Joseph Dale Perry Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph Dale Perry Memorial Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	15% 9% 3 8% 5% 4 4 4 6% 7% 35%	131596
	Night	16% 9% 3 8% 5% 4 4 3 5% 7% 36%	130235
Transparency	Day	47% 5% 10% 23% 14% 2	135294
	Night	47% 4 10% 22% 15% 2	134607
Seeing	Day	22% 11% 25% 22% 14% 6%	16764
	Night	21% 9% 23% 29% 16% 2	26919

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	17% 19% 7% 13% 6% 3 3 2 4 5% 22%	5258
	11th to 20th	Day	15% 16% 7% 12% 6% 3 3 2 3 3 30%	5280
	21st to end	Day	18% 12% 4 10% 7% 4 3 3 4 6% 27%	5780
February	1st to 10th	Day	20% 13% 4 9% 5% 4 3 3 4 5% 30%	1896
	11th to 20th	Day	23% 11% 4 9% 4 4 3 3 4 5% 29%	2048
	21st to end	Day	21% 13% 4 6% 4 3 4 3 5% 5% 31%	1730
March	1st to 10th	Day	19% 15% 3 7% 5% 4 4 3 5% 5% 32%	4880
	11th to 20th	Day	13% 7% 3 6% 5% 4 4 4 6% 6% 42%	5090
	21st to end	Day	17% 8% 2 6% 5% 5% 4 4 5% 7% 36%	5586
April	1st to 10th	Day	16% 9% 2 7% 5% 5% 4 3 5% 7% 37%	2211
	11th to 20th	Day	17% 7% 3 7% 3 4 4 4 4 6% 42%	2492
	21st to end	Day	17% 8% 2 7% 5% 4 5% 3 5% 7% 37%	2600
May	1st to 10th	Day	17% 10% 3 8% 6% 6% 5% 4 7% 8% 27%	5862
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 30%	5861
	21st to end	Day	14% 6% 3 7% 6% 5% 6% 5% 7% 9% 32%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2600
	11th to 20th	Day	10% 6% 2 7% 5% 6% 5% 5% 7% 10% 39%	2600
	21st to end	Day	10% 5% 2 6% 4 5% 6% 4 7% 10% 40%	2600
July	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 9% 43%	5900
	11th to 20th	Day	11% 6% 2 8% 5% 5% 5% 5% 8% 11% 34%	5900
	21st to end	Day	14% 8% 3 9% 6% 6% 6% 4 7% 8% 29%	6424
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 10% 37%	4234
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 8% 9% 38%	4174
	21st to end	Day	6% 3 2 5% 3 3 4 4 6% 9% 55%	4512
September	1st to 10th	Day	14% 6% 2 4 4 4 4 3 5% 6% 49%	2200
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 8% 47%	2200
	21st to end	Day	18% 7% 2 6% 4 3 4 4 5% 5% 42%	2200
October	1st to 10th	Day	13% 4 2 6% 3 3 3 4 5% 7% 50%	2140
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 4 6% 49%	2000
	21st to end	Day	21% 7% 3 6% 4 3 3 2 3 6% 42%	2200
November	1st to 10th	Day	13% 6% 2 6% 3 3 3 4 5% 5% 50%	1877
	11th to 20th	Day	15% 12% 3 6% 4 3 3 4 5% 6% 40%	1890
	21st to end	Day	17% 12% 4 6% 3 3 4 4 4 5% 38%	1890
December	1st to 10th	Day	20% 12% 3 9% 4 4 4 3 3 4 34%	3020
	11th to 20th	Day	23% 14% 4 7% 4 4 4 3 5% 5% 26%	3463
	21st to end	Day	18% 15% 4 11% 6% 6% 5% 4 4 5% 22%	4565
January	1st to 10th	Night	18% 14% 5% 13% 5% 4 4 3 4 5% 25%	7635
	11th to 20th	Night	19% 12% 5% 11% 5% 4 3 3 4 5% 31%	7680
	21st to end	Night	20% 11% 4 9% 5% 4 4 3 4 5% 32%	8401
February	1st to 10th	Night	20% 11% 4 8% 5% 5% 4 4 5% 6% 29%	2719
	11th to 20th	Night	24% 12% 4 9% 4 3 4 3 5% 5% 28%	2458
	21st to end	Night	23% 13% 4 8% 4 4 4 3 4 5% 29%	2076
March	1st to 10th	Night	20% 12% 3 6% 4 4 4 3 5% 6% 36%	5179
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 5% 8% 46%	5090
	21st to end	Night	14% 9% 3 7% 6% 5% 6% 4 5% 8% 34%	5588
April	1st to 10th	Night	14% 9% 2 8% 5% 4 5% 3 5% 7% 38%	2212
	11th to 20th	Night	15% 7% 3 5% 5% 3 4 4 6% 7% 41%	1871
	21st to end	Night	15% 8% 3 6% 4 4 5% 5% 5% 7% 38%	1800
May	1st to 10th	Night	20% 8% 2 8% 5% 5% 5% 4 6% 8% 29%	4033
	11th to 20th	Night	15% 8% 3 8% 5% 5% 6% 4 6% 8% 33%	4032
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 3 6% 8% 34%	4421
June	1st to 10th	Night	10% 5% 2 5% 5% 5% 4 4 7% 9% 44%	1800
	11th to 20th	Night	7% 5% 2 6% 4 5% 5% 5% 7% 11% 44%	1800
	21st to end	Night	10% 5% 1 5% 4 4 5% 5% 7% 11% 44%	1800
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	4058
	11th to 20th	Night	10% 7% 3 7% 5% 4 6% 4 6% 10% 38%	4057
	21st to end	Night	12% 8% 3 9% 6% 5% 6% 4 6% 8% 34%	4427
August	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	2915
	11th to 20th	Night	11% 6% 2 7% 4 4 4 5% 6% 8% 43%	2875
	21st to end	Night	5% 4 2 4 3 3 4 4 7% 8% 57%	3347
September	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 48%	2200
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 3 5% 7% 49%	2200
	21st to end	Night	14% 6% 3 6% 5% 5% 4 3 5% 5% 45%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 54%	2260
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 4 6% 48%	2400
	21st to end	Night	19% 8% 2 5% 4 3 3 3 4 5% 43%	2640
November	1st to 10th	Night	13% 6% 3 4 4 4 3 3 4 5% 50%	2636
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 4 6% 42%	2730
	21st to end	Night	17% 12% 3 6% 4 3 3 3 5% 5% 38%	2730
December	1st to 10th	Night	19% 14% 4 9% 5% 3 4 2 4 4 33%	4380
	11th to 20th	Night	20% 13% 7% 8% 4 4 3 2 4 5% 29%	4952
	21st to end	Night	18% 16% 4 10% 6% 5% 5% 4 5% 6% 22%	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	67% 3 9% 16% 6%	5259
	11th to 20th	Day	61% 2 6% 17% 12% 2	5280
	21st to end	Day	59% 3 8% 23% 6% 1	5780
February	1st to 10th	Day	58% 4 8% 19% 11% 1	1896
	11th to 20th	Day	58% 4 7% 19% 11% 2	2058
	21st to end	Day	56% 2 8% 20% 11% 3	1730
March	1st to 10th	Day	55% 3 7% 18% 13% 5%	4880
	11th to 20th	Day	41% 3 8% 23% 21% 3	5090
	21st to end	Day	47% 4 9% 24% 14% 1	5586
April	1st to 10th	Day	47% 4 10% 22% 14% 3	2211
	11th to 20th	Day	43% 3 10% 22% 18% 4	2492
	21st to end	Day	47% 5% 11% 22% 12% 3	2600
May	1st to 10th	Day	53% 5% 12% 19% 10% 1	5897
	11th to 20th	Day	52% 5% 11% 22% 9% 1	5865
	21st to end	Day	45% 7% 13% 21% 12% 2	6447
June	1st to 10th	Day	38% 7% 13% 25% 17% 1	2600
	11th to 20th	Day	38% 8% 14% 20% 18% 2	2600
	21st to end	Day	37% 7% 14% 23% 16% 3	2600
July	1st to 10th	Day	37% 7% 13% 26% 16% 1	5835
	11th to 20th	Day	41% 9% 15% 27% 7% 1	5831
	21st to end	Day	51% 6% 10% 21% 11% 1	6240
August	1st to 10th	Day	40% 7% 15% 28% 9% 1	4221
	11th to 20th	Day	39% 5% 12% 30% 14% 1	4213
	21st to end	Day	25% 5% 14% 34% 20% 2	4512
September	1st to 10th	Day	36% 5% 10% 23% 21% 6%	2200
	11th to 20th	Day	35% 5% 11% 29% 19% 1	2200
	21st to end	Day	44% 4 10% 22% 18% 2	2200
October	1st to 10th	Day	28% 4 8% 24% 35% 2	3491
	11th to 20th	Day	27% 2 10% 31% 25% 5%	3240
	21st to end	Day	32% 2 10% 30% 22% 4	3538
November	1st to 10th	Day	36% 4 8% 26% 23% 3	1877
	11th to 20th	Day	45% 3 8% 23% 18% 3	1890
	21st to end	Day	49% 3 9% 23% 15% 1	1890
December	1st to 10th	Day	55% 3 7% 22% 12% 1	3020
	11th to 20th	Day	61% 2 8% 17% 10% 1	3460
	21st to end	Day	64% 4 9% 18% 5%	4565
January	1st to 10th	Night	63% 3 10% 17% 7% 1	7636
	11th to 20th	Night	58% 2 5% 18% 14% 2	7680
	21st to end	Night	56% 2 9% 21% 10% 1	8401
February	1st to 10th	Night	56% 4 9% 19% 12% 1	2719
	11th to 20th	Night	59% 3 8% 18% 11% 1	2459
	21st to end	Night	59% 3 9% 15% 11% 3	2076
March	1st to 10th	Night	50% 3 5% 23% 16% 3	5179
	11th to 20th	Night	37% 4 8% 27% 22% 2	5090
	21st to end	Night	49% 5% 9% 22% 15% 1	5588
April	1st to 10th	Night	46% 5% 10% 20% 17% 3	2212
	11th to 20th	Night	41% 4 10% 23% 15% 6%	1871
	21st to end	Night	44% 6% 11% 22% 16% 2	1800
May	1st to 10th	Night	52% 6% 12% 18% 10% 1	4060
	11th to 20th	Night	49% 6% 13% 21% 10% 1	4033
	21st to end	Night	46% 7% 14% 19% 12% 2	4429
June	1st to 10th	Night	35% 6% 15% 22% 18% 3	1800
	11th to 20th	Night	34% 8% 17% 18% 20% 4	1800
	21st to end	Night	33% 8% 15% 23% 16% 4	1800
July	1st to 10th	Night	38% 8% 13% 23% 16% 2	4015
	11th to 20th	Night	39% 8% 16% 24% 11% 2	4006
	21st to end	Night	46% 7% 12% 21% 13% 2	4297
August	1st to 10th	Night	41% 7% 13% 27% 11% 1	2904
	11th to 20th	Night	38% 5% 12% 27% 18% 1	2902
	21st to end	Night	24% 5% 13% 32% 24% 3	3347
September	1st to 10th	Night	34% 5% 13% 22% 18% 8%	2200
	11th to 20th	Night	34% 5% 10% 22% 26% 3	2200
	21st to end	Night	40% 5% 10% 22% 22% 2	2200
October	1st to 10th	Night	27% 3 7% 25% 33% 4	3689
	11th to 20th	Night	29% 3 7% 26% 26% 9%	3899
	21st to end	Night	32% 3 6% 25% 26% 8%	4252
November	1st to 10th	Night	37% 3 9% 23% 24% 5%	2636
	11th to 20th	Night	44% 3 7% 21% 20% 5%	2730
	21st to end	Night	48% 3 8% 24% 15% 2	2730
December	1st to 10th	Night	56% 3 9% 21% 10% 1	4380
	11th to 20th	Night	58% 3 9% 20% 9% 1	4954
	21st to end	Night	63% 4 9% 16% 7%	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	30% 23% 32% 10% 4 1	330
	11th to 20th	Day	38% 25% 27% 9% 1	330
	21st to end	Day	34% 25% 31% 9% 2	363
February	1st to 10th	Day	33% 18% 34% 11% 3	330
	11th to 20th	Day	45% 21% 25% 5% 2 1	330
	21st to end	Day	36% 17% 38% 9%	273
March	1st to 10th	Day	34% 17% 29% 16% 5%	461
	11th to 20th	Day	30% 18% 33% 14% 4	540
	21st to end	Day	29% 19% 32% 15% 5%	594
April	1st to 10th	Day	27% 17% 31% 18% 7% 1	504
	11th to 20th	Day	24% 18% 31% 16% 9% 2	492
	21st to end	Day	25% 14% 33% 20% 6% 2	500
May	1st to 10th	Day	26% 16% 31% 16% 7% 3	503
	11th to 20th	Day	21% 12% 26% 24% 13% 4	527
	21st to end	Day	16% 9% 24% 24% 17% 9%	573
June	1st to 10th	Day	12% 7% 25% 29% 18% 8%	530
	11th to 20th	Day	12% 7% 19% 32% 20% 10%	530
	21st to end	Day	11% 7% 27% 24% 21% 11%	530
July	1st to 10th	Day	13% 4 18% 23% 25% 17%	525
	11th to 20th	Day	10% 4 20% 31% 23% 12%	524
	21st to end	Day	14% 4 16% 29% 26% 11%	576
August	1st to 10th	Day	13% 4 15% 34% 22% 12%	526
	11th to 20th	Day	15% 4 16% 30% 24% 12%	519
	21st to end	Day	10% 2 15% 33% 28% 13%	583
September	1st to 10th	Day	16% 4 14% 27% 27% 12%	530
	11th to 20th	Day	13% 3 12% 32% 28% 12%	530
	21st to end	Day	23% 6% 22% 25% 17% 7%	530
October	1st to 10th	Day	16% 6% 22% 30% 21% 6%	530
	11th to 20th	Day	18% 10% 29% 26% 12% 5%	530
	21st to end	Day	27% 10% 28% 23% 11% 1	583
November	1st to 10th	Day	20% 13% 33% 24% 8% 2	355
	11th to 20th	Day	30% 15% 31% 16% 7% 1	330
	21st to end	Day	31% 15% 31% 17% 5% 1	330
December	1st to 10th	Day	34% 18% 30% 12% 5% 1	330
	11th to 20th	Day	30% 17% 34% 14% 4 1	330
	21st to end	Day	34% 18% 29% 14% 4	363
January	1st to 10th	Night	29% 17% 29% 20% 5%	840
	11th to 20th	Night	34% 17% 27% 19% 3	840
	21st to end	Night	27% 17% 32% 21% 3	924
February	1st to 10th	Night	28% 12% 32% 26% 2	840
	11th to 20th	Night	33% 17% 30% 18% 1	823
	21st to end	Night	33% 16% 32% 18% 2	692
March	1st to 10th	Night	25% 15% 30% 25% 4	840
	11th to 20th	Night	26% 16% 31% 20% 6%	840
	21st to end	Night	23% 13% 33% 24% 6%	924
April	1st to 10th	Night	21% 14% 31% 27% 7%	805
	11th to 20th	Night	18% 12% 30% 28% 11% 1	633
	21st to end	Night	21% 7% 38% 26% 8% 1	600
May	1st to 10th	Night	22% 9% 27% 29% 13% 2	600
	11th to 20th	Night	19% 7% 25% 31% 17% 1	591
	21st to end	Night	13% 4 19% 35% 24% 5%	650
June	1st to 10th	Night	12% 3 18% 39% 26% 3	600
	11th to 20th	Night	9% 3 15% 42% 27% 5%	600
	21st to end	Night	12% 2 17% 36% 29% 3	600
July	1st to 10th	Night	13% 3 10% 34% 32% 7%	585
	11th to 20th	Night	7% 3 14% 32% 38% 6%	582
	21st to end	Night	12% 2 9% 35% 36% 6%	657
August	1st to 10th	Night	13% 3 8% 40% 32% 4	597
	11th to 20th	Night	10% 3 10% 35% 35% 6%	594
	21st to end	Night	11% 2 9% 36% 34% 8%	678
September	1st to 10th	Night	12% 3 11% 33% 34% 7%	800
	11th to 20th	Night	12% 2 14% 28% 38% 5%	800
	21st to end	Night	17% 4 19% 31% 26% 3	800
October	1st to 10th	Night	14% 4 17% 33% 28% 4	800
	11th to 20th	Night	18% 8% 25% 30% 17% 2	800
	21st to end	Night	23% 10% 23% 30% 14% 1	880
November	1st to 10th	Night	16% 8% 25% 33% 15% 2	820
	11th to 20th	Night	23% 13% 25% 28% 10% 1	840
	21st to end	Night	25% 11% 23% 31% 10% 1	840
December	1st to 10th	Night	33% 12% 26% 25% 5%	840
	11th to 20th	Night	26% 12% 30% 27% 6% 1	840
	21st to end	Night	30% 15% 26% 25% 4	924

Page generated at 2026-03-30 02:18:44. Version ac07

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