ATMS 451: Instruments and Observations

MWF 11:30 AM – 12:20 PM310c ATG

TuTh 10:30 AM – 12:20 PM108 or 610 ATG** (be prepared for changes)

Instructors

Becky AlexanderAssist. Professor, Atmospheric Sciences306 ATGbeckya@uw.edu

Robert WoodAssoc. Professor, Atmospheric Sciences718 ATGrobwood2@uw.edu

Course Materials and Logistics

1. No required textbook (I can suggest some)

2. Course materials on website http://www.atmos.washington.edu/~robwood/teaching/451/

3. Buy a laboratory notebook from UBS

4. Form groups of 2 by Wednesday

5. Determine if you can host a weather station

Learning goals

1. Assess and understand the relevance of good, quantitative observational data

2. Experience how such information is obtained, analyzed, and expressed in scientific and technical communications

Topics and Related Activities

1. Analyzing and Quantifying Measurement Quality

2. Practicalities of Making Measurements

3. Concepts and Realities of Common Sensors

4. Scientific Communication: Report Writing

Relevance to Past and Future Work

1. Basic Science: Science: a connected body of agreed upon truths based on OBSERVABLE facts, classified into laws (theories)

Observation

Hypothesis

ExperimentalTest (measure)?Uncertainty?

?Uncertainty?

Gravitational Lensing – Einstein Rings

D.E.D 1916 – measured bend angle by sun:2 +/- 0.3”

Relevance to Past and Future Work

2. Applied Science/Engineering

Price of Gold

A treasure hunter is hawking a 1 kg royal crown she has found, claiming it is solid gold.

Your assistant measures its density to be 15 +/- 1.5 g/cm3. The density of pure gold is 15.5 g/cm3. The price of gold is currently $51,500 per kg. What do you do?

Measurement Uncertainty Issues

1. For multiple measurements of the same quantity, what exactly is the “best estimate” of the true value?

2. X x x implies a range within which we are “confident ” the true value exists

3. How do we determine the value of x?

Significant Figures – Avoid Significant Embarrassment

After a series of measurements and calculations you determine the acceleration due to gravity on Earth.

The answer on your calculator/computer is:

g = 9.82174 m/s2,

and the uncertainty estimate is

0.02847 m/s2.

How do you report your result?

Measurement Comparison Issues

1. Need to manipulate uncertainties through mathematical operations ERROR PROPAGATION

2. Comparisons (two measurements or measurement vs predictions) come down to a range over which we are “confident” about our conclusions

3. As important to know how the estimate of uncertainty was made as it is to know the uncertainty

Measurement, Error, Uncertainty

• Measurement: determination of size, amount, or degree of some object or property by comparison to a standard unit

• All measurements carry uncertainty, often called “errors” – NOT a mistake, and cannot be avoided!

• Error = Uncertainty (here) – cannot be known exactly, only estimated, must explain basis of estimation

Uncertainty

• An indefiniteness in measurements of a system property, and any quantities derived from them, due to sensor limitations, problems of definition, and natural fluctuations due to the system itself.

Sources of Measurement Uncertainty

Analytical Sampling

Types and Sources of Measurement Uncertainty

Analytical Sampling To Minimize

Random Electronic noise from

sensor circuitry

Turbulence Repetitive measurements

Systematic

Gradations on

thermometer off by 20%

Measure air T over

blacktop parking lot

Calibrate sensor to known

standard; place sensor in

appropriate location

Accuracy and Precision

Distribution of N Measurements and of Means

-10 -8 -6 -4 -2 0 2 4 6 8 10

Pro

ba

bil

ity

xt

x

x = x/10

N = 10; blue

N = 10 performed many times, distribution of means; black

Normal Error Integral

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

t

P(t)

2

21

2

t z

t

P t e dz

68% w/in 1 95.4% w/in 2

Normal vs Student’s t-distribution

Question

Two different weathernuts living in adjacent towns (town A and B) measure the air temperature in their town during a brief period. Both want to claim their town was colder than the other during this time. Does either one have a valid claim?

Town A T Measurements: 10.2 11.5 13.4 15.1 12.2 oC Town B T Measurements: 9.8 10.2 12.8 14.6 11.7 oC

Average uncertainty in any one of Nut A’s or Nut B’s individual measurements = 0.5 oC.