1. Weather
Chapter 12

2. Describing Weather
Lesson 1

3. Key Concepts
1: What is weather? 2: What variables are used to describe weather? 3: How is weather related to the water cycle?

4. Vocabulary
Weather Air Pressure Humidity Relative Humidity Dew Point Precipitation Water Cycle

5. Meteorologist: is a scientist who study and predict weather.
What Is Weather
Weather: the atmospheric conditions, along with short-term changes, of a certain place at a certain time.
Temporary behavior of the atmosphere (what’s going on at any certain time)
Small geographic area
Can change rapidly
Everyone talks about weather. “Nice day, isn’t it” , “How about that weather”, talking about weather has gotten so common that we even use weather terms to describe unrelated topics like “That homework assignment was a breeze” or “I’ll take a rain check”.
If you have ever been in caught in a rainstorm on what began as a sunny day, you know the weather can change quickly – sometimes in a just a few hours, but other times you might have the same weather for several days in a row.
Some of the first things that come to mind when you talk about weather are temperature and rainfall. These are important, because they are essential for how you plan your day – what you will wear (temperature) and what you will do (rainfall), but those are just two variables used to describe weather.
Meteorologists are scientists who study and predict weather using several specific variables that describe a variety of atmospheric conditions. Some variables include, air temperature, air pressure, wind speed/direction, humidity, cloud coverage, and precipitation.
Meteorologist: is a scientist who study and predict weather.

6. Weather Variables: Air Temperature
Air Temperature: the measure of average kinetic energy of molecules in the air
Temperature is high – molecules have a high kinetic energy
Molecules in warm air move faster than molecules in cold air
Varys with time of day, season, location and altitude

7. Weather Variables: Air Pressure
Air Pressure: the force that column of air applies on the air or a surface below
Air pressure decreases with altitude
Barometric pressure
Measured using a barometer
Unit is millibars (mb)
The force that a column of air applies on the air or a surface below is called air pressure. So is air pressure higher at the top of the atmosphere or down near Earth’s surface?
You might have heard the term barometric pressure while watching the weather report – barometric pressure refers to air pressure. The tool used to measured air pressure is called a barometer. Air pressure is measured in millibars (mb)
Why do you think knowing the barometric pressures of different areas could help meteorologists predict the weather?

8. Weather Variables: Wind
Wind: air moving from areas of high pressure to low pressure
Wind is noted with direction and speed
Weather vane used to denote direction
direction = where it is blowing from
Anemometer is used to measure wind speed
As air moves from areas of high pressure to areas of low pressure it creates wind. When meteorologists describe weather they usually talk about the direction it is coming from and how fast it is movng in mph. For example a south wind, a south is blowing from the south, the westerlies blow from west to east.

9. Weather Variables: Humidity
Humidity: the amount of water vapor in the air
Measured in (g/m³)
Higher humidity = more water vapor in air
Humidity is what makes your skin feel sticky.
The amount of water vapor in the air is called humidity. Humidity is measured in grams of water per cubic meter of air. When humidity is high, there is more water vapor in the air. On a day with high humidity, your skin might feel sticky and sweat might not evaporate from your skin as quickly.

10. Weather Variables: Relative Humidity
Relative Humidity: the amount of water vapor in the air compared to the maximum amount of water vapor the air could hold at that temperature
When air is saturated, it holds the maximum amount of water vapor possible at that temperature
Measured using an psychrometer
Recorded as a percent
Think about how a sponge can absorb water. At some point, it becomes full and can’t absorb any more water. When air is saturated, it contains as much water vapor as possible . Temperature determines the maximum amunt of water vapor the air can hold.
Warm air can hold more water vapor than cold air.
Relative humidity is measured using an instrument called a psychrometer and is given as a percent. For example, air with a relative humidity of 100 percent cannot contain any more moisture and dew or rain will form.
When air is staturated and can’t contain any more water vapor you get precipitation.

11. Weather Variables: Dew Point
Dew Point: the temperature at which air is saturated and condensation can occur
When air near the ground is saturated, water vapor condenses into liquid
Temperature is above 0° C dew forms
Temperature is below 0° C ice crystals or frost forms
When the sponge becomes saturated with water, the water starts to drip from the sponge. Similarly, when air becomes saturated with water vapor, the water vapor will condense and form water droplets.
,When air near the ground becomes saturated, the water vapor in the air will condense to a liquid.
If the temperature isi above 0 C dew forms. If the temperature is below 0 C, ice crystals, or frost form. Higher in the atmosphere, clouds form.
When temperatures decrease, the air can hold less moisture.

12. Clouds: Formation
As warm air rises in the atmosphere, it cools.
When the air cools enough that the dew point is reached, small droplets of water form.
Thousands of small droplets together block and reflect light – forming clouds.
When you exhale outside on a cold winter day, you can see the water vapor in your breath condense into a foggy cloud in front of your face. This also happens when warm air containing water vapor cools as it rises in the atmosphere. When the cooling air reaches its dew point, water vapor condenses on all those small particles in the atmosphere. Those particles reflect light which is what allows the droplets to be visible and form clouds.
Clouds are water droplets or ice crystals suspended in the atmosphere. Clouds can have different shapes and be present at different altitudes within the atmosphere.
Clouds: water droplets or ice crystals suspended in the atmosphere.

13. Fog: a cloud that forms near Earth’s surface.
Clouds: Fog
Fog: a cloud that forms near Earth’s surface.
Suspension of water droplets or ice crystals close to Earth’s surface.
Reduces visibility

14. Clouds: Stratus Flat, white, and layered Altitude up to 2,000 m
Stratus clouds are uniform gray clouds that usually cover the entire sky. They can form when very weak, upward vertical air currents lift a thin layer of air high enough to initiate condensation. Stratus clouds also form when a layer of air is cooled from below to its dewpoint temperature and water vapor condenses into liquid droplets. Stratus clouds look like a layer of fog that never reaches the ground. In fact, fog that "lifts" off the ground forms a layer of low stratus clouds. Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes accompany stratus clouds.

15. Clouds: Cumulus Fluffy, heaped, or piled up Altitude 2,000 to 6,000 m
Cumulus clouds form as water vapor condenses in strong, upward air currents above the earth's surface. These clouds usually have flat bases and lumpy tops. Cumulus clouds are usually very isolated with large areas of blue sky in between the clouds. Most cumulus clouds form below 6,000 feet and are relatively thin and associated with fair weather.

16. Clouds: Cirrus Wispy Altitude above 6,000 m
Cirrus clouds are thin, wispy clouds that usually form above 18,000 feet. These clouds are blown by strong westerly winds aloft into streamers known as "mares' tails" Cirrus clouds generally move from west to east across the sky and usually "point" to fair weather. Cirrus clouds form when water vapor undergoes deposition and forms ice crystals. Cirrus clouds are thin because they form in the higher levels of the atmosphere where little water vapor is present.

17. Clouds: Nimbus Low level Bring steady rain or snow
Usually combined with other clouds
Nimbus clouds often mean that a thunder storm is brewing: there may soon be thunder and lightning. Get out of the swimming pool! Nimbus really just means a cloud that already has rain or snow falling out of it.

19. Precipitation
Precipitation: is water, in liquid or solid form, that falls from the atmosphere.
Rain: precipitation as droplets of water
Snow: precipitation as solid, frozen crystals of water.
Sleet: precipitation as melted snow that refreezes or freezing rain
Hail: precipitation as large pellets of ice
Recall that droplets in clouds form around small solid particles in the atmosphere. These particles might be dust, salt, or smoke. Precipitation occurs when cloud droplets combine, and become large enough to fall back to Earth’s surface. Examples of precipitation are rain, snow, sleet, or hail.
Rain is precipitation that reaches Earth’s surface as doplets of water.
Snow is precipitation that reaches Earth’s surface as solid, frozen crystals of water.
Sleet may originate as snow. The snow melts as it falls through a layer of warm air and refreezes when it passes through a layer of below-freezing air. Other times it is just freezing rain.
Hail reaches Earth’s surface as large pellets of ice. Hail starts as small piece of ice that is repeatedly lifted and droped by an updraft within a cloud. A layer of ice is added each lifting. When it finaly becomes too heavy for the updraft to lift, it falls to earth.

20. snow falling aloft goes through a warm layer where it melts into rain
snow falling aloft goes through a warm layer where it melts into rain. After exiting this warmer layer, the raindrops then refreeze into pellets of ice as they fall into a deep and or very cold layer of sub-freezing air near and just above the surface of the earth

22. The Water Cycle
Water Cycle: the series of natural processes by which water continually moves among oceans, land, and the atmosphere.
Most water vapor enters the atmosphere when water at the ocean’s surface is heated and evaporates. Water vapor cools as it rises in the atmosphere and condenses back into a liquid. Eventually, droplets of liquid and solid water form clouds. Clouds produce precipitation, which falls to Earth’s surface and later evaporates continuing the cycle.

23. Describe humidity in your own words.
Reflection
Describe humidity in your own words.
Using five complete sentences, describe humidity.