Tobey Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tobey Memorial Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 11% 4 9% 5% 5% 4 4 5% 7% 30%	132197
	Night	16% 12% 4 10% 6% 5% 4 3 5% 6% 29%	130590
Transparency	Day	51% 8% 13% 18% 10% 1	135790
	Night	54% 8% 12% 17% 9% 1	135083
Seeing	Day	22% 16% 29% 20% 9% 3	16391
	Night	22% 15% 29% 24% 9% 1	25890

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	16% 20% 7% 16% 9% 5% 4 2 4 4 14%	4661
	11th to 20th	Day	11% 23% 8% 21% 9% 5% 4 3 3 3 9%	5084
	21st to end	Day	15% 15% 8% 16% 10% 9% 6% 3 4 4 9%	5780
February	1st to 10th	Day	17% 19% 4 13% 8% 8% 4 3 4 5% 14%	1890
	11th to 20th	Day	14% 18% 7% 16% 9% 5% 5% 3 4 4 16%	1843
	21st to end	Day	19% 16% 5% 12% 8% 4 5% 4 5% 5% 18%	1630
March	1st to 10th	Day	17% 13% 6% 12% 6% 5% 4 3 4 5% 28%	5042
	11th to 20th	Day	11% 17% 4 8% 5% 6% 4 3 5% 6% 30%	5090
	21st to end	Day	20% 13% 3 10% 4 4 4 3 5% 6% 29%	5586
April	1st to 10th	Day	18% 15% 4 8% 5% 4 4 3 5% 6% 29%	2503
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 4 6% 34%	2515
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 5% 5% 34%	2730
May	1st to 10th	Day	22% 10% 3 8% 4 4 4 3 5% 6% 30%	6018
	11th to 20th	Day	16% 10% 4 9% 5% 5% 5% 4 5% 7% 31%	6395
	21st to end	Day	19% 8% 3 6% 4 4 4 4 6% 7% 37%	6949
June	1st to 10th	Day	16% 6% 3 6% 4 5% 5% 4 6% 7% 39%	2800
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 10% 38%	2800
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 7% 8% 41%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 44%	6360
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 40%	6360
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 9% 46%	6497
August	1st to 10th	Day	17% 6% 2 5% 4 4 5% 4 6% 8% 38%	4234
	11th to 20th	Day	13% 5% 2 8% 5% 5% 5% 4 7% 11% 34%	4174
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	4424
September	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 7% 7% 42%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 45%	2320
	21st to end	Day	14% 8% 3 7% 5% 6% 5% 4 6% 8% 33%	2135
October	1st to 10th	Day	20% 6% 2 7% 5% 4 5% 4 4 7% 36%	2000
	11th to 20th	Day	19% 10% 4 10% 5% 4 5% 3 5% 6% 29%	2000
	21st to end	Day	25% 13% 4 9% 6% 4 4 3 4 5% 23%	2200
November	1st to 10th	Day	15% 11% 3 9% 6% 5% 5% 4 5% 5% 31%	1823
	11th to 20th	Day	15% 14% 4 10% 5% 4 5% 3 5% 8% 28%	1680
	21st to end	Day	20% 18% 4 9% 6% 5% 4 3 5% 6% 20%	1680
December	1st to 10th	Day	20% 20% 5% 11% 6% 5% 5% 3 5% 5% 13%	2680
	11th to 20th	Day	21% 18% 5% 10% 5% 4 3 3 5% 5% 20%	3066
	21st to end	Day	23% 20% 6% 12% 7% 6% 3 3 3 4 12%	4048
January	1st to 10th	Night	16% 18% 7% 15% 8% 6% 4 3 4 4 16%	8234
	11th to 20th	Night	12% 20% 8% 18% 7% 6% 5% 3 4 4 12%	7784
	21st to end	Night	17% 16% 6% 16% 9% 7% 6% 3 4 4 12%	8401
February	1st to 10th	Night	17% 18% 5% 12% 7% 5% 4 3 4 5% 19%	2730
	11th to 20th	Night	15% 16% 5% 12% 7% 5% 5% 4 5% 5% 21%	2663
	21st to end	Night	17% 16% 4 14% 6% 5% 4 3 5% 6% 19%	2172
March	1st to 10th	Night	18% 12% 5% 9% 8% 4 4 3 3 5% 28%	5018
	11th to 20th	Night	15% 13% 3 7% 4 4 5% 3 5% 7% 34%	5089
	21st to end	Night	19% 10% 3 9% 5% 4 4 3 4 6% 33%	5588
April	1st to 10th	Night	19% 12% 3 9% 6% 4 4 3 4 5% 30%	2116
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 4 5% 6% 34%	2071
	21st to end	Night	17% 9% 4 7% 3 4 6% 4 5% 6% 35%	1890
May	1st to 10th	Night	25% 11% 4 8% 4 4 3 3 4 5% 29%	4143
	11th to 20th	Night	12% 8% 3 7% 6% 6% 5% 4 6% 8% 35%	3628
	21st to end	Night	18% 7% 3 7% 4 4 4 4 6% 7% 37%	3926
June	1st to 10th	Night	12% 6% 3 6% 6% 6% 6% 5% 7% 8% 35%	1600
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 5% 6% 7% 42%	1600
	21st to end	Night	13% 5% 2 7% 4 6% 4 4 5% 9% 41%	1600
July	1st to 10th	Night	10% 6% 3 7% 4 5% 4 3 7% 9% 43%	3600
	11th to 20th	Night	10% 5% 3 6% 4 4 5% 4 5% 9% 45%	3600
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 7% 50%	4337
August	1st to 10th	Night	10% 6% 3 6% 5% 4 5% 4 6% 9% 42%	2915
	11th to 20th	Night	15% 6% 3 7% 4 4 4 4 5% 8% 40%	2875
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	3439
September	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 6% 6% 46%	2000
	11th to 20th	Night	12% 4 1 5% 3 4 4 3 5% 6% 52%	2076
	21st to end	Night	16% 8% 3 6% 5% 4 5% 4 5% 7% 37%	2265
October	1st to 10th	Night	15% 8% 3 6% 4 4 4 4 5% 7% 40%	2400
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 31%	2400
	21st to end	Night	24% 11% 3 8% 4 4 4 4 5% 6% 28%	2640
November	1st to 10th	Night	16% 11% 3 7% 4 4 4 3 4 7% 37%	2692
	11th to 20th	Night	15% 13% 4 8% 6% 5% 5% 4 5% 7% 28%	2940
	21st to end	Night	18% 15% 5% 9% 5% 5% 5% 4 5% 6% 22%	2940
December	1st to 10th	Night	18% 22% 7% 11% 6% 5% 4 4 4 6% 14%	4720
	11th to 20th	Night	18% 22% 5% 10% 5% 4 4 3 4 4 21%	5348
	21st to end	Night	25% 17% 6% 11% 6% 6% 4 3 4 5% 14%	7150

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	73% 7% 10% 7% 3	4662
	11th to 20th	Day	79% 7% 5% 5% 3 1	5118
	21st to end	Day	78% 6% 10% 4 2	5780
February	1st to 10th	Day	72% 7% 11% 9% 1	1890
	11th to 20th	Day	71% 8% 10% 10% 2	1852
	21st to end	Day	67% 8% 11% 9% 4	1630
March	1st to 10th	Day	57% 7% 14% 14% 8%	5045
	11th to 20th	Day	50% 9% 18% 15% 7% 2	5090
	21st to end	Day	53% 9% 9% 20% 8% 1	5586
April	1st to 10th	Day	56% 6% 11% 17% 9% 1	2503
	11th to 20th	Day	50% 6% 11% 18% 13% 1	2515
	21st to end	Day	50% 7% 12% 17% 12% 2	2730
May	1st to 10th	Day	52% 7% 12% 15% 10% 4	6027
	11th to 20th	Day	49% 10% 10% 18% 12% 1	6399
	21st to end	Day	44% 8% 12% 21% 12% 2	6949
June	1st to 10th	Day	41% 9% 14% 20% 15% 2	2800
	11th to 20th	Day	40% 8% 15% 22% 13% 1	2800
	21st to end	Day	37% 9% 15% 23% 14% 1	2800
July	1st to 10th	Day	34% 7% 14% 27% 17% 1	6290
	11th to 20th	Day	36% 7% 15% 27% 14% 1	6285
	21st to end	Day	32% 8% 15% 27% 17% 1	6311
August	1st to 10th	Day	42% 9% 15% 21% 12% 1	4221
	11th to 20th	Day	42% 8% 16% 24% 9% 1	4213
	21st to end	Day	35% 7% 13% 33% 11% 1	4424
September	1st to 10th	Day	38% 8% 15% 26% 12% 1	2400
	11th to 20th	Day	34% 8% 15% 29% 14% 1	2320
	21st to end	Day	46% 9% 15% 20% 9% 1	2135
October	1st to 10th	Day	36% 7% 16% 26% 14% 1	3260
	11th to 20th	Day	46% 6% 16% 19% 12%	3240
	21st to end	Day	51% 6% 15% 20% 7%	3538
November	1st to 10th	Day	52% 8% 13% 17% 8% 1	1823
	11th to 20th	Day	54% 8% 16% 14% 6% 1	1680
	21st to end	Day	65% 9% 13% 11% 3	1680
December	1st to 10th	Day	70% 8% 15% 6% 1	2680
	11th to 20th	Day	66% 9% 10% 9% 6% 1	3066
	21st to end	Day	76% 7% 9% 6% 2	4048
January	1st to 10th	Night	71% 8% 10% 9% 2	8234
	11th to 20th	Night	71% 9% 8% 7% 4	7825
	21st to end	Night	74% 6% 8% 7% 5%	8401
February	1st to 10th	Night	67% 9% 9% 12% 3	2730
	11th to 20th	Night	64% 8% 13% 11% 3	2665
	21st to end	Night	64% 8% 12% 11% 5%	2172
March	1st to 10th	Night	58% 6% 12% 17% 8% 1	5028
	11th to 20th	Night	47% 9% 16% 19% 8% 2	5090
	21st to end	Night	50% 6% 12% 20% 11% 1	5588
April	1st to 10th	Night	57% 6% 9% 17% 10% 1	2116
	11th to 20th	Night	49% 6% 11% 19% 12% 2	2071
	21st to end	Night	48% 8% 11% 18% 13% 2	1890
May	1st to 10th	Night	55% 7% 9% 13% 11% 5%	4150
	11th to 20th	Night	43% 10% 16% 19% 12% 1	3629
	21st to end	Night	43% 9% 12% 20% 15% 1	3927
June	1st to 10th	Night	43% 9% 11% 20% 16% 2	1600
	11th to 20th	Night	38% 9% 13% 21% 18% 2	1600
	21st to end	Night	40% 7% 15% 19% 17% 2	1600
July	1st to 10th	Night	37% 6% 13% 23% 19% 2	3560
	11th to 20th	Night	35% 7% 14% 28% 15% 1	3552
	21st to end	Night	32% 6% 14% 24% 22% 1	4208
August	1st to 10th	Night	36% 8% 16% 26% 13% 2	2904
	11th to 20th	Night	42% 7% 15% 26% 11%	2902
	21st to end	Night	32% 8% 15% 31% 13% 1	3439
September	1st to 10th	Night	36% 7% 15% 26% 14% 1	2000
	11th to 20th	Night	31% 6% 13% 31% 17% 1	2076
	21st to end	Night	45% 8% 13% 21% 13% 1	2265
October	1st to 10th	Night	39% 7% 10% 25% 19% 1	3920
	11th to 20th	Night	48% 6% 14% 20% 12%	3899
	21st to end	Night	45% 6% 16% 24% 8% 1	4252
November	1st to 10th	Night	47% 8% 12% 21% 10% 2	2692
	11th to 20th	Night	54% 9% 14% 15% 7% 1	2940
	21st to end	Night	60% 10% 15% 13% 2	2940
December	1st to 10th	Night	70% 9% 13% 7% 1	4720
	11th to 20th	Night	66% 7% 8% 11% 7% 1	5348
	21st to end	Night	72% 7% 10% 8% 2	7150

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	26% 32% 25% 11% 5% 1	220
	11th to 20th	Day	31% 29% 27% 10% 2 1	300
	21st to end	Day	27% 26% 29% 14% 3	363
February	1st to 10th	Day	31% 29% 28% 11% 1	330
	11th to 20th	Day	28% 30% 34% 8% 1	330
	21st to end	Day	38% 20% 32% 10% 1	332
March	1st to 10th	Day	27% 21% 33% 14% 4	540
	11th to 20th	Day	31% 18% 33% 15% 3 1	540
	21st to end	Day	29% 19% 30% 17% 4 1	594
April	1st to 10th	Day	28% 22% 30% 15% 5% 1	540
	11th to 20th	Day	33% 21% 22% 16% 6% 2	532
	21st to end	Day	27% 16% 34% 15% 7% 1	540
May	1st to 10th	Day	26% 15% 31% 18% 8% 3	543
	11th to 20th	Day	20% 13% 30% 24% 10% 3	551
	21st to end	Day	17% 12% 29% 25% 12% 4	573
June	1st to 10th	Day	18% 17% 32% 25% 7% 2	530
	11th to 20th	Day	15% 15% 28% 28% 12% 3	530
	21st to end	Day	17% 10% 33% 29% 9% 2	530
July	1st to 10th	Day	10% 8% 26% 34% 20% 4	525
	11th to 20th	Day	15% 9% 27% 28% 16% 5%	524
	21st to end	Day	11% 7% 33% 30% 14% 4	576
August	1st to 10th	Day	13% 7% 23% 32% 18% 8%	526
	11th to 20th	Day	12% 6% 25% 30% 19% 8%	519
	21st to end	Day	12% 8% 29% 28% 15% 8%	583
September	1st to 10th	Day	14% 11% 24% 27% 16% 8%	530
	11th to 20th	Day	10% 8% 20% 25% 24% 13%	530
	21st to end	Day	17% 13% 29% 23% 12% 6%	530
October	1st to 10th	Day	20% 14% 26% 20% 15% 5%	530
	11th to 20th	Day	24% 20% 31% 18% 5% 2	530
	21st to end	Day	34% 19% 26% 16% 5%	583
November	1st to 10th	Day	25% 21% 32% 14% 6% 2	365
	11th to 20th	Day	30% 25% 26% 13% 4 2	220
	21st to end	Day	32% 27% 29% 11% 1	220
December	1st to 10th	Day	33% 23% 30% 10% 4	220
	11th to 20th	Day	32% 26% 29% 11% 2	220
	21st to end	Day	31% 34% 22% 9% 2 1	242
January	1st to 10th	Night	29% 24% 31% 12% 4	840
	11th to 20th	Night	31% 24% 29% 14% 2	840
	21st to end	Night	26% 20% 33% 17% 4	924
February	1st to 10th	Night	26% 25% 30% 16% 3	840
	11th to 20th	Night	24% 24% 32% 17% 3	823
	21st to end	Night	28% 21% 32% 17% 2	692
March	1st to 10th	Night	23% 18% 33% 21% 4	840
	11th to 20th	Night	26% 19% 31% 19% 4 1	840
	21st to end	Night	24% 14% 33% 21% 7% 1	924
April	1st to 10th	Night	25% 18% 33% 21% 3	840
	11th to 20th	Night	24% 15% 31% 22% 7% 1	827
	21st to end	Night	21% 10% 34% 27% 9%	630
May	1st to 10th	Night	21% 10% 27% 29% 12% 2	630
	11th to 20th	Night	15% 7% 31% 38% 8%	411
	21st to end	Night	13% 9% 27% 36% 14% 1	433
June	1st to 10th	Night	13% 7% 34% 35% 11%	400
	11th to 20th	Night	14% 8% 32% 34% 13% 1	400
	21st to end	Night	14% 6% 31% 40% 9%	400
July	1st to 10th	Night	8% 6% 25% 42% 19% 1	390
	11th to 20th	Night	10% 6% 28% 37% 18% 1	388
	21st to end	Night	11% 3 26% 39% 20% 1	618
August	1st to 10th	Night	9% 6% 17% 43% 23% 2	597
	11th to 20th	Night	10% 5% 21% 34% 28% 2	594
	21st to end	Night	12% 5% 23% 35% 22% 3	785
September	1st to 10th	Night	15% 7% 23% 33% 20% 3	800
	11th to 20th	Night	13% 5% 19% 30% 28% 5%	800
	21st to end	Night	18% 10% 23% 29% 16% 4	800
October	1st to 10th	Night	19% 9% 29% 25% 17% 2	800
	11th to 20th	Night	26% 17% 27% 22% 7% 1	800
	21st to end	Night	31% 15% 27% 23% 4	880
November	1st to 10th	Night	24% 16% 29% 21% 9%	820
	11th to 20th	Night	23% 20% 32% 16% 7% 1	840
	21st to end	Night	27% 22% 30% 16% 5%	840
December	1st to 10th	Night	33% 23% 25% 16% 4	840
	11th to 20th	Night	27% 25% 31% 14% 3	840
	21st to end	Night	34% 19% 31% 14% 2	924

Page generated at 2026-05-14 01:58:59. Version ac07

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