	630
May	1st to 10th	Night	21% 8% 33% 29% 9% 1	630
	11th to 20th	Night	22% 5% 27% 32% 13% 1	621
	21st to end	Night	12% 3 23% 40% 20% 2	683
June	1st to 10th	Night	11% 5% 25% 41% 16% 2	600
	11th to 20th	Night	10% 4 22% 45% 18% 2	600
	21st to end	Night	11% 4 23% 42% 20% 2	600
July	1st to 10th	Night	9% 3 14% 41% 29% 4	585
	11th to 20th	Night	10% 2 17% 43% 24% 5%	582
	21st to end	Night	9% 2 13% 42% 30% 3	657
August	1st to 10th	Night	8% 2 13% 45% 27% 4	597
	11th to 20th	Night	12% 2 13% 36% 33% 5%	594
	21st to end	Night	7% 3 13% 39% 32% 5%	660
September	1st to 10th	Night	12% 3 17% 34% 30% 5%	773
	11th to 20th	Night	11% 2 13% 34% 34% 6%	800
	21st to end	Night	18% 6% 20% 31% 21% 4	800
October	1st to 10th	Night	14% 5% 24% 34% 20% 3	800
	11th to 20th	Night	20% 10% 29% 29% 10% 1	800
	21st to end	Night	25% 11% 31% 26% 7%	880
November	1st to 10th	Night	17% 11% 32% 28% 12% 1	820
	11th to 20th	Night	22% 17% 33% 21% 6% 1	840
	21st to end	Night	27% 14% 31% 23% 5%	840
December	1st to 10th	Night	25% 16% 35% 21% 4	840
	11th to 20th	Night	25% 16% 36% 19% 4	840
	21st to end	Night	26% 17% 34% 20% 3	924

Page generated at 2026-05-29 01:53:28. Version ac07

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