3.5 - Sensible Temperature and Temperature Measurement

Sensible Temperature

We have already talked about how temperature and heat are related but not the same. Temperature is the average speed of molecular movement within an object or substance. We can measure temperature with a thermometer. But as humans, our standard of comparison is our skin. This is the sensible temperature. It  is the temperature perceived by a person as opposed to what it says on the thermometer. Many things affect our comfort level and therefore,  our sensible temperature.

A major factor is evaporation. Evaporation is the process whereby liquid water becomes a gas. It is a cooling process. Our bodies rely on the evaporation of perspiration from our skin as our main cooling mechanism.  Therefore, a hot, dry day when perspiration is evaporating freely from our skin is going to feel much cooler than the same temperature with higher humidity where less evaporation occurs.  The heat index measures our body's response to changes in temperature and humidity.

Wind is another factor affecting sensible temperature. Wind increases the rate of evaporation by constantly replacing warm air with cooler air. This is wind chill. On a warm day, the wind enhances our body's cooling mechanism, increasing evaporation of perspiration off of our skin. It can make a hot day more comfortable. On a cold or even a cool day, the wind can make even moderate temperatures deadly. Being out all night without a jacket at 40°F with still air is a very different experience from doing so with a 30 mph wind. The Wind Chill Chart shows us the impact of wind and temperature on the human body.

Measuring Temperature

To measure temperature we use thermometers. The standard liquid-in-glass thermometer functions because when the liquid, usually mercury or alcohol, is heated it expands more than the glass and when it cools it contracts more than the glass. Because of this relative expansion and contraction, the height of the liquid column in the glass tube is a measure of the surrounding temperature. 

For meteorological purposes, where and how you place a thermometer is important. There are "rules".

• It should be shaded from direct sunlight
• It should be shielded from radiating surfaces like cement or buildings
• It should be enclosed in a protective case or box that has good ventilation and is light in color
• It should be a minimum of 3 feet off the ground
• It should be placed over grass or another natural surface

Well, that's a lot of rules. Question is, do you think that  records going back 10's and in some cases, 100's of years reflect measurements being taken following all these rules? Have measurements been always taken in the same location year to year, decade to decade? The answer is, probably not.

Using San Francisco as an example, temperature records have been kept since 1847 but the location of the official SF weather station has moved 11 times since then. Its been downtown, in the Avenues and even in the Mission (the part of SF with the sunniest weather). We are often not comparing apples to apples when we look at this kind of data. We have to be careful how we use temperature data. The media is quick to conclude that a record breaking heat wave is an indicator of a change in climate. In fact, it may not even be record breaking. More importantly, a really hot day or spell may or may not be a sign of climate change. To determine that, we need to see change over a much longer period of time than a single hot summer.

Temperature Scales

Three different scales are commonly used to measure temperature: Fahrenheit, Celsius and Kelvin. The Fahrenheit scale was developed in 1717 by a German physicist, Gabriel Fahrenheit, who designated the temperature of a bath of ice melting in a solution of salt as the zero point on his scale. The upper point of the scale is based on the boiling point of water, designated as 212°F whereas the melting point of ice is 32°F.

The Celsius scale was developed in 1742 by the Swedish astronomer, Anders Celsius. It is based on the melting  and boiling points of water under normal atmospheric conditions. Today, it is divided into 100 increments with the boiling point of water at 100°C and the freezing point of water at 0°C

Lord Kelvin, developed the Kelvin scale in 1848. His scale uses molecular energy to define the extremes of hot and cold. Absolute zero, or 0 K, corresponds to the hypothetical point at which molecular energy ceases. This is a theoretical condition and Kelvin is general used primarily in scientific applications.

To convert temperatures in degrees Fahrenheit to Celsius, subtract 32 and multiply by .5556 (or 5/9).

• Example: (50°F - 32) x .5556 = 10°C

To convert temperatures in degrees Celsius to Fahrenheit, multiply by 1.8 (or 9/5) and add 32.

• Example: (30°C x 1.8) + 32 = 86°F