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 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	16% 11% 4 8% 5% 5% 4 4 5% 7% 31%	105179
	Night	17% 12% 4 9% 5% 5% 4 4 5% 6% 30%	104175
Transparency	Day	49% 9% 14% 18% 9% 1	108783
	Night	52% 8% 13% 17% 9% 1	108659
Seeing	Day	21% 16% 29% 21% 10% 3	14689
	Night	22% 15% 29% 24% 10% 1	24203

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% 21% 8% 12% 7% 5% 4 2 4 4 17%	2597
	11th to 20th	Day	12% 22% 7% 22% 10% 4 4 3 3 3 9%	2836
	21st to end	Day	23% 19% 9% 12% 7% 5% 5% 3 4 4 10%	3133
February	1st to 10th	Day	17% 20% 4 13% 7% 7% 4 3 5% 5% 15%	1710
	11th to 20th	Day	13% 18% 7% 16% 9% 5% 5% 3 4 4 17%	1663
	21st to end	Day	20% 15% 5% 12% 9% 4 4 4 5% 5% 18%	1478
March	1st to 10th	Day	18% 13% 5% 10% 6% 5% 4 3 4 5% 28%	3432
	11th to 20th	Day	10% 21% 4 9% 5% 6% 4 3 5% 6% 28%	3480
	21st to end	Day	17% 11% 3 9% 4 4 4 3 6% 7% 31%	3815
April	1st to 10th	Day	18% 15% 4 8% 4 4 4 3 5% 5% 30%	2265
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 4 6% 35%	2273
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 5% 5% 34%	2470
May	1st to 10th	Day	19% 9% 3 7% 5% 4 5% 3 5% 7% 33%	4108
	11th to 20th	Day	15% 10% 3 8% 5% 5% 5% 4 5% 8% 30%	4391
	21st to end	Day	11% 7% 2 5% 4 4 4 4 6% 8% 46%	4837
June	1st to 10th	Day	16% 6% 3 6% 4 4 5% 4 6% 7% 38%	2660
	11th to 20th	Day	12% 5% 3 7% 5% 5% 5% 4 7% 10% 39%	2660
	21st to end	Day	15% 5% 2 6% 4 4 4 3 7% 8% 41%	2660
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 10% 45%	4440
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 7% 10% 40%	4440
	21st to end	Day	8% 5% 3 6% 5% 5% 5% 5% 7% 9% 42%	4511
August	1st to 10th	Day	18% 7% 2 5% 4 4 5% 4 6% 8% 37%	4104
	11th to 20th	Day	13% 5% 3 8% 5% 5% 5% 4 7% 11% 34%	4044
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	4289
September	1st to 10th	Day	13% 8% 2 6% 4 4 4 4 7% 7% 41%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	2204
	21st to end	Day	15% 8% 3 7% 5% 6% 5% 4 6% 8% 33%	2028
October	1st to 10th	Day	20% 7% 2 7% 5% 4 5% 4 5% 7% 34%	1900
	11th to 20th	Day	20% 10% 4 10% 5% 4 5% 3 5% 6% 29%	1900
	21st to end	Day	25% 14% 4 9% 6% 4 4 3 5% 5% 22%	2090
November	1st to 10th	Day	14% 11% 3 9% 6% 5% 5% 4 5% 5% 32%	1735
	11th to 20th	Day	15% 14% 4 10% 5% 4 5% 3 5% 8% 28%	1600
	21st to end	Day	20% 18% 4 9% 6% 5% 4 3 5% 6% 20%	1600
December	1st to 10th	Day	21% 19% 5% 11% 6% 5% 5% 3 6% 5% 13%	2600
	11th to 20th	Day	22% 18% 5% 10% 5% 4 3 3 5% 5% 20%	2986
	21st to end	Day	23% 20% 6% 12% 7% 6% 3 3 3 4 12%	3960
January	1st to 10th	Night	18% 20% 6% 11% 6% 5% 4 3 4 4 18%	4569
	11th to 20th	Night	13% 20% 8% 16% 8% 6% 5% 3 4 4 12%	4341
	21st to end	Night	25% 20% 6% 13% 7% 5% 4 2 3 3 11%	4546
February	1st to 10th	Night	18% 18% 6% 12% 7% 4 4 3 4 4 19%	2470
	11th to 20th	Night	15% 16% 5% 12% 8% 6% 5% 4 5% 5% 21%	2403
	21st to end	Night	18% 16% 4 14% 6% 5% 4 3 4 6% 19%	1972
March	1st to 10th	Night	21% 13% 4 9% 5% 3 4 3 3 4 31%	3408
	11th to 20th	Night	18% 16% 3 8% 4 4 4 3 5% 6% 28%	3479
	21st to end	Night	16% 9% 2 8% 4 4 4 3 5% 6% 40%	3817
April	1st to 10th	Night	19% 13% 3 8% 6% 4 3 3 4 5% 31%	1914
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 4 5% 6% 35%	1871
	21st to end	Night	17% 9% 4 7% 3 5% 6% 4 5% 6% 34%	1710
May	1st to 10th	Night	22% 10% 3 7% 4 4 3 3 5% 6% 32%	2833
	11th to 20th	Night	13% 7% 3 7% 6% 6% 5% 4 6% 9% 34%	2500
	21st to end	Night	12% 6% 3 5% 4 4 4 4 6% 9% 44%	2738
June	1st to 10th	Night	12% 6% 3 7% 6% 6% 6% 5% 7% 8% 35%	1520
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 5% 6% 7% 42%	1520
	21st to end	Night	13% 6% 2 7% 4 6% 4 4 5% 9% 40%	1520
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 4 3 6% 9% 43%	2520
	11th to 20th	Night	11% 6% 2 6% 3 4 4 4 5% 9% 46%	2520
	21st to end	Night	10% 4 2 5% 4 4 5% 4 6% 8% 47%	3024
August	1st to 10th	Night	11% 6% 3 6% 5% 4 5% 4 6% 9% 42%	2825
	11th to 20th	Night	15% 6% 3 7% 4 4 5% 4 5% 8% 40%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	3334
September	1st to 10th	Night	13% 7% 2 5% 4 4 3 4 5% 6% 46%	1900
	11th to 20th	Night	13% 4 2 5% 3 4 4 3 5% 7% 51%	1972
	21st to end	Night	16% 9% 3 6% 5% 4 5% 4 5% 7% 36%	2152
October	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 7% 38%	2280
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 4 5% 6% 30%	2280
	21st to end	Night	25% 11% 3 8% 4 4 4 4 5% 6% 26%	2508
November	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 4 7% 38%	2560
	11th to 20th	Night	15% 13% 4 9% 6% 5% 4 4 5% 7% 28%	2800
	21st to end	Night	19% 15% 5% 9% 5% 5% 5% 4 5% 6% 23%	2800
December	1st to 10th	Night	18% 22% 8% 11% 6% 4 4 4 5% 6% 14%	4580
	11th to 20th	Night	19% 22% 5% 10% 5% 4 4 3 4 4 21%	5208
	21st to end	Night	25% 17% 6% 10% 6% 6% 4 3 4 4 14%	6996

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	69% 11% 8% 9% 3	2598
	11th to 20th	Day	78% 10% 6% 5% 1	2836
	21st to end	Day	77% 9% 10% 3 1	3133
February	1st to 10th	Day	71% 7% 11% 9% 1	1710
	11th to 20th	Day	70% 8% 9% 10% 2	1672
	21st to end	Day	67% 8% 11% 9% 4	1478
March	1st to 10th	Day	55% 8% 13% 18% 6%	3435
	11th to 20th	Day	52% 11% 17% 14% 6% 1	3480
	21st to end	Day	47% 11% 12% 21% 7% 1	3815
April	1st to 10th	Day	55% 6% 12% 17% 9% 1	2265
	11th to 20th	Day	49% 7% 12% 18% 13% 2	2273
	21st to end	Day	51% 7% 11% 17% 12% 2	2470
May	1st to 10th	Day	48% 8% 14% 17% 11% 2	4120
	11th to 20th	Day	48% 12% 14% 18% 7% 1	4395
	21st to end	Day	33% 10% 14% 25% 16% 3	4837
June	1st to 10th	Day	41% 9% 14% 19% 15% 2	2660
	11th to 20th	Day	39% 8% 16% 23% 14% 1	2660
	21st to end	Day	38% 9% 15% 23% 14% 1	2660
July	1st to 10th	Day	33% 8% 14% 27% 17% 2	4370
	11th to 20th	Day	35% 8% 16% 29% 12% 1	4365
	21st to end	Day	35% 10% 17% 22% 15% 1	4367
August	1st to 10th	Day	43% 9% 15% 21% 12% 1	4091
	11th to 20th	Day	42% 8% 16% 24% 8% 1	4083
	21st to end	Day	35% 6% 14% 33% 12% 1	4289
September	1st to 10th	Day	38% 8% 15% 25% 12% 1	2280
	11th to 20th	Day	35% 8% 15% 27% 14% 1	2204
	21st to end	Day	46% 9% 15% 21% 9% 1	2028
October	1st to 10th	Day	37% 7% 17% 25% 14% 1	3160
	11th to 20th	Day	46% 6% 17% 19% 12%	3140
	21st to end	Day	51% 6% 16% 21% 6%	3428
November	1st to 10th	Day	52% 8% 13% 17% 8% 1	1735
	11th to 20th	Day	54% 9% 16% 14% 6% 1	1600
	21st to end	Day	65% 9% 12% 11% 3	1600
December	1st to 10th	Day	70% 8% 15% 5% 1	2600
	11th to 20th	Day	66% 9% 10% 8% 7% 1	2986
	21st to end	Day	76% 7% 9% 6% 2	3960
January	1st to 10th	Night	67% 13% 11% 7% 2	4569
	11th to 20th	Night	70% 14% 9% 5% 1	4341
	21st to end	Night	76% 9% 8% 5% 2	4546
February	1st to 10th	Night	68% 9% 9% 11% 2	2470
	11th to 20th	Night	64% 9% 13% 11% 4	2405
	21st to end	Night	63% 8% 12% 12% 5%	1972
March	1st to 10th	Night	55% 7% 12% 20% 6%	3418
	11th to 20th	Night	53% 11% 15% 15% 6% 1	3480
	21st to end	Night	41% 7% 14% 24% 13% 1	3817
April	1st to 10th	Night	56% 6% 10% 18% 10% 1	1914
	11th to 20th	Night	49% 6% 11% 19% 13% 2	1871
	21st to end	Night	48% 8% 11% 17% 13% 2	1710
May	1st to 10th	Night	50% 9% 11% 17% 12% 2	2840
	11th to 20th	Night	43% 11% 18% 18% 8% 1	2501
	21st to end	Night	32% 10% 13% 23% 20% 2	2739
June	1st to 10th	Night	43% 9% 11% 20% 16% 1	1520
	11th to 20th	Night	38% 9% 12% 21% 18% 2	1520
	21st to end	Night	41% 8% 15% 19% 16% 2	1520
July	1st to 10th	Night	37% 6% 13% 23% 19% 2	2480
	11th to 20th	Night	34% 7% 16% 29% 13% 1	2472
	21st to end	Night	33% 8% 18% 23% 17% 2	2927
August	1st to 10th	Night	37% 7% 15% 26% 13% 2	2814
	11th to 20th	Night	42% 7% 15% 26% 10%	2812
	21st to end	Night	32% 8% 15% 32% 13% 1	3334
September	1st to 10th	Night	37% 7% 16% 24% 14% 1	1900
	11th to 20th	Night	32% 6% 13% 29% 18% 1	1972
	21st to end	Night	44% 9% 13% 21% 13%	2152
October	1st to 10th	Night	40% 7% 11% 24% 18% 1	3800
	11th to 20th	Night	48% 6% 14% 20% 11%	3779
	21st to end	Night	45% 6% 17% 24% 8% 1	4120
November	1st to 10th	Night	47% 8% 12% 21% 11% 2	2560
	11th to 20th	Night	54% 9% 14% 15% 7% 1	2800
	21st to end	Night	60% 10% 15% 13% 2	2800
December	1st to 10th	Night	70% 9% 13% 6% 1	4580
	11th to 20th	Night	66% 8% 8% 10% 7% 1	5208
	21st to end	Night	72% 8% 10% 8% 2	6996

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% 30% 25% 13% 5% 1	200
	11th to 20th	Day	28% 29% 29% 11% 3 1	273
	21st to end	Day	24% 25% 30% 16% 4	300
February	1st to 10th	Day	31% 30% 29% 9% 1	270
	11th to 20th	Day	26% 31% 34% 8% 1	270
	21st to end	Day	38% 17% 33% 10% 1	278
March	1st to 10th	Day	25% 21% 35% 15% 4	460
	11th to 20th	Day	27% 18% 35% 16% 3 1	460
	21st to end	Day	29% 17% 30% 18% 4 2	506
April	1st to 10th	Day	26% 21% 31% 16% 5% 1	460
	11th to 20th	Day	31% 20% 22% 17% 7% 2	452
	21st to end	Day	27% 16% 35% 14% 7% 2	460
May	1st to 10th	Day	25% 13% 30% 19% 9% 3	463
	11th to 20th	Day	19% 11% 30% 25% 11% 3	487
	21st to end	Day	15% 12% 29% 26% 13% 5%	529
June	1st to 10th	Day	18% 16% 31% 25% 8% 2	490
	11th to 20th	Day	15% 14% 27% 30% 12% 3	490
	21st to end	Day	17% 10% 32% 29% 9% 2	490
July	1st to 10th	Day	10% 7% 26% 33% 21% 4	485
	11th to 20th	Day	14% 9% 27% 28% 16% 5%	484
	21st to end	Day	11% 7% 32% 31% 14% 5%	532
August	1st to 10th	Day	14% 7% 23% 30% 17% 8%	486
	11th to 20th	Day	12% 6% 26% 29% 19% 8%	479
	21st to end	Day	12% 8% 29% 28% 16% 8%	539
September	1st to 10th	Day	13% 11% 25% 27% 15% 8%	490
	11th to 20th	Day	11% 8% 20% 24% 24% 12%	490
	21st to end	Day	18% 13% 28% 22% 12% 6%	490
October	1st to 10th	Day	21% 15% 26% 19% 14% 4	490
	11th to 20th	Day	24% 21% 30% 18% 6% 2	490
	21st to end	Day	35% 19% 25% 16% 5%	539
November	1st to 10th	Day	25% 20% 32% 15% 7% 2	337
	11th to 20th	Day	29% 27% 26% 14% 3 2	200
	21st to end	Day	31% 28% 29% 12% 1	200
December	1st to 10th	Day	32% 24% 31% 9% 5% 1	200
	11th to 20th	Day	32% 27% 28% 12% 2	200
	21st to end	Day	28% 36% 23% 10% 2 1	220
January	1st to 10th	Night	29% 24% 31% 12% 4	800
	11th to 20th	Night	31% 23% 29% 15% 2	800
	21st to end	Night	25% 19% 33% 18% 4	841
February	1st to 10th	Night	26% 26% 30% 15% 2	760
	11th to 20th	Night	24% 24% 33% 17% 3	743
	21st to end	Night	29% 21% 31% 18% 2	628
March	1st to 10th	Night	23% 18% 34% 21% 5%	760
	11th to 20th	Night	26% 18% 31% 20% 5% 1	760
	21st to end	Night	22% 14% 33% 22% 8% 1	836
April	1st to 10th	Night	25% 17% 33% 22% 3	760
	11th to 20th	Night	23% 15% 32% 22% 7% 1	747
	21st to end	Night	21% 9% 32% 27% 9%	570
May	1st to 10th	Night	20% 9% 27% 29% 13% 2	570
	11th to 20th	Night	15% 7% 30% 40% 8% 1	379
	21st to end	Night	11% 9% 27% 36% 14% 1	411
June	1st to 10th	Night	13% 7% 35% 33% 12%	380
	11th to 20th	Night	13% 7% 31% 34% 14% 1	380
	21st to end	Night	14% 7% 30% 39% 9%	380
July	1st to 10th	Night	8% 6% 25% 41% 19% 1	370
	11th to 20th	Night	10% 6% 29% 36% 17% 1	368
	21st to end	Night	11% 3 26% 39% 19% 2	587
August	1st to 10th	Night	9% 6% 18% 42% 22% 2	567
	11th to 20th	Night	10% 5% 22% 34% 28% 2	564
	21st to end	Night	12% 5% 23% 35% 22% 3	746
September	1st to 10th	Night	15% 7% 23% 33% 19% 3	760
	11th to 20th	Night	14% 5% 20% 30% 27% 5%	760
	21st to end	Night	18% 10% 24% 28% 16% 4	760
October	1st to 10th	Night	19% 10% 29% 24% 16% 2	760
	11th to 20th	Night	27% 17% 25% 23% 8% 1	760
	21st to end	Night	32% 16% 27% 22% 4	836
November	1st to 10th	Night	24% 15% 29% 22% 9% 1	780
	11th to 20th	Night	23% 20% 32% 16% 8% 1	800
	21st to end	Night	27% 22% 30% 16% 5%	800
December	1st to 10th	Night	33% 23% 25% 16% 4	800
	11th to 20th	Night	27% 24% 31% 15% 3	800
	21st to end	Night	33% 19% 31% 14% 3	880

Page generated at 2025-01-19 03:06:49. Version ac07

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