Chapter 6

Atmospheric Measurements

Introduction

This is the chapter that provides the ‘nuts and bolts’ content for the environmental technician.

This unit assumes that you will be gainfully employed as a field technician whose job it is to acquire the raw data for all aspects of environmental regulation and legislation, and more often than not, that requires the collection (and analysis) of meteorological data.

Why? After all, it is only wind and the like…how could that be important?

Good question! It turns out that nearly all ambient air analysis and noise measurements (which are two huge fields under the environmental banner) require good, accurate, micro-scale meteorological measurements.

In fact without these measurements, the air and noise data can be ‘thrown out of court’ (as you shall find out in a later unit).

We will discuss atmospheric measurements by asking five simple questions;

What is it?

What measurements are there?

How do we measure it?

What values do we expect to find?

What can go wrong with the measurements?

So what do we commonly measure?

From a meteorological perspective, we only measure a (relatively) few atmospheric parameters, as most are calculated or derived from the parameters that are measured.

Understand that measurements can be made at ground level, and, via the use of weather balloons, at various altitudes through a vertical slice of the atmosphere.

The most common measures include temperature(s), pressure, and the speed and direction of the wind.

It is from these few measurable parameters that we infer our weather and other specific environmental information.

It must be said again that other parameters are commonly measured, such as rainfall, solar intensity, snow depth and the like, and it really depends on the reason the meteorological station is being used; for weather forecasting, air pollution studies or noise studies.

Temperature Measurements

What is temperature

You would know of temperature as the hot and cold of something, but temperature is actually a property of matter.

In fact, temperature is the key to the subject of thermodynamics.

Temperature is something that matter exhibits and can be easily measured.

What is temperature

On the microscopic scale, temperature is related to the motion of atoms in matter.

On the macroscopic scale, temperature is the unique physical property that determines the direction of heat flow between two objects placed in thermal contact.

If no heat flow occurs, the two objects have the same temperature; otherwise heat flows from the hotter object to the colder object.

These two basic principles are stated in the zeroth law and second law of thermodynamics, respectively.

How do we measure temperature?

Temperature is measured with an apparatus called a thermometer that may be calibrated to a any common scale such as Celcius or Kelvin.

There are three major classes of thermometer; physical, mechanical and electrical, and each class can have many types of thermometer.

Note that images of common lab equipment have been used below as modern temperature sensors in weather stations have been reduced to boring ‘black boxes’.

Physical Thermometers

These include the traditional liquid in glass thermometers (alcohol and mercury) which work because the liquid expands significantly compared to the expansion of the glass containing it and can be calibrated against a primary thermometeric procedure.

Infra-red detectors (and laser based equipment) use the properties of balckbody emission (see chapter 2) and a detector to generate a voltage which is equivalent to the actual temperature.

Mechanical Thermometers

These include bi-metallic strips which curve with an increase in heat, which inflects a needle pointing to a calibrated scale.

Electrical Termometers

Thermocouples which use a unique property of metal called the thermoelectric effect to produce a voltage difference which is equivalent (somehow) to the temperature.

Note that these are not considered to be very precise or accurate. Thermistors are conceptually similar to thermocouples.

What temperature measurements are there?

Direct measures of temperature include;

The dry bulb temperature (T), which measures ambient air temperature, and is usually measured in the shade, but can be done in direct sun for certain reasons.

The wet bulb temperature (Tw) which is used for determining the relative humidity (RH) amongst other derived measures. See the moisture section below for uses.

Both the minimum temperature and maximum temperature can be recorded via use of an old style Six’s thermometer or a computer controlled data system.

The dewpoint temperature (Td) was usually a derived temperature, but modern stations can employ a cooled mirror which directly measures the Td.

Derived measures of temperature include;

Dewpoint, as mentioned above has historically been determined mathematically.

Virtual temperature (Tv) is calculated to make equal the density of a parcel of both dry and moist air. Because the density changes with temperature, we ask the question “to what temperature must we raise the dry air so that its density equals that of a moist parcel of air?”

Potential temperature (Tp) is calculated to determine the temperature that a parcel of aloft air would be if it was brought down to the altitude where the pressure is 1000hPa.

What units are involved?

Celcius scale (°C)

The Celcius scale is (and was originally called) a centigrade scale, it was derived from Anders Celcius thermometer which used the freezing and boiling points of water (0°C and 100°C respectively) as the calibration points.

This unit is used by both scientific and leyman in most countries. You will notice in Figure 6.4 below that 0 K = -273.15°C.

Kelvin scale (K)

The Kelvin, after Lord Kelvin, is the thermodynamic temperature which is just the Celsius scale shifted downwards so that 0 K = −273.15 °C, or absolute zero.

All scientific fields use the kelvin scale for thermodynamic (physics) calculations.

You will notice in Figure 6.5 below that 0 K = -273.15°C.

Fahrenheit scale (°F)

Fahrenheit is the temperature scale named after its inventor, Daniel Fahrenheit.

The freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (which gave a difference of 180, which was apparently significant at the time somehow). Absolute zero is −459.67 °F.

You will notice in Figure 6.4 below that 0 K = -459.67°C.

The fahrenheit scale will not be used at all in these notes.

Unit Conversions

Converting between these units is often required.

We shall go through them here given the ultimate significance of temperature in both the study and practice of meteorology.

Refer to the notes for the particular equations

Celcius, Kelvin and Fahrenheit Scales

-500

-400

-300

-200

-100

0

100

200

300

400

0 50 100 150 200 250 300 350

What values do we expect to find?

The Australian atmosphere has experienced temperature extremes from ~ -23°C to ~ 51°C.

Industrial applications obviously experience much higher (and lower) temperatures, and you may encounter some of these if you work as a stationary emission technician (smoke stack testing).

Moisture (humidity) measurements

How hard could it possibly be to measure the amount of moisture in the air…? Surely it’s a simple case of …umm…just…Yeah right! So,

how could you do it?

You could in fact perform a gravimetric analysis, and draw a sample of air through a desiccating material such as silica, and then determine how ‘wet’ the desiccant has become. Simple!

Except….how exactly did you calculate the volume? Trust me, it is much easier to calculate the humidity rather than measure it directly (although there are modern devices that do just that!).

What is humidity?

Because of the difficulty associated with performing direct moisture measurements on air, scientists have found a number of different ways of expressing the amount of moisture in the air, and all of them can be calculated from simple measures of temperature, with the help of one or two scientific constant values to help them along the way.

The most common ‘measures’ of atmospheric moisture are;

Absolute Humidity

This is a measure of density, m/V.

The water vapor is expressed as the mass of water vapor contained in a given volume of air.

A problem with using absolute humidity is that an air parcel changes volume as the ambient temperature and pressure change. 

This means that the absolute humidity changes when the volume changes.

How do you conveniently measure the mass and the volume?

Specific Humidity

This is the vapor content of the air using the mass of the water vapor for a given mass of air. 

The kilogram of air measured includes the water vapor present.

Unlike absolute humidity, specific humidity doesn't change as the air parcel expands or is compressed.

Mixing Ratio

This measures the mass of water vapor for a given mass of dry air.

Since water vapor comprises only a small percentage of the mass of air, the values for specific humidity and mixing ratio are very close for a given parcel of air.

Mixing ratio is not affected by changes in pressure and temperature. 

This is a commonly used measure by meteorologists.

Vapor Pressure

Vapor pressure measures the water vapor content of the air using the partial pressure of the water vapor in the air.

The gases in the atmosphere exert a certain amount of pressure which we call atmospheric pressure, the average of which is 1013.25 hPa.

Since water vapor is one of the gases in air, it contributes to the total air pressure, but is obviously variable. 

The contribution by water vapor is rather small, since water vapor only makes up a few percent of the total mass of a parcel of air.

Saturation

The term saturation refers to the mass of water vapour per unit mass of air (including the water vapour).

It is another measure of the actual water vapour content of the air,

It describes the point at which no more water vapor can exist in the air without condensing out to form water droplets.

Saturated Vapor Pressure

The actual water vapour pressure when the air is saturated.

Relative Humidity

The term most frequently used to express the amount of moisture in the air is relative humidity (RH).

The relative humidity is the ratio of the actual amount of water vapour in a sample of air compared to the total amount of water vapour the same sample can hold before condensation begins (i.e., it becomes saturated with water vapour) at a given temperature and pressure,

but RH only gives us a relative sense of the amount of moisture in the air, not the actual amount.

How do we measure humidity?

Psychrometry (wet – dry bulb) This is a very traditional way of finding the relative humidity.

It is very simple, and reasonably accurate.

Two liquid in glass thermometers are used in this technique where one is dry and measures the ambient (or dry) temperature of the air and the other has a cloth material (can be glass fibre, muslin or cotton etc) wrapped around the glass bulb that houses the liquid (alcohol or mercury) that is moistened with water (not saturated).

The dry thermometer measures the dry temperature (T) and the wet thermometer measures the wet bulb temperature (Tw) in whatever units the thermometers are scaled in.

Psychrometry

There are two common varieties of the technique.

The most common for the layman is the wall mounted type where the two thermometers are positioned in a housing mounted on a wall with the wet bulb material connected via a wick to a store of water.

The other type is called a sling psychrometer

Psychrometry

Psychrometers work by determining the wet bulb depression, which is simply the difference between the dry bulb and wet bulb temperatures.

It is called a depression, because the wet bulb temperature will always be cooler than the dry bulb temperature if there is moisture in the air (if the air is saturated with water vapor, then both thermometers will read the same temperature).

Why is it cooler? Because of the latent and sensible heat used up in the evaporation of the water (which changes from a liquid to a gas), thus lowering the temperature as a result.

Once you have established the wet bulb depression value (T – Tw), you then look up a psychometric table (or humidity table) where you will find the dry bulb temperature in the rows and the wet bulb depression in the columns.

The value that you arrive at is the relative humidity.

It is that simple!

Hair hygrometer

It turns out that hair – yes the stuff on your head - gets longer and shorter depending on the humidity,

This can be used to measure humidity by attaching a calibrated hair system to a needle or pen which writes against a scaled paper roll–

This produces a hydrograph, or a continuous plot of humidity

Tranducers

This is the modern day ‘small black magic box’ approach where electronics perform all the work.

Transducers simply (in a very complex manner) transform one measured (or sensed) parameter, in this case water vapour, into an electrical signal, which is then turned into the relative humidity reading (or other reading) by calculation.

Common Transducer

Remote Sensing

Satellites can use infrared technology to view the humidity of the Earth from space. See image below

Calculations

Usually from Dewpoint temperature readings

What is Pressure?

Pressure is not only due to the mass of the molecules, but also the motion of the molecules bombarding our body.

If you want to feel how much pressure your body is actually under from the force of the atmosphere, simply go to a diving pool and touch the bottom.

Being ten meters under water applies the equivalent force as one atmosphere!

What pressure measurements are there?

Although pressure can be difficult to comprehend, the measurements are few are far between.

Atmospheric pressure is measured as both the true pressure, and as corrected pressure.

Standard Pressure

The standard for pressure is called sea level (or more accurately, the Mean Sea Level Pressure (MSLP), and has a value of 101.325 kPa.

But what is sea level? It is the average height (or imaginary line) of the ocean measured over about 19 years for a specific place.

True/real/actual surface pressure

This is simply a raw pressure reading at any point or altitude prior to the value being corrected to the MSLP (if it is corrected at all).

This is what you will measure if you use an aneroid or mercury barometer, as most modern instruments will use an algorithm to correct, unless they offer both raw and corrected readings.

The unit problem

The problem with pressure is not the number of measures that are available; it is with the number of units that have been invented, so we shall provide a comprehensive overview of atmospheric pressure units here that will help you ‘cope’ with the ‘problem’.

The international system of units (SI) stipulates that the standard unit of pressure is the Pascal (Pa), which is called a derived unit, as the Pascal exhibits the base units of kg/ms2, or Newtons/m2. The use of the Pascal leads to large cumbersome numbers, so we often employ the use of a metric prefix such as kilo (kPa) or hecto (hPa). Other units of pressure include;

mmHg (Torr)

atmosphere

bar

psi

dyne/cm2

Unit Conversions

If you ever need to convert between units, then tables such as the one below from the SI Chemical Data Handbook are invaluable tools to get the job done.

Your teacher will explain to you how to use it.

Table of Conversion Factors

pascal atm mmHg bar dyne/cm2 psi

Pascal 1 9.87x10-6 7.5x10-3 10-5 10 1.45x10-4

atm 1.013x105 1 760 1.013 1.013x106 14.7

mmHg 133.3 1.32x10-3 1 1.33x10-3 1333 1.93x10-2

bar 105 0.9869 750.1 1 106 14.5

dyne/cm2 10-1 9.87x10-7 7.5x10-4 10-6 1 1.45x10-5

psi 6895 6.8x10-2 51.71 6.89x10-2 6.89x104 1

How do we measure atmospheric pressure?

Pressure at the surface and aloft is measured with a barometer.

While there are many types of barometers, the most commonly used barometers for meteorological purposes are the aneroid and mercury barometers, but there are many others.

Pressure measurements at higher altitudes (such as on a plane or weather balloon can be measured with either a barometer, transducer or via a simple mechanical ‘Pitot tube’ set up which measures pressure by comparing the static and absolute pressure.

Ancient school…

The water barometer This invention was water based vessel with a narrow spout,

and as the atmospheric pressure changed, the water in the narrow spout moved up and down, indicating high and low pressure.

From this simple device, the prediction of stormy weather could be made, and hence such devices were known as ‘storm’ or ‘thunder’ glasses.

These are no more than home decorations these days, but do provide an opportunity for students to make there own barometer using a beaker filled with coloured water and a capillary tube!

Old School…

The mercury barometer

A properly calibrated mercury barometer is extremely accurate.

These barometers consist of a glass tube filled with liquid mercury and closed on one end.

The tube stands on end with the closed end up and the open end submerged in a reservoir of mercury that is exposed to the air.

As the air pressure rises, it pushes on the liquid in the reservoir.

The level of the mercury rises in the glass tube to compensate for the additional pressure exerted on the exposed reservoir.

Your parents used..

The aneroid barometer

An aneroid barometer is a mechanical device which used a specially constructed chamber that is partially evacuated.

As the atmospheric pressure changes, it either ‘pushes’ or ‘pulls’ the wall of the chamber which is connected to a needle and scale, thus providing a measure of pressure.

They are calibrated against a known pressure and as such can be quite accurate.

Aneroid barometers needles can be connected to pens rather than scales, and record changes in pressure over time, in which case the instrument is called a barograph.

What you will use…

These devices are varied and the transducers are complex in design and as with the humidity transducers, are visually disappointing, and often appearing as little black boxes hidden away inside a casing.

You will often need to refer to the manufacturer’s handbook to find out exactly how your modern barometer will work.

What pressure values do we expect to find?

To answer this you must remember the how pressure changes around the Earth, with temperature and with altitude. The lowest ever recorded mean sea level pressure (MSLP)

was 870 hPa during Typhoon Tip

and the highest value is about 1080 hPa (debatable), which is an approximate variation of 210 hPa.

Standard pressure also changes with altitude where the average MSLP = 1013.25 hPa,

whereas on top of Mount Everest it is approximately 320 hPa.

In Australia, the pressure variation will obviously not be as dramatic, but we are not far off these extremes.

Our highest mainland peak is ‘hill’ Kosciusko at a meagre 2228 meters, where the pressure is approximately 780 hPa.

Wind Measurements

Wind is the horizontal movement of air which results from the presence of a pressure gradient and creation of the horizontal pressure gradient force between areas of high and low pressure.

The pressure differences results from the heating of the Earth from the Sun which creates convective lifting or air, creating a low pressure system relative to the air around it.

This process starts a chain reaction of high and low pressure systems which occur on all the scales from global to micro – the result of which is wind.

What wind measurements are there?

Two important measurements of the wind are the direction and speed of the wind.

Wind speed obviously refers to the velocity of wind.

Wind direction obviously refers to the direction in which the wind is blowing, but is not given in reference to the direction in which they are blowing, but rather the direction from which the wind comes from.

A westerly wind blows from west to east. A northerly wind blows from north to south.

Units of Measure

Wind direction is obviously measured via either compass points (i.e. North, South etcetera) or by degrees bearing (i.e. 335°N).

Wind speed on the other hand has a variety of units that can be employed, the most important being the SI derived unit, which is m/s or other metric prefix such as km/hr. Other common units include;

Miles per hour (mph)

Knots (nautical miles per hour)

Unit Conversions

1 m/s = 3.6 km/h

1 mph = 1.609 km/h

1 knot = 1.852 km/h = 0.514 ms-1

How do we measure wind?

The most common way to measure the wind direction and speed at the earth's surface is with wind cups and vanes.

The vane gives the direction (in conjunction with a compass of some sort) while the cup catches the wind and rotates giving an indication of speed.

Any device that measures the wind speed is called an anemometer, of which there are many types.

There are many types of anemometer available;

Cup

Windmill

Hot wire

Sonic

Laser Doppler

Even a manometer can be used