Meteorology

1. Meteorology Moisture in the Atmosphere

2. Humidity • Humidity - amount of moisture in the air • When an object (including air) is heated, it expands. • The expansion is creating “empty” space that can be filled with water vapor. Water Holding Capacity Air Temperature

3. Relative Humidity • Relative Humidity - a comparison of the actual amount of water vapor in the air to the amount it can hold at a given temperature. • Expressed as a percent (%) Relative Humidity BrainPop

4. Effect of Temperature on Relative Humidity • Relationship: as air temperature increases, and the amount of moisture remains the same, relative humidity will decrease. Relative Humidity Air Temp.

5. Warm Air & Relative Humidity Air warms and expands Warmer air has a larger water vapor holding capacity As air temperature increases, its ability to hold more water increases, and its relative humidity decreases.

6. Effect of Time of Day on Relative Humidity • If relative humidity decreases as air temperature increases then: • The highest relative humidity will occur at the coolest time of day 5:00 a.m. • The lowest relative humidity will occur at the warmest time of day 3:00 p.m.

7. Dew Point Temperature • Dew Point Temperature - The temperature to which air must be cooled to reach saturation. • If the air temperature and the dew point temperature are equal, the relative humidity is 100%. • If the air temperature falls below the dew point temperature, condensation forms.

8. Sling Psychrometer • Consists of two thermometers. One is the dry thermometer (dry-bulb) and the other has a wet cloth on the end (wet-bulb). • The sling psychrometer is swung around and the wet cloth dries (evaporation) if the air is NOT saturated. • Evaporation absorbs heat and is a cooling process so the wet-bulb temperature should always be equal or cooler than the dry-bulb.

9. Relationships • The drier the air, the faster/more evaporation will occur resulting in greater/more cooling. In turn, the difference in temperature between the dry-bulb and wet-bulb will be greater/more. • The more humid the air, the less evaporation will occur resulting in less cooling of the wet-bulb thermometer. In turn, the difference in temperature between the dry-bulb and wet-bulb will be smaller. • At saturation, the temperature difference between the dry-bulb and wet-bulb would be zero.

10. Cloud Formation Clouds - Tiny droplets of liquid water or tiny ice crystals suspended in air. BrainPop Clouds - Cloud Animation - • Conditions needed for cloud formation: 1.Moisture in the air 2.Cooling air temperature 3. Condensation nuclei - aerosols or dust particles in the atmosphere which provide a surface for water molecules to condense on.

11. Cooling in the Atmosphere • As air rises, the atmospheric pressure surrounding the parcel of air decreases. Cloud • Therefore, the parcel of air expands in volume. 50°F 3000ft • As it expands, it becomes cooler. 2000ft 53°F • When the temperature of the air falls below the dew point temperature, the water vapor will condense and a cloud forms. 1000ft 57°F

12. Examples of Condensation - Dew • Dew is moisture that condenses at the ground level. • The layer of air within about a meter above the ground experiences rapid changes in temperature from day to night. Therefore the temperature may be cooler on the ground than the air above. • Consequently, the air temperature close to the ground may cool to below the dew point temperature for condensation to form. • Moisture, therefore, is observed on grass and on the surface of some objects such as those with metal surfaces.

13. Condensation - Frost • Frost is observed as ice crystals mostly on grass. It forms when dew freezes because the air temperature falls below the level necessary for freezing (from liquid to solid form of moisture). • Frost may also form under the process of deposition when water vapor is cooled so rapidly it forms ice (from gas to solid form of moisture).

14. Condensation - Fog • Fog can be described as a cloud forming very close to or at ground level. It requires air with sufficient moisture to cool or to be cooled to a temperature to achieve condensation. • Fog is more common in valleys and near creeks or streams. It also most commonly occurs overnight and during the morning.

15. Atmospheric Phenomenon - Contrails • Contrails - a trail of condensed water from an aircraft or rocket at a high altitude, seen as white streak across the sky. • The condensation trail left behind jet aircrafts are called contrails. Contrails form when hothumid air from jet exhaust mixes with environmental air of low pressure and low temperature. Cloud formation by a mixing process is similar to the cloud you see when you exhale and "see your breath.”

16. Atmospheric Phenomenon - Rainbows • Rainbows appear in seven colors because water droplets break sunlight into the seven colors of the spectrum. You get the same result when sunlight passes through a prism. The water droplets in the atmosphere act as prisms, though the traces of light are very complex. • When light meets a water droplet, it is refracted at the boundary of air and water, and enters the droplet, where the light is dispersed into the seven colors. The rainbow effect occurs because the light is then reflected inside the droplet and finally refracted out again into the air. BrainPop Rainbows

17. Atmospheric Phenomenon -Crepuscular Rays • Rays of sunlight that appear to radiate from a single point in the sky • Crepuscular rays are produced when hazescatters light. • Rays are most commonly seen when the Sun shines through a break in the clouds.

18. Atmospheric Phenomenon - Sunrises & Sunsets • Sunlight is composed of a multicolored spectrum, just like a rainbow. Combined together, its different wavelengths are perceived as white light when they enter the Earth's atmosphere. • That atmosphere is made up of a mixture of gaseous molecules. These molecules, clumped more densely close to Earth where the atmosphere is thickest, create tiny obstacles for traveling light waves to navigate.

19. Sunrises & Sunsets Continued The light at the longest wavelengths (red, orange, and yellow) sails more easily over these atmospheric speed bumps, while the shorter blue and violet rays get bounced left and right as they journey towards us, in a process called "scattering."