In general, most people have their own concept of air temperature. Typical definitions of temperature includes "how hot or cold it is" relating to how they feel. They also associate sunny days with warmer weather and cloudy or rainy days with cooler. Therefore, an improved concept of temperature and associated observation become important.
The temperature of the air is directly related to the amount of energy which is derived from the sun's solar radiation. The higher the temperature, the higher the amount of energy in the air. Temperature is measured using a thermometer with units in degrees Celsius (or Fahrenheit under the imperial system). The higher the temperature, the higher the measurement. Measurements of temperature of the air are taken away from direct sunlight or its reflected radiation. Meteorologists place thermometers in Stevenson's screens that consist of louvers allowing air but minimal sunlight to flow. In other words, air temperature is that measured in the shade. If you stand in sunlight, what you are feeling is the combination of the sun's radiation and the temperature of the air which is warmer than the actual temperature of the air. Try standing on the edge of a shadow and you will experience this difference.
The temperature of the air constantly changes despite the fact that a thermometer seems to remain constant. Think of when you go swimming in calm water such as a pool. As you move through the water, you may notice that certain areas are warmer than others. However, the air temperature does not vary as much because it can mix more rapidly than water.
Temperature can be estimated by how comfortable or uncomfortable you feel at certain temperatures. By associating what you feel with what you observe on a thermometer. However, you must consider the following situations which can affect your perceptions of what the actual temperature.
On days with high solar radiation, it may feel that the temperature is higher than what is actually measured. Conversely, days that are cloudy are generally associated to be cooler. However, it is possible for temperatures to reach above 30 degrees Celsius on cloudy days. The amount of moisture in the air and the amount of wind affect the comfort level and therefore also affects peoples perceptions of temperature. This will be discussed in the section on humidity. The higher the percentage of moisture in the air, the warmer it may feel. Wind affects perceptions of temperature particularly with lower temperatures. The higher the wind strength, the colder it may feel. A person riding a bicycle on a cold calm morning will experience a 'relative breeze' that will make it feel colder than when not moving.
The rate of change in temperature (how fast or slow temperature decreases or increases) on particular days can make you perceive that the temperature is higher than what you estimate. For instance, if the temperature rises rapidly it will feel warmer than what you may estimate. On a day when the temperature remains fairly constant throughout the day, it may feel that the temperature is actually falling. You normally will obtain the best estimates of temperature on days where the temperature rises and falls normally.
Other observations associated with temperature includes observations related to the effects of temperature. For instance, air heated by warmer surfaces causes a shaky, blurred background image. When any forms of ice such as frozen lakes, frost on grass, ice on windows or snow are observed, they indicate a cold environment. Since hail can fall after hot days, it contradicts this rule although the air may have cooled somewhat. Plants and animals are also affected by temperature and therefore will react to more extreme temperature conditions.
Depending on the climate, temperature can change very quickly taking people by surprise. It is not uncommon for temperatures to drop more than 10 degrees Celsius under certain conditions. In the United States, some places have recorded conditions changing from warm and sunny to blizzard conditions within a short time span.
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