Thermometers

>> Types of Thermometers

Figure 1: This figure illustrates the basic parts of a liquid-in-glass thermometer.

There are three basic types of thermometers.  The liquid-in-glass thermometer is the most commonly used and probably the most familiar type of thermometer.  This type of thermometer works because the liquid enclosed in the glass tube will change its volume as the temperature changes in a known fashion.  A basic liquid-in-glass thermometer can be seen in Figure 1 to the right.

Figure 2: This figure shows the basic structure of a bimetallic thermometer.  Notice the bimetallic coil used to determine the temperature.

Another type of thermometer is a bimetallic thermometer.  A bimetallic thermometer is made up of a coil of two different metals and a dial on which the temperature can be read.  The bimetallic coil of the thermometer bends due to the different expansion rates of the two metals of the coil.  The temperature on the dial is determined by the amount that the bimetallic strip bends.  An illustration of a bimetallic thermometer can be seen to the left in Figure 2.

Figure 3: The picture above shows the instrument package from a weather balloon, or radiosonde.  Included in this instrument package is an electronic thermometer.

The third type of thermometer is known as an electronic thermometer.  Included in the instrument package of a weather balloon is an electronic thermometer.  An electric current is monitored, with the amount of current directly related to the temperature of the thermometer.  A picture of the radiosonde instrument package can be seen in Figure 3.

>> Site Selection

Thermometers are very useful tools for meteorologists, but the placement of a thermometer will make a world of difference when taking temperature measurements.  In order to obtain an accurate temperature reading from a thermometer, it must be placed in a well ventilated and shaded area at a fixed height above the ground.  An example of good thermometer placement can be seen in Figure 4.

Diurnal Range of Temperatures

>> Mirrors annual cycle
>> Change in Sun angle
>> Maximum temperature lags maximum solar radiation

Figure 5: The image above depicts the diurnal cycle in both temperature and incoming solar radiation.  Notice that the maximum temperature lags the maximum incoming solar radiation.

The daily change in temperature is very similar to the annual temperature cycle.  Both cycles depend on the amount of solar radiation hitting the surface of the earth at any given time.  Throughout one day the amount of incoming solar radiation changes due to the change in the sun angle as it rises and sets.  The impact of the large change in the angle of the sun can be seen in the flashlight demonstration from Lecture Two.

The diurnal cycle of temperaure and the annual cycle both show a lag between maximum incoming solar radiation and maximum temperature.   While the maximum solar radiation occurs when the sun is nearest to directly overhead (solar noon), the maximum temperature doesn't occur until a few hours later.  This relationship can be seen in Figure 5.

Effect of Cloudiness on Diurnal Temperature Range

>> Presence of clouds decreases the diurnal range of temperatures
>> Less solar radiation reaches the surface: lower daytime temperature
>> Terrestrial (IR) radiation absorbed and emitted by clouds at night: higher nighttime temperature

Figure 6:  The two daily temperature cycles found above illustrate the impact of cloud covered skies on daytime and nighttime temperatures.

As we all know from living in State College, not every day is guaranteed to be perfectly clear and sunny.  As a result of this, the diurnal temperature cycle can vary greatly.  One of the largest impacts on the diurnal temperature cycle is the presence of clouds.  As mentioned above, clouds keep daytime temperatures cooler and nighttime temperatures higher.  The modulation of the diurnal temperature cycle due to the presence of clouds can be seen in Figure 6 to the left.

Seasonality

>> Greater temperature change occurs over land than over water
>> Higher heat capacity of water

Figure 7: The graph above shows the mean monthly temperatures for Saint Louis, Missouri, and San Francisco and Sacramento, California.  Notice that Saint Louis, which is furthest inland, has the greatest variation of annual temperature, while coastal San Francisco experiences less seasonality.

As discussed in Lecture Three, water takes a much longer time to warm or cool than land.  Thus, land areas that are close to bodies of water tend to have smaller annual temperature ranges than areas further inland.  This difference is illustrated in the graph found in Figure 7.

>> Prevailing Wind Direction

The prevailing wind direction can also have a large impact on the annual temperature cycle for any given location.  A location where the prevailing wind blows from a body of water will have a smaller annual temperature range than if the wind blows predominantly from a large land mass.

>> Ocean Currents

Warm and cold ocean currents also play a part in the annual cycle of temperature in coastal locations.  The effect, of course, depends on what type of ocean current is just off the coast.  In areas where a warm ocean current is present, warmer waters and, therefore, higher temperatures will prevail year-round as compared to a location with a cold ocean current nearby (at any given latitude). Still, prevailing wind direction will strongly determine how much of an effect the nearby ocean has on any given location.  A map of the major ocean currents can be found in Figure 8.