S c i e n c e s Weather, Atmosphere and Meteorology Key words: Atmosphere, Ozone, Water vapor, solar radiation, Condensation, Evaporation, Humidity, Dew-Point Temperature, Cirrus Clouds, Stratus Clouds, Cumulus Clouds,
Composition of the Atmosphere 10000 km 700 km 80 km 50 km 8-18 km Ozone layer T -90 C 0 C 20 C 50 C 100 C
Moisture in the Atmosphere Pressure (atm) Water Vapor (g/m3) 80 km 50 km 8-18 km Ozone layer 10-4 10-3 10-2 10-1 10 0 0 20 40
Moisture in the Atmosphere Condensation is the change of the physical state of matter from gaseous phase into liquid phase. Evaporation is the process by which water molecules in liquid water escape into the air as water vapor.
Moisture in the Atmosphere Humidity of the air is the amount of water in it. Warmer air contains more water than colder one. When the air reaches the maximum capacity, it is saturated. Relative humidity measures the percentage of water (%) in the air. Air Temperature C 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 Relative Humidity (%) 40 45 50 55 60 65 70 75 80 85 90 95 100 Extreme Danger: Exertional Heatstroke highly likely Danger: heat cramps or heat exhaustion likely, heatstroke possible Extreme caution: Heat cramps or heat exhaustion possible Exercise caution; Dehydration likely if people do not drink adequate fluids
Moisture in the Atmosphere Relative humidity measures the percentage of water (%) in the air How to calculate the Relative humidity Psychrometer method The Psychrometer is an instrument composed of two thermometers one thermometer measures the air temperature (the Dry-bulb). The other thermometer has a wet wick on the bottom of the bulb (Wetbulb). Water evaporating from the wet-bulb removes energy and cools the thermometer. If the air is humid, less water evaporates from the bulb and the two thermometer read the same temperature. Dry-bulb The more humid is the air, the closer is the reading between the two thermometers. If the temperature are the same, the relative humidity is 100% Wet-bulb
Moisture in the Atmosphere Measure the dry bulb temperature. Measure the wet bulb temperature. Calculate the difference between the dry bulb and wet bulb temperatures. Find the difference between the wet and dry bulb temperatures on the table. The relative humidity will be at the intersection of the row and column. How to calculate the Relative humidity Psychrometer method Example TDry-bulb = 24 C TWet-bulb = 30 C The difference between Dry-bulb and Wet-bulb temperatures is T=6 C. So the corresponding Relative humidity would be RH=55%
Moisture in the Atmosphere The Dew-Point (DpT) is the temperature at which water vapor condenses into liquid water. The condensed water is called dew. If the Dew-Point Temperature is below 0 C, the air frosts instead of condensing and it is called Frost point. The DpT is found in a similar manner to that of relative humidity.
Moisture in the Atmosphere Measure the dry bulb temperature. Measure the wet bulb temperature. Calculate the difference between the dry bulb and wet bulb temperatures. Find the difference between the wet and dry bulb temperatures on the table. The DpT will be at the intersection of the row and column. How to calculate the Dew Point Temperature Psychrometer method Example TDry-bulb = 24 C TWet-bulb = 30 C RH = 55% The difference between Dry-bulb and Wet-bulb temperatures is T=6 C. The corresponding Dew Point Temperature would be DpT =14 C
Moisture in the Atmosphere Why is it important to calculate the Dew Point Temperature? The dew point is associated with relative humidity. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is almost saturated with water. One of the phenomena associated to the DpT is the formation of fog and clouds. Fog is the condensation and accumulation of water droplets suspended in the air near the Earth's surface. Fog starts to form when the difference between temperature and dew point is generally less than 2.5 C and it can create a visibility hazard.
Clouds Clouds form when the air is cooled down to its DpT. The air cools down as it rises up from the Earth surface. if DpT > 0 C The cloud is composed of water drops if DpT < 0 C The cloud is composed of ice crystals
Clouds Warm air is lighter and less dense than the surrounding air, so it rises and takes on the shape of a round "parcel." Several things happen as the air rises up through the atmosphere: 1 - The atmospheric pressure drops down and the parcel increases in size (PV=nRT). 2 - As soon as the air expands it gets cooler. The parcel cools at a rate of 10 C per 1000 mt of rise. This is called the dry adiabatic lapse rate. 3 - Now the cooling parcel is no longer able to hold the water vapor. The excess water vapor begins to condense into liquid droplets (moisture), this means the air parcel is saturated and has reached its Dewpoint temperature. The higher the parcel rises, the more it expands, the cooler it gets, and the greater the amount of moisture that condenses out of it.
Clouds The air parcel expands and cools The air reaches the DpT and the vapor starts to condense The atmospheric pressure applied on the parcel decreases DpT Level Warmed air rises with shape of a round parcel The Sun heats the ground air
Clouds Clouds form wen the parcel rises up and cools to its DpT There are four situation that make the air to rise: 1 - Convection 2 - Topography 3 - Air convergence 4 - Weather front s Lifting
Clouds Clouds formations fall into three categories Cirrus clouds are very high and are made of ice crystals. They are thin and feathery clouds Stratus clouds are layered and they are found at lower altitudes Cumulus clouds are puffy and cotton-like and develop vertically
Precipitation In order for the water vapor to condense and precipitate, the air must: 1.Cool down till the DPT. 2.Find a condensation nucleus (particle). These nuclei can be dust, salt, bacteria or pollutants (acid rains). In meteorology, precipitation is a product of the condensation of atmospheric water vapor that falls under gravity when the droplet becomes too heavy to be maintained on the air. There are several forms of precipitation as rain, snow and hail. The type of precipitation that forms depends on the air temperature. If it is above 0 C, rain forms; if it is under 0 C, snow forms.
Precipitation RAIN Inside the cloud the air moves the drops of comndensing water up and down constantly. As the droplets collide against each other, they aggregate and get larger. When the drops become too heavy to stay up in the cloud, they fall down to the Earth. The stronger is the updraft the bigger would be the forming drop and the resultant precipitation. Raindrops have sizes ranging from 0.1 mm to 9 mm, above which they tend to break up.
Precipitation SNOW Snowflakes are conglomerations of frozen ice crystals which fall through the Earth's atmosphere. They begin as snow crystals which develop when droplets freeze. Snowflakes come in a variety of sizes and shapes.
Precipitation HAIL Like other precipitation, hail forms in clouds when water freezes on contact with condensation nuclei. 1. Hail forms in a tall cloud with strong updrafts. 2. The frozen droplet moves through the cloud collecting other droplets at the lower part of the cloud. 3. As the hail-stone rises up in the cloud, the outer layer freezes and increases its size. 4. This process continues till the hail-stone falls into the ground. The cross section of a hail-stone shows the history of the formation. Hailstones can grow to 15 cm and weigh more than 5 kg Accretion strata Condensation nucleus