Temperature and Dew Point Tutorial

By Nickolas McColl

Index


Temperature

Basic Explanation

Temperature is defined as: "The degree of hotness or coldness of a body or environment"

Restated, temperature gives us a way to express our hotness or coldness in terms of numbers instead of subjective terms (cold or hot). Thus, temperature is commonly used in the realm of weather.

The normal variation of temperature with respect to a given day is shown by the following graph.Shows the daily variations of temperature As you can see, temperature has a minimum in the morning hours and has a maximum in the early evening. This of course is based on the amount of sun that a given area receives during the day. During daytime hours the sun is shining and the temperature increases. But after the sun sets the temperature slowly starts to lower until the sun comes back up the next day. The magnitude of the highest and lowest temperature is based on the time of year. In the summer, high temperatures occur and in the winter low temperatures occur. It is important to note: that this only occurs in the Northern Hemisphere. The Southern Hemisphere's seasons are reversed. So when we are having our winters they are enjoying their summers.

Temperature Fact

Lowest Recorded Temperature occurred on July 21, 1983 in Vostok, Antarctica. They had a low of -129°F!!!

How it is Measured

Comparison of different temperature scalesTemperature can be found many ways, from classical thermometers (red liquid in glass container), to sensing it from satellites. Most home users use less accurate thermometers mounted to their porch or some other fixed location. Scientist on the other hand use highly accurate thermometers. These thermometers are digital instead of the classic kind.Image of a Digital thermometer Digital thermometer operate on the principle that a wire's resistance is temperature dependent. Measure resistance and the temperature can be aquired.

It is also important to note that temperature is measured in the shade!! That is right, when it is 110°F outside that is in the shade!

Units

All measurable quantities contain units; it is a way to show the user how many or how much value a number has. In meteorology, we use three temperature scales: Kelvin, Celsius and Fahrenheit. The one most users are familiar with is Fahrenheit. But only America actually has Fahrenheit as its official unit for temperature. All other Countries use the Celsius(Metric) scale.

Note:

The Kelvin scale is used only for scientific studies. The reason is because the Kelvin scale only has positive number. Which makes mathematical equations easier to work with.

Here is a comparison of the two temperature scales:

A comparison of the two temperature scales

Usefulness

With all of this said, we need to consider the usefulness of this information. First most people in the US know Fahrenheit but not Celsius. Sad to say the US is the only country that uses Fahrenheit so the rest of the world is using the Celsius scale. Thus we need to format our site accordingly.

Another area that users are not familiar with is the normal daily temperature. For example: If someone from Brownsville, TX views our pages and it says that the current temperature is 50°F and it is in the middle of summer. How are they to know if that is hot? For all they know we are having a heat wave. So we need to convey to the people the expected highs and lows for our location.

More information on temperatures:

  1. If temperature is falling rapidly(10 degrees in an hour is rapidly), then cold front is passing by. You can expect to receive precipitation.
  2. If temperature has risen steadly and it is warmer then normal, then warm front has passed by. You can expect warm weather until the cold front passes.
  3. Clear nights(no clouds) are colder then cloudy nights.

Dew Point

Basic Explanation

Example of dew on some grassDew point temperature is defined as: "the temperature at which dew begins to form." Dew is the water you find on your grass or car early in the morning. If the temperature reaches the dew point temperature then dew will forms. Some things to know about dew point:

  1. The current dew point will never be higher then the current temperature.
  2. If the temperature is at the dew point and the temperature falls, the dew point must follow.
  3. Higher the dew point temperature more moisture that is in the air. Example: Death Valley(desert) California has low dew points, thus feels dry. Key West Fl. has high dew points, thus feels sticky.

How it is Measured

A device that measures RHDew point is measured exactly how it is defined. It is simply the temperature when dew forms. So weather stations have a device that captures air and holds it in. It then decreased the temperatures until dew forms on its sensor mirror. When that happens it records the temperature and that is the dew point temperature. A more common way to measure dew point is to actually measure Relative Humidity (Rh) and then through mathematical equations figure out the dew point.

RELATIVE HUMIDITY

The ratio of the amount of moisture in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

Units

Since dew point is based off of the temperature at which dew forms the units are the appropriate temperature unit. Relative humidity (RH) as mentioned above is just a percent; there are no units for RH.

Usefulness

Dew point is a very important variable and the most confusing of them all. Maybe with the exception of wind direction. The main thing we need to convey to the users is that RH just tells how full the air is of moisture. IT DOESNT TELL YOU HOW MUCH MOISTURE IS IN THE AIR, NOR DOES IT TELL YOU IF THERE IS LARGE AMOUNTS OF MOISTURE. Dew point is the indicator as to how much moisture is in the air. High numbers (55°F to 75°F+) tells the users that it is "sticky" outside. Values like 30°F or lower show that it is dry out.

Some more facts about dew point and RH

  1. High dew points occur in the tropics (Equator) and low dew points are found in deserts and polar areas.
  2. RH near 100% is an indication of dew, frost (frozen dew) and fog.
  3. In the US, high dew points usually occur ahead of cold fronts.
  4. RH normally reaches 100% during periods of rain.

Tables and Indexes

Heat Index

Have you ever noticed how some days are just plain hot and other days it is hot and sticky, but the temperature is the same?? There is a very simply explanation as to why this occurs. The body is designed to cool itself by sweating. We all know this but what we might not know is that if the sweat does not evaporate then it serves no purpose. That is because evaporation is a cooling process. When water (or other liquids) evaporates it takes some heat with it and thus leaves the original surface cooler. The reason why some days it feel sticky is the water is not evaporating this is caused by the fact that there is already a high amount of moisture in the air so it is hard to evaporate more into the air. So some water stays on the skin and we feel sticky. This is a serious enough that the NWS(National Weather Service) uses something called the Heat Index. The heat index shows what it feels like because of the lack of evaporation off our skin. It is based off the temperature and dew point (measure of moisture). If both variables are low then there is a low heat index, but if both are high then the heat index is also high.

Here is a chart showing the different combinations:

Heat index table

Wind Chill

Another dangerous combinations is that of temperature and wind. The body keeps itself warm by having a warm layer of air surround the skin. This air is held in place by our hair or even our clothes, but if there is wind blowing across our skin then the warm air is constantly being striped away. This cools our body, which is good if it is summer, but if this is combined with low temperature then body could be cooled to dangerous levels. So the NWS also uses what is called a wind chill index. Here is the wind chill table:

Windchill table