Morrow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morrow Observatory clear sky chart from 2005-11-06 to 2024-08-30.

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 6% 4 4 4 4 5% 7% 38%	95469
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 38%	93557
Transparency	Day	44% 5% 10% 23% 15% 2	95228
	Night	46% 5% 10% 22% 15% 2	93404
Seeing	Day	18% 13% 27% 23% 14% 5%	15254
	Night	19% 11% 28% 27% 14% 2	25036

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% 6% 8% 4 4 4 3 4 6% 30%	2527
	11th to 20th	Day	18% 11% 4 12% 6% 5% 3 3 5% 5% 28%	2530
	21st to end	Day	26% 17% 7% 7% 4 3 3 3 4 4 23%	3020
February	1st to 10th	Day	17% 13% 5% 8% 4 4 3 2 5% 6% 31%	1900
	11th to 20th	Day	16% 10% 4 8% 5% 4 4 3 5% 6% 35%	1848
	21st to end	Day	17% 14% 5% 8% 5% 4 4 2 5% 6% 32%	1570
March	1st to 10th	Day	23% 15% 3 6% 3 3 3 3 3 4 33%	3120
	11th to 20th	Day	14% 7% 3 6% 4 4 3 3 5% 6% 45%	3342
	21st to end	Day	15% 9% 4 7% 6% 5% 5% 4 6% 7% 34%	4005
April	1st to 10th	Day	14% 10% 3 6% 5% 5% 4 3 6% 8% 38%	2280
	11th to 20th	Day	17% 10% 3 6% 4 4 4 3 5% 6% 39%	2254
	21st to end	Day	19% 7% 3 6% 4 5% 4 3 4 7% 37%	2405
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 4 7% 8% 29%	4082
	11th to 20th	Day	21% 7% 2 6% 3 4 4 3 5% 7% 37%	4081
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	4507
June	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 4 5% 8% 43%	2660
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 4 7% 8% 46%	2660
	21st to end	Day	10% 6% 2 5% 5% 4 5% 5% 7% 9% 42%	2660
July	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 6% 10% 39%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 48%	4440
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 6% 8% 11% 30%	4631
August	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 5% 6% 11% 44%	2457
	11th to 20th	Day	10% 5% 2 5% 5% 6% 5% 4 7% 9% 42%	2272
	21st to end	Day	6% 5% 2 5% 4 5% 5% 3 6% 9% 50%	2485
September	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 4 4 6% 46%	2138
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 3 6% 8% 48%	1980
	21st to end	Day	12% 4 2 5% 4 3 3 3 5% 7% 52%	1980
October	1st to 10th	Day	11% 4 2 6% 4 4 3 3 5% 7% 51%	1980
	11th to 20th	Day	13% 7% 3 5% 5% 4 3 3 5% 8% 44%	1863
	21st to end	Day	20% 9% 3 5% 4 4 4 3 5% 5% 38%	1782
November	1st to 10th	Day	10% 5% 2 4 4 3 4 3 4 6% 54%	1672
	11th to 20th	Day	16% 9% 4 7% 4 3 4 3 5% 7% 39%	1710
	21st to end	Day	20% 9% 2 6% 4 3 3 3 4 5% 41%	1710
December	1st to 10th	Day	20% 14% 4 7% 4 3 4 3 3 4 33%	1710
	11th to 20th	Day	16% 15% 5% 8% 4 4 3 3 4 6% 31%	1985
	21st to end	Day	24% 19% 6% 7% 4 4 4 3 4 5% 20%	2783
January	1st to 10th	Night	18% 15% 6% 9% 4 3 5% 4 5% 5% 27%	4428
	11th to 20th	Night	15% 16% 6% 13% 6% 6% 4 2 3 4 24%	4440
	21st to end	Night	26% 18% 5% 6% 3 3 3 2 3 4 25%	4625
February	1st to 10th	Night	21% 11% 4 9% 5% 4 4 3 5% 5% 28%	2280
	11th to 20th	Night	15% 11% 3 11% 6% 4 3 3 5% 6% 33%	2218
	21st to end	Night	21% 11% 4 8% 4 3 5% 3 5% 4 31%	1884
March	1st to 10th	Night	23% 15% 3 6% 3 3 3 2 4 5% 32%	3733
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 4 6% 48%	3617
	21st to end	Night	16% 8% 3 7% 5% 5% 4 4 5% 6% 38%	3640
April	1st to 10th	Night	13% 9% 2 7% 4 4 4 4 6% 6% 41%	1900
	11th to 20th	Night	16% 9% 2 5% 4 4 3 3 5% 7% 42%	1875
	21st to end	Night	17% 8% 2 6% 4 4 5% 4 5% 7% 38%	1773
May	1st to 10th	Night	18% 12% 4 8% 5% 4 4 3 5% 7% 30%	2813
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 6% 8% 40%	2812
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 6% 10% 41%	3032
June	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 7% 10% 47%	1520
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 50%	1520
	21st to end	Night	6% 3 1 4 4 5% 3 5% 8% 10% 51%	1520
July	1st to 10th	Night	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	2530
	11th to 20th	Night	5% 3 2 4 4 3 4 4 6% 9% 56%	2530
	21st to end	Night	9% 6% 2 8% 6% 6% 6% 5% 7% 10% 36%	2924
August	1st to 10th	Night	7% 5% 2 5% 5% 4 4 4 6% 9% 50%	1701
	11th to 20th	Night	7% 3 2 5% 4 5% 5% 5% 7% 10% 47%	1845
	21st to end	Night	5% 3 1 5% 3 3 4 3 6% 10% 56%	2080
September	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 5% 7% 49%	1804
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 5% 9% 54%	1980
	21st to end	Night	12% 5% 2 4 3 3 3 3 5% 7% 52%	1980
October	1st to 10th	Night	9% 4 2 5% 3 3 3 3 5% 7% 57%	1980
	11th to 20th	Night	15% 7% 3 5% 3 4 4 4 4 6% 46%	2092
	21st to end	Night	18% 8% 3 6% 4 4 4 2 4 6% 42%	2574
November	1st to 10th	Night	12% 7% 2 5% 4 3 4 3 6% 6% 50%	2406
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 4 7% 43%	2470
	21st to end	Night	20% 10% 3 6% 4 4 3 3 4 6% 37%	2470
December	1st to 10th	Night	18% 12% 4 7% 5% 4 4 3 5% 6% 32%	2470
	11th to 20th	Night	18% 13% 4 8% 4 4 3 3 5% 5% 32%	3207
	21st to end	Night	23% 19% 5% 9% 6% 5% 4 3 4 4 20%	4884

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	57% 4 9% 21% 9% 1	2528
	11th to 20th	Day	59% 3 10% 18% 9% 1	2530
	21st to end	Day	65% 4 7% 19% 4 1	3020
February	1st to 10th	Day	54% 3 9% 22% 11% 2	1900
	11th to 20th	Day	50% 4 9% 22% 13% 2	1858
	21st to end	Day	54% 4 9% 20% 10% 3	1570
March	1st to 10th	Day	57% 3 5% 19% 11% 4	3120
	11th to 20th	Day	40% 4 9% 28% 16% 2	3342
	21st to end	Day	48% 6% 12% 22% 11% 1	4005
April	1st to 10th	Day	45% 5% 11% 22% 15% 1	2280
	11th to 20th	Day	46% 4 9% 24% 14% 3	2254
	21st to end	Day	48% 4 9% 24% 13% 3	2405
May	1st to 10th	Day	49% 7% 14% 21% 8% 1	4120
	11th to 20th	Day	47% 6% 9% 24% 13% 1	4085
	21st to end	Day	42% 7% 11% 17% 19% 3	4520
June	1st to 10th	Day	39% 6% 10% 25% 17% 3	2660
	11th to 20th	Day	33% 7% 14% 24% 20% 3	2660
	21st to end	Day	36% 6% 15% 21% 17% 4	2660
July	1st to 10th	Day	41% 6% 11% 21% 19% 2	4371
	11th to 20th	Day	28% 7% 12% 32% 19% 2	4365
	21st to end	Day	46% 8% 14% 17% 13% 1	4484
August	1st to 10th	Day	32% 9% 14% 28% 16% 2	2444
	11th to 20th	Day	37% 7% 12% 27% 17% 1	2272
	21st to end	Day	29% 6% 13% 27% 18% 6%	2485
September	1st to 10th	Day	39% 5% 11% 24% 17% 5%	2138
	11th to 20th	Day	33% 5% 11% 27% 19% 4	1980
	21st to end	Day	32% 5% 8% 26% 25% 3	1980
October	1st to 10th	Day	33% 5% 8% 26% 25% 2	1980
	11th to 20th	Day	38% 5% 8% 25% 22% 3	1863
	21st to end	Day	48% 4 8% 23% 16% 2	1782
November	1st to 10th	Day	32% 4 10% 29% 20% 5%	1672
	11th to 20th	Day	45% 4 9% 22% 17% 3	1710
	21st to end	Day	47% 3 9% 23% 16% 1	1710
December	1st to 10th	Day	55% 4 10% 20% 11% 1	1710
	11th to 20th	Day	55% 4 7% 22% 11% 1	1982
	21st to end	Day	66% 4 9% 13% 7% 1	2783
January	1st to 10th	Night	58% 5% 10% 17% 8% 1	4428
	11th to 20th	Night	65% 3 9% 15% 8% 1	4440
	21st to end	Night	64% 3 7% 16% 9% 1	4625
February	1st to 10th	Night	58% 3 11% 19% 7% 1	2280
	11th to 20th	Night	52% 3 10% 21% 13% 1	2219
	21st to end	Night	56% 4 10% 18% 11% 2	1884
March	1st to 10th	Night	55% 3 6% 22% 12% 1	3733
	11th to 20th	Night	36% 5% 12% 25% 21% 1	3617
	21st to end	Night	46% 6% 9% 26% 11% 1	3640
April	1st to 10th	Night	43% 4 11% 25% 14% 3	1900
	11th to 20th	Night	42% 5% 10% 19% 20% 5%	1875
	21st to end	Night	45% 5% 9% 24% 14% 3	1773
May	1st to 10th	Night	53% 8% 11% 20% 7% 1	2840
	11th to 20th	Night	42% 6% 12% 28% 10% 1	2813
	21st to end	Night	36% 8% 14% 23% 14% 4	3040
June	1st to 10th	Night	33% 6% 10% 26% 21% 5%	1520
	11th to 20th	Night	30% 6% 12% 24% 22% 6%	1520
	21st to end	Night	25% 8% 17% 24% 20% 7%	1520
July	1st to 10th	Night	36% 7% 9% 25% 19% 3	2490
	11th to 20th	Night	24% 7% 13% 33% 21% 3	2482
	21st to end	Night	41% 6% 15% 23% 12% 2	2831
August	1st to 10th	Night	29% 7% 16% 28% 17% 2	1690
	11th to 20th	Night	29% 8% 15% 27% 19% 2	1845
	21st to end	Night	23% 6% 14% 30% 22% 6%	2080
September	1st to 10th	Night	33% 6% 8% 28% 18% 7%	1804
	11th to 20th	Night	28% 6% 9% 26% 26% 5%	1980
	21st to end	Night	32% 5% 9% 24% 25% 5%	1980
October	1st to 10th	Night	28% 4 10% 26% 27% 5%	1980
	11th to 20th	Night	39% 4 7% 25% 21% 4	2092
	21st to end	Night	45% 3 7% 22% 18% 4	2574
November	1st to 10th	Night	36% 3 9% 25% 22% 4	2406
	11th to 20th	Night	43% 2 9% 22% 20% 4	2470
	21st to end	Night	49% 3 10% 22% 15% 1	2470
December	1st to 10th	Night	53% 4 11% 20% 11% 1	2470
	11th to 20th	Night	54% 4 8% 20% 12% 1	3209
	21st to end	Night	68% 4 8% 12% 7% 1	4884

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	28% 21% 30% 17% 3 1	270
	11th to 20th	Day	33% 25% 30% 10% 2	270
	21st to end	Day	28% 27% 32% 11% 2	297
February	1st to 10th	Day	28% 28% 31% 10% 3	270
	11th to 20th	Day	26% 29% 32% 11% 1 1	270
	21st to end	Day	30% 21% 37% 11% 1	225
March	1st to 10th	Day	26% 27% 32% 12% 2	270
	11th to 20th	Day	27% 23% 33% 12% 3 1	374
	21st to end	Day	30% 20% 31% 14% 6%	506
April	1st to 10th	Day	23% 20% 30% 20% 6% 1	460
	11th to 20th	Day	22% 24% 24% 21% 8% 2	452
	21st to end	Day	24% 20% 34% 14% 7% 1	460
May	1st to 10th	Day	22% 16% 32% 19% 7% 2	463
	11th to 20th	Day	23% 12% 29% 22% 9% 5%	487
	21st to end	Day	13% 8% 30% 25% 16% 8%	547
June	1st to 10th	Day	13% 8% 27% 28% 19% 5%	680
	11th to 20th	Day	11% 7% 26% 34% 18% 5%	680
	21st to end	Day	11% 8% 30% 27% 17% 6%	680
July	1st to 10th	Day	13% 4 22% 24% 24% 14%	671
	11th to 20th	Day	7% 5% 24% 33% 22% 9%	668
	21st to end	Day	10% 4 21% 35% 22% 8%	632
August	1st to 10th	Day	9% 4 18% 31% 27% 11%	486
	11th to 20th	Day	15% 5% 22% 29% 24% 6%	479
	21st to end	Day	8% 4 15% 32% 28% 12%	532
September	1st to 10th	Day	12% 4 19% 30% 28% 8%	450
	11th to 20th	Day	10% 3 17% 30% 28% 11%	450
	21st to end	Day	14% 7% 23% 26% 23% 7%	450
October	1st to 10th	Day	10% 8% 28% 31% 16% 5%	450
	11th to 20th	Day	18% 15% 37% 22% 8% 2	381
	21st to end	Day	31% 14% 36% 18% 1	297
November	1st to 10th	Day	13% 19% 33% 24% 8% 3	270
	11th to 20th	Day	24% 19% 31% 19% 6% 1	270
	21st to end	Day	29% 12% 34% 21% 5%	270
December	1st to 10th	Day	23% 28% 27% 16% 5% 1	270
	11th to 20th	Day	28% 18% 36% 13% 4 1	270
	21st to end	Day	33% 23% 30% 11% 3	297
January	1st to 10th	Night	23% 20% 33% 21% 4	950
	11th to 20th	Night	27% 20% 34% 17% 2	950
	21st to end	Night	22% 19% 35% 20% 2	943
February	1st to 10th	Night	27% 15% 37% 20% 1	760
	11th to 20th	Night	24% 19% 35% 20% 1	743
	21st to end	Night	29% 16% 37% 17% 1	628
March	1st to 10th	Night	23% 18% 34% 22% 3	760
	11th to 20th	Night	24% 15% 34% 19% 6%	760
	21st to end	Night	24% 13% 34% 24% 5%	736
April	1st to 10th	Night	17% 15% 34% 26% 7% 1	570
	11th to 20th	Night	21% 13% 29% 26% 11%	563
	21st to end	Night	23% 10% 35% 26% 6% 1	570
May	1st to 10th	Night	21% 8% 33% 27% 11% 1	570
	11th to 20th	Night	17% 7% 26% 30% 17% 3	561
	21st to end	Night	10% 3 25% 34% 23% 5%	617
June	1st to 10th	Night	12% 4 22% 38% 22% 2	570
	11th to 20th	Night	9% 3 21% 44% 22% 1	570
	21st to end	Night	7% 3 22% 40% 26% 2	570
July	1st to 10th	Night	13% 2 12% 34% 30% 10%	555
	11th to 20th	Night	6% 3 15% 36% 35% 5%	552
	21st to end	Night	8% 3 15% 37% 33% 4	624
August	1st to 10th	Night	9% 1 13% 36% 36% 5%	567
	11th to 20th	Night	6% 2 14% 40% 34% 4	564
	21st to end	Night	6% 1 14% 35% 38% 6%	624
September	1st to 10th	Night	13% 4 13% 32% 35% 4	544
	11th to 20th	Night	8% 3 17% 32% 35% 5%	720
	21st to end	Night	14% 5% 23% 29% 25% 3	720
October	1st to 10th	Night	10% 6% 24% 32% 23% 5%	720
	11th to 20th	Night	17% 12% 29% 27% 14% 1	720
	21st to end	Night	22% 13% 32% 23% 9%	792
November	1st to 10th	Night	17% 9% 32% 28% 13% 1	740
	11th to 20th	Night	21% 15% 30% 26% 8% 1	760
	21st to end	Night	25% 14% 27% 26% 7%	760
December	1st to 10th	Night	25% 15% 33% 22% 5%	760
	11th to 20th	Night	28% 12% 31% 24% 5% 1	878
	21st to end	Night	32% 16% 32% 17% 3	1045

Page generated at 2024-08-27 09:49:37. Version ac07

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