Weather Measuring Change

Lesson Objectives In this lesson you will learn:  identify the function of the following weather instruments used in a weather station: thermometer, psychrometer, aneroid barometer, anemometer, and rain gauge

Measuring Change Measuring Temperature  Temperature is one of the weather conditions measured on a regular basis.  Temperature is defined ad the measure of the average kinetic energy of a sample of matter.  The rate of motion of these particles results in their kinetic energy and is measured as temperature.

1. Thermometer  A thermometer is a device used to measure the outside air temperature.  A thermometer is a device used to measure the outside air temperature.  How does a thermometer work?  The thermometer is based on the idea that matter expands when heated and contracts (shrinks) when cooled. The liquid inside the thermometer behaves this way.  The thermometer is based on the idea that matter expands when heated and contracts (shrinks) when cooled. The liquid inside the thermometer behaves this way.  As the thermometer is heated the liquid inside warms up and expands (takes up more space). Since there is no where to go but up, the liquid rises up the thin tube.  When the thermometer is cooled, the liquid then contracts (shrinks) and moves back down into the bulb.

 Does it matter if the thermometer is placed in the sun or shade?  The thermometer must be placed in the shade. If it is placed in the sun you will not measure the outside air temperature accurately. The direct sunlight will cause the liquid in the thermometer to warm up giving you a false reading.

Measuring Wind Speed Measuring Wind Speed  In winter a high wind speed will make us feel much colder than on a day when the wind is very calm even though the temperature is the same in both cases. This is known as the "wind chill factor."

2. Anemometer  An anemometer (annie mom meeter) is used to measure the speed of the wind.  An anemometer (annie mom meeter) is used to measure the speed of the wind. Weather forecasters' anemometers have the propeller connected to a small generator and computer that can accurately count and calculate the revolutions per minute into kilometers per hour. (km/h).

3. Psychrometer  A psychrometer (sike ro meeter) is a device that measures the amount of relative humidity in the air.  A psychrometer (sike ro meeter) is a device that measures the amount of relative humidity in the air.  Humidity is the measure of the amount of moisture (water vapour) in the air.

Measuring Relative Humidity  Consider that our sample of air is saturated. This means that it is holding its maximum amount of water. Under this condition, the air sample would have a relative humidity of 100%. If we could remove half of the water from our air sample, it would no longer be saturated. The relative humidity would now be 50% since it now holds half the maximum amount of water that it could hold when saturated.

Measuring Relative Humidity  To make this a little simpler we can use an analogy:  Alex has big hands. He can hold 100 jelly beans when his hands are cupped together. If he has 83 jelly beans in his hands, he is holding 83% of the maximum number that he could possibly hold!  Alex has big hands. He can hold 100 jelly beans when his hands are cupped together. If he has 83 jelly beans in his hands, he is holding 83% of the maximum number that he could possibly hold!  This is just like the air. The air can hold a maximum amount of water. If we can measure how much water is actually in the air we can then figure out its percentage or relative humidity.

Measuring Relative Humidity  Consider the following problem: If your sample of air when saturated (relative humidity of 100%) held 5 grams of water (in the form of vapour), how many grams would be in your air sample, if the relative humidity was reduced to 20%?  If the relative humidity is reduced to 20%, there would be only 1 gram of water vapour present in the air sample.

What does humidity have to do with the weather?  Warm air can hold more water than cold air. The warm air is more spread out than cold air and because of this there is more space for water vapor between the warm air molecules. The more water that is in the air the greater the chance for precipitation (rain or snow). So warm, wet air is usually associated with wet, stormy weather. The hygrometer can detect a change in humidity and thus predict a change in the weather.  Warm air can hold more water than cold air. The warm air is more spread out than cold air and because of this there is more space for water vapor between the warm air molecules. The more water that is in the air the greater the chance for precipitation (rain or snow). So warm, wet air is usually associated with wet, stormy weather. The hygrometer can detect a change in humidity and thus predict a change in the weather.

4. Aneroid barometer An aneroid (an er oyd) barometer, commonly called a barometer is a device used to measure surrounding air pressure. Air pressure is the force that air pushes down on the earth. Cold air is denser (thicker and heavier) than warm air. The colder the air the more we can squeeze into the same size air mass. As a result cold air is heavier than warm air. So a high barometric pressure is usually associated with cool, clear, dry air – fine weather.

 If the air pressure begins to drop that means a low pressure system is moving in. A low pressure system is usually warmer, moist air - poor weather on the way.

However, if we experience a sudden change in elevation (altitude) such as when we travel in an airplane, we may notice a pain against our ear drums, or even pain in our sinuses. This pain is due to the imbalance of the air pressure. The pain usually goes away as soon as the pressure balance is once again equalized. Sometimes our ear pressure is balanced suddenly as our ears "pop."

5. Wind vane  A wind vane (weather vane) is a tool that measures the direction of the wind. A typical wind vane would have pointer that can spin with one end larger than the other and compass bearing.

Weather Balloons  Weather vanes can only measure wind direction a few meters off the ground. Large, helium-filled weather balloons are used to measure winds high above the earth's surface. The balloons move with the same speed and direction of the wind. Using computer tracking devices the speed and direction of the balloon and thus the wind can be measured.

Activity  Place a small quantity of water into a can and place on a hot plate until the water begins to boil. The water vapour (steam) will drive out the air from within the can. Place a lid on the can to prevent air from entering. Remove the can from the hot plate and allow the can to cool. As the can cools the water vapour (steam) will condense to liquid water. Observe what happens to the can as it cools. Write a paragraph in which you attempt to explain why the can behaved in the way it did. How is this lab experience related to atmospheric pressure?